Impact of the formation of NpO{sub 2} and Np{sub 2}O{sub 5} on the Np solubility in YM waters

The objective of this activity is to evaluate the impact of different Np solids on the solubility of neptunium in YM waters. Special interest is on the impact of Np{sub 2}O{sub 5} found in solubility experiments as the Np solid phase. The formation and stability of Np{sub 2}O{sub 5} is discussed and its stability calculated in J-13 based upon new solubility data. Clearly, the results show that Np{sub 2}O{sub 5} is the most stable Np(V) solid phase that determined the solubility of neptunium under aerobic conditions. Both solubility experiments and geochemical modeling show a decrease of the neptunium solubility in J-13 of about two orders of magnitude (compared to previously reported results) based on the new solubility data. Under anaerobic conditions NpO{sub 2} is the most stable Np bearing solid phase. Its formation would decrease the Np solubility by several orders of magnitude