Note: Descriptions are shown in the official language in which they were submitted.
~ss~
IMMOBILIZATION OF RADWASTES IN GLASS
CONTAINERS AND PRODUCTS FORMED THEREBY
BACKGROUND OF THE INVENTION
The disposal of large quantities of toxic materials
such as high level radioactive wastes stored in spent
reactor fuel storage pools, or generated in the
reprocessing of spent nuclear power reactor fuel, or
generated in the operation and maintenance of nuclear
power plants, is a problem of considerable importance to
10 the utilization of nuclear power. It is generally
accepted that the most promising approach is to convert
these radioactive wastes to a dry solid form which would
render such wastes chemically, thermally and radio-
lytically stable.
The problem of dry solid stability of radioactive
wastes is closely related to the safety of human life on
earch for a period of more than 20,000 years. For
example, radioactive wastes usually contain the isotopes
Sr90, PU239, and csl37 whose half lives are 28 years,
20 24,000 years, and 30 years, respectively. These isotopes
alone pose a significant threat to life and must be put
into a dry, solid form which is stable for thousands of
years. The solid radioactive waste form must be able to
keep the radioactive isotopes immobilized for this length
25 of time, preferably even in the presence of an aqueous
environment. The radioactive wastes are produced in high
volumes and contain long-lived, intermediate-lived, and
short-lived radioactive ions and some non-radioactive
ions. These solutions can be highly corrosive and it is
30 difficult, if not impractical, to reduce them to
concentrated forms for further processing or storage.
The two most popular types of commercial reactors
both of which produce low level wastes are the Boiling
112552~3
Water Reactor (B.W,R,) and the Pressurized Water Reactor
(P.W.R.). In a typical Pressurized Water Reactor (P.R.W.),
pressurized light water circulates through the reactor
core (heat source) to an external heat sink (steam
generator). In the steam generator, where primary and
secondary fluids are separated by impervious surfaces to
prevent contamination, heat is transferred from the
pressurized primary coolant to secondary coolant water
to form steam for driving turbines to generate electricity.
In a typical Boiling Water Reactor (B.W.R,), light water
circulates through the reactor core (heat source) where
it boils to form steam that passes to an external heat
sink (turbine and condenser). In both reactor types,
the primary coolant from the heat sink is purified and
i5 recycled to the heat source.
The primary coolant and dissolved impurities are
activated by neutron interactions. Materials enter the
primary coolant through corrosion of the fuel elements,
reactor vessel, piping, and equipment. Activation of
these corrosion products adds radioactive nuclides to
the primary coolant. Corrosion inhibitors, such as
lithium, are added to the reactor water. A chemical
shim, boron, is added to the primary coolant of most
P.W.R.Is for reactivity control. These chemicals are
activated and add radionuclides to the primary coolant.
Fission products diffuse or leak from fuel elements
and add nuclides to the primary coolant. Radioactive
materials from all these sources are transported
around the system and appear in other parts of the plant
through leaks and vents as well as in the effluent
streams from processes used to treat the primary coolant.
Gaseous and liquid radioactive wastes (radwaste) are
~2-
l~ZSS~3
processed within the plant to reduce the radioactive
nuclides that will be released to the atmosphere and to
bodies of water under controlled and monitored conditions
in accordance with federal regulations.
The principal methods or unit operations used in the
treatment of liquid radwaste at nuclear power plants are
filtration, ion exchange, and evaporation.
Liquid radwastes in a P.W.R. are generally segregated
into five categories according to their physical and
chemical properties as follows: clean waste, dirty or
miscellaneous waste, steam generator blowdown waste,
turbine building drain waste, and detergent waste.
Liquid radwastes in a B.W.R. are generally segregated
into four categories according to their physical and
chemical properties as follows: high purity waste, low
purity waste, chemical waste, and detergent wastes.
The liquld radwastes from both types of reactors
are highly dilute solutions of radioactive cations, and
other dissolved radioactive materials as well as
undissolved radioactive particles or finely divided
solids.
A practical process for disposing of radioactive
materials in a dry solids form having high resistance
to leaching and other forms of chemical attack would not
only be suitable for the disposal of radioactive nuclear
wastes, but also for the fabrication of radioactive
sources useful in industry, medicine, and in the
laboratory.
Heretofore, there did not exist any practical,
foolproof means for the safe disposal, storage and
immobilization of pernicious radioactive waste material.
Present day storage containers do not provide sufficient
-3-
~ ZS15~isolation and immobilization of such radioactive material,
sufficient long-term resistance to chemical attack by the
surroundings, and sufficient stability at high temperature.
Currently low level radioactive waste, that is
radioactive waste generated at reactor sites, is disposed
of in the following manner:
A) The dead ion exchange resin containing radio-
active waste is mixed with cement and cast in forty
gallon barrels.
B) The bottoms from evaporators which contain the
radioactive contaminated boric acid and the solutions
used to regenerate the ion exchange columns are mixed
with cement powder and cast in forty gallon metal or
plastic barrels.
C) The filters containing particulate forms of
radioactive waste are usually encased in cement in metal
or plastic barrels.
These cement barrels are transported to low level
radioactive waste sites and buried six feet deep in
the ground. At least one of the sites is in the United
-~ States Eastern States and exposed to substantial rain-
fall. In Europe, these barrels are buried at sea. In
both cases water will first corrode the metal then the
cement and will relatively quickly expose the radio-
active ions for leaching into the ground water or sea
water. Because the U.S. burials are only a few feet
deep, the contaminated water can readily intermix with
streams, lakes and rivers, thus, entering the ecosphere.
The rationale for this practice is the assumption that
upon sufficient dilution the radioactivity becomes
harmless.
Some of the most serious nuclear wastes are cesium
_4_
1~2';i52~
and strontium which are blologically similar to sodium and
calcium, They have thirty year half lives indicating that
they should be isolated from the ecosphere for at least
three hundred years (ten half lives). At Bikini, the
experts assumed that dilution had made the island
inhabitable after decades in which no atomic explosions
were performed, yet when the population was returned to
the island its health was deleteriously effected. It
has since been realized that plants and animal life
biologically reconcentrate these radioactive elements
back up to dangerous levels.
Thus, the "safe" concentration of radioactive waste
must be much lower than accepted values and a more
durable substitute for cement is needed. The present
invention presents a safe alternative to the cement-
solidification of low level waste.
Another route heretofore suggested is the so-called
dry solids approach which involves the fixation of the
waste materials in glasses via mixing with glass-forming
compositions and melting to form glasses. This approach
offers some improvement regarding isolation and decrease
in the rate of release of radioactive elements when the
outer envelopes or containers are destroyed. Further,
such glasses remain relatively more stable at high
temperatures than plastic and are generally more
chemically durable in saline solutions than are metals.
Glasses with high chemical durability and low alkali ion
conductivity suitable for this prior art technique are
formed at very high temperatures, e.g., 1800C and higher.
Prior processes utilizing such high melting glass-forming
compositions are economically unsound and moreover, cause
a dangerous problem due to the risk of volatilization
~5-
1~2St;~
of pernicious radioactive materials. Furthermore, thisprior procedure is restricted to dry solid radioactive
wastes and provides no solution to the high volumes of
liquid radioactive wastes produced by the operation and
5 maintenance of nuclear reactors, by the current practice
of storing spent fuels in pools of water, and by spent
reactor fuel recovery systems.
In view of the overall difficulties of handling
radioactive material, and especially in view of the danger
of volatilization of radioactive material into the
atmosphere, attention has been directed to using glass
compositions having relatively low melting temperatures,
that is to say, using glass compositions with SiO2
contents as low as 27 weight percent. While the problem
of volatilization of radioactive materials is reduced,
it is not completely controlled. Moreover, the resultant
glass composition exhibits greatly reduced chemical
durability and increased ion diffusion rates for the
radioactive materials present therein. The greater this
diffusion rate, the lower is the ability of the glass to
keep the radioactive materials immobilized in its matrix.
For long-term containment of radioactive waste, demanded
under present day standard, these prior glass compositions
are inadequate.
Unlike melting glass containment procedures, the
methods of the present invention provide for the control
of radioactive materials that are prone to volatilization
at high temperatures employed in the containment procedure,
thereby providing for elimination of environmental hazards
due to the possible escape of volatilized radioactive
material in the atmosphere and avoiding the necessity of
.. . . .
1~25S~B
providing elaborate recapture and/or redisposal procedures
and equipment.
SUMMARY OF THE INVENTION
The invention broadly relates to the concentration
and immobilization of toxic solids, such as, mercury,
cadmium, tellurium, lead, insecticides and poisons, and
especially radioactive materials and the like for
extremely long periods of time.
The invention more specifically contemplates novel
glass articles containing said toxic solids and having
high mechanical strength and high chemical durability to
aqueous corrosion and having sufficiently low radio-
isotope diffusion coefficient values to provide protection
to the environment from the release of radioactive
material such as radioactive isotopes, nuclear waste
materials, etc., and which are concentrated, immobilized
and encapsulated therein and are suitable for burial
underground or at sea. The glass articles are made by
depositing the radioactive solids in a glass container
collowed by heating the container to drive off non-
radioactive volatiles and to drive off non-radioactive
decomposition products The glass container may be made
of porous glass and may or may not contain a porous or
non-porous glass packing which can preferably be
particulate or can be relatively large as a single or
few glass rods. The glass articles of this invention
have a composition characterized by a radiation activity
illustratively above one microcurie, generally above one
millicurie, preferably greater than one curie, per cubic
centimeter of said article. (When highly dilute
radwastes are treated pursuant to this invention for the
purpose of concentrating and immobilizing the radwaste
1~25~,Z8
for storage, the radiation activity of the resulting glass
articles may not reach the level of one millicurie per cubic
centimeter of the glass article and may remain below
1 microcurie per cc., when it becomes expedient for other
reasons to collapse and seal the glass container. In
concentrating and immobilizing radioactive materials in
diluate radwastes, the glass container can be loaded up
to 10 microcuries per cc. or more but usually is loaded
up to 1 microcurie per cc. of said glass article.) The
radioactive material is in the form of radioactive
- solids that are sealed within the glass container. In
one aspect, the amount of radioactive material contained
in the glass articles is at least 1 ppb (part per billion
based on weight), in solid form of a plurality of
radioactive elements, generally at least five, and
preferably at least ten of the radioactive elements
listed hereinafter. Preferably the novel glass articles
should contain at least 75 mol percent SiO2, most
preferably greater than 89 mole percent SiO2.
From a practical standpoint, the upper limit of
radioactive material contained in the glass articles
will be governed, to a degree, by such factors as: the
concentration, form and type of radioactive material
encapsulated in teh glass article, by the volume fraction
of pores, if any, in the glass container, by the amount,
if any, of glass packing in the glass container, by the
various techniques employed to encapsulate the radio-
active material in the glass container and other factors.
Radioactive materials which can be concentrated,
encapsulated and immobilized in the glass container
pursuant to this invention include radioactive elements
(naturally occurring isotopes and man-made isotopes
--8--
1~2SS2~
existing as liquids or solids dissolved or disposed in
liquids or gases), in combined or uncombined form (i.e.,
as anions, cations, molecular or nonionic, or elemental
form) such as rubidium, strontium, the lanthanides, e.g.,
La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb,
Lu, cobalt, cadmium, silver, zirconium, molybdenum,
technetium, niobium, ruthenium, rhodium, palladium, the
tellurium, cesium, barium, francium, yttrium, radium
and actinides, e.g., Ac, Th, Pa, U, Np, Pu, Am, Cm, Bk,
Cf, Es cations and elements. Especially suitable in the
p~act~ce of the invention are radioactive wastes from
nuclear reactors, spent reactor fuel reprocessing,
spent fuel storage pools or other radioactive waste
producing processes.
The invention can be practiced in many ways.
Illustratively, one facile yet highly effective way is to
deposit the radioactive materials, e.g., radioactive
nitrates, as a solid in a non-porous glass container,
such as a glass tube, having at least one opening, and
~0 then followed by heating the container to drive off
water and/or other non-radioactive volatiles, if present,
and then to collapse the walls of the tube and seal it
around the deposited radioactive solids. The heating
step can be carried out in such manner that solids, such
as nitrates, deposited in the tube decompose to provide
non-radioactive gases, such as nitrogen oxides, which
are removed from the glass tube before sealing.
