A document prepared for WASTE MANAGEMENT '95 CONFERENCE-PAPER FOR PUBLISHED PROCEEDINGS ONLY-ABSTRACT WAS APPROVED 10-13-94. at Tucson from 02/26/95 -03/02/95. DOE Contract No. DE-AC09-89SR18035This paper was prepared in connection with work done under the above contract number with the U. S. Department of Energy. By acceptance of this paper, the publisher and/or recipient acknowledges the U. S. Government's right to retain a nonexclusive, royalty-free license in and to any copyright covering this paper, along with the right to reproduce and to authorize others to reproduce all or part of the copyrighted paper. DISCL.RMERThis report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or-usefklness of any information, apparatus, product, or pro~ess disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, ABSTRACTInvestigations have been performed to determine the suitability of glass as a host for surplus fissile material removed from nuclear weapons. The U.S. Department of Energy -Office of Technology Development has sponsored research at the Savannah River Site to develop durable glass compositions that are compatible with high concentrations of plutonium and uranium. These investigations also being performed to provide baseline actinide glass durability and processing data.Two glass forming systems are being evaluated. One of the systems is a commercial borosilicate glass and the other an iron phosphate glass. Both glass systems have a very high degree of compatibility with actinide oxides and are considerably more durable than conventional high-level waste glasses. The iron phosphate glass has a melting temperature in the 1 100°C range and has the higher uranium and plutonium solubility. The borosilicate has a melting temperature in the 1425°C range and is the more durable (on the order of fused silica) glass.
This paper presents results of an investigation of the microstructures and durabilities of glasses for immobilization of excess Pu, Am, and Cm at the Savannah River Site (SRS). The glasses investigated had compositions based on commercial lanthanide glasses developed in the 1930's. All the glasses were prepared remotely in shielded cells and the analyses performed using instruments in radio hoods or benches. Durabilities were measured using the ASTM C-1285 standard leach test (PCT). Results for three glasses are presented. Two glasses contained 15 and 7 wt.% Pu, respectively. The third glass contained Am and Cm. The 15 wt% Pu glass was not completely amorphous and contained crystals of undissolved PuO2 and as well as dissolved PuO2. The 7 wt% Pu glass was completely amorphous. The Am/Cm glass contained <1 wt% actinides and was homogenous. The PCT tests for the Pu glasses indicated that B and Ba were leached congruently. Sm and Pu were leached at slower rates. In some cases release rates for specific Ba, Sm, and Pu isotopes were measured by analyzing the leachates by mass spectroscopy. For the Am-Cm glass release rates for B and Ba were equal indicating congruent dissolution. All three glasses were 25 to 50 times more durable than the borosilicate glasses developed at SRS for immobilization of the fission product HLW resulting from reprocessing operations at SRS.
SummaryA phase separated (amorphous) glass has been developed which allows very effecient recovery of -1-4 valence actinides. The total amount of crystal formation in a heat treated vycor-type glass can be controlled with time, temperature and loading. Heat treatments at lower temperatures and for less time inhibit crystal formation while still allowing significant phase separation. If the Thorium loading exceeds 10 weight percent oxide, crystal formation during heat treatment may not be avoided. The total amount of crystal growth has a direct affect on thorium leachability. An increase in crystal formation limits the Th recovery significantly. High thorium loaded glasses (15 weight percent) with heat treatments (increased crystal forniation) leach at approximately the same rate as non-heat treated glasses.A phase separated (amorphous) glass has been produced using thorium as a surrogate for neptunium. Two different homogeneous vycor compositions targeting 10 and 15 weight percent thorium oxide have been processed, heat treated and leached with concentrated nitric acid at 110" C. Thorium recovery rates have been shown to be considerably' better when the glass has been heat treated inducing phase separation that is relatively crystal free. Non-heat treated and crystalline (due to heat treatment) glasses have similar Th recovery rates with respct to surface area. Phase separated amorphous samples were found to have significantly higher thorium concentrations in the leachate compared to non-heat treated and crystalline glasses for all mesh sizes. All glasses had increased thorium concentration in the leachate as surface area increased.Thorium leach rates of greater than 90 percent have been achieved by contacting a large mesh size, heat treated glass with nitric acid at 110" C for 4 hours. The heat treatment was for 7 hours at 650" C . The glass contained very few crystals after the heat treatment. Thorium leach rates for a slightly more crystallized sample was -20 percent less. Non-hear treated glasses show -40 percent less recovery compared to the sample with very few crystals. INon-heat treated glasses exhibited ve.ry low silica releases. lrypical levels are at or below 5 ppm of silica dissolved in solution for large mesh sizes and greater than 15 ppm for small mesh sizes. Large glass fragments of phase separated (amorphous) samples had a silica release also below 5 ppm. However, glasses containing amorphous phase separation and crystalline phases did show a ten fold (lox) increase in total silica dissolved as the surface area increased.
Immobilization by vitrification is one potential disposition option for a portion of the United States' excess plutonium inventory. Research has been performed at the Savannah River Site (SRS) to determine the optimum composition of a lanthanide borosilicate frit for the vitrification of plutonium using a Plackett-Burman design and simplex algorithm as a statistical tool. This technique uses various response variables to rank and optimize a composition. The variables used in this study correspond to homogeneity, durability, actinide solubility and devitrification after heat-treatment.The optimized frit composition was determined using a constant ThO2 loading of 20 wt%. No noticeable trends were followed with respect to the individual components which may indicate a relatively robust system able to accommodate variations in the feed.Batches containing various loadings of ThO2 were melted to determine if actinide solubility was improved in the optimized composition compared to that of a similar lanthanide borosilicate glass. No noticeable improvement in ThO2 solubility was realized as a result of using this optimization technique.
SummaryA lanthanide borosilicate (LaI3S) glass composition, being evaluated by the Savannah River Technology Center (SRTC) for plutonium disposition as part of the US. Department of Energy's (DOE) Office of the Fissile Materials Disposition (OFMD) program , has been processed with greater than 15 elemental weight percent thorium (a plutonium surrogate) without the presence of lead oxide in the glass. The glass composition is a result of several efforts to remove the lead by replacing it with strontium and sodium. The initial melts that included sodium and considerably lower aluminum resulted in visible phase separation. t Two homogeneous lead free melts have been processed. The first one replaced one-fifth cf the total lead with Sr on a mole percent basis. -Qther changes included slightly less aluminum with increases in boron and silica. The second glass composition was exactly the same as the first with an additional 1: 1 replacement of barium with strontium on a mole percent basis. This last composition contains no elements considered to be hazardous as defined by the Resource and Conservation Recovery Act (RCRA elements). The Product Consistency Test (PCT) results of these two homogeneous glass compositions show them to be slightly more durable than the original lanthanide borosilicate glasses (with lead) loaded with thorium.
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