Note: Descriptions are shown in the official language in which they were submitted.
51
Background of the Invention
The disposal of large quantities of high level radio-
active wastes generated in the reprocessing of spent power
reactor fuel is a problem of considerable importance to 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 radiolytically stable. This
problem of dry solid stability is closely related to the
safety of human life on earth for a period of over 20,000
years. For example, radioactive waste contains the isotopes
Sr9, Pu240, and Csl37 whose half lives are 29 years, 66,000
years, and 30 years respectively. These isotopes alone pose
a significant threat to life and must be put into a dry,
solid form which are stable for thousands of years. The solid
radioactive waste form must be able to keep the radioactive
isotopes immob~lize~ for this length of time, preferably even in the
presence of a water environment.
A process for fixating 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.
There does not presently exist any practical, fool-proof
means for the safe disposal, storage and immobilization of
pernicious radioactive waste material. Present day storage
containers do not provide sufficient isoiation and
immobilization of such radioacti-~e material, sufficient long-
term resistance to chemical attack by the surroundings~ and
aufficient stability at high temperature.
Si
One present route is the so-called dry solids approach
which involves the method of fixation of was~e materials in
glasses via melting glass procedures. 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. ~urther, glasses remain relatively
more stable at high temperatures than plastics and are generally
more chemically durable in saline solutions than are metals.
Glasses with high chemical durability and low alkali ion
conductivities are melted at very high temperatures, e.g.,
1800C and higher. Such high melting processes are economically
unsound and moreover, cause a dangerous problem due to the
volatilization of pernicious radioactive materials. In view
of the overall difficulty of handling radioactive material,
and especially in view of the danger of volatilization of
radioactive material into the atmosphere, attention was
directed to using glasses having relatively low melting
temperatures, that is to say, using glasses with SiO2 content
as low as 27 weight percent. While the problem of volatilization
of radioactive materials was reduced, it was not completely
controlled. Moreover, the resultant glass composition
exhibited 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 standards,
these glass compositions were inadequate. Additionally,
none or very small amounts of gaseous radioactive materials
are trapped by the foregoing procedures.
--2--
1~1~51
As will be apparent hereinafter from the various aspects
of applicants' contributions to the art, there are provided
novel methods to obtain novel compositions and articles for
the containment of pernicious and dangerous radioactive
material over extraordinarily long periods of time. Unlike
melting glass containment procedures, the methods of the
invention need not involve any steps which would expose radio-
active material to temperatures above about 900C thereby
eliminating the environmental hazard due to volatilization of
radioactive material into the atmosphere. In addition, there
are provided novel methods for the fixation and immobilization
of radioactive gas wastes such as Kr. I, Xe, Ra, etc., in
a glass matrix.
Summary of the Invention
The invention contemplates novel non-porous glass
compositions or articles with high chemical durability to
aqueous corrosion and with sufficiently low radioisotope
diffusion coefficient values to provide protection of the
environment from the release of radioactive material such as
radioactive isotope, nuclear waste material, etc., encapsulated
or entrapped therein. Such glass compositions are characterized
by at least 75 mol percent of SiO2 and by a radiation activity
above one millicurie, preferably greater than one curie,
per cubic centimeter of said glass compositions. In one
aspect, the amount of radioactive material contained in the
novel glass compositions is at least 0.1 mol percent, generally
in the form of the oxide of at least five and preferably at
least ten o~ the radioactive elements listed hereinafter.
Preferably the novel glass compositions should contain at least
82 mol percent SiO2, most preferably greater than 89 mol
percent SiO2.
--3--
~ t~5 1
From a practical standpoint, the upper limit of radio-
active material contained in the novel non-porous glass
composltion will be governed, to a degree, by such factors
as: the SiO2 concentration in the glass, by the concentration
and type of other ingredients which may be present in the glass
such as B2O3, Al2O3, TiO2, zirconia, alkali metal oxides, ana
ao~' by the concentration and type of radioactive material
encapsulated in the glass, by the volume fraction of the porous
structure in the porous glass preform, by the various techniques
employed to encapsulate the radioactive material in the glass,
and other factors.
Radioactive material which can be encapsulated and
immobilized in the glass matrix include radioactive elements
(naturally occurring isotopes and man-made isotopes and
existing as liquids, solids or gases), generally in the form
of the oxide, such as carbon, rubidium, strontium, lanthanides
cobalt, cadmium, silver, zirconium, molybdenum, technetium,
niobium, ruthenium, rhodium, palladium, tellur1um~ cesium,
barium, francium, yttrium, radium, actinides and the gases of
krypton, iodine, bromine, xenon and radon. Especially
suitable in the practice of the invention are radioactive
wastes from nuclear reactors or other waste producing processes.
One embodiment of the invention is directed to novel
glass articles with enhanced containment properties and
characterized by an outer clad whose composition is at least
90 mol percent silica, preferably greater than 95 mol percent~
and whose inner core contains the radioactive materials.