Alternatively, a non-porous glass container can be
closed at one end and at least partially filled with a
3Q packing such as porous glass particles, such as, porous
glass powder, or tiny glass spheres or silica gel in
particulate or other form. The fluid containing radio-
_, __ ~,, ., , _ _ _ , .. .. .. _ .. _ _ .. , _ ... . _ . _ ._ _ ._ . . .. _ , _ _ , .. , ,, , ,, ,
,_ _ ,
1125iS~B
active material is then poured into the container to fillthe interstices between the glass particles followed by
heating to drive off non-radioactive volatiles with or
without decomposition of components, such as nitrates,
in the fluids and ultimately to seal the glass container
around the radioactive solids deposited on the glass
particles and in the pores of porous glass particles, if
present, contained by the container. In this instance
the contained glass particles provide surfaces on which
the solids can be deposited and also act to control the
volatilization to prevent eruption of fluid out of the
tube during the heating step. The porous glass particles
provide additional interior surfaces within the pores of
the particles for deposit of additional dissolved solids
in the fluid as well as external surfaces for deposit
of dispersed solids.
In another embodiment a non-porous glass container
having open upper and lower ends can be filled with porous
or non-porous glass particles which are held in the glass
container by means of a porous structure, such as glass
~ool or a porous glass disc or rod in the lower portion
of the container to support the glass particles in the
container. The fluid containing dissolved and/or
dispersed radioactive sol~ds is then poured into the
upper or lower end of the container and passes through
the bed of glass particles which act as a filter to
remove dispersed radioactive solids from the fluid. The
glass particle bed can contain glass particles having
silicon-bonded cation exchange groups, such as, alkali
metal oxide or ammonium oxide groups.
The porous cation exchange glass particles remove
dissolved radioactive cations from the fluid. The fluid
-lQ-
~zss~
can he passed through one or more such beds usingconventional techniques for multibed filtration and/or ion
exchange until the fluid has been cleansed of radio-
activity to the desired level. When the filtration-ion
exchange glass particles become loaded or when, for some
other reason, it is no longer desired to further utilize
them, the beds and the container containing them can be
heated to drive off water and/or other non-radioactive
volatiles or gases such as decomposition products, e.g.,
nitrogen oxides, and then to collapse the pores of the
porous glass particles containing the radioactive cations,
to fuse the glass particles together thus entrapping
radioactive solids and/or cations deposited on the inner
and outer surfaces of the particles, and then to collapse
the glass container and seal it around all of its
contents to encapsulate the entire mass into a
substantially solid leach-resistant structure suitable
for long-term storage.
In stiil another embodiment the glass container
itself can be made of porous glass and the radioactive
fluid is introduced into the interior of the container
and caused to permeate through the pores of the glass
from the interior walls to the outer walls of the glass
container. The insoluble radioactive solids originally
dispersed in the fluid are deposited on the interior wall
of the container and the dissolved radioactive solids are
disposed in the pores of the glass container where they
can be deposited by various techniques, such as those
taught in U.S. Patent No. 4,110,096. The glass container
can then be heated to drive off volatiles as described
above, to collapse the pores of the glass container and
ultimately to collapse the glass container and seal it
--11--
112~S;~3
thereby encapsulating the radioactlve solids within the
glass structure. Prior to heating, the outer wall
surfaces of the container can be washed to remove deposited
radioactive solids from the outer surface layer of the
glass container so that ultimately a radioactive-free outer
clad is provided after heating to collapse the pores and
the container.
The non-porous glass compositions when used herein
for the glass container and/or for the glass packing
wlthin the container are of any suitable type, but
preferably are strong, durable, leach-resistant and
chemical-resistant. Any glass composition having these
properties can be used such as high silica glasses, for
example, Vycor and Pyrex. Suitable glasses contain at
least about 70%, preferably at least about 80~, most
preferably at least about 93% silica.
Sultable glass compositions which may be utilized as
porous glass compositions in the novel methods generally
contain sio2 as a major component, have a large surface
area. ln the practice of various embodiments of the
invention the sio2 content of the porous glass or silica
gel desirably is at least about 75 mole percent SiO2,
preferably at least about 82 mole percent SiO2. Such
glasses are described in the literature, see U.S. Patent
Numbers 2,106,744; 2,215,036; 2,221,709; 2,272,342;
2,326,059; 2,336,227; 2,340,013 and 4,110,096.
The porous silicate glass compositions can also be
prepared in the manner described in U.S. Patent No.
3,147,225 by forming silicate glass frit particles,
dropping them through a radiant heating zone wherein they
become fluid while free falling and assume a g~nerally
spherical shape due to surface tension forces and there-
,12~
SZ~
after cooling them to retain their glassy nature and
spherical shape.
In general, the porous silicate glass ean be made by
melting an alkali-borosilicate glass, phase-separating it
into two interconnected glass phases and leaching one of
the phases, i.e., the boron oxide and alkali metal oxide
phase, to lea~e behind a porous skeleton eomprised mainly
of the remaining high silieate glass phase. The principal
property of the porous glass is that when formed it
contains a large inner surface area eovered by silicon-
bonded hydroxyl groups. We prefer to use porous glass
made by phase-separation and leaehing because it can be
made with a high surface area per unit volume and has
small pore sizes to give a high eoncentration of silicon-
bonded hydroxyl surface groups, and because the processof leaching to form the pores leaves residues of hydrolyzed
siliea groups in the pores thus increasing the number of
silieon-bonded hydroxyl surfaee groups present. The
porous borosilicate glass when used as packing may be in
the form of powder as for use in chromatography eolumns
or in a predetermined shape sueh as plates, spheres or
eylinders.
It is preferable to utilize a glass eomposition in
the container whieh will produee a elad or envelope that
~5 is low in leaehable eomponents, sueh as, alkali metals
or boron. In the event that this is not possible or
practical it is preferred to then insert the glass
eontainer before or after eollapse into a seeond glass
container which has a composition eontaining no or low
amounts of alkali metals or boron or other leachable
eomponents. It is most preferred, that very high silica
-13-
1~255;2~3
glasses are employed in both the glass container and the
glass packing.
When it is desired to avoid cracking of the glass
container in the case where a glass packing, such as glass
particles, spheres or a glass rod, are disposed within the
glass container, it is preferred to utilize, as the
container glass, a glass, which, after the decomposition
step, has a glass transition temperature of up to 100C
higher than the glass transition temperature of the glass
formed from the glass packing and solids deposited in and
on the glass packing. It is also preferred for this
purpose to utilize as the glass container a glass which,
after the decomposition step, has a thermal expansion
coefficient which is up to about two times 10-6 per
degree Centigrade less than the thermal expansion
coefficient of the glass resulting from sintering of
the glass packing and solids deposited in or on said glass
packing. In determining the glass transition temperatures
and thermal expansion coefficients, the amount and type
of solids deposited in the pores of the glass container,
when a porous one is used, and of solids deposited in
the pores and on the outer surfaces of porous glass
packing when used and of solids deposited on the outer
surfaces of non-porous glass packing, when used, can
have a considerable effect on the glass transition
temperatures and thermal expansion coefficients and
should be taken into consideration. It is also preferred
to regulate the cooling of the composite glass container
and contents, resulting from the depositions and
sintering step, such that the rate of cooling is as
nearly the same as possible throughout the composite
glass container and contents. While cracking has been
-14-
552~
observed in certain instances, it has not prevented theaehievement of the objects of this invention, namely, the
immobilization and isolation from the environment of
radioactive solids from radioactive wastes containing such
solids in dissolved and undesolved form.
According to one aspect of this invention there is
provided emthod of preventing the dissemination of toxic
material to the environment which eomprises providing an
admixture of toxie material and glass packing in a hollow
glass eontainer of high siliea eontent, and heating said
- glass eontainer to collapse the surfaces thereof and to
provide a monolithic glass eladding completely about the
said admixture.
In aecordanee with another aspeet of this invention
there is provided a glass article eomprising a non-porous
glass core portion and a non-porous non-radioaetive doped
glass elad portion enveloping said eore portion, said core
portion eontaining radioaetive materials entrapped and/or
immobilized therein, and said elad portion having a
thermal expansion eoeffieient lower than the thermal
expansion eoefficient of said core portion.
In aceordanee with another aspect of this invention
there is provided a method of preventing the dissemination
of toxie material to the environment which comprises
introdueing toxie material into a hollow glass eontainer
of high silica eontent, heating said glass eontainer to
eollapse the surfaees thereof and to provide a monolithie
glass cladding eompletely about the said toxie material.
DETAILED DESCRIPTION OF THE INVENTION
In one method of the present invention, a radioaetive
material is deposited as a solid in a hollow glass
eontainer having at least one opening. The radioactive
-15-
1~2'5S2~
material is deposited from a fluid which passes
continuously through the glass container or which is placed
batch-wise into the container. The fluid may contain
dissolved radioactive materials, particluate radioactive
materials, or both types of radioactive materials. The
fluid may either be a gas or a liquid or both. The radio-
active materials, whether particulate or dissolved in the
fluid to be treated, can be deposited on a non-porous
glass or on a porous glass having an interconnected porous
structure, When a porous glass is used, the pores are
usually smaller than particulate radioactive materials
in the fluid thereby preventing passage of the particulates
into the interconnected pores. In this case, the
particulates are deposited on the inner wall surfaces of
the porous glass container. Radioactive materials which
are dissolved in the fluid or which are gaseous radio~
active materials, pass into the pores of the glass and
are entrapped within the porous structure by either
reacting with the glass, undergoing a cation exchange
reaction with the glass, or by precipitating within the
pores of the glass. Whether a porous glass or a non-
porous glass is used, after the radioactive material is-
deposited in the hollow glass container, the deposited
radioactive materials are sealed within the glass matrix
by collapsing the walls of the container. Collapse of
the walls of the container is achieved by heating the
container while: a) applying a vacuum to the inside
of the container, b) applying external pressure to the
container, for example by placing a weight on the
container or by increasing the gas pressure outside of
the container, and c) by combinations of methods a) and
b). Where radioactive material is deposited in the pores
-16-
~2;;~SZ~
of a porous glass, the container is heated to collapsethe pores prior to collapse of the walls of the container.
The glass container is hollow and has at least one
opening. The most preferred container for processing
liquids is one in a shape of a test tube. Where the
container has more than one opening, as in the case of a
hollow glass rod (or a tube) one or more of the openings
may be plugged with a glass stopper to prevent the fluid
from escaping during filling in a batch operation. For
continuous operation, a glass tube having an opening at
each end is preferred. In the latter case, both
openings may be plugged with a porous glass stopper. The
greater the volume of the fluid to be treated, the more
preferable the continuous operation becomes. Examples
of other configurations of the glass container which are
suitable for the purpose of the present invention are
U-shaped, beaker-shaped, box-shaped, etc.
The simplest embodiment of the present invention
merely involves deposition of the radioactive materials
in a non-porous glass container followed by collapse of
the walls of the container and burying the resulting
glass article underground or at sea. For example, the
glass container can be test-tube shaped and made of a
non-porous glass such as a Vycor* glass (trademark for a
heat and chemical resistant, low thermal expansivity
glass of Corning Glass Works). In another embodiment
of the present invention, the radioactive material is
deposited in a porous glass container. In still
another embodiment, the radioactive material can be
deposited in a non-porous glass container having a
second glass, e.g., a glass packing, disposed within
*trademark
-17-
~lZ'S~Z~
the container. In yet another embodiment of the present
invention, the radioactive material is deposited in a
porous glass container having a second glass or glass
packing disposed within it. In the last two mentioned
embodiments, the second glass may be a non-porous glass
or a porous glass. The second glass, whether porous or
non-porous, may be a glass preform of any suitable shape
(e.g., rod shaped, rectangular shaped, particulate,
spheroid, etc.) for fitting into and at least partially
filling the glass container. The second glass, however,
is preferably in the form of particles such as spheres.
A preferred embodiment of the present invention utilizes
a non-porous glass tube having porous glass particles
therein.
