Dissolution of Used Nuclear Fuel Using a TBP/N-Paraffin Solvent

Abstract

The dissolution of unirradiated light water reactor fuel pellets pretreated for tritium removal by an oxidizing gas was demonstrated using a solvent containing tributly phosphate which is used in the PUREX (Plutonium Uranium Reduction Extraction) process. Dissolution of pretreated fuel in tributyl phosphate could potentially combine fuel dissolution with two cycle of solvent extraction required for separating the actinide and lanthanide elements from other fission products. Simplified flowsheets which reduced the equipment footprint and facility size are needed to improve the economic viability of complete recycle of used nuclear fuel. Dissolutions were performed using used fuel surrogates prepared from both uranyl nitrate and uranium trioxide produced from the pretreatment process by adding selected actinide and stable fission product elements. In the laboratory-scale experiments, the U dissolution efficiency ranged from 80-99+% for surrogates prepared from either the nitrate or oxide forms of pretreated fuel. On average, 80% of the Pu and 50% of the Np and Am in the nitrate surrogate dissolved; however, little of the Np, Pu, or Am oxides which were added to the used fuel surrogate containing uranium trioxide dissolved. Approximately 60-90+% of the lanthanide elements (Nd, Eu, and Ce) in the 3+ oxidation state (as both nitrates and oxides) dissolved; however, little of the Ce in the 4+ oxidation state (as cerium oxide) was solubilized, which was consistent with the transuranic oxides. Only small amounts of Sr (0-1.6%) and Mo (0.1-1.7%) and essentially no Cs, Ru, Zr, or Pd in either used fuel surrogate dissolved.

Preliminary Result from the Dissolution of Neodymium and Erbium Oxide in a Tributyl Phosphate Solvent

“…Simplified flowsheets that reduce the equipment footprint and facility size are needed to improve the economic viability of complete recycle. To address this issue, a hybrid process was demonstrated on a laboratoryscale that combines the dissolution of pretreated UNF (for tritium removal) in a tributyl phosphate (TBP) containing solvent with two cycles of solvent extraction required for the recovery of the actinide and lanthanide elements [2]. The hybrid process would be used to dissolve the lanthanide fission products and actinides directly into TBP.…”

“…Laboratory data are available for the distribution of the actinide and lanthanide fission product elements between a TBP solvent and undissolved solids [2], however, the kinetics for the dissolution are not well established, especially for the lanthanide fission products. To address this gap in the data, a methodology was demonstrated to measure the rate of dissolution of selected elements across the lanthanide series.…”

“…[2]. The aqueous phase has not yet been sampled but based on the observed color and that no Nd2O3 powder could be seen in solution, most likely contains Nd(NO3)3 in dilute nitric acid.…”