The high silica content of the clad imparts to the articles
a considerably greater chemical durabllity. The inner core
has a lower silica concentration of the order set forth
hereinbelow, i.e., at least 75 mol percent silica, preferably
82 mol percent 7 and most preferably 89 msl percent.
l~lQ~5i
Various aspects of the invention are directed to novel
methods for producing the aforesaid novel compositions or
articles. The broad method is directed to encapsulating
or entrapping radioactive material in a glass matrix which
o~mprises impregnating the pores of a porous glass preform
with radioactive material, drying when necessary, heating
said porous glass preform until collapse of the porous
structure occurs, thereby trapping said material in the
resultant chemically inert, non-porous glass product.
The radioactive isotopes, as indicated previously,
exist as gases, liquids or solids. The liquid isotopes
are generally compounded into salts or other compounds,
which, as with the solid isotopes, are dissolved into solution.
Below, we treat such solutions separately from the gaseous
isotopes. In the case of the solutions, two embodiments of
the invention are described.
The literature adequately describes the preparation of
porous glass preforms. Suitable preforms which may be utilized
in the novel methods generally contain SiO2 as a major component.
In the practice of various embodiments of the invention the
concentration of SiO2 desirably is at least 75 mol percent
SiO2, preferably at least 82 mol percent SiO2, and most preferably
at least 89 mol percent SiO2 (and preferably less than l mol
percent alkali metal oxide). Such glasses are described in the
literature, see U.S. Patent Numbers 2,106,744, 2,272,342
and 2,326,059, for example. According to the first embodiment,
the dopant consisting of the radioactive isotopes in the form
of salts, oxides or other compounds are diffused into the
pores of the preform; the preform is then removed from the
dopant solution and is dried. Sintering of the "molecular
stuffed" porous glasses allows for decomposition of the salts
--5--
S~
into oxides and incorporation into the glass matrix. Sintering
is conducted at temperatures generally far lower than glass
melting temperatures and thus minimizes the chances of radio-
active material evaporation into the atmosphere. The resultant
non-porous glass thus produced has a finite concentration of
dopant throughout. According to the second embodiment,
the dopant solution containing the radioactive compounds is
diffused into the pores of the preform; the preform is
removed from the dopant solution and immersed in one or several
solutions of solvent such as water, methanol, ethanol, acetone
and nitric acid; the preform is then removed from the solvent
solution(s) after a sufficiently thick, substantially dopant-
free region has been developed at the surface of the preform
and is dried. Sintering as in the method of the first
embodiment ensues to form a non-porous glass with an inner core
region containing the radioactive dopant, and an outer cladding
region with a silica content of at least 90 mol percent
preferably greater than 95 mol percent, and essentially
containing no radioactive material.
Two embodiments of the invention are likewise set forth
hereinafter for entrapping radioactive gases into the glass
matrix. According to the first embodiment the porous glass
preform is dried and then exposed to the radioactive (waste)
gas at a pressure determined by the desired trapped gas
concentration in the glass, allowing the gas to dissolve
into the glass matrix, and the pores are sintered at high
temperatures in the presence of the radioactive gas. According
to the second embodiment, the porous glass preform is first
immersed in a solution containing one or more other dopant compounds
desi~ably in the form of their salts, prior to drying. These dopant
compo~nds may be chosen to increase the surface area which in turn
increases the soiubility of the radioactive gas in the glass or
1~10~51
to lower the glass transition temperature. The concentration of
dopant compounds in the glass may be varied if desired. After
precipitating said dopant compounds, the preform is dried and,
if necessary, further heated to decompose the dopant compounds
generally to their oxide form. The preform is then exposed to
the radioactive waste gas at a pressure determined by the
desired trapped gas concentration in the glass, allowing the
gas to dissolve into the glass matrix, and the pores are sintered
at high temperatures in the presence of the radioactive waste gas.
l~ The variation in dopant concentration in the glass
preform prior to drying and incorporation of the radioactive
waste gas may be desirable for several reasons. If the dopant
~ is another radioactive waste product, it will be desirable
; to lower its concentration as low as possible near the
~urface of the glass preform to further reduce its chance of
leaking to the environment. Secondly, if the dopant used
~ increases the solubility of the gas in the porous structure
c of the preform, it will also be desirable to decrease its
concentration as low as possible near the surface of the
preform since this will result in a lowered dissolved radio-
active gas concentration near the surface of the glass and
q~ thus decrease its chance of leakage into the environment.
Thirdly, if the dopant lowers the glass transition temperature
y of the glass matrix, and its concentration is varied to have
a maximum at the interior of the preform and a minimum in a
layer ad~acent to the surface, then corresponding differences
in the concentratlon of the dissolved radioactive gases will
be obtained.
In the earlier fiber optics patent, U.S. Patent No.
3,938,974, we have limited the range of dopants used because
high optical transmission was necessary and those dopants
which caused the glass to attenuate light could not be used.
51
However, in the radioactive fixation contemplated herein
no such criteria need be applied.
