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The melt-dilute treatment technology program is focused on the development and implementation of a treatment technology for diluting highly enriched (>20% ²³⁵U) aluminum spent nuclear fuel to low enriched levels (<20% ²³⁵U and qualifying the LEU Al-SNF form for geologic repository storage. Typically, many domestic and foreign research reactor fuel assemblies were manufactured using highly enriched uranium-aluminum alloys. These assemblies have been irradiated and the burn-up levels range from 30-70%. In order to reduce the enrichment of these assemblies prior to ultimate geologic repository disposal, the melt-dilute technology proposes to melt these SNF assemblies and then dilute with additions of depleted uranium. Dilution levels of < 20 % are desired. Benefits accrued from this process when compared to the direct disposal option include the potential for significant volume reduction, reduced criticality potential, and the potential for enhanced SNF form characteristics.

The current emphasis for this project has been on full-scale surrogate testing at the Savannah River Technology Center and on bench-scale irradiated coupon testing at Argonne National Laboratory. The major focus of both of these sets of experiments has been fission product volatilization under representative melt-dilute operating conditions and off-gas system development.

A critical technology element in the development of the melt-dilute process is the development of offgas system requirements. The volatilization of radioactive species during the melting stage of the process primarily constitutes the offgas in this process. Several of the species present following irradiation of a fuel assembly have been shown to be volatile or semi-volatile under reactor core melt-down conditions. Some of the key species that have previously been studied are krypton, iodine, and cesium. All of these species have been shown to volatilize during melting experiments however, the degree to which they are released is highly dependent upon atmosphere, fuel burnup, temperature, and fuel composition. With this in mind an analytical and experimental program has been undertaken to assess the volatility and capture of species under the melt-dilute operating conditions.

The initial step in examining volatilization with respect to the melt-dilute treatment technology has been to use several different analytical/modeling approaches to generate a list of key species of interest for the process. The initial approach was to examine the boiling points (BP) for the fission products, light metals, and actinides as generated through an ORIGEN code run for a typical Al-based spent fuel assembly. Knowing that the upper bound operating temperature for the process was 1000°C, all species with boiling points greater than 1000°C were initially eliminated. However, since it is possible for the vapor phase to be in equilibrium with the solid at temperatures below the boiling point, the vapor pressure of all the species were also examined to determine if any of the BP eliminated species needed attention. Following this a simple ideal solution approach was used to predict the amount (gms) of species volatilized under melt-dilute processing conditions. Additionally, fundamental thermodynamic modeling was performed to determine if any compounds could be formed between any of the radiological and non-radiological species expected to be present in the melt. From all of these analytical calculations and modeling, the one constant regardless of approach used, is that the major melt constituent of concern with respect to volatilization is cesium.

Based on the outcome of the analytical modeling that cesium is the major concern an experimental program was begun to develop an offgas system to effect the capture of cesium from the off-gas stream. Bench-scale laboratory scoping studies were conducted using non-radioactive cesium. The outcome of these tests indicated that two absorber bed media, activated alumina and zeolite 4A, were acceptable for cesium trapping. Following these tests a full-scale integrated melting and off gas apparatus was constructed. This apparatus was capable of melting a single full-size ( 3" x 3" x 36") surrogate fuel assembly. Three different absorber bed designs using zeolite 4A as the trapping media were evaluated. The goal with each design concept was to completely capture cesium vapor by maximizing residence time and velocity. A design concept termed the concentric ring design produced the most consistent result with a nominal system efficiency of 99.9%. Further melt-dilute tests using irradiated SNF coupons were also performed at Argonne National Laboratory. The results form these tests validated the analytical modeling and laboratory studies. The outcome of these experiments was two-fold in that other than the noble gases and iodine only Cs-137 was shown to volatilize from the melt and a zeolite 4A absorber bed allowed zero breakthrough of cesium during testing. Based on these laboratory testing with fuel surrogates and irradiated coupon test results, a full-scale irradiated fuel melting facility is being designed and assembled at the Savannah River Site.

In summary, the analytical and experimental tests conducted at both SRTC and ANL, has identified cesium as the melt constituent of most concern with respect to volatilization. Experimental tests using both cesium surrogates and radioactive cesium have shown that zeolite 4A is an effective cesium trap and as a result a preliminary offgas system concept has been developed employing dry zeolite 4A absorber beds as the primary cesium trapping medium. Final, validation of this offgas concept will occur during full-scale irradiated testing in FY2000.