Note: Descriptions are shown in the official language in which they were submitted.
1 ~9,555
ENCAPSULATING SPHEROIDS CONTAINING
NUCLEAR WASTE
BACKGROUND OF THE INVENTION
Approximately 25 million gallons of high-level
nuclear waste has accumulated at the Savannah River Labor-
atory (SRL) of the Department of Energy (DOE) from the
production of defense materials during the past 25 years.
One procedure under consideration for disposing of this
nuclear waste is to encapsulate it in glass or ceramic
; spheroids. The spheroids are then coated to reduce the
leachability of the nuclear waste from the glass material.
The coated glass or ceramic spheroid waste form concept is
one having particular appeal because the spherical waste
form is produced directly from a liquid in a dustless
process that is especially amenable to remote operation.
However, the procedures plannPd for coating the spheroids
pose some serioud difficulties because three separate
coatings are to be applied by chemical vapor deposition at
temperatures greater than 1100C. First, pyrolytic carbon
will be deposited by thermal decomposition of acetylene in
a fluidized bed reactor. Then, two alumina layers will be
-3~3
~ 49,555
applied by uslng gaseous aluminum tetrachloride and hydro-
gen in a rotating drum furnace. The use of highly com-
bustible gases and high temperatures in combination with
nuclear waste creates a danger of an explosion or fire
with the potential release of radionuclides. Also, com~
plex manipulatlons must be performed by remote operations,
and hydrochloric acid by-product solutions are produced
which are difficult to treat.
SUMMARY OF THE _VENTIONS
We have discovered a method of applying an
amorphous coating to glass or cera~lic spheroids containing
nuclear waste. The coating of this invention i5 of very
low leachability and can be made to have about the same
thermal expansion as the spheroids do, so that cracking
does not occur with changes in temperatura. The coating
of this invention can be applied and cured at temperatures
of about 400 to 500C, but~if desired, the coating can be
heated at about 850C to give a dense, amorphous coating
o~ the same quality as coatings produced conventionally at
much higher temperatures.
DESCRIPTION OF THE INVENTION
-
Figure 1 is a diagrammatic side view in section
illustrating a certain presen-tly preferred embodiment of a
spraying process for coating spheroids according to the
process of this invention.
Figure 2 is a diagrammatic side view in section
showing a certain presently preferred em~odiment of a
two-liguid immersion process for coating spheroids accord-
ing to this invention. Alternatively, this two-liquid
immersion process could also be utilized ln reverse pro-
vided the specific gravities of the constituen~ fluids
were properly matched.
In Figure l~a sphere dispenser 1 drops spheroids
2 one at a time into chamber 3. At the top portion of the
chamber, spray nozzles 4 spray a coating composition ac-
cording to this in~Tention onto spheroids 2. In the lower
portion of chamber 3 a heating coil 5 heats the coatings 6
on the spheroids and hardens them.
. .
3'~3~3
3 ~9,555
In Figure 2~a sphere dispenser 7 drops spheroids
8 one at a time into trough 9. The trough 9 contains a
hydrolyzed alkoxide coating composition 10 in its upper
portion, an immiscible liquid layer 11 in its middle
portion, and a hardening agent 12 in its lower portion.
The immiscible liquid layer, which separates the
coating composition from the hardening agent, is necessary
because otherwise the two fluids will mix and the entire
coating composition will become hard. A silicone oil
layer about 1/8 to about 1/4 inch thick has been found
suitable for this purpose. It is preferable to heat the
hardening fluid up to about 75C to accelerate the harden-
ing process. Higher temperatures, however, should be
avoided as they may result in bubbles in the coating. A
pump 13 circulates the hardening fluid through line 14
which forces the coated spheroids up tube 15 onto moving
belt 16 into heated chamber 17. Excess hardening fluid is
collected in vessel 18 where it passes through line l9 to
the pump for recirculation. In heated chamber 17 the
coated spheroids are calcined to glassify the coating.