A dopant is a compound which is deposited in the
porous glass. Said compound, or in the case of gases,
element or molecule, may be incorporated in the glass during
collapsing as deposited, or after conversion to the appro-
priate oxide. In the case of metals it is often introduced
as a nitrate in solution, or precipitated as a nitrate
crystal. Upon heating, it decomposes to an oxide crystal,
which upon collapsing of the structure, is incorporated in
the silica structure.
Illustrative dopants include, of course, the radio-
active material contemplated and described, the alkali
metals, the alkaline earths, boron, germanium, aluminum,
titanium, lead, bismuth, phosphorous and the rare earths, as
nitrates, carbonates, acetates, phosphates, oxides, borates,
arsenates and silicates in either hydrated or unhydrated form
or mixtures thereof.
Detailed Description of the Invention
Fabrication of the porous glass matrix may follow any
of the available methods used by one practiced in the art to
form porous glass in any desired shape, preferably cylindri-
cal or rectangular, with a composition greater than 75%
silica. We prefer to form our porous glass matrices in the
shape of cylinders according to the methods disclosed in
U.S. Patent ~o. 4,110,096. Briefly, a glass is melted con-
taining silica, boron trioxide and two alkali metal oxides
~such as Na2O and K2O) and drawn into long rods. By suitable
heat-treatment, these rods are phase separated into two
phases; one phase, a silica-rich phase containing also, B2O3
and alkali metal oxide and a
-- 8
Sl
silica-poor phase which contains greater amount of B203 and
alkali metal oxide. The heat-treated rods are then immersed
ln 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 preform characterized by a'SiO2 content greater than
90 mol percent which is ready for use as a matrix for the
encapsulation of the radioactive material. Herein several
procedures are followed depending upon whether the radioactive
materials to be encapsulated are in the form of solids and
liquids, or gases.
A. Solids and Liquids
In the case of solid or liquid radioactive material
including nuclear waste materials, the materials are generally
available as an aqueous salt or other compound mixture of
the various materials to be encapsulated. High level waste
generally consists mainly of the aqueous raffinate from the
first cycle of the well known Purex Solvent Extraction Process
as well as other miscellaneous waste streams. Significant
variation in the waste composition can occur. A typical
composition is given in Table I below. Solvent is added to
the waste material until all components have substantially
gone lnto solution. Some processing plants also extract
individual components from the waste mixtures such as Cs and
Sr for specific use.
~ ~c- ~o,~
l~lQ~51
TABLE II
Replacements in stuffing
Fl_sion ProductsConcentration (atomic %) solution
Rb 1.0 Rb(N03)
Sr 5.2 Sr(N03)2
rare earths* 17.8 rare earth nitrate mixture
Zr 20.3 Zr(N03)4 5H2O
Mo 18.1
Tc 2.1 - Fe(N33)2 6H2
Ru 11.4
Rh 1.0 Co(~03)3 5H20
Pd 6.2 Ni(N03)2 6~20
Te 2.3
Cs 5.2 Cs(NO3)
Ba 5.2 Ba(N03)2
actinides 4.2 rare earth nitrate mixture.
~ e use rare earths and lanthanides indistinguishably to
mean the series of elements consisting of lanthanum, cerium,
praseodymium, neodymium, promethium, samarium, europium,
gadolinium, terbium, dysprosium, holmium, erbium, thulium,
ytterbium and lutetium.
In the non-wasteapplications the radioactive elements
may occur singly, or as groups or compounded into salts or
other compounds. In any case, they must be dissolved into
solution or melted.
Porous glass substrates (in convenient rod shape) are
submerged in the dopant solutions containing the radioactive
materials for a period long enough to allow diffusion of the
dopant material solution (stuffing solution) into the pores.
This time depends upon the temperature of the stuffing solution,
the pore structure, and the si~e of the porous glass host.
--10--
Q51
Various methods may be followed once the stuf~ing
solutlon has filled tne pores with waste material or other
dopant. According to one method, the porous glass preform is
removed from the stuffing solution; then the dopant is caused
to precipitate within the preform by any of several routes
such as evaporating the solvent from the solution, decreasing
the temperature of the solution, by chemical means, etc. If
desired, the preform may be given a brief wash to clean its
surface area. Optionally, the preform may then be immersed in
washing solution such as a solvent with low solubility for
the dopant for several hours of soaking. Thereafter, the preform
is dried, e.g., in a desiccator, vacuum chamber, etc., preferably
at relatively low temperatures. This method when completed by
the drylng and sintering procedure described hereinafter, yields
non-porous glass compositions with an essentially homogeneous
concentration of waste materials.
According to a second method, the porous glass preform is
removed from the stuffing solution; the dopant is allowed to
begin precipitation, and thereafter the preform is immersed
in a solvent with intermediate solubility for the dopant
materials for several hours. This latter soaking procedure
allows the formation of a thick skin or clad at the outer surface of
the ~eform by virtue of dopant being redissolved into the solvent.
Soaking in several solvents with decreasing solubility for the
dopants allows the removal of dopant from this skin region.
Once accomplished, the preform is dried as illustrated above.