- Nuclear Waste in a Non-Porous Tube
A non-porous hollow glass container made of Vycor or
silicate glass is at least partially filled with a fluid
containing radioactive materials. The container
preferably has one opening therein which is plugged with
20 a porous-glass plug. In the case where the fluid is a
liquid, e.g., water, the glass container is then heated
to evaporate the fluid to dryness so as to precipitate
the radioactive materials on the inside walls of the
glass container. Temperatures slightly above the boiling
point to about 50C above the boiling point of the fluid
can be used. Lower temperatures can be used to dry the
fluid when a vacuum is applied to the interior of the
glass container. The glass container is further heated
and at about 400 C the radioactive salts originally in
the nuclear waste, e.g~, the radioactive metal nitrates
decompose or are calcined to form the corresponding
oxides, e.g., the radioactive metal oxides. The non-
-18-
5;~3
radioactive gaseous decomposition products, e.g,, nitrogenoxides are driven off by the heating and the porous plug
acts as a barrier to keep the nuclear waste from leaving
the glass container. The glass container is further heated
until it collapses to trap the precipitated, crystalline
nuclear waste within the sealed container. Before adding
the fluid to the glass container, silica and alumina can
be added to the fluid to create a calcined material
upon heating. Calcining of nuclear waste materials in
metallic containers is well known. The procedures and
operating conditions utilized in calcining in a metallic
container are also applicable when calcining in the glass
containers of the present invention and such teachings
are incorporated herein by reference. The glass tube
typically collapses at around 1300C. Further details
on the drying procedure, the trapping of radioactive
decomposition products, and collapsing of the glass
container are presented below.
Nuclear Waste in a Porous Glass Container
Fabrication of the porous glass container used in the
process of the present invention may follow any of the
available methods used by one practiced in the art to form
porous glass in any desired shape, such as cylindrical or
rectangular. It preferably has a composition containing
more than 75% silica. We prefer to form the porous
glass according to the methods disclosed in U.S. Patent
4,110,096. For example, a glass composition containing
silica, boron trioxide and two alkali metal oxides (such
as Na2O and X2O) is melted and drawn into long rods or
tubes. By suitable heat-treatment, these rods or tubes
are phase-separated into two phases; one phase, a silica-
rich phase containing also small amounts of B2O3 and
--19--
1~2SSZ3
alkali metal oxide and a silica-poor phase which contains
greater amounts of B2O3 and alkali metal oxide. The heat-
treated rods or tubes are then immersed in a suitable
leaching solution in order to dissolve and remove the phase
containing the lower silica concentration. Removal of
this phase and subsequent washing yields a porous glass
characterized by a sio2 content greater than 90 mole
percent which is ready for use as the glass container for
the encapsulation of the dissolved or gaseous radioactive
material pursuant to this invention.
The invention is further described with reference to
a hollow porous glass container which is test-tube
shaped. A solution containing dissolved radioactive
materials and particulate radioactive materials such as
metallic precipitates of the platinum metal family which
are typically present in nuclear waste solutions from
spent nuclear fuel reprocessing stations is poured into
a porous glass test tube. The solution impregnates the
walls of the tube and in thls way disposes dissolved
radioactive materlal in the form of dopant within the
walls of the porous glass test tube. On the other hand,
the particulate radioactive material because of their
particle size do not go into the walls of the glass
container but instead are deposited on the inner wall of
the tube by settling, or filtration. Deposition upon
the inner walls of the tube occurs by filtration and by
subsequent evaporation of the fluid.
The radioactive material that was originally in
solution on the other hand, is disposed inside the pores
of the glass tube in solution, as a nitrate for example.
The opening in the tube than can preferably be plugged
with a porous or non-porous plug. Then, the dissolved
-20-
1~2~1~2~
radioactive material is deposted ln the pores byprecipitation out of solution by methods such as those
disclosed in U.S. Patent 4,110,096. Thus, the precipitation
may be caused by cooling the glass container (thermal
precipitation), by chemical precipitation and combinations
thereof. Chemical precipitation includes precipitation
by the common ion effect to reduce solubilities, and
cause precipitation, of the dissolved radioactive materials.
It also includes the exchange-of-solvents technique for
reducing solubilities to cause precipitation. In this
method, the porous glass test tube can be immersed in a
solvent in which the soluble radioactive material in the
pores is less soluble. The addition of a suitable
precipitant which reacts with the dopant, or dissolved
radioactive materials, in the pores or causes a suitable
change in pH, is also a means of chemical precipitation.
Precipitation can also be caused by evaporation of the
fluid from the pores, preferably under vacuum and at
temperatures around room temperature or below.
Precipitation methods other than that involving the
evaporation of solvent as the sole means of precipitation
are used when it is desired to obtain higher strength
glasses consistently. Combinations of precipitation
techniques can be used. A preferred combination of
precipitation techniques is thermal precipitation and
precipitation by exchange of solvents.
Deposition of the dissolved radioactive materials
within the pores can also be accomplished by a cation
exchange reaction with the glass. Manufacture of
suitable glasses for the cation exchange reaction
together with a detailed description of the cation
exchange process are disclosed in the above-identified
-21-
1~2SSiZ~
concurren~ly ~iled application entitled: I'Fixation ByIon Exchange of Toxic Materlals In A Glass Matrix", herein
incorporated by reference. A porous glass container having
cation exchange capabilities is particularly suited to a
continuous process. The solution is passed into the
interior of the container, through the porous walls for
the cation exchange and entrapment of the dissolved
radioactive material, and then the remainder of the
solution, i.e., the solvent such as water, passes out
through the exterior wall of the container.
Subsequent to the deposition step, the outer wall of
the porous glass tube can be washed so that the precipitated
radioactive material disposed within the pores of the
outer surface layers of the porous tube is removed. The
washing step is preferred when it is desired to produce a
radioactive article free of, or has a lower amount of,
radioactive material in its outer surface layers and is
not essential in the broad sense of this invention. The
solvent of the solution is then removed preferably without
migration of the radioactive material within the pores.
This can be accomplished by the drying procedure described
in u.s. Patent 4,110,096. Typically, the porous glass
container is placed in a drying oven and heated to an upper
drying temperature under a vacuum at a rate less than
100C/hour. However, in some circumstances it is desirable
to use a hlgher heating rate to increase throughput of
articles through the processing system.
~ fter the tube is dried, two forms of deposited
radioactive materials are obtained: 1) the originally
undissolved particulates which are disposed in the interior
space defined by the inner walls of glass tube, and 2) the
originally dissolved materials which are deposited in the
-22-
~lZS52~
pores of the walls of the glass tube.
Following the drying step, continued heating of theglass container causes decomposition of the radibactive
materials both deposited in the pores and outside of the
pores. For example, the radioactive material goes from
its nitrate form (or whatever its original form was) to
its oxide or phosphate or silicate form with nitrogen oxide
decomposition products being driven off. If it is desired
to encapsulate radioactive gases, e.g., krypton or iodine,
these can be introduced into the pores of the porous glass
at a temperature, e.g,, 50 to 150C, below the glass
transition temperature, Tg, of the glass tube including
the deposited materials.
The heating is continued until the pores of the porous
glass collapse. Upon collapse of the pores, the radio-
active material which was deposited from solution and
includlng radioactive gases are totally trapped within the
matrix of the glass. It is chemically bonded to the glass
and physically enclosed within the glass. The heating
can be continued without great risk of losing the radio-
active material within the glass by vaporization because
it is now buried inside the glass itself. While
continuing to heat the tube, a small imposed pressure
difference between the inside of the tube and the outside
of the tube can be used to collapse the tube. The pressure
inside the tube is made a little lower than the pressure
outside of the tube tby means of a vacuum) for
progressively collapsing the tube into a rod. External
- pressure from a weight placed upon the tube for example,
can also be used to collapse the walls of the container.
Upon collapse of the tube (or other glass container) the
radioactive material which was originally particulate in
-23-
~12~C;2~
nature is trapped inside the resulting sealed glass vessel.The originally undlssolved particulate radioactive material
which is now trapped within the glass vessel can be in
the elemental form of a metal, a metal salt, metal oxide
or other metal forms. The particulate radioactive material
which decomposes as described below, would be in the oxide,
phosphate, or silicate form. The collapsed tube is in rod
form wherein two kinds of radioactive material are trapped:
1) one which was originally soluble in the solution and
which eventually became chemically attached to or entrapped
- within the glass, and 2) an insoluble, solid radioactive
material which eventually became encapsulated by the glass.
Thus, upon collapse of the hollow glass article, a glass
article is obtained which comprises originally undissolved
solid radioactive materials entrapped in its core. The
originally dissolved radioactive materials are entrapped
and immobilized ln the glass matrix surrounding the core.
Utilization of a hollow, porous glass container,
particularly one which is tube-shaped, permits several
advantages over use of a porous glass rod. For example,
one advantage of a porous glass test tube-shaped container
is that two surfaces (an inner wall and an outer wall
surface) are utilized. The solution which contains the
nuclear waste is placed inside the tube to trap the
particulate radioactive materials and permeate the pores
with the solution containing dissolved radioactive materials.
A solution which will cause dissolved radioactive material
to precipitate within the pores can be introduced through
the outside surface of the porous tube. For example,
potassium silicate reacts with many of the nuclear wastes
(e.g., iron). The high pH caused by the potassium silicate
causes precipitation (e.g., iron oxide) within the pores.
-24-
~'~ 2S5Z~
In the case of ruthenium, ruthenium sillcate precipitatesand so forth. By bringing in the material from the outer
surface of the porous tube, control of the precipitation
of the nuclear waste inside the pores of the porous glass
is enhanced.
A further advantage of the tube configuration is
that dissolved radioactive materials which do not
precipitate during the precipitation step can be sucked-
to the inner wall of the tube. Thus, the tube is filled
completely with the fluid containing the radioactive
- material. It i~ then placed in clean water or a second
solution and a vacuum is applied to the inside of the tube.
In so doing, some dissolved radioactive material may not
be precipitated by a decrease in temperature, by
insolubllity in the second solvent or by chemical activity
with the second solvent, The dissolved radioactive
material which does not precipitate for any of these
reasons is sucked to the inner wall of the tube, with the
outer wall staying clean. This flow to the inner wall
causes a distribution of the dopant, i.e., originally
- dissolved radioactive material, which is higher on the
inside surface of the tube. When the tube is finally
collapsed into a rod this region of high concentration
of radioactive materials is entrapped into the total glass
system. A high concentration of nuclear waste on the
outside of the glass article can be thereby avoided.
Another advantage of the tube configuration is the
entrapment of any radioactive gases produced by
decompositionofthe radioactive materials during the
drying step. For example, if ruthenium tetraoxide is
produced from the radioactive materials within the pores:
it can be aspirated from the center of the tube, out of
-25-
~2~cj2~
~he d~ying/collapsing furnace and into another tube ofporous glass which is at a lower temperature. The radio-
active gases are filtered by the second porous glass tube.
The fumes react with the silicate of the glass and
precipitate within the pores of the glass. For example,
rubidium will be reduced from the tetraoxide to a lower
oxidation state and precipitate within the pores. The
second tube serves as a micropore filter. Its pores are
t~en collapsed followed by the collapse of the walls of
the tube to thereby entrap and immobilize the gaseous
radioactive materials in a sealed glass matrix.
~ s can be seen, the porous glass tubes can be used to
absorb radioactive gases from nuclear waste disposal
systems which do not utilize porous glass containers.
Thus, radioactive fumes from other nuclear waste processing
systems can be treated in a porous glass filter. The
radioactive fumes may contain particulate radioactive
materials and gaseous radioactive materials. Non-radio-
active materials, e.g., water, etc., will pass right
2~o through the porous glass filters whereas the nuclear waste,
for example, ruthenium tetraoxide or cesium would be
trapped inside the porous glass filter. When the filters
begin to lose efficiency, the filter itself is heated to
collapse the pores and the walls of the porous glass
25 filter. Both the particlate radioactive materials which
were suspended in the gas and the gaseous radioactive
materials are thus entrapped and immobilized in the glass
matrix. When using the porous glass container as a filter,
it need not be in the test tube configuration. Flat
configurations, for example, are equally as good, or
even better in some cases, than the test tube configuration.
-26-
5~3
~ nother ~dvantage of using a porous glass container
~as opposed to a porous glass rod~ is that processing time
is reduced for a given thickness of porous glass. For
example, the time required to fully impregnate a porous
glass rod with a 10cm radius is the same as that required
to fully impregnate a porous glass container whose inner
radius is 10cm and outer radius is 30cm. The wall of the
glass container is 2Ocm thick but impregnation occurs from
both the inside and outside surface, to a depth of 10cm.