The spheroids then pass through opening 20 where they are
collected for disposal.
In the two liquid-immersion process the harden-
ing fluid may be either a setting ("electrolytic") agent
or a dehydrating agent. Sodium hydroxide, ammonium hy-
droxide, hydrochloric acid, acetic acid, and hot water,
are suitable setting agents. Sodium hydroxide is pre-
ferred bec~use it promotes the most rapid hardening,
although it does contaminate the coating somewhat with
sodium ions. The purpose of the hardening agent is to
make the coating sufficiently hard and non-tacky so that
the spheroids will not stick together and the coating ~ill
not be damaged in handling. The amount of setting agent
required to produce hardening of the coating may vary from
3S about 0.03 to about 0.1 moles per mole of alkoxide ~lass
forming constituent. Generally, equal volumes of alkoxide
coating solution and setting agent are present in the
~ ~9,555
system when sodi1lm hydroxide is used as the setting agenk.
With the other setting agents which take a longer time for
hardening, about 3 to 4 times as much as the coating
solution is used to give an increased travel time.
The dehydrating agent works by removing water
rom the alkoxide composition, thus further cross-linking
the silicon and aluminum oxides. The preferred dehydrat-
ing agent i5 t.richloroethylene as it has been found to
work well, although 2-ethyl-l-hexanol or octanol may also
be used. When a dehydratin~ agent is used, a molecular
siev0 should be inserted into the recirculating line
leading ~rom the pump to remove the water captured by the
dehydrating agent. It is preferable to use a dehydrating
agent instead of a setting agent as dehydrating agents do
not introduce contaminant species such as sodium into the
coating, even though they promote less rapid hardening o
the coating.
The final step in the process of this invention
~, is to cure the coatin~ which can be accomplished by heat-
20 ing the coated spheroids to about 400 to about 500C in
air for about one hour. At this temperature the cured
coating contains closed porosity. Heating to about 800C
will completely densify the coating. To avoi~ thermal
shock it is desirable to expose the coated spheroids to a
temperature o~ about lOO~C before exposing them to the
higher temperature.
The process of this invention may be repeatedusing the same spheroids in order to enhance the thickness
of the coating to the desired level. Generally, a coating
of about 0.5 mm thick is considered desirable for the
disposal of nuclear waste. Each coating cycle, however,
should not add more than about 0.1 mm to the thickness of
the coating in order to avoid cracks in the coating.
Preerably, each cycle should add about 0.05 mm to the
coating thickness. This normally means about 5 to 10
passes are required to produce a coating of a desired
thickness.
!
33
49,555
The nu_]ear waste contained in the spheroids may
take a variety of forms~such as a sludge consisting of a
mixture of complex hydroxides or hydrolyzed oxides of
aluminum, iron, magnesiu~" manganese, silicon, calcium,
sodium, potassium, ruthenium, mercury, nickel, cesium,
strontium, uranium, molybdenum, the transuranics, and
other elements. Defense nuclear wasce can also include up
to about 10 percent by weight sulfate, phosphate, nitrate,
or mixtures thereof, and up to about 95 percent by w~ight
water. The radioactive elem~nts in nuclear waste may
include uranium, thorium, cesium, ruthenium, iodine, and
stront.ium. The nuclear waste in the spheroids may also be
the result of fuel reprocessing which produces an aqueous
nitrate solution of many of thesP elements.
The nuclear waste is contained in various types
of glass or ceramic, usually of a borosilicate type by
processes well known in the art. See, for example, the
reference "Ceramics in Nuclear Waste Management," Proceed-
ings of an International Symposium held in Cincinnati,
Ohio, April 30-May 2, 1979, sponsored by the American
Ceramic Society and the U.S. Dept of Energy, pp. 73-122.