This method, when completed by the drying and sintering procedures
described hereinafter, yields non-porous glass compositions with a
skin of substantially lower concentration of dopant at the outside
3o
--11--
lllQ~l
surface of the preform. Since it contains a lower dopant
concentration, this skin acts to protect the interior of the
glass from chemical attack and acts to further delay diffusion
of radioactive materials from the glass interior to the surface
where they may react with the environment.
Illustrative drying and ~intering procedures including
keeping the preform prepared by either of the aforesaid illustrated
two methods under vacuum at low temperatures for approximateiy
a day and then heating them, essentially at a rate of 15C/hr
still under vacuum, to a temperature above the decomposition
temperature of the dopant and approximately 100C below the
glass transition temperature of the dopant-free porous glass
matrix. At this point, any desired redox conditions for the
dopant may be accomplished by var~ous routes such as by further
heating to the sintering temperature of the preform under vacuum
for a reduced valence state of the dopant, or heating to the
sintering temperature of the preform under a hydrogen-containing
atmosphere for a highly reduced valence state of the dopant,
or by heating to the sintering temperature of the preform under
~0 an oxygen-containing atmosphere of suitable pressure for a
partially or totally oxidized dopant.
This ability to alter oxidation-reduction states of the
radioactive materials allows a fine control of the resistance
of the glass to chemical attack since many of the radioactive
materials, especially the actinides, are less soluble in water
when in a reduced state. The ability to alter the oxidation-
reduction state of such dangerous materials provides an
additional safeguard against premature dispersion of such
materials int~ the environment.
After sintering at high temperatures the products are
cooled to ambient temperature whereby they form a non-porous
~l~Q6~51
glass completely encapsulating and immobilizing the radioactive
material in its glass structure.
In a further embodiment, the invention provides for the
storage and immobilization of radioactive contaminated boric
acid aqueous solutions resulting during its use as a heat
transfer media for pressured watér nuclear reactors. Such
contaminated solu~ions contain various radioactive impurities
including Co, Cs, Mn, Cr, Sr. and Zn as well as inert impurities
such as Fe. Presently such contaminated solutions are disposed
by fixation in cement forms and then buried a few feet beneath
the ground. This disposal technique is very unsatisfactory
since the cement has a low loading factor because the boric
acid at room temperature has about a 5% solubility. Furthermore,
cement is not especially insoluble and leaches into the water
table. Thus, even though one is dealing with the disposal of
low level nuclear fuel cycle wa~tes, the detrimental impact
on the environment and the population is obvious.
The novel methods disclosed herein provide for the
stuffing of such low level radioactive contaminated boric acid
solutions into the porous glass preform. Such stuffing
procedures can be effected at temperatures which will afford
loadings several times greater than that achieved in the cement
fixation technique. For example, the solubility of boric acid
in water at 100C is approximately 30 weight percent which
permits about six fold greater loadings than are achieved at
room temperature using the cement fixation technique.
The novel non-porous glass compositions of this
latter embodiment are characterized by the encapsulation and
immobilizaticn of low level radioactive contaminated boric
acid wastes (from pressurized water nuclear reactors), such
- 111~51
waste being fixed in the glass matrix in the form of oxides
of boron and the radioactive contaminants including cobalt
and chromium. Other contamlnants such as strontium, cesium,
manganese and/or zlnc may also be present in oxide form. Such
compositions generally contain less than one weight percent of
radioactive contaminants (in thé form of their oxide).
However, in serious leakage problems in the pressurized water
nuclear reactor, the boric acid wastes can contain much higher
levels of contaminants.
;J In Example~ I and II, the dopant solution contains only
cesium nitrate in order to demonstrate simply the behavior of
dopant and porous glass system. Cesium is chosen because of
its high concentration in the waste material and because by
virtue of being an alkali metal its diffusion coefficient
through glass is higher than other constituents except for
r. ~c~
rubidium (another alkali metal) which, hocvcr, is present at
low concentrations. All compositions are on a mole basis.
Cesium poses a further problem because of its high vapor pres~ure
at high temperatures normally required to melt glasses, which
we circumvent by the practice of the novel methods contemplated
herein. Thus, of all the radioactive isotopes in reactor
waste, Cs probably poses the most problems in containment in
glass and the greatest dangers in the near term. Accordingly,
a procedure which will immobilize cesium in a glass matrix
will also solve the containment problems associated with other
radioactive isotopes. Accordingly, a method which will result
in the immobilization of Cesium will also immobilize other
radioactive isotopes.
3~
-14-
51
In Examples III to VI below, the molecular "stuffing"
process of Example I is followed. In Example III, the stuffing
solution contains SrNO3 which like CsNO3 is present in
pernicious waste streams. The solution in Example IV stimulates
possible low level waste products in boric acid. The solutions
in Examples V and VI resemble the actual unseparated waste
material with Example VI simulating the major components of
typical radioactive wastes generated in reprocessing fuel
elements fr ~n utility-company reactors.