In the case of the glass rod, the eross sectional area of
the glass is only 100 ~r (or about 314) sq. cm. However,
for the glass eontainer, the cross sectional area of the
porous glass is 800 lr or approximately 2514 sq. cm. or 8
times greater than that of the rod. Thus, in the same
amount of time, a much greater amount of glass ean be
impregnated. This advantage also eomes into play during
the drying step, and during deeomposition. For
eomparison purposes, it is assumed that there is zero
shrinkage for the porous glass. When the glass eontainer
is eollapsed, the final diameter of the resulting rod is
approximately 28 em. On the other hand, the final
diameter of the glass rod whieh had a 10 em radius is
approximately 20 em. Thus, a larger end produet
eontaining more radwaste is obtained in the same amount
of proeessing time when the glass eontainer is used.
Particulate Glass Inside Glass Container
A particulate glass whieh is either a non-porous
glass or a porous glass having an intereonneeted pore
structure or mixture of both can be disposed within the
glass eontainer. Formation of non-porous particles is
well known in the art, Non-porous glass particles of
either conventional compositions or modifications thereof
-27-
~ . _.. ~ ... . , . _.. _ _ . . _ . _._ , _ . .. .. , .. . _ . ,.. ... _ . . .. .. _ ~ . , . _ _ , _. .. .. .
. . . . ... . ...
1~2~5Z~
can be used. Poro~s glass particles ca~ be made fromglasses having compositions simiiar to those used to make
the porous glass container. The porous glass is
preferably formed according to the compositions and
methods disclosed in U.S. Patent 4,110,096.
To make a powdered glass, the molten glass can
be poured direc-tly into cold water to crack it and break
it into little pieces. Alternatively, the molten glass
can be pulled into rods or cast into any desired shape.
In the latter situation, the glass is broken in a
- milling machine. The glass pieces are sieved to provide
glass particles of the desired particle size. The
sieved glass particles can then be passed through a flame
to form them into little spheres. The advantage of
making spheres instead of just using the sieved glass
particles which have random irregular shapes is that it
provides a more uniform and greater density of packing
in the glass container. Thus, two kinds of glass
particles can be produced. One is just broken and
sieved and therefore has elongated grains of random
irregular shapes. The other is broken, sieved, and
remelted by going through a hot zone and then rapidly
cooled to produce glass particles in the shape of tiny
spheres.
The particles thus obtained are non-porous and can
be used in the embodiment wherein the radioactive
material is deposited on non-porous glass particles
within the glass container. To make the non-porous
glass particles porous, they are reheated at approximately
550C for about two hours. They are thus phase-
separated and then are leached; see U.S. 4,110,096.
The finished product is a wet, particulate porous glass
-28-
~lZ55~3
with pores interconnected throughout each particle whichcan be used in the process of the present invention.
However, by heating the porous glass ln excess of 100 C,
the water is removed and a dry particulate porous glass
is obtained which is a flowable, powdery product.
If one uses elongated grains, the packing of the
glass container may not be very efficient, typically
60% may be voids and the particles (or grains) may
represent only 40% of the interior volume of the glass
container. On the other hand, if one uses round spheres
they tend to pack better and one can get packing rates
of 60% or more. The ideal for packing of the spheres
- would be close to a packing rate of 80%. Improvement
in packing can be obtained by applying pressure. Glass
particle slzes ranging from 5 micron to 5 mm, preferably
50 micron to 1 mm, can be used.
The loading of the glass container can be accomplished
in any suitable manner desired other than those disclosed
hereinabove.
There are several approaches that can be used in
the drying step. First, some glass wool or a porous
glass disc or some other kind of porous top can be placed
on top of the glass particles to prevent them from moving
vertically when gases are driven off from the contained
radwaste. Also, an adequate space can be left at the
top of the container so that the glass pa~ticles can
move up as the gases are driven off and then eventually
down again after gas flow has stopped. It is also better
to dry the contained and glass particles by having a
relatively small heat zone that is brought downwards
from the top to progressively drive off the gases from
top to bottom. Otherwise if heat is very deep or applied
-29-
~255Z~
at the bottom bolling can occur inside the container nearthe bottom which can cause the glass particles to be
blown out at the top. Desirably, the upper part of the
container should be kept above 100C This procedure can
go relatively fast so that by heating from the top, and
bringing it down, all the water in the container can be
eventually evaporated. Then, when a layer of porous
glass particles at the top is dried, it is kept in the
temperature range of 100-150C to prevent the escape of
other poisonous gases through it. Non-radioactive
nitrate decomposition vapors in the container can escape
through the dry porous layer while the cesium and the
sodium a~d other radioactive isotopes such as cadmium
are caught in this porous sieve. Once the water has
been vaporized from the whole column, it can be heated
to a temperature of the order of 400C fast enough to
prevent the distillation of radioactive nitrates. At
temperatures of this order, decomposition starts and
~itrous oxide fumes are driven off. Again, ruthenium
tetraoxide can be a problem for it must be kept from
escaping the top of the container by keeping the porous
glass particle layer in the top hot enough so that
steam will escape but cold enough that Ru04 will stay
down in the container while the nitrate decomposition is
going on. As long as the nitrate decomposition fumes
keep coming off, the material will be under high
oxidizing conditions and there is not much chance that
ruthenium will be reduced to lower, less volatile,
oxidation states.
Once the decomposition of the nitrates is complete,
a vacuum can be applied to the interior of the container
while maintaining the elevated temperature, thus
-30-
s~
reducing the vaPor pressure of oxygen low enough so that
ruthenium tetraoxide spontaneously decomposes to lower
oxidation states which have a high temperature
characterization or a very low vapor pressure, thereby
permenantly trapping the ruthenium in the glass. The
vacuum should be applied before the porous glass actually
starts to close its pores under heat because under such
conditions one can also reduce the amount of dissolved
gases in the final product. In effect, by reducing the
amount of soluble gases in the glass is lowered.
Thereafter, the temperature is raised and whenever there
is a pressure jump vacuum is applied until the pressure
comes down again quietly. Around 1300C, the exact
temperature depends on furnace configuration, container
bore size, the type of particulate glass, etc., the
tube collapses. If the walls of the container are
thin, they will collapse to a flat or elliptical cross
section forming more of a ribbon than a rod. If the
walls are thick, a rod-like cross section is emphasized.
2Q Another way of favoring the rod-like cross section is to
pull on the container while heating so that it stretches
while it collapses. For convenience in packing the
finished glass articles, it may be easier to pack a rod-
like cross section for storage. If there is a major
heat transfer problem, however, it may be more
convenient to work with flat ribbon-like cross sections
in order to alleviate such heat transfer problems. By
using a narrow heat zone and moving it up from the
bottom, a region near the top is reached where there are
no glass particles left and the glass container collapses
on itself to provide further improved sealing of the
nuclear waste.
-31-
_,-- __ _ _ _ _. ____.. ~ .. _ __ _.__ ~ ... , . _ .. .. ,.. _. _ . ,. _ --_._.. _ __ . . . .... _ _ .. . _ ,.. ... _ ~
. . .. _ .
1~2~
If the degasing is performed properly, there will be
only a very minute amount of bubbles and there results a
finished product which has an envelope of low temperature
expansion coefficient, radiation free glass enclosing a
high temperature expansion coefficient glass. This
provides compression on the outer glass layers and
tension on the inner core of glass. If the inside glass
is relatlvely free of bubbles, it will support the
tension and make the final article a strong prestressed
material having a modulus of rupture considerably in
excess of annealed glass. The advantages of keeping
the finished product monolithic are the following:
1~ the outer surface area of monolithic glass is much
smaller than if it is discontinuous and, since the
amount of leaching is proportionate to the surface
area, the risk of leaching is reduced considerably,
2) in the case where there is no nuclear waste in the
outer layers of the container, there is no nuclear
waste available to be leached in the initial period of
leaching conditions until, if ever, the leaching is
able to continue through the thickness of the radwaste-
free outer layers of the collapsed container. This can
be designed to be a long period compared to the short
half-life of the radioactive isotopes encapsulated
inside the container thus encapsulating them for the
life of their radioactivity and no radioactivity is
exposed to the biosphere. Furthermore, the processing
of nuclear waste according to this invention has the
advantages that it utilizes no furnace electrode which
can be corroded by the molten glass, no fumes of radio-
active elements are expelled, and in general a very clean
operation is possible. In the event that a glass
-32-
. . .
1~2~ LjZ~
containe~ breaks, the glass can be disposed of bycomminuting lt into particles or by remelting it and
processing it into particles as explained above and
disposing said particles into another glass container.
Thus, no new waste is produced requiring a separate
disposal system~
A monolithic (not particulate) porous glass rod or
similar preform containing a radwaste solution tends to
break when heated because internal pressures build up
because of the boiling away of internal water. If
violent enough the internal pressures can become great
enough to cause the glass preform to break. Also, after
the bulk of the liquid has been removed, as the preform
dehydrates, it shrinks and, if the dehydration has been
uneven, unequal stresses are developed when one side has
shrunk more than the other which can cause the preform
- to break. Furthermore, at the slightly higher
temperatures used to decompose the salts, such as nitrates,
gases, e.g., nitrous oxides are given off. Again, too
fast an evolution of such gases can break the preform.
In addition, if the material is deposited unevenly
in the monolithic porous glass rod or similar preform
the dopant increases the thermal expansion coefficient
of the silica component of the glass preform and, upon
collapsing of the pores by heating, the uneven
expansion coefficient can lead to breakage. The dopant
distribution profile in the monolithic glass preform
has to be very well controlled in order to avoid
breakage. These problems are greatly reduced or
eliminated when using porous glass particles in a
glass container. The individual particles are so
small that the stresses built up in them during heating
-33-
i~2~5~
heating are not great enough to break them and if a fewdo break it causes little or no problem and the heating
can be accomplished much faster. Also, the cross
sectional dopant distrlbution profile in the final product
can be important. In the case of a non-porous glass
container the outer layers of the final product will have
the initial thermal expansion coefficient of the container
which can be made with a lower thermal expansion
coefficient than the glass particles inside. Thus, the
final product, in this case, exhibits compression at the
surface which makes it stronger.
Moreover, the use of a glass container containing
porous and/or non-porous glass particles has the further
advantage of providing distribution of the deposited
radioactive solids throughout the interior of the tube
rather than just on the interior surface walls of the
container in the case of a glass container in which no
glass packing is used or on the exterior surface of a
porous glass rod when that is used. Also, when a glass
partlcle-filled, non-porous glass container is employed,
the resulting~clad is free of radioactivity thus
providing essentially no radiation contamination risk to
the environment.
In the processing of glass containers pursuant to
this invention, gases can escape from the container through
the open end. A convenient way to control these gases
is to insert a layer of porous glass in the open end of
the container. It will act as a molecular sieve and
because of its very large initial surface area, e.g.,
hundreds of square meters per gram, the gases attempting
to pass out of the container are trapped by it. By
controlling the temperature of the porous glass layer,
-34-
55'~
the passage of water, non~radioactive nitratedecomposition products, and other non-radioactive fumes
that it is desirable to get rid of, can be permitted
while at the same time trapping in the container the
ruthenium, cesium, cadmium, and other radioactive
materials. The differences in temperature along the
container can be used to advantage in driving off the
non-radioactive volatiles while preventing escape of
radioactive materials.
It is also advantageous to be able to collapse the
container into a rod of smaller dimensions or into a
tape having one small dimension, i.e., its thickness,
and a larger width. The smaller dimension facilitates
more uniform heat removal, i.e., it redùces the
temperature gradients in the resulting glass and avoids
or considerably reduces cracking.
Non-Porous Glass Packing
The glass container can be packed with non-porous
glass particles in addition to or in place of the above-
mentioned porous glass particles. The non-porous glass
particles can be made from any suitable glass forming
composition using the operational procedures described
hereinabove relative to the porous glass particles
except, of course, the phase-separating and acid-leaching
steps are not necessary in the case of non-porous glass.