There are saveral basic processes for containing the
nuclear waste in spheroids. One process is a gel precipi-
tation process. In this process the ~uclear waste is
mixed with a gel-support additive such as polyvinyl chlor-
ide, methyl cellulose, and/or formamide. Drops of the
mixture are then permitted to fall into a gelation agent,
such as ammonium hydroxide, which hardens the drops into
small spheroids. The spheroids are then collected and are
heatad to remove the organics and to densify them, result-
ing in ceramic spheroids about 0.5 to about 3 mm diameter.
Another process for containing the nuclear waste
in spheroids is a marble process. In this process nuclear
wastes are mixed with glass frit containing such constitu-
ents as sodium oxide, silicon dioxide, and boron oxide.The mixture is then melted and cast into molds. This
process produces glass spheroids about 10 to about 25 ~m
in diameter.
6 ~9,555
The spheroids shouid be cleaned before they are
used in the process of this invention. Cleaning may be
accomplished by immersion in trichloroethylena, ethyl
alcohol, one molar (1 M) nitric acid, or other cleaning
fluids.
The composition used to coat the ~lasses is a
mixture o alcohol and partially hydrolyzed alkoxides.
The first glass-forming component of the composition is
prepared from a silicon compound having the general form-
ula SiRm(OR')nXp or Si(OSiR3~ where each R is independ-
ently selected from alkyl to ClO and alkenyl to C10, each
R' is independently selected from R and aryl, each X is
independently selected from chlorine and brornine, m is O
to 3, n is O to 4, p is O to 1, and m + n + p equals 4.
The SiRm(OR')nXp compounds are preferred due to their
availability, stability, and compatibility with the other
glass-forming constituents. The R' group is pr~ferably
alkyl to C4 with n = 4 because these alkoxides are the
most suitable starting compounds.
Examples of appropriate compounds which fall
within the scope of the general formula include:
Trimethylethoxysilane (C~3)3Si(OC2~5)
Ethyltriethoxysilane C2H5Si(oc2H5)3
Tetrapropoxysilane Si(OC3H7)4
Tetraethylorthosilicate Si(OC2H5)~
Tetratriethysiloxysilane Si[OSi(CH3)2C2~5]4
Triethylchlorosilane (C2H5)3SiCl
Vinyltriphenoxysilane CH2:CHsi(oc6H5)3
The preferr0d silicon compound is tetraethylorthosilicate
because lt is relatively i~expensive, readily available,
stable, and easy to handle. ~efore the silicon compound
is added to the composition, it is partially hydrolyzed
because it:s rate of hydrolysis is slower than the other
compounds, and preferential precipitation may result if
3~3~
7 ~9,555
t.,~ components are hydrolyzed after they have been combin-
ed. Partlal hydrolyzation may be accomplished by the
addition of water to the silicon compound, where either
the water, the silicon compound, or both, have been dilut-
ed with alcohol. The molar ratio of a silicon compound tothe alcohol can range from about 0.2 to about 2. The
alcohol is preferably the same alcohol that is prodwcsd
during hydrolyzation so that it is not necessary to separ-
ate two different alcohols. The mole ratio of the silicon
compound to the water can range from about 0.1 to about 5.
It is occasionally necessary to use up to about six drops
of concentrated nitric acicl per mole of water to aid in
the hydrolyzation reaction.
The second component of the composition is an
lS aluminum compound which has a general formula ALR'~(OR)rXS
or Mg(Al(OR)4)2 where each R is independently selected
from alkyl to C10 and alkenyl to C10, each R' is indepen-
dently selected from R and aryl, q is 0 to 3, r is 0 to 3,
s is 0 to 1, and q + r ~ s is 3. The AlR'q~OR)rXs com-
pounds, where r is 3 and R is alkyl to C4 are preferred asthey are the most stable and available and are easiest to
handle. Examples of suitable aluminum compounds include:
Trimethyl Aluminum Al(CH3)3
Triethyl Aluminum Al~C2H5)3
Triethoxyaluminum Al~OC2H5)3
Aluminum Isopropropate Al~ 3 7)3
Aluminum Secondary Butoxide Al( C4 9)3
Triphenyl Aluminum A ~ 6 5)3
Aluminum Magnesium Ethoxide Mg[Al(0c2Hs)4]2
Diethylaluminum Chloride (C2H5)2AlCl
The ~referred aluminum compound is aluminum
secondary butoxide because it is stable, available, and
does not require special handling. These compounds ara
hydrolyzed prior to addition to the composition to avoid
~43~
8 ~9,555
inhomogeneities. Hydrolysis can be accomplished using a
molar ratio of alumi~um compound to water of about 0.0007
to about 0.03 and using about 0.03 to about 0.1 mole of lM
HN03 per mole of AlO(OH) produced. The water is prefer-
ably heated to about 70 to about 100C.