Examples I and II
The porous glass preform is obtained by melting a glass
having the composition: 60.2 mol percent SiO2, 32.8 mol percent
B2O3, 3.6 mol percent Na2O and 3.4 mol percent K2O, in a
platinum cruclble near 1400C, homogenizing the melt with a
platinum stirrer, thereafter pulling the melt into rods using
a glass bait, cutting into cylinders or rods 7 mm. diameter x 30.5
cm. long, heat-treating these rods at 550C for 1.5 to 2 hours,
annealing the glass, washing in 2.5% aqueous solution of
HF for 60 seconds, leaching in 3N HCl and 15 to 20% aqueous
NH4Cl solution, and then washing in water. The resulting porous
glass preform is characterized by an interconnective porous
structure and has the following composition: 95 mol percent
SiO2, 5 mol percent B203, and less than 0.1 mol percent alkali
metal oxide.
~ porous glass preform rod so prepared is then placed
into a so-called "stuffing" solution containing the waste
material which in this case is an aqueous solution of CsNO3
(con~entration is 65 weight percent CsNO3) for at least three
hours while maintaining the solution at 105C. In Example I,
the stuffing solution of CsN03 is then replaced by a mixture
51
of cold wate~ and methanol for a few minutes un~,il precipitation
of CsN03 occurs in the porous structure, then the rod is
soaked ln methanol f'or three hours. In Example II, the
stuffing solution is replaced by cold water and methanol for
less than 2 minutes whereupon precipitation of CsNO3 o~curs.
Then the glass rod is soaked in water for 3 hours at 2C,
followed by soaking in methanol for a~ least another 3 hours
at 0C. These two soakings produce a thin skin or clad of
the order of 1 mm. a,round the cylindrical surface of the rod.
The skin was devoid of CsNO3.
In both Examples I and II, after maintaining the glass
rods for 24 hours at 0C under vacuum (of the order of 0.1 mm.
of Hg), the rods are dried under vacuum at increasing temperatures
ranging from 0C to 625C to remo~e solvent, water and
methanol, and to decompose the cesium nitrate dopant to its
oxide. A heating rate of 15C/hour is used. Further heating
to 840C, which is the sintering temperature of the porous
glass rod, is conducted under an oxygen atmosphere. Once the
porous glass structure collapses, consolidation is considered
to be complete and the sintered non-porous rod is removed from
the furnace and brought to ambient temperature.
Thereafter, both non-porous rods obtained i'rom Examples I
and II were tested for cesium concentration and associated
refractive index profile. The composition Or the center of the
rods from both Examples I and II was 92.1 mol percent Sio2, 4.9
mol ~ercent B2O3~ and aPProximately 3 mol percent Cs2O (which
corresporlds to an in~ex Or refraction Or 1.487) with trace amount
of alkali metal oxide. In Example I, this concentration remaine~l
essentiall,y the same up to a very thin layer (or skin~
appreciably less than 1 mm. at the surrace. Analysis of the
non-porous rod rrom Example II gave a rerractive index value
-16-
S~
of 1.458 in a 1 mm. thick surface layer about the rod. This
value index indicates no measurable Cs2O dopant content in
this layer.
The sole Figure is a plot of the fraction of waste
material released into the environment after 20,000 years
of storage for a sintered glass, product from Example II
compared with a waste product obtained by the melting glass
technique from Battelle Pacific Northwest Laboratory, Richland,
Washington, U.S.A.
In order to obtain estimates of the diffusivity of
cesium ions to allow estimates of the loss of radioactive waste
as a function of time, electrical conductivity measurements
were made on the two non-porous glassproducts obtained via the
teachings of Examples I and II above and on a glass product
(melting process) obtained from Battelle Pacific Northwest
Laboratory. The values obtained are listed in Table II below.
TABLE II
CONDUCTIVITY AT TWO TEMPERATURES
Glass Temp. Conductivity Dif2fusivity
Produ~t C (ohm-m)~l (cm /sec)
Example I 100 8 x 10-18 1 x 10-24
Example II( ) 100 9 x 10 16 1.4 x 10 22
Battelle(3)100 3.5 x 1o-13 5.3 x 10 20
Example I 400 1 x 10 9 3 x 10 16
Example II(2) 400 3 x 10 1 x 10-14
Battelle(3)400 7 x 10-7 2 x 10 13
(1) These are calculated using the Stokes-Einstein relationship
and are therefore only estimated values.
QSl
(2) The data reported for Example II are for the surface
layer only. The central region has the same values as
reported for Example I.
(3) Battelle Pacific Northwest Laboratory, Richland, Washington.
This glass product contained less than one-half the cesium
concentration of dopant encapsulated in glass products from
Example I and II.
In Table II above, diffusivity is estimated from conductivity
values using the Stokes-Einstein relationship. These data
10 establish that the non-porous glass product of Examples I and II
when compared with the glass product (a melting procedure
product from Battelle Pacific Northwest Laboratory) at room
temperature, exhibit diffusivity values which are lower by
several orders of magnitude notwithstanding the fact that
the former products contained more than twice as much dopant
as the latter product. From the data, the theoretical weight
loss after 20,000 years of exposure to a perfect sink
(material which does not allow buildup of diffused molecules
or atoms at the boundary--which is the most pessimistic
estimate) is shown in the sole Figure plotted as a function
of storage temperature for a non-porous rod product made
according to the method outlined in ~xample II.