Non-porous glass particles thus can be in the form of
spheres, elongated grains or any other suitable shapes
and function in the glass container in essentially the
same way as the porous glass particles except that there
are no pores into which dissolved radioactive materials
permeate. Therefore, the radioactive materials, both
dissolved and undissolved, are deposited on the
-35-
1~2~SZ~3
peripheral or outer surfaces of the particles and in thesubsequent heating step the oxide forms of the radioactive
materlal react with the molten non-porous glass particles
and become an integral part of the final glass product
while other forms are entrapped deep within the final
glass product. In many cases, it is preferred to use a
moving heat zone with a differential pressure produced by
evacuati~g the interior of the container or by applying
greater pressure externally as by mechanical means or by
gaseous means.
It can also be advantageous in using a moving heating
zone to progressively collapse the container from the
bottom up. If the container is very long it may not be
able to support its weight if supported only at its upper
reglons and can be supported also at the bottom so that
it will not elongate during collapsing. On the other hand
if it is desired to stretch the tube so that it will
collapse into a rod rather than a flat slab, a small
pulling force (in addition to gravity) can be applied
from the bottom of the container and will produce a
rod-shaped object.
In order to prevent cracking of the glass container
enclosing glass packing, the glass container must have a
lower thermal expansion coefficient than the resulting
enclosed glass which is obtained when the glass container
and contents containing the deposited radioactive
materials are heated to sinter the glass packing thereby
providing an enclosed glass doped with radioactive
materials. Silicate glasses with or without small
amounts of boron, e.g., Vycor*, have low thermal
*trademark
-36-
l:lZS'~
eXpansion coefficients and, as alkali metal content is
increased, the expansion coefficient materially increases.
It is preferred that the container not collapse prematurely,
even when the inside is under vacuum and the outside is
under atmospheric pressure at temperatures at which the
enclosed glass packing begins to melt so that it remains
a container which will contain the enclosed glass until
it is advantageous to collapse the container. In that
respect, it is preferred to use a container having a
higher glass transition temperature (silicate glass and
Vycor are advantageous). When the particles of the glass
packing are heated, they will degas as solid objects below
the glass transition temperature or even just above the
glass transition, as long as the glass is not so hot
that the particles coalesce with each other. At a slightly
higher temperature, the glass particles melt into each
other, i.e., they coalesce, and become a unitary glass
body which, if done properly, is bubble-free. If the
container collapses at a temperature which is slightly
higher than the TG of the interior molten glass (including
deposited solids), a bubble-free final glass product
containing the nuclear waste results. However, if the
container is not collapsed until a much higher
temperature is reached, there is the risk that the
solubility of gases, such as oxygen in the interior
molten glass will decrease to a point where the content
of the gas, e.g., oxygen, exceeds its solubility in the
molten glass because it is under the vacuum used to
collapse the container. When the gas (oxygen) content
exceeds its solubility at the temperature and reduced
pressure of the interior molten glass before the
container collapses, bubbles and foam can form in the
-37-
1~2~
interior molten glass. Once the tube collapses, theinterior is no longer subject to vacuum but at that time
is subject of the external pressure; thus, solubility of
the gases in the interior molten glass increases and the
danger of bubble or foam formation is relieved. Prevention
of bubble or foam formation requires fairly accurate
selections of the container glass transition temperature
and the interior glass transition temperature of the
interior glass composition including the deposited
radioactive solids. ~gain, if the interior glass
composition is too soft for Vycor or a fused silica glass
container, the collapsing temperature of the container
can be lowered by using a container glass such as Pyrex.
Of course, the manufacture of compositions having any
desired glass transition temperature is well within the
skill of the art and any means available can be used to
provide glass compositions having suitable TGIs for the
container and the interior glass packing. The container
g~ass composition should be higher melting than the
interior glass composition including deposited radioactive
solids; i.e., it should have a higher transition
temperature and should be able to collapse only after
the interior glass composition has sintered. The presence
of interior bubbles is not intolerable in many cases;
however, if the absence or reduction of bubbles or foam
is desired the TGIs of the glass compositions used should
be selected as explained above.
~ hen using high silica (> 90 mole ~ SiO2) low alkali
(< 0.5 mole ~ Na2O) porous glass as a packing: (a) Pyrex*
tubes collapse at too low a temperature to permit sintering
*trademark
-38-
5~
of the packing; and (b~ Vycor* tubes have the following
disadvantages:
ti) The thermal expansion coefficient ls so low
that it can only be matched by the core glass
when the loading is very low (e.g., less than
5 weight % for the U~ composition, see
~xample 25~.
Because of the high collapsing temperature
(about 1300 ~ 1400C~ it may cause
volatilizatlon of Cs and other nuclear wastes.
~hile Pyrex and Vycor nuclear waste containers are
suitable for many of the applications as shown in the
examples, other compositions have prelerred properties.
The preferred container is produced by: a) producing a
porous glass container, such as a tube, as described in
U.S~ Patent 4,110,096 at column 10, line 50 to column 16,
l~n~ 36, and b) doping said porous glass container with
at least one dopant such as cesium, rubidium, strontlum,
and copper. The doping could be accomplished by either
of two methods:
1) The preform is immersed in a solution
containing the dopant ions at a pH between 9 to
13.5, preferably between 10 and 13, for a
time which depends on the wall thickness
and the desired concentration of dopants.
Typically, the immersion time is between
1 hour and 7 days. The pH of the solution
is preferably adjusted with NH40H. For
maximum speed of ion exchange,- the solution
is saturated with the desired dopant ions.
*trademarks
-39-
5Z~3
Usually the dopants are introduced into the
solution as nitrate compounds. However, chloride
and carbonates can be used.
2~ The porous preform is immersed in a solution
of dopant or a dopant compound. After the
dopant concentration is uniform throughout
the preform, the dopant is precipitated by
dropping the temperature. The preform is
immersed in a solution free of dopant. The
dopant is allowed to partially dissolve and
diffuse out of the matrix. Only the dopant
precipitated near the outer surface is removed
in this step.
In both illustrative methods, the doped porous
~S preform is then dried and heated to the collapsing
temperature of the pores. The drying should not
substantially change the dopant distribution in accordance
with the teachings in U.S. Patent 4,110,096 nor the
shape of the container. Upon collapse of the pores,
2Q the container changes in appearance from opalescent to
clear without a substantial change in shape other than
the shrinking of its linear dimensions by about 20~, In
addition, the dopant compound is used in an amount so as
to result in a dopant concentration range of from 0.5 to
6 mole percent in the form of its oxide in the resulting
shrunken glass product. The porous glass preform
usually contains up to 8 mole percent B2O3 including
other components, e.g., alumina (if any). Under these
conditions, the resulting shrunken container will be
characterized by a minimum SiO2 content of 86 mole
percent, In a preferred aspect, said container will
be characterized by at least about 90 mole percent
-40-
sio2 thus enhancing the chemical durability of theglass
Of the above two methods for introducing the dopant
into the porous giass, method 1 is preferred. The dopant
concentration is very uniform throughout the cross-section
of a preform doped according to method 1. This high
uniformity permits further preparing of the container
by conventional glass blowing techniques. In Example 27,
for example, the glass tube produced by method 1 (the
ion exchange method) is heated and one end is closed
without breakage.
Since the preferred nuclear waste container should
have both lower viscosity (lower collapsing temperature)
a~d higher expansion coefficient than a 96% SiO2 glass,
~5 the addition of alkali dopants seems appropriate. We
have discovered that at concentrations higher than 85
mole % SiO2 and lower than about 5 mole ~ alkali, the
chemical durability of Cs or Rb glasses is superior than
that of the ~a or K glasses of comparable composition.
At room temperature, for 2 mole % alkali dopant, sodium
glass is 1000 times less durable than cesium glass, and
for cesium and rubidium at 100C, rubidium is 10 times
better than cesium glass. The chemical durability for
the cesium and rubidium glasses were measured by a
leaching rate measurement in water of pH roughly 5.6
and 20C. The leaching rates were found to be below
10-9 gm of silica per square cm of exposed surface of
the sample per day after 20 days soaking time. This is
an excellent chemical durability. However, while high
chemical durability is obtained with a rubidium dopant,
a cesium dopant is preferred because of the much lower
coast of cesium. Divalent elements that can be
-4i-
112~i5~
advantageously incorporated together with Cs and/or Rbare Sr and Cu.
In choosing the dopant and the concentration, one
must not only consider the chemical durabllity but also
the matching of thermal expansion coefficient and container
collapse temperature to the sintering temperature of the
nuclear waste powder. One ordinarily skilled in the art
can obtain such a matching by independently adjusting the
following variables: composition of nuclear waste,
loading of nuclear waste in core material, dopant
compositions and concentrations of dopant in container.
However, some of the product may still crack, permiting
the core to be exposed to the outside. Because of the
large surface area of the core glass which is still
covered by container glass (cladding) thereis still a
very major reduction in leaching rates of nuclear waste
material into water notwithstanding the presence of said
cracked cladding. Thus, we still consider t~is to be
sealed.
The present invention, which includes porous cation
exchange particles in a glass container, can be employed
to remove dissolved and undissolved radioactive solids
from highly dilute solutions of same. For example,
solutions containing as little as l ppt (part per
trillion) based on solution weight, i.e., 1 wt. part per
10l2 wt. parts solution of radioactive cations can be
purified. Dilute solutions having less than 0.01
microcurie radioactivity per ml as well as more
concentrated solutions, e.g., those having 1 curie or
more radioactivity per ml and those solutions between
0.01 microcurie and l curie radioactivity per ml, are
- efficiently treated by this invention.
-42-
~Z55,Z~
I~ a typical nuclear reactor there are several
sources of radwaste as described hereinabove that must
be safely con-tained. These include highly dilute liquid
waste streams which can contain dispersed radioactive
solids as well as dissolved radioactive solids, e.g.,
cations; concentrated liquid wastes which can contain
radioactive cations, radioactive anions and radioactive
solids (such wastes are the result of the boiling down
of primary coolant containing boric acid initially used
in the coolant as a chemical shim and the boiling down of
used regeneration solutions from the regular ion exchange
beds customarily used); and/or radioactive gases such as
radioactive krypton and/or radioactive iodine. Therefore,
our invention includes a total radwaste disposal system
wherein particulate porous glass or silica gel having
silicon-bonded alkali metal oxy, Group Ib metal oxy, and/
or ammonium oxy groups is packed into a cation exchange
column which preferably is a fusible glass column. The
glass or silica gel particles can be held in the column
by means of a porous closure such as glass wool or a
porous disc in its lower end and, if desired, in its
upper end also. In addition, the porous and/or non-
porous glass particles can be mixed with the ion exchange
glass or silica gel particles in the column to provide
additional external surface on which dispersed, unsettled
solids can settle out. It is preferred that the porous
glass or silica gel by finely divided and sieved to a
suitable size to maximize the rate of flow of the radwaste
stream through and between the particles of the porous
glass or silica gel and to also minimize the ion
exchange time. First, the dilute radwaste stream is
passed through the column and the radioactive cations
-43-
1~25SZ'~
in solution are cation exchanged with the alkali metal,Group Ib metal and~or ammonium cations in the porous
glass or silica gel to chemically bond the radioactive
cations to the glass or silica gel. If the dilute radwaste
stream is to be reused as the primary coolant, it is
conventional to add lithium ions as a corrosion inhibitor.
Therefore, it can be advantageous to utilize a porous
glass or silica gel having silicon-bonded lithium oxy
groups so that lithium ions (which do not become radio-
active as do sodium ions~ are released to the coolantstream as radioactive cations are removed from it.
Additionally, dispersed, undissolved radioactive solids
in the dilute radwaste stream can be mechanically
filtered on the porous glass or silica gel particles in
the column as the stream percolates through and between
the particles. In order to maintain the ratios of solids
in the radwaste stream to the porous glass or silica gel
- small enough to maintain the filtering action as the
solids accumulate on the porous glass or silica gel
particles, fresh porous glass or silica gel particles
can be added to the column.