In addition, the composition may contain alicox-
ides of boron or sodium whirh may be needed to match the
thermal expansion of the coat:ing with the thermal expan-
sion of the spheroids. EIowever, preferably no boron or
sodium compounds are present as they increase the leach-
ab.ility of the coating. Sod:Lum compound~ which could be
used have a general formula NaOR or NaZR'3 where each R is
independently selected from alkyl to C10 and alkerlyl to
C10, each R' i5 independently selected from R and aryl,
and Z is carbon or boron. The NaOR compounds where R is
alkyl to C4 are preferred as they are more stable and
compatible. The sodium compounds should be hydrolyzed
prior to being mixed into the composition to avoid differ-
ential hydrolyzation. A molar ratio of a sodium compound
~0 ts water of about 0.003 to about 0.1 may be used for
hydroly7ation. Suitable sodium compounds which fall
within the scope of the general formula include:
Sodium Methylat~ NaOC~3
Triphenylmethylsodium MaC(C6H5)3
Triphenylborylsodium NaB(C6H5)3
Sodium methylate is preferred as it is easier to
handle and i5 readily available.
30ron compounds which can be used have a general
formula BR'~(OR~rXs where each R is independently selected
30 from alkyl to C1~ and alkenyl to C10, each R' is independ-
ently selected from R and aryl, q is O to 3, r is O to 3,
s is O to 1, and q ~ r ~ s is 3. The compounds where R is
alkyl C4 and r is 3 are preferred as they are relatively
available and well~matched with the other constituents. A
molar ratio of a boron compound to water of about 0.1 to
l.O may be used. Dilution in the same alcohol as that of
9 49,55~
the boron compound at a level of about lS to 25 moles
alcohol to boron compound is pre~erred for a more homogen-
eous hydrolyzation. Suitable boron compounds which fail
within the scope of the general formula include:
Trimethyl Boron B(CH3)3
Triethyl Boron B(C2H5~3
Trimethyl Borate B(OC~3)3
Triethyl Borate B(OC~H5)3
Triisobutyl Borate B(OC4~9)3
Triisopropyl Borate B(OC3H7)3
Triisobutylborine B(C~9)3
Dimethyloxyboron Chloride (CH3o)2Bcl
Diphenyl Boric Acid (C6H5)2BH
Trimethylborate and triethylborate are pr~ferred
as they are relatively available and are compatible and
re~uire very little special handling.
Finally, the composition preferably includes up
to about 2 percent of a surfac-tant to increase the adhe-
sion of the coating to the spheroid. A suitable sur-
factant i5 octylphenoxy~lyethoxyethanol sold under the
trade designation Triton~ -102 by Rohm and Haas of Phila-
delphia, Pennsyl~ania.
EXAMPLE 1
Aluminum_Alkox-de PreParation - batch I
To 108 g of deioni2ed water at 85C was added
13.014 g Al(OC4~9)3) while stirring. Subse~uently, 7.8 ml
of lM HNO was introduced, and the resulting solution
(actually a colloidal dispersion) was aged for 12 hours ~t
85C.
Aluminum Alkoxide Preparation - batch II
The same procedure as above was followad with
the exception that the weight of Al(OCaHg)3 was double~.