Example III
A porous glass preform ~in the shape of a rod) prepared
as described in Example I was immersed in a solution containing
95 grams os Sr(N03)2 per lO0 ml. of water at 60C. The rod
was allowed to soak in this solution for 4 hours, then the
rod was remo~ed from this solution and placed in cold isopropyl
alcohol. Precipitation of SrN03 occurred slowly within the
3Q porous structures, taking 2 to 3 hours to reach completion.
The rod was then soaked in a solution consisting of 66% ethanol
and 34% water at 0C for 5 hours to remove the SrNO3 dopant
Q51
from a cladding region at the surface of the rod. Finally,
the rod was soaked in a cold 90% ethanol-10% water mixture
and then in a cold isopropanol bath. Thereafter, the rod was
dried and collapsed (sintered) as in Examples I and II,
beginning with a stage in vacuum for 24 hours at 0C. The
rod when withdrawn from the furnace after sintering contained
strontium incorporated into the glass structure in a core
region of the rod; the cladding region was essentially free
of strontium.
Example IV
A porous glass preform rod prepared in Example I is
soaked in a solution at 100C containing 25 grams H3BO3,
0.27 gram Fe(NO3)3 9 H2O, 0-27 gram Cs(N03), 0.27 gram Sr(NO3)2,
0.27 gram Co(NO3)2 6 H20, 0.27 gram Cr(NO3)3 9 H2O, 80 ml
f water, and 1 ml of HNO3 (13.4 molar concentration) for 3
hours. The rod is removed from the solution and it is then
immersed in 0C water for 30 minutes, and dried in vacuum at
0C for 24 hours. The rod is subsequently heated at a rate
of 15C/hr to 625C under vacuum to remove remaining solvent
and to decompose the metal nitrate dopants, and then heated
at a rate of 50C/hr to 850C under vacuum to collapse the
pores and consolidate the glass. The final product ls non-
porous glass solid containing SiO2, B203, and the oxides of
Cs, Sr, Fe, Co, and Cr as an integral part of the matrix.
The water immersion step after the soaking (stuffing) step
decreases the dopant concentration near the surface of the rod.
The rod may also be aoaked in methyl acetate or acetone after
removal of same from the stuffing solution in order to form
a cladding region at the surface of the rod with lower dopant
concentrations. However, rods prepared in this latter manner
--19--
ll~Sl
exhibit a tendency to split along its length during the drying
and collapsing steps. However, such splitting of the rods is
not detrimental to the fixation and immobilization of the
radioactive material therein.
Example V
A solution is prepared comprising:
12.9 grs Sr(NO3)2
18 grs Rb(NO3)
15.9 grs Ba(N03)2
104.1 grs Zr(NO3)4 5H20
153.6 grs Fe(NO3)3 9H2O
4.2 grs Co(NO3)2 6H20
21.3 grs Ni(No3)2-6H2o
12.3 grs Cs(NO3)
89.4 grs rare earth nitrate mixture
and sufficient water to cause the above metal dopants to
dissolve therein at approximately 100C. A porous glass
preform (in the shape of a rod) made as described in Example I
was soaked in the above (stuffing) solution for 3 hours. The
rod was then removed from the solution and inserted into a
solution of 3N HNO3 at 0C for 10 minutes. The rod was then
soaked ln acetone for 2 hours at 0C and was then placed in
a drying chamber maintained at 0C with suction pumping means
operating a period of 2g hours (for withdrawing solvent from
the rod~, Thereafter, the rod was heated under vacuum up to
625C at a rate of 15C/hour. The rod was held at 625C
for 24 hours under vacuum, then heated slowly to 840C,
also under vacuum to obtain a highly reduced state for the
waste material dopants contained in the porous structure of
the preform. The rod was sintered near 840C and formed a
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~11C~51
solid non-porous glass product with the waste material as an
lntegral part of lts structure.
Example VI
A solution containing 10 major components from a
typical radwaste composition often denoted as PW8a is used
in this Example. The relative concentration of these components
is representative of the waste stream composition with a
reconcentration factor of three times. The solution contains
the following:
148.8 grs Fe(N3)3 9H20
21.3 grs U02(N03)2-6 H20
33.6 grs NaN03
18.6 grs Zr(N03)4 5H20
19.8 grs Nd(N03)3 5H20
9.0 grs Ce(N03)3 6H2o
4.2 grs La(N03)3 6H2
4.2 grs CsN03
2.4 grs Sr~N03)2
3.0 grs Ba(~03)2
264.9 grs HN03
150 ml H20
Sufficient water and nitric acid in ~mo~lts i^,~ c~ted -,ov~
we~e added to maintain the above metal nitrates in solution.
The preparation of the solution was accomplished stage-wise at
varying temperature levels; the final temperature being 99C.