After the column has been exhausted of its ion
exchange capacity by the dilute liquid radwaste stream,
it can be dried and the concentrated liquid radwaste
tcontaining concentrated boric acid, for example, at a
temperature 100C) can be added to the column. Thus,
the pores of the porous glass or silica gel can be
stuffed with the radioactive solids, cations and anions
contained by the concentrated radwaste. Excess boric
acid then can be washed from between the particles of
the porous glass or silica gel using cold water (less
than 30C~ and the particles can be dried to deposit the
-44-
1~25S~
radioactive solids, cations and anions within the poresof the porous glass or silica gel using techniques taught
in U~S. Pat~nt 4,110,096. Thereafter, the column can be
first evacuated to remove decomposition gases. Then
radioactive gases can be introduced into the glass column,
and the column can be heated to collapse the pores of the
porous glass or silica gel and to collapse the glass column
thereby immobilizing and containing the exchanged
radioactive cations, the radioactive solids on the
exterior of the porous glass or silica gel particles,
the radioactive solids, anions and/or cations deposited
in the pores of the porous glass or silica gel and the
radioactive gas contained by the glass column. Suitable
pressure differentials can be used to facilitate the
collapsing of the glass column. Heating can be continued
to cause the porous glass or silica gel particles to
stick to each other to further trap interstitial radio-
active solids between the particles. Upon cooling there
results a highly durable solid which effectively contains
the radioactive waste introduced into the glass column.
Because some of the nuclear reactor streams may be
basic, some elements in the radwaste appear as anions,
e.g., chromium, molybdenum, praseodymium and certain
anions, which, of course, have to be immobilized also.
One way to accomplish this is to pass the basic radwaste
stream through a customary anion exchange resin column.
The column is regenerated with non-radioactive base,
e.g., ammonium hydroxide. The effluent from said
regeneration contains a hlgher concentration of radio-
active elements and is boiled down in a boiler toprovide a reduced volume of basic radwaste. When the
concentrated baslc radwaste in the bottom of the boiler
-45-
15 ~2S52B
is acidified under reducing conditions, some of theanions, e.g., Cr, Mo, Ce and Pr become cations which can
be ion-exchanged with and removed by the above-mentioned
porous glass columns. The boiler bottoms are defined as
S the concentrated solution or raffinate which remains
after boiling down the solution and it may contain solids.
It can be molecularly stuffed into the porous glass to
become a highly durable solid waste product.
There are many other industrial wastes which have
to be eliminated from waste streams which, although not
radioactive, are very poisonous to humans. For example,
it has been well publicized that water bodies have been
contaminated in the past with mercury, cadmium, thallium,
lead, other heavy metals, insecticides, and organic
~5 polsons. Often the concentration of such toxic
` substances in the waste streams is very low, thus
presenting the problem of treating large volumes of water
containing small amounts of toxic substances. Nevertheless,
overall, large quantities of such contaminants do enter
the ecosphere. The present invention can be used to
purify such waste streams.
This invention can be employed for concentrating
and immobilizing radioactive cations in glass for
extremely long time storage. For example, the sintered,
silicate glass loaded with radioactive solids can be
appropriately packaged in containers and buried beneath
the earth's surface or at sea. Alternatively, the
radioactivity of the sintered glass product containing
the radioactive solids can be utilized in suitable
devices or instruments for a variety of purposes, such as,
destroying microorganisms, e.g., in the preservation of
food, or in sterilizing sewage sludge or for any other
-46-
S,~i3
purpose where radioactivity can be employed constructively.
~ typical range of radioactive solids content of theglass products of this invention resulting from the
treatment of low level waste is about 1 ppb to 20,000 ppm
of the glass product. The radioactive solids content of
glass products resultlng from the treatment of high level
radwaste is upward to about 30 weight percent or more,
e.g., from about 2 weight percent to about 30 weight
percent. Glass products of this invention which are to
be used as radioactive sources can have solids contents
falling the above-mentioned ranges.
In general, the glass articles of this invention
comprise a first non-porous glass portion and a second
non-porous glass portion surrounding the first portion.
The first portion contal-s radioactive materials
entrapped and immobilized therein and the second portion
contains further radioactive materials entrapped and
immobilized therein. The radioactive materials in one
of said portions is derived from radioactive materials
which were soluble in a nuclear waste (radwaste) solution
and the radioactive materials in the other portion is
derived from radioactive materials which were insoluble
in said nuclear waste solution. For example, the radio~
active materials in the first portion are derived from
materials which were insoluble in the radwaste.
As another example, the radioactive material in
the first portion is derived from the radioactive
materials which were soluble in the radwaste.
Furthermore, the glass articles of this invention
can include a third non-porous glass portion which
surrounds the second portion, and the third portion is
free of radioactive materials. The radioactive materials
-47-
. ~
l~S15~3in the novel glass articles are described above. Also,
the insoluble radioactive materials can be metallic
precipitates of the platinum metal family. The glass
article can be rod-shaped, tape-shaped or any desired
shape.
The following examples are presented. Unless
otherwise specified all solutions are aqueous solutions.
The "aqueous ammonium hydroxide" or 'INH40H'' used in the
- Examples contained about 28% NH3, ppm means parts per
million parts of solution, ppb means parts per billion
parts of solution, ppt means parts per trillion parts of
solution, all parts and percentages are on a weight basis
and all temperatures are given in degrees Centigrade.
For reasons of safety all simulated radwaste solutions
used in the Examples were actually non-radioactive;
however, radioactive solutions of the same kind can be
substituted and concentrated and encapsulated in
accordance with the following Examples.
EXAMPLE 1
Preparation of Glass
Particles and Tubes
A. A molten glass was formed in a platinum
crucible at 1400C from sand, boric acid, sodium carbonate
and potassium carbonate, the glass having a nominal
composition of 3.5 mole percent Na2O, 3.5 mole percent
K2O, 33 mole percent B2O3 and 60 mole percent SiO2.
The molten glass was vertically updrawn and solidified
into rods having a diameter of about 0.8 cm and a
length of about 100 cm which were then crushed in a
stainless steel cylinder with a stainless steel rod.
The resulting powder was sieved and the fraction between
-48-
_ _ _ , . . _ _ . .. . _ .. _, .. _ _, _ .. , .. ... . _ ~ .. _ _ .. . . _ , _ . , . ~ _ . _ ... .. _ .. . . .
_ _ , . . ~ _ . _ .
l~ZS52~3
32 and 150 mesh screens was selec~ed for use in certain
of the following Examples.
B, Tubes were formed by pulling the above-described
molten glass and applying a small internal pressure. Tubes
that were sealed at one end were formed by turning off
the internal pressure during the drawing operation. Tubes
open at both ends were formed by maintaining the internal
pressure through the drawing and cut-off operation.
The tubes were formed with an outside diameter of about
1 cm and a wall thickness of about 0.15 cm and were cut
to about 5 cm long.
E~AM;PLE 2
Preparation of
Porous Glass Tubes
A base glass tube having one sealed end and one open
end was prepared as descrlbed in Example ls. The tube
was then heat-treated at 550C for 110 minutes in an
electric furnace to induce suitable phase separation. The
tube after heat-treatment was annealed by cooling slowly
down to room temperature, and was leached to form a
porous tube by soaking it in a 3N CHl solution saturated
with NH4Cl at 95C for two days. The porous tube was
then soaked in hot water for one day to wash out residue
from the leaching operation and was then kept in a
dessicator until the pores were dry of the washing water.
The resulting porous glass tube had a nominal composition
of 95 mole percent SiO2, 5 mole percent B2O3 having
interconnected pores, and an internal surface of about
100 m2/gr. The surface of the resulting porous glass
tube was saturated with ~ SioH groups.
-49-
l~ZSS;2~
EXAMP~E 3
Preparation of
Porous Glass Powder
. ~
- Glass rods were prepared as described in Example lA.
Before crushing the glass rods, they were heat-treated
at 550C for 110 minutes and then crushed to form glass
powder. Next the glass powder was sieved and the fraction
passing through a 32 mesh screen but not through a 150 mesh
screen was leached in a 3N HCl solution at about 95C
for about six hours. The glass powder was washed with
deionized water for about 24 hours at about 25C. The
resulting porous glass powder had a nominal composition
of 95 mole percent SiO2; 5 mole percent B2O3, had
interconnected pores, and had an internal surface of
about 100 m2/gr. The resulting glass surface was
saturated with sio H groups. The porous glass powder was
dried in a beaker on a hot plate at about 150C.
EXAMPLE 4
Use of a Porous Glass Tube
to Concentrate and Encapsulate
A dry porous tube having one end open and one closed
end, prepared as described in Example 2, was impregnated
with a solution containing dissolved CsNO3 and A12O3
particles simulating a nuclear waste fluid. The CsNO3
solution contained 67 grs of CsNO3 (which could be radio-
active) dissolved in 23 ml water at 100C and 10 grs of
A12O3 representing suspended solids (which could be
contaminated with radioactive isotopes). The interior of
the tube was filled with the dopant solution, and the
solution was allowed to penetrate into the pores. Some
of the solution in the tube was allowed to pass through
the tube walls to theoutside of the tube and was
collected for use in other tubes. This was continued
-50-
1~2~
until the interior of the tube was essentially empty ofthe solution. The A12O3 solids suspended in the solution,
however, being much larger than the pore size of the
tube walls were retained in the interior of the tube.
Also, the solution containing the dissolved CsNO3 filled
the pores of the glass tube walls. The resulting laden
porous tube was then inserted in methanol at 0C to cause
the dissolved CsNO3 in the solution in the pores to
precipitate in the pores. The inner and outer surfaces
of the laden tube were soaked in clean methanol at 0C
for 24 hours, while changing the methanol often, resulting
in thin layers on both the outside and inside surfaces
of the tube in which the concentration of the precipitated
CsNO3 was lower than the concentrations of precipitated
CsNO3 deeper in the glass. (That is the inner and outer
surface layers or regions contained approximately one
fifteenth of the CSNO3 concentration of regions located
deeper in the tube wall.)
The porous tube was then removed from the 0C
methanol bath and placed into a larger diameter (3.5 cm),
substantially non-porous, fused silica glass tube having
an open end and was dried under vacuum at 0C for 24
hours. The fused silica glass tube containing the laden
porous tube was then allowed to warm under vacuum to room
temperature and was put into a furnace where it was
slowly heated at 15C/hr up to 625C. This heating
period allowed the pores of the glass to dry further.
The laden porous tube inside the non-porous tube was held
at 625 C for 16 hours to ensure that all the CsNO3 was
decomposed and the resulting nitrogen oxides were
expelled leaving Cs2O. It was then heated to 875C
still under vacuum in order to fuse the pores and sinter
~51-
1~2~Si~
the glass structure of the porous glass tube thusconverting it into a substantially non-porous glass tube
with the cesium (Cs2O) trapped as a part of the glass
structure. The solid (A12O3) remained deposited on the
tube interior. The tube is placed horizontally on a
graphite block in a ceramic tube furnace with another
graphite block resting on top of it. It is heated to
about 1350C and the tube sags under the weight of the
upper graphite block causing the interior surfaces of
the tube to fuse and seal together, thus immobilizing
and encapsulating both the Cs2O from originally dissolved
CsNO3 and the originally dispersed A12O3 solids.
EXAMPLE 5
Use of Porous Powder in Non-
15 Porous Tube to Encapsulate
A non-radioactive aqueous solution simulating a
radwaste stream projected for an existing spend nuclear
fuel reprocessing plant and containing 3.06 grs (Fe(NO3)~
9H2O, 1.68 grs Ce(NO3)3~6H2O, 0.78 grs La(NO3)3 6H2O,
0.78 grs CsNO3, 3.88 grs Nd(NO3)3-5H2O, 0.52 grs Ba(NO3)2,
2,72 grs Zr(NO3)4, 0-42 grs Sr(N3)2~ 0-34 grs Y(NO3)3-
5H2O and 5 ml water, with all elements in solution
except Zr(NO3)4 which was present as a precipitate, was
poured into a 50 ml beaker which contained 5 grs of
porous glass powder made as described in Example 3. The
excess solution was decanted and the beaker was heated
to 200C on a hot plate to dry the glass powder and
deposit the dissolved nitrates in the pores of the glass
powder and the undissolved Zr(NO3)4 on the outer surfaces
of the glass powder.- The laden glass powder was then
placed in a Vycor tube (Corning 743170-4381) having a
nominal composition of 96% SiO2 and 4% B2O3, an inside
-52-
1~2~5;~
diameter of 7 mm, an outside diameter of 9 mm and alength o~ 50 cm. The tube was sealed at one end and was
co~nected to a vacuum pump. The tube containing the
laden porous glass powder was then inserted into a
furnace at room temperature under vacuum and heated at
15C/hr up to 600C to evaporate any remaining water
or other volatiles and to decompose the nitrates present
into the corresponding metal oxide and nitrogen oxides
and to expel the nitrogen oxides. After holding at
600C for 24 hours, the tube was transferred to a second
furnace capable of providing higher temperatures. Upon
transferring from one furnace to the other, the temperature
dropped to 530 C.
