Investigation of molybdate melts as an alternative method of reprocessing used nuclear fuel

Abstract

An investigation of molybdate melts containing sodium molybdate (Na 2 MoO 4) and molybdenum trioxide (MoO 3) to achieve the separation of uranium from fission products by crystallization has been performed. The separation is based on the difference in solubility of the fission product metal oxides compared to the uranium oxide or molybdate in the molybdate melt. The molybdate melt dissolves uranium dioxide at high temperatures, and upon cooling, uranium precipitates as uranium dioxide or molybdate, whereas the fission product metals remain soluble in the melt. Small-scale experiments using gram quantities of uranium dioxide have been performed to investigate the feasibility of UO 2 purification from the fission products. The composition of the uranium precipitate as well as data for partitioning of several fission product surrogates between the uranium precipitate and molybdate melt for various melt compositions are presented and discussed. The fission products Cs, Sr, Ru and Rh all displayed very large distribution ratios. The fission products Zr, Pd, and the lanthanides also displayed good distribution ratios (D>10). A melt consisting of 20 wt% MoO 3-50 wt% Na 2 MoO 4-30 wt% UO 2 heated to 1313 K and cooled to 1123 K for the physical separation of the UO 2 product from the melt, and washed once with Na 2 MoO 4 displays optimum conditions for separation of the UO 2 from the fission products.

Cathodic Processes in Uranium Containing Molybdate Melts

“…Excess MoO 3 is an essential component required for dissolving added oxides if the molybdate melts are to be used for SNF reprocessing. [26][27][28] Nagai et al 23 also observed two cathodic processes separated by approximately 0.6 V, which were attributed to deposition of unspecified molybdate compounds (higher potential cathodic peak) and lithium molybdenum oxides Li 2 MoO 3 and Li 6 Mo 2 O 7 .…”

“…Alternatively, UO 2 or UO 2 -PuO 2 mixtures can be electrodeposited from molybdate melts. A possible application of sodium molybdate melts containing excess molybdenum oxide (the composition can be referred to as Na 2 MoO 4 -Na 2 Mo 2 O 7 or Na 2 MoO 4 -MoO 3 ) was subsequently considered in more recent works 26,28 where, in particular, distribution of fission products in the recrystallization step was considered in detail.…”

A Review of Opportunities and Methods for Recovery of Rhodium from Spent Nuclear Fuel during Reprocessing

“…The PGMs tend to form small inclusions [89] of metallic alloys and distinct oxide phases separate from the bulk glass phase [90], as occurs during irradiation of nuclear fuel, sometimes containing non-metallic elements like tellurium [86], though this phase separation can be managed with careful control of temperature during the vitrification process [91]. Although not displaying the same volatility challenges presented by Ru [1,92], Rh nonetheless poses some difficulties in HLLW vitrification due to this tendency to phase partition. The use of alternative glass chemistries, such as iron, or zirconium phosphate systems, have been demonstrated to improve the solubility of the PGMs in glassy wasteforms [93,94] while alternative techniques such as alloying out the PGMs with metallic Sn have also been reported [95].…”

Structural investigation of the efficient capture of Cs+ and Sr2+ by a microporous Cd–Sn–Se ion exchanger constructed from mono-lacunary supertetrahedral clusters

“…In most cases, the ion exchange method will produce high-quality treated effluent, and it will not cause secondary pollution due to the safe landfill disposal of used adsorbents. Inorganic porous materials with high radiation and thermal stabilities have been extensively employed for the removal of radioactive Cs + and Sr 2+ , as confirmed by the great development made in research on zeolites, 26,27 clays, 28,29 titanium silicates, 30,31 phosphates, 32,33 molybdates, 34 etc. However, these oxide exchangers usually lose their activities in acidic/alkaline or highly salty environments, and thus cannot be used in the extreme conditions in the practical treatment of nuclear liquid wastes.…”