Six porous rods prepared as in Examples I and II were
immersed in the above solution maintained at g3Oc for 3 hours,
rinsed in 20C water for 5 seconds, and then placed in a
desiccator r~aintained at 20C for 16 hours (hereinafter
"non-clad" rods). Three non-clad rods were removed frorn the
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l~lOQSl
desiccator and were subjected to the following immersion
treatments to produce a cladding region at the surface of the
rod havlng a lower concentration of dopants (waste materials~
than the lnner core of the rod (herelnafter "clad" rods):
30 mlnutes ln isopropanol containing 8 weight percent Uo2(No3)2-6H2O,
4 weight percent Ce(NO3)2 6H2O and 2 weight percent La(NO3)3 5H2O,
and an additional 30 minutes in another solution of 80%
chloroform and 20% isopropanol. These three clad rods were
then dried for 24 hours at 0C, at Cl mm. of Hg; thereafter,
they were put into a furnace with the remaining 3 non-clad rod
rods at room temperature and heated at 15C/hr to 625C under
vacuum to remove solvents and to decompose the metal nitrate
dopants to metal oxides. The rods were grouped into three pairs,
one clad and one non-clad. Each pair was then heated at
50C/hr under different en~ironments, namely (a), (b), or (c) below:
(a) 95% N and 5% H at one atm. (high reducing
condi~ion); 2
(b) Under vacuum, i.e., ~1 mm. of Hg ( a moderate
reducing condition); and
(c) One atm. of air (an oxidizing condition).
At about 860C sintering and collapse of the porous structure
of the rods occurred.
Analysis of the resulting 3 pairs of non-porous glass
rod products showed the metal oxide dopants to be in appropriately
varying oxidation states. It is quite apparent that by the
practice of this invention there can be obtained novel non-
porous, chemically inert, glass products containing radioactive
materials (in their lower state of oxidation),e.g., the actinides,
encapsulated and immobilized therein for extraordinary long
periods of time, e.g., centuries.
5~
B. GASES
Various embodiments of the invention, as noted herein-
above, are directed to novel methods for the encapsulation and
lmmobilization of radioactive gases in glass.
One such novel method is illustrated by Examples VII
and VIII wherein the porGus glass rod, as the host, is
partially dried at 0C under vacuum for about 24 hours,
then further dried under vacuum by heating progressively to
an upper drying temperature which is 50C to 150C below the
1~ glass transition temperature of the undoped consolidated
glass used as the host. This drying temperature is about
625C for the porous glass rod employed. At 625C, one or
several radioactive gases are introduced into the enclosed
chamber containing the rod. The temperature is held at 625C
for approximately 24 hours, then the rod is heated at
approximately 50C/hour up to 835-900C where sintering occurs
under an atmosphere of the radioactlve gases, thereby trapping
the dissolved gases within the glass matrix. The heating
rates and holding temperatures may be readily altered, if
desired. Rapid heating may in some cases break the rod. This
breakage, however, has no effect on the fi~ation and
immobilization of the radioactive gas within the glass host.
In a second novel method, the porous glass rod is
immersed in a solution containlng one or several dopants.
If desired, these dopants may be chosen to increase the
radioactive waste gas solubility in the rod. After the dopants
have diffused into the pores of the glass rod, the rod
is removed from the dopant solution and immersed in a dopant-
free solvent. If desired, the concentration of dopant within
the pore structure may be varied by various routes. First,
for no dopant concentration variation in the glass rod,
the solvent bath will be chosen to cause precipitation of
~1~51
the dopants evenly within the pores of the rod. Then the
solvent is removed and the rod dried. Secondly, (see
Example IX) if a dopant-free layer forming the surface of
the rod is desired, the dopant is precipitated as in the first
case, but then the rod is transferred to another solvent or
solvents whereby the dopant in ,the surface layer is redissolved
and allowed to diffuse out of the rod, before drying. Thirdly,
if a graded variation of dopant is desired with the structure
of the rod, the rod is first soaked in a solvent which does
not precipitate the dopant and allows its diffusion out of
the pore structure. Then it is transferred to another solvent
or solvents which induce precipltation and thereafter the
rod is dried.
If desired, the drying step can begin at 0C in vacuum.
The rods are then slowly heated, i.e., rate of 15C/hr, to
6~5C under vacuum to remove remaining solvent and allow
~or dopant decomposit~on. At 625C, one or several
radioactive gases are introduced into the heating chamber
containing the porous rods, and the temperature is held at
625C for about 24 hours. Then the rods are heated at a rate
of 50C/hr up to ~35-~00C where sintering occurs thereby
incorporating the dissolved gas into the glass matrix.