The temperature in the second furnace was increased
gradually from 530C to 1340C over a period of three
hours and 25 minutes. The tube was removed and was found
to have collapsed above the level of the glass powder
which had been impermeated with the simulated nuclear
waste solution. This occurred because the furnace had
a relatively large temperature gradient across it, and
the tube had been inserted too far. Nevertheless, the
final product was a partially collapsed tube completely
sealing within it the glass powder with no cracks present
in the tube. The uncollapsed lower portions of the tube
contained the impermeated glass some of which was a loose
powder, some of which had melted into chunks and some
of which had melted and stuck to the interior walls
of the tube. There were no breaks in the tube walls
and no stress of the tube walls was observed under
crossed polaroids. The resulting product effectively
encapsulated the metal oxides resulting from the metal
nitrates in the initial simulated nuclear waste stream
-53-
~2SS2~3
and isolated them from the environment,
EXAMP~E 6
Encapsulation of Calcined
5Nuclear Waste in a Vycor
Tube for Burial
About 1.5 ml of a non-radioactive aqueous solution
simulating a radwaste stream projected for a spent nuclear
fuel reprocessing plant and as described in Example 5
were placed in a 50 cm long Vycor tube which also is
described in Example 5~ The solution included dissolved
nitrates as well as precipitated Zr(NO3)4 as described
in Example 5. No glass powder was added. The tube was
connected to a vacuum pump by a rubber hose. In order
not to have excessive bubbling, the tube was placed in
an ice bath at 0C and pumped overnight to dry its
contents. The next day the temperature of the tube was
28C and the interior pressure was 20 m Torrs. The tube
was transferred to a furnace where it was heated under
~acuum according to the heating schedule given in Table 1
beloW-
TABLE l
Time Temperature, CPressure, m Torr
tHours:~inute ?
12:45 70 137
13:40 80 40
13:50 130 140
14:05 155 50
14:25 190 79
14:50 190 25
15:15 290 50
15:30 340 80
15:40 350 55
16:05 450 34
17:05 600 16
18:10 850 16
' 20:00 1340 14
-54-
1~2~iZ3
At 20:00, after seven hours and 15 minutes of heating,
the tube which had collapsed during heating, was removed
from the furnace. From the data in the above Table, it
can be seen that pressure maxima occurred at 12:45, 13:50
and 14:25. This appears to have been due to the evaporation
of water still ln the tube when it was placed in the
furnace and appears -to have occurred each time when the
temperature was significantly raised. If the temperature
is held constant as at 13:40, 14:05 and 14:50, the pressure
is reduced as the water vapor is taken off by the vacuum.
Another maximum occurs around 15:30 at about 300-400C
which is apparently due to the decomposition of nitrates
to form nitrogen oxides.
The final product was a collapsed and sealed Vycor
tube with calcined simulated nuclear waste (i.e., the
oxides Fe, Ce, Ha, Cs, Nd, Ba, Zr, Sr and Y~ encapsulated
inside the collapsed and sealed tube. The surface of the
collapsed and sealed tube showed no cracks. When the
tube was examined under polarized light it was found to
be free of stress. The resulting product was suitable
for burial in the ground or sea and can be packaged with
other like products in larger containers for such purposes.
EXAMPLE 7
Use of Non-Porous Glass Powder
in a Non-Porous Glass Tube for
Encapsulating Nuclear Waste
for Burial
Pyrex glass (Corning 234030-510) having a nominal
composition of 81~ SiO2, 2~ A12O3, 13% B2O3 and 4~ Na2O
(given in wt. ~'s) was crushéd in a stainless steel
cylinder using a stainless steel rod. The crushed ~lass
was sieved and the fraction which passed through 60 mesh
and was caught on lS0 mesh was selected for use. 9.5 Gms
-55-
, . , ______ _ . ., ... . , _ , .,, ._ __ _ _, .. _ ... _ .. . , . ., .. , , ~ , , ,._, . _.
1~2~5~'~
of the selected fraction of Pyrex powder were mixed with0.5 gm of porous glass powder impregnated with simulated
nuclear waste stream and dried as described in Example 5.
The mixed powder was further dried in a beaker on a hot
plate at 110C for about two hours. Part of this mixed
powder was then placed in a 50 cm long Pyrex tube having
the nominal composition given above, a 9 mm O.D. and a
7 mm I.D., so that it formed a column 10 cm high. Also,
a piece of platinum wire, 1 cm long and 1.5 cm in
diameter was added to the powder in the tube. The open
end of the tube was attached to a vacuum pump and
placed in a furnace where it was gradually heated from
about 25C to about 830C in about four hours and
35 minutes. The finished product developed some cracks
after it was pulled out of the furnace. The cracks
appeared to be internal and did not extend to the outside
surface of the collapsed Pyrex tube. The resulting
product effectively encapsulated the glass powder
containing simulated radioactive waste materials and
platinum which represented the platinum group metals
such as Pd, Ru and Rh that are commonly dispersed solids
in nuclear waste streams. The cracking can be eliminated
by more closely matching the thermal expansion coefficient
of the tube and of the contents. The final product can
be suitably buried underground or at sea, preferably
with other like products and packaged in a larger
container for convenience.
EXAMPLE 8
Trapping Radioactive Vapors in
a Porous Glass Rod
The purpose of this examp~e is to show that gas
products e~anating from the simulated nuclear waste being
-56-
l~Z~5,~
heated in a glass tube can be trapped in a porous glass
rod, 6 Gms of porous glass powder prepared as descrihed
in Example 3, was mixed in a beaker with 2,76 gms of
Cs~O3, 3.17 gms of Cu(~O3)2, 73 ml of H2O and 25 ml of
~H40H for 20,5 hours and washed for 24 hours, The
impregnated porous glass powder was dried on a hot plate
at a low temperature (about 200 C for about one hour).
Then, the sample was placed in a Pyrex glass tube
identical to the one described in Example 9 and having
one end closed and a constricted neck located about 11 cm
from the closed end, The powder formed a 4 cm high
column in the tube. A 12,5 cm long porous glass rod,
as prepared in Example lA, having a diamter slightly
less than 7 mm was inserted into the tube. The inner end
~5 of the rod has been ground down to a taper shape (which
then was washed in a HF solution to free the pores) so
that a fairly good seal was made between this end of the
rod and the constricted neck section of the tube, The
tube was placed upright partly inside a furnace so that
the upper half of the rod was outside the furnace.
Heating was carried out according to the time, temperature
and pressure schedule shown in Table 2 below, At the
end of the heating cycle, the tube was removed from the
furnace, The bottom portions of the tube had collapsed
up to 1 cm below the tapered end of the porous rod, The
5 cm section of the rod which was half inside and half
outside the furnace was slightly yellow in color
indicating the condensation of copper vapors, while all
the other parts of the tube and the rod were substantially
colorless, This indicates that the simulated radioactive
vapor, i.e., copper vapors, escaping from the impregnated
porous glass powder during the heating process were
-57~
1~'255~
txapped in the approximately 5 cm section of the porous
rod and prevented from leaving the tube. The resulting
collapsed tube product effectively encased the simulated
radwaste in a strong glass structure.
TABLE 2
Time Temperature, CPressure, m Torrs
(Hours:Minute~
2:30 20 5
2:31 95 22
102:52 95 17
3:13 95 13
3:43 150 13
4:21 260 24
9:30 580 8
1510:20 750 12
The pressure maxima at 2:31 is due to water being
expelled from the porous glass powder and the maxima at
4:21 is due to the nitrogen oxides produced by the
decomposition of the cesium and copper nitrates.
EXAMPLES 9-13
Each of the Examples 4 through 8 are repeated,
except that corresponding radioactive nitrates are used
in place of the corresponding non-radioactive nitrates
specified in Examples 4 through 8 and radioactive by
contaminated A12O3 is used in place of non-radioactive
A12O3 specified in Example 4. In each instance, the
radioactive material is immobilized and encapsulated
within the resulting glass product.
EXAMPLE 14
The porous glass powder made in Example 3 is then
immersed in an appropriate 3.2 molar sodium nitrate-
ammonium hydroxide a~ueous solution for three days and
-58~
s~
then is xinsed in water until the p~ of the rinse water
is reduced to about 8. The resulting powder ls then
placed in an ion exchange column made of the Vycor glass
as described.in Example 5~ A radioactive primary
coolant from a pressurized water nuclear reactor plant
utilizing UO2 fuel clad in stainless steel (containing
4.9 weight percent 235u) is passed through the column.
The primary coolant has the composition given in Table 3
below which lists the radionuclide, the probable source,
the probable form and the average concentration in
microcuries per milliliter. The cationic radionuclides
ion~exchange with sodium cations bonded to silicon
through oxy groups in the porous silicate glass powder
~59
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l~Z~5~
The radioactive cations of the radionuclides llsted
in Table 3, cation-exchanye with sodium catlons bonded to
silicon through oxy groups in the porous glass thereby
binding the radionuclides to the porous glass through
said silicon-bonded oxy groups and releasing non-
radioactive sodium cations to the coolant solution. The
insoluble radioactive solids in the coolant also filter
out on the external surfaces of the porous glass
particles. Additional porous glass particles can be
added to increase the filtering capacity of the ion
exchange column as the insoluble solids build-up on
the column.
The anionic radionuclides are not substantially
removed in the column and pass with the coolant through
the column. The anionic radionuclides can be
subsequently removed by treatment with conventional
anion exchange resins, Upon regeneration of the
conventional anion exchange resin after it becomes
loaded, the regenerant solution containing the anionic
2Q radionuclides can be concentrated by evaporation and the
resulting concentrate can be molecularly stuffed pursuant
to the procedures described in U.S. Patent 4,110,096
into the pores of the porous glass in the ion exchange
column after said porous glass had become substantially
loaded with silicon-bonded radionuclide cation oxy
groups. It is preferred to first dry the loaded porous
glass so that the anionic radionuclide concentrate can
readily enter the pores of the porous glass. The
anionic radionuclides can be precipitated or deposited
within the pores of the porous glass by the careful
drying procedures disclosed in U.S. Patent 4,110,096~
Thereafter,columns containing the porous glass particles
~64-
,, . _ _ .. . __ . .. _ _ . . . . . , .. . , . . . . _ _.. , _ _ _ _ _
>S2~
can be heated to drive off volatiles, to decomposedecomposables and drive off non-radioactive decomposition
products, to collapse the pores of the particles and
sinter same into a unitary mass and to collapse the Vycor
glass column around the sintered mass thereby en~eloping
the filtered solids and the sintered mass glass particles
containing the cationic and anionic radinuclides within
the collapsed Vycor glass column. While the glass column
cracks because of differential thermal contraction it
~0 still contains and further immobilizes the radioactive
materials and forms a product that is many times more
durable than cement or metal drum presently in use.
There is thus provided a durable package of concentrated
radionuclides whieh is highly resistant to leaching by
~5 water or other fluids.
EXAMPLE 15
Use of Porous Powder in Non Porous
Tube to Eneapsulate
~ non-radioaetive nitrate mixture was used to
2~ similate the United Kingdom UKM-22 eommereial waste whose
eomposition is reported in terms of oxides in Table 4.
Various amounts of nitrates were mixed together in sueh
a proportion as to yield the appropriate oxide
eoneentrations given in Table 4. Appropriate amounts
of nitrates whose total weight eorresponds to a total of
2g oxides were plaeed in a 250ml beaker; 20ml H2O was
added; the solution was stirred and heated up slowly to
80C at whieh temperature a light brown solution
eontaining some undissolved salts was obtained. 18g of
porous glass prepared as in Example 3 was then added to
the solution as to give a 10~ loading of waste oxides
with respeet to the final glass. The volume ratio of
~65-
_ _. _ . ~ , , , __ ___ . .. .. ,. . .. __ ., , . _ . __ _ . .... . ... . . . . . _ . _ . _ .
