Americium-Curium Vitrification Process Development (U)
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ABSTRACT The successful demonstration of sequentially drying, calcining and vitrifying an oxalate slurry in the Drain Tube Test Stand (DTTS) vessel provided the process basis for testing on a larger scale in a cylindrical induction heated melter. A single processing issue, that of batch volume expansion, was encountered during the initial stages of testing. The increase in batch volume centered on a sintered frit cap and high temperature bubble formation. The formation of a sintered frit cap expansion was eliminated with the use of cullet. Volume expansions due to high temperature bubble formation (oxygen liberation from cerium reduction) were mitigated in the DTTS melter vessel through a vessel temperature profile that effectively separated the softening point of the glass cullet and the evolving oxygen from cerium reduction. An increased processing temperature of 1470'C and a two hour hold time to fine any remaining bubbles successfully reduced bubbles in the poured glass to an acceptable level. The success of the preliminary process demonstrations provided a workable process basis that was directly applicable to the newly installed Cylindrical Induction Melter (CIM) system, making the batch flowsheet the preferred option for vitrification of the americium-curium surrogate feed stream.
INTRODUCTION The Savannah River Technology Center (SRTC) is developing the equipment design bases
and process operating parameters to vitrify a nitric acid solution containing isotopes of americium and curium (Am-Cm). The final glass form will be placed in interim storage until it can be transported to the U.S. Department of Energy's Oak Ridge National Laboratory for recovery of the Am-Cm from the glass. Technical problems associated with efforts to directly vitrify the nitric acid feed solution in a slab-type bushing melter led to scoping studies of a batch operation that included a new pretreatment step as well as a new melter design. Operating results and difficulties of the continuous feed Am-Cm slab melter pilot system have been documented by Smith, et.al.[I] The alternative of batch vitrification and the simplification of the melter system are described by Marra, et.al.[21 While a new induction-heated cylindrical melter was being installed, vitrification experiments with the product slurry from a new pretreatment step were carried out in a resistance-heated platinum melter to validate the technical feasibility of the new flowsheet. This paper describes the preliminary process and discusses the test results of initial scoping studies.
Oxalic Pretreatment Process Description An oxalic acid precipitation of the nitric acid feed stream will be the only "in-cell" pretreatment step before the feed material is introduced to the melter vessel. The precipitation pretreatment flowsheet is shown in Figure 1. The pretreatment includes a precipitation with the addition of 8 weight percent oxalic acid. The precipitated oxalates (primarily rare earth oxalates) 367 Mat. Res. Soc. Symp. Proc. Vol. 556 © 1999 Materials Research Society
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