Combine Studies Pertaining to the Solubility of Neptunium in Oxidizing Aqueous Systems

The report combines two separate studies dealing with different aspects of the behavior of neptunium in oxidizing aqueous systems. The goal of both studies is to obtain a better understanding of what would control the concentration of neptunium in oxidizing groundwaters upon release from a geologic high-level waste repository. {sup 237}Np has a very long half-life, and consequently tends to appear as a component of concern in long-term dose calculations. Part A examines the formation of NpO{sub 2} [Np(IV) oxide] from aqueous Np(V) under oxidizing conditions, using elevated temperature as a means of accelerating putative slow kinetics. According to thermodynamic data, NpO{sub 2} should be the stable Np solid, even though Np is generally thought to be mainly some form of Np(V) under oxidizing conditions. The first observed precipitation of this solid from aqueous solution under any conditions is reported. This result suggests, but does not prove, that long-term very slow formation of NpO{sub 2} might be an important long-term control on neptunium migration. Even at 200{sup o}C, the kinetics can be sluggish on typical experimental time scales. Though not reported in Part A, a similar situation may exist for Pu, and NpO{sub 2} and PuO{sub 2} may potentially formmore » solid solutions. Under reducing conditions, UO{sub 2} might join in. Part B examines the interaction of Np(V) with the uranium mineral schoepite. Is neptunium incorporated into the crystal structure as a solid-solution component, or does it just sorb onto the mineral surface? The results presented here suggest that it basically just sorbs onto the surface, which would be a less effective process in limiting neptunium migration. This is not a surprising result, as schoepite is a relatively simple solid of U(VI). There is no obvious means to compensate for the charge difference between U(VI) and Np(V) without introducing unfavorable energetics. However, the results reported here do not rule out a possible substitution of Np(VI) in schoepite or other U(VI) minerals. This may be significant especially at higher pH, where the formation of Np(VI) species is more favored relative to Np(V). Both studies reported here suggest that there is much to learn about the actual long-term behavior of Np in the geologic environment.« less