S2~,
solution to glass powder was close to 1:1. The mlxturewas dried at 90C. Approximately 3g of the dry mixture
was heated under vacuum in a Vycor tube stmilar to the
one descrlbed in Example 5 according to the following
schedule:
Time T ( C1 Pressure, m Torrs
(hour:minute
9:45 AM 25 25
10:15 AM 65 30
1011:15 AM 278 26
11:30 AM 342 38
11:40 AM 383 32
11:50 PM 403 68
12:05 PM 520 44
153:20 PM 1300 36
3:45 PM 1310 16
4:15 PM 1310 16
Th~e finished glass product showed that the pores of
the po~der and the grains inside the tube were well
~- 20 sintered. In addition, the tube was completely collapsed
but cracked during air quenching. The finished product
was powdered to increase its surface area and was
subjected to a leaching test at pH 5.6 and at 70 C for
various exposure times. The results as reported in
Table 5 show that the glass sample possesses an exceilent
chemical durability.
~66-
. _ . . ... , _ ., . , , . _ . _ . . .. .. . . . , , .. .. _ . .. ... ... ., . . _ . . .... . .
_ . ..
l~Z;;~5~Z~
TABLE 4
.
United Klngdom UK~-22 composition
S~ated
Oxide Reported wt% Slmulated wt% Oxide Reported wt% ~t~
.
5 A12O319.89 19.89 ZrO25.57 5.57
Rb2O 0.43 0.43 PO40.93 0.93
Cs2O 3,00 3~00 Cr2O32,18 2.18
M~O 24.68 24.68 M~O36.89 6.89
SrO 1.25 1,25 Fe2O310.6310,63
~Q BaO 1.48 1,48 RuO22.65 2.65
Y2O3 0.66 0.66 NiO21.40 1.40
La2O31.71 1.71 PdO1.71 1.71
Pr6O111.67 ~ ZnO1.71 1.71
Nd2O37.08 7.08 U3O8 C'eO
CeO23.85 3.85 SO40.39 0.39
TABLE 5
Chemical Durability of Product Obtained In
Example 15 in D~ionized water Having an
Initial pH of 5.6*-
Sample SiO2 Ln*** Fe Na Cs Sr
Core and Clad 295 32 ~1 <4 c20 ~1
Powdered
Core 127 42 11 173 8
25 Powdered
. . . _
* Data taken between Day 12 and Day 15, 70C, 71 hrs.
** Leach rates are in ng of waste dissolved per cm2 of
surface area of powdered product per day.
*** Includes all lathanites.
30 The leach rates reported in Ta~le 5 above and in
Tables 6 and 8 below have been normalized by the a~ount
of the co~ponent present in the glass. Thus, they
-67-
1~2~5~
represent the leach rate the glass would have if the
measurement was made only on that component. The glass
is dissolving at the silica leach rate, The sodium,
strontium and cesium diffuse to the surface and are
initially leached at a faster rate. Iron and lanthanites
concentrate at the surface. Eventually, the whole glass
w~ll leach at the silica rate.
EXAMPLE 16
Use of Porous Powder in Non-
Porous Tube to Encapsulate
A non-radioactive nitrate mixture similar to the one
described in Example 15 to simulate the UKM-22 waste
was prepared. However, in the preparation of this
nitrate mixture, Zr(NO3)4 and K2MoO4 was dissolved
separately from the other nitrates with sufficient
amount of concentrated HNO3, the others being dissolved
in a 3MHNO3 solution or in water. The two solutions
were then mixed together and no precipitate was observed.
Phosphoric acid and sulfuric acid were subsequently
added to the solution to yield appropriate amounts of
PO4= and SO4 . A white gelatinous precipitate
appeared and did not dissolve upon heating up to 70 C.
About 50% of the nitrates precipitated out when the
solution was evaporated down to about 15 ml. Eight
grams of porous glass prepared as in Example 3 were
then added to the solution to give a 20% loading of
waste oxides with respect to the final glass The
volume ratio of solution to glass powder was about 1:1.
The mixture was dried at 90C for about 16 hours.
Approximately 3g of the dry mixture was placed under
vacuum in a Vycor tube having an outside diameter of
13mm and a wall thickness of 1~5mm. The mixture was
-68~
_ . _ _ ... _ .. _ . . _ . .... . , .. , _ . . . _ _ _ _ _ .. _ . . . . . .
dried at 90C for about 16 hrs. ~pp~oximately 3g of the
dry mixture was placed under v~cuu~ in a V~cor tube having
an outside diameter of 13mm and a wall thickness of 1.5mm.
The mixture was heated to 600C at 50C/hr. After
holding at 600C for 48 hrs, the tube was subjected to
a temperature jump to 12~0C where the pores and the
grains inside the tube were well sintered. The tube,
however, did not collapse and bubbles were formed in
the waste-glass matrix. Moreover, the tube cracked
during air quenching. Leaching tests were performed on
the core of the sample. The results reported in Table 6
show that it has an excellent chemical durability.
T~BLB 6
Chemical Durability of Product Obtained
In Example 16 in Deionized Water Having
an Initial pH of 5.6*
Glass Component and Leach Rate**
Time
(Days~ sio2Fe Ln*** Na Sr Cs
0.34 6,19011507373.61 x 105 3,260 ~1000
1.3 963120 344c2,500 6,340 300
2.2 55030 400~2,500 2,200 <300
3,3 37049 550<2,500 2,300 1,400
5.7 20012 80<2,500 ~.,400 120
9.3 260~13 50<2,500 680 ~ 320
12.2 2203 210~2,500 1,900 150
15.2 230~13 56 ----- 2,000 ~320
* Data taken at 70 C
** Leach rates are in ng of waste dissolved per cm2 of
surface area of powdered product per day.
*** lncludes all lanthanites.
-69-
_ _ _ _ . _ _ __ __, , .. ,_ . , . .. , __ . _. . ~ ~_ _ . _ ~ . _ _._ .. . ~ . , . .. . . , _ _ . , .
~ ,_ , .. __,
1~255;2~
~XAMPLE 17
Use of Porous Powder in Ion-Exchanyed Tube
to Encapsulate
A mixture containtng non-radioactive nitrates and
porous glass was prepared as in Example 16 but with
only 5~ loading of oxides with respect to the final
glass. Approximately 3g of the dry mixture was
introduced in an ion-exchanged tube which was prepared
as follows: an opened porous tube having an outside
lQ diameter of 10mm, a wall thicnkess of ^~ lmm and a
length of 20c~ was prepared as in Example 2. The porous
tube was then soaked in a solution containing 20Oppm
Cs with enough NH40H to give a pH of 10 for 18 hrs,
and washed in room temperature water until a pH of 7
was obtained. The Cs exchanged tube was subsequently
dried under vacuum and was heated from room temperature
to 600C at 15C/hr and from 600 C to 870C at 50C/hr
to collapse the pores. One end of the tube was then
sealed using a torch prior to the introduction of the
mixture of simulated wastes and porous glass. The
mixture was then heated under vacuum in the tube
according to the following schedule.
Time T(C) Pressure, m Torrs
~hour:minute) ----- -----------------
.
25 11:00 AM 22 15
11:20 AM 180 100
11:25 AM -200 100
11:35 AM 252 50
12:02 PM 330 48
30 12:10 PM 470 36
12:53 PM 547 28
1:00 PM 775 25
1:20 PM 875 24
1:30 PM 927 24
1:35 PM 1010 24
1:47 PM 1075 24
2:00 PM 1100 24
The finished glass article showed that the
collapsing of the tube was complete and there were no
-70-
.. ___ . _ _ . , _. . . -- , , . , . . . _ .. , , _, . _ _ . .. ... ... . . . _ . . _ . ..
l~Z~
cracks. The grains inside the tube, however, did not
completely sinter. Hexe the thermal expansion co-
efficients of the tube and powder were matched. However,
complete sintering was not achieved because the
collapsing temperature of the tube (about 1100C~ was
too low for the nuclear waste composition and loading
level utilized.
The composition of the ion exchange tube was
measured to be 0.5 weight per cent Cs.
lQ EXAMPLE 18
Use of Porous Powder in Non-Porous
Tube to Encapsulate
A non-radioactive nitrate mixture was used to
simulate the West-Valley PW-8a waste whose composition
1~ is reported in terms of oxides in Table 7. Various
amounts of nitrates were first dissolved separately
in 3M HNO3 or in water and then were mixed in such a
proportion as to yield the appropriate oxide concentrations
giVen in Table 7. A solution containing appropriate
amounts of nitrates plus some undissolved salts whose
total weight corresponds to a total of 4g oxides was
evaporated down to near dryness and was then mixed with
16g of porous glass prepared as in Example 3 as to yield
a loading of 20~ waste oxides with respect to the final
glass. The mixture was subsequently dried at 90C.
Approximately 3g of the dry mixture was heated under
vacuum in a Vycor tube similar to the one described in
Example 5. The mixture was heated to 600C then was
subjected to a temperature jump to about 1250C at
which temperature the waste porous glass mixture
sintered completely. The Vycor tube did not fully
collapse and cracked during air ~uenching. Leaching
-71-
_ ~ _. ~ ~, ,_ __ . ,. , ~. . _ _ .. _, ... . _, ._ . ., ~ ~ _ _ .. . ~ . _ _.. . ._ _ _ , _ . _. _ _ _ .. _
1~2;~
tests were performed on the core of the sample. The
results reported i~ Table 8 show t~at it has an excellent
chemical durability.
TABL~ 7
West~Valley PW-8a Composition
Reportéd Simulated Reporte~ ~imulated
Oxide wt% wt% Oxide wt% wt%
Na2O 16,62 16.62 TeO20.86
; Fe2O3 34.29 34.29 cs2o1.14 1.14
10 Cr2O3 1.36 1.36 BaO1.85 1.85
Nio 1 . 741.74 Y2O30.05 0.05
P2O5 1.58 1.58 La236.05 6.05
Rb2O 0.21 0.21 CeO212.09 12.09
SrO 1.25 1.25 Pr6O111.06 1.06
15 ZrO2 5.84 5.84 Nd233.62 3.62
MoO3 7.54 7.54 Sm230.64 0.64
Rh23 0.36 0.36 EU230.17 0.17
Ag2O 0.1040.104 Gd230.43 0-43
CdO 0.15 0.15
2~
~72
. __ _ ... . , _ .... , . . .. .. _ , . . ... __ ..
l~ZSSZ~
TA~LE 8
.
Chemical purability of Product
Obtai~ed in Example 18 ln
Deionized Water ~aving an
~nitial pH of 5~6*
Glass Component and Leach Rate**
Time
(Days) SiO Fe Ln*** Na Sr Cs
2 -- _
0.34 2800 62 c326500 560 223
1.3 905 8 3702500 2000 ~630
2.2 550 25 4401400 240 370
3.25 430 12 4401360 1200 670
5,7 200 100 150870 880 340
9,3 280 ~25 150780 600 630
15 12.2 313 ~ 1 200780 780 770
15.2 300 3 120840 ~ 620
* Data taken at 70 C.
** Leach rates are in ng of waste dissolved per cm2
of surface area of powdered product per day.
*** ~ncludes all lanthanites.
EXAMPLE 19
The porous powder mixed with nuclear waste described
in Example 18 was used in a tube made according to
Example 27. The mixture was heated in vacuum to 600C
then subjected to a temperature jump to about 1100C
at which temperature the waste porous glass mixture
sintered completely. The ion exchanged tube did
collapse completely. However, it cracked during air
quenching. Upon examination of the core material it
was found that it had been completely sintered and
that it was a good quality glass. Thus, by increasing
the loadi~g level of nuclear waste from Example 17,
~e were able to lower the sintering temperature to below
-73-
. _ .. .. ~ . , , _ _, . . .. . , . , .. . , . _ ~ . .. . . . ...
S~
the collapsi~g temperature of the ion exchanyed tube.
However, we put an excessive amou~t o~ nuclear waste
in this experiment and the expansion coefficient was
slightly too high causing a small number of cracks.
To achieve a completely sintered uncracked product
~ith ion exchanged tubes used in Example 17 and 19,
intermediate loading levels should be used. For
example, loading levels between 8 and 12%.
lQ
~5
2Q
-74/75-