1~ ~9,555
Silicon Alkoxid~_Pr~e~ration
To 104~ Si(OC2H5)4 was added 90 g of absolute
ethyl alcoho , followed by 9 g of cieionized ~ater and l
drop of concentrated HN03.
Combining this hydrolyzed silicon alkoxlde batch
with batch I of the hydrolyzed aluminum alkoxide corre-
sponded to a 90 SiO2:lO Al203 ratio on an oxide basis.
The use of batch II of the hyd~olyzed aluminum gave a 80
Si2 2 Al23 oxide ratio-
Generally, in most of the coating experiments
better results were obtaine~ with the addition of a wet-
ting agent to the hydrolyzed alkoxide mixture to ~nhance
the application of the coating on the simulated waste-
glass spho~oids ~or o~er ~lass shapes ) . P~ter c~nsider-
'~`' 15 able screening, Trito -102, a Rohm and ~as product, wa~
~o~nd to be most e~ective at a concentration ~ about O.S
volume percent, although acceptable coatings were obtained
with up to ~2 v/o (% by volume).
A syst~m, represen~ed by Figure 1, was assembled
which allowed either of the silicon-aluminum alkoxide
mixtures to be atomized onto the waste-glass spheroids,
followed by heating. This heating to cure the coating was
performed as depicted in Figure l, or alternatively, by
pre-heating tha spheroids to about 250C before spray
application of the coating. In this case a wetting agent
(surfactant) was not required. Subsequent heating to
500C removed all water of hydration and residual alkyls
to yield an adherent, amorphous coating. Although the
coating produced at 500C was porous, the porosity was not
joined and the coating served as a durable barrier against
leaching. Haating to 800-850C produced a totally dense,
amorphous coating identical to that which would be ob-
tained by melting at much higher temperatures using con-
~entional processes.
The coating process was repeated a number of
times to increase the thickness o the coating and to
ensure against imperfections such as connected porosity or
11 49,555
cracks. Even whetl the coating composition did not exactl~
match that of the spheroid, cracks or spalling of the
coating due to diferences in thermal expansion coeffi-
cients were not observed. Composltional anal;sis by EDAX
across the spheroid-coating interface indicated some
short-range "diffusion" of sodium in particular from the
spheroid into the coating. Perhaps this i5 promoted by
the reactivity, resemb].ing chemical etching, of the alk~
oxides with respect to the glass spheroid (substrate).
EXAMPLE 2
The system illustrated in Figure 2 was built to
enable the spheroids to be individually dispensed into the
alkoxide coating f].uid and, then, through a solution which
caused curing of the coating into a stiff gel before the
spheroids came into contact with one another. The two
liquids wera separated by an immiscible silicone oil
manufactured by Dow Corning Co. Using the silicon- _
aluminum batch II alkoxide mixture containing Triton~)
surfactant prepared in Example 1 in the top of the column
(Figure 2), and sodium hydroxide of about 0.5 M in the
bottom segment, waste glass spheroids properly cleaned in
trichloroethylene with ultrasonic agitation were success-
fully coated. The sodium hydroxide produced rapid and
uniform curing o~ the coating. Other ayents such as
ammonium hydroxide, hydrochloric acid, and acetic acid
required somewhat longer times for curing and, thus, a
longer travel time in them was needed before khe spheroids
could be collected. Final heating to 500 or 800C pro-
duced the quality vitreous coatings described in Example
1.
EXAMPLE 3
An alternative to using a "setting" agent in the
bottom stage of the column in Figure 7, was to use a fluid
which would cause curing of the coating by means of de-
hydration. Such a liquid was trichloroethylene or octanolor 2 ethyl-1 hexanol. The solubility of water in these
fluids was sufficient, particularly when they were warmed
12 49,555
to ~75C, to cause the coatiny to harden. Al~hough the
hardening did not occur as rapidly as with the sodium
hydroxide setting agent, a high quality coating as de
scribed in Example 1 could nevertheless be obtained. In a
continuous system, the water would be extracted by molecu-
lar sieve, for example, and circulated back to the column.
