CA1182993A - Encapsulating spheroids containing nuclear waste - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Disclosed is a method of encapsulating spheroids containing nuclear waste. The spheroids are coated with a composition of about 30 to about 85 percent by weight, calculated as SiO2, of a partially hydrolyzed silicon alkoxide, and up to about 30 percent by weight calculated as Al2O3 of a partially hydrolyzed aluminum alkoxide. The coating on each spheroid is then individually hardened, and is cured by heating to about 500°C to produce a har-dened amorphous coating on the spheroids.

Description

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.
!

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.

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).

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.

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.

Claims (19)

CLAIMS:

1. A method of encapsulating glass or ceramic spheroids containing nuclear waste comprising:
(A) coating said spheroids with a composition which comprises:
(1) about 30% to about 85% by weight, cal-culate as SiO2, of a partially hydrolyzed silicon compound having the general formula SiRm(OR')nXp or Si(OSiR3)4, where each R is independently selected from alkyl to C1O and alkenyl to C10, each R' is independently selected from R and aryl, each X is independently selected from chlorine and bromine, m is 0 to 3, n is 0 to 4, p is 0 to 1, and m + n + p equals 4;
(2) a partially hydrolyzed aluminum compound in an amount up to about 30% by weight, calculated as Al2O3, said partially hydrolyzed aluminum compound having the general formula AlR'q(OR)rXS
or Mg(Al(OR4)2, where each R is independently selected from alkyl to C10 and alkenyl to C10, each R' is in-dependently selected from R and aryl, each X is inde-pendently selected from chlorine and bromine, q is 0 to 3, r is 0 to 3, s is 0 to 1, and q + r + s equals 3, where R is (3) about 30 to about 50 percent by weight of an alcohol;

(B) preventing said coated spheroids from touching while hardening said coatings; and (C) curing the coatings on said spheroids.

2. A method according to Claim 1 wherein said spheroids are coated by spraying said composition onto them as they fall.

3. A method according to Claim 2 wherein the coatings on said spheroids are hardened by heating them as they fall.

4. A method according to Claim 1 wherein said spheroids are first heated to ~250°C and are coated by spraying said compositions onto them.

5. A method according to Claim 1 wherein said spheroids are coated by dropping them into said composition.

6. A method according to Claim 1 wherein the coatings on said spheroids are hardened by immersion in a setting agent.

7. A method according to Claim 6 wherein said setting agent is an aqueous solution of MOH where M is selected from the group consisting of alkali metals, ammonium, and mixtures thereof.

8. A method according to Claim 6 wherein said agent is heated to about 75°C.

9. A method according to Claim 1 wherein the coatings on said spheroids are hardened by immersion in a dehydrating agent.

10. A method according to Claim 9 wherein said dehydrating agent is trichloroethylene.

11. A method according to Claim 9 wherein said dehydrating agent is heated to about 75°C.

12. A method according to Claim 1 wherein said composition is hardened by coating said spheroids with a setting agent prior to coating them with said composition.

13. A method according to Claim 1 wherein said spheroids are about 1/2 to about 25 mm in diameter.

14. A method according to Claim 1 wherein steps (A), (B) and (C) are repeated about 5 to about 10 times.

15. A method according to Claim 1 wherein said spheroids are coated and hardened by dropping them into a vessel containing said composition on top and a hardening fluid on the bottom, said composition and said hardening fluid being separated by an immiscible liquid.

16. A method according to Claim 15 wherein said immiscible liquid is a silicon oil.

17. A product produced according to the method of claim 1.

18. A method according to Claim 1 wherein said silicon compound is tetraethylorthosilicate.

19. A method according to Claim 1 wherein said aluminum compound is aluminum secondary butoxide.