In a third novel method, one can develop a radial
variation in the concentration of the radioactive gases
dissolved within the glass structure. Here the porous glass
rod is doped with a material which will reduce its glass
transition temperature such as ~tassium in the form of potassium
nitrate. After the dopant has completely diffused into the
pores of the rod, the dopant is precipitated within the pores
either thermally by cooling the rod or by introducing a solvent
with very low solubility for the dopant. The rod is then
immersed into a solvent with from 5 to 10% solubility for the
dopants in order to remove dopant fro~ a layer ad~acent to
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1~5~
the surface of the rod. After drying of the solvent, the rod
will have a high concentration of dopant in the interior and
a low concentration in the surface layer thereof. As
a result, the rod will have a higher sintering temperature
for its surface layer and a lower sintering temperature for
lts interior. The sample is dried under vacuum at 0C for
about 24 hours. It is then heated to 600C under vacuum at
15C/hr. At 600C one or several radioactive gases are
introduced into the chamber containing the rod. The temperature
is then slowly raised until the inner region of the porous
glass rod is sintered, while remaining below the temperature
at which the outer region of the rod sinters. Now the
radioactive gases are removed from the environment of the rod
and the temperature raised to sinter the surface layer. By
this method the inner region of the rod will contain dissolved
radioactive gases while the surface layer will be essentially
free of dissolved radioactive gas content.
Example VII
A porous glass preform prepared as described in Example
I was immersed in methanol for 3 hours at ambient temperature.
It was then transferred to a drying chamber maintained at 0C
for 24 hours with suction pumping means operating to dry the
rod. The rod was heated to 650C at a rate of 50C/hr to
complete drying. At 650C, Kr gas was introduced into the
chamber containing the rod at 1 atmosphere. The rod was
then kept at 650C for 18 h~urs in a Kr environment and heated
to 880C to achieve sintering in the Kr environment, thus
trapping the dissolved Kr gas in the glass matrix.
l~l~Sl
Example VIII
The procedure of ~xample VII is repeated except that
lnstead of krypton, iodine gas was introduced into the chamber
at 650C by boiling iodine pellets to achieve an atmosphere
of iodine at 1/10 bar of pressure. The rod was kept for 18
hours at 650C in this iodine environment and was heated to
880C to achieve sintering of the pores of the rod thus
trapping the dissolved iodine gas into the glass matrix as
evidenced by a pink color of the resulting non-porous glass
rod product.
Example IX
This example illustrates incorporating both a dissolved
solid and a gaseous dopant into the glass matrix.
A porous glass preform prepared as described in Example I
was immersed in a 67 weight percent CsN03 solution at 105C
for 4 hours. The rod was then transferred to, and immersed
into, pure water at 0C to induce precipitation of the cesium
nitrate within the pores of the rod. Total precipitation
took 30 seconds. The rod was left in water at 0C for 3 hours
to induce a variation in cesium nitrate concentration within
the pores, resulting in a cylindrical layer free of cesium
nitrate ad~acent to the surface. The rod was then transferred
to and immersed in methanol at 0C where it was kept for 3
hours to further reduce the cesium nitrate concentration
at the surface. Drying began immediately thereafter by
inserting the rod into a vacuum chamber at ~C for 24 hours.
Thereafter, the rod was heated to 625C at a rate of 15C/hr
to complete drying and to induce decomposition of the cesium
nitrate to cesium oxide. ~he rod was maintained under vacuum
at 625C for 24 hours. Krypton gas was then introduced into
-26-
Q5~
the chamber until a pressure of one atmosphere was reached.
The rod was maintained at 625C for an additional 24 hours in
this Kr environment, followed by heating at 50C/hr to the
sintering temperature of the rod to thus encapsulate and
immobilize dissolved Kr as well as Cs in the glass matrix.
As a result of the improved encapsulation of radioactive
material especially radioactive nuclear waste by this invention,
these radioactive materials can be put to beneficial use for
mankind instead of posing an irrepressible threat to life on
earth. Some examples of useful applications are discussed below.
After sintering the porous glass, the radioactive
elements are completely encapsulated within the glass matrix.
These radioactive elements are chemically bonded and immobilized
within the novel glass article and in such form represent a
safe, extraordinarily long-term containment of pernicious and
hazardous radioactive nuclear waste streams. The novel glass
articles may be transported with little risk of leakage of the
radioactive isotopes into the environment. They can be
deposited into subterranean vaults for long-term storage.
These vaults would be preferably more than l,000 feet below
the surface of the ground.
Some radioactive isotopes, including some which do not
come from radioactive wastes can be used for beneficial
applications rather than storage. Such applications include
sterilization of foodstuffs and drugs by destroying bacteria,
viruses and other microorganisms; uses in medical and industrial
instrumentation, in educational devices, and in research.
For sterilization, glass articles containing selected isotopes
including radioactive cesium, strontium and cobalt can be
used to preserve foods and to recover the nutrients in sewage
sludge for inclusion into the food cycle. These applications
1~1(~5~
are not widely used commercially today because suitable
containers must be developed to prevent contamination of the
foodstuffs by radioactive isotopes and to prevent the associated
high probability of radiation poisoning which would occur
from their ingestion. Our invention will allow the use of
radioactive materials which aré produced in large quantities
in nuclear reactors by safely encapsulating them as described
herein to sterilize food products.
The safe encapsulation of radioactive waste materials
in glass articles by our invention also allows their use in
medical, industrial, educational and research instrumentation
where radiation sources are required for ionization, sterilization,
irradiation of patients, quality control and miscellaneous
equipment for the detection of flaws in materials by gamma
ray irradiation.
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