Sorption of Np by UO2 under repository conditions

This work is a part of the joint Russian - American Program on Beneficial Use of Depleted Uranium. The production of nuclear fuels results in the accumulation of large quantities of depleted uranium (DU) in the form of uranium hexafluoride (UF6), which is converted to uranium oxides. Depleted uranium dioxide (DUO2) can be used as a component of radiation shielding and as an absorbent for migrating radionuclides that may emerge from casks containing spent nuclear fuel (SNF) that are stored for hundreds of thousands of years in high-level wastes (HLW) and SNF repositories (e. g. Yucca Mountain Project). In this case DU oxides serve as an additional engineered chemical barrier. It is known that the primary radioisotope contributor to the calculated long-term radiation dose to the public at the Yucca Mountain SNF repository site boundary is neptunium-237 (Np-237). This paper describes the sorption of Np-237 in various media (deionized water and J-13 solution) by DUO2. Samples of DUO2 used in this work originated from the treatment of UF6 in a reducing media to form UO2 (DUO2-1 at 600 degrees C, DUO2-2 at 700 degrees C, and DUO2-3 at 800 degrees C). All species of DUO2 sorb Np(V) and Np(IV) from aqueous media. Equilibrium was achieved in 24 hours for Np(V) and in 2 hours for Np(IV). Np(V) sorption is accompanied with partial reduction of Np(V) to Np(IV) and vice versa. The sorption of Np(V) onto DUO2 surfaces is irreversible. The investigations on DUO2 transformations were performed under dynamic and static conditions. Under static conditions the solubility of the DUO2 samples in J-13 solution is considerably higher than in DW. When the pre-treatment temperature is decreased, the solubility of DUO2 samples raises regardless of the media. The experiments on interaction between DUO2 and aqueous media DW and J-13 solution) under dynamic conditions demonstrated that during 30-40 days the penetration/filtration rate of DW and J-13 solution through a thin DUO2 layer decreased dramatically, and then slowed and stabilized. The filtration rate of J-13 solution through DUO2 layer is several times lower than that of DW. If Np were sorbed by UO2 in SNF rather than being transported to the site boundary, the site boundary dose would be more than an order of magnitude lower than that calculated. If this is so, the remaining primary contributors to site boundary dose would be I-129 and the decay products of U-238. One could provide an additional barrier, a chemical barrier, to radionuclide release by surrounding the SNF with UO2. The experiments also indicate that a relatively thin layer of UO2 essentially stops water penetration. Such a barrier would deny water access to the SNF. Such a repository SNF configuration would provide reduced radionuclide transport from the drift as well as sorbing released nuclides from failed SNF pins.