Neptunium(V) and neptunium(VI) solubilities in synthetic brines of interest to the waste isolation pilot plant (WIPP)

The solubility of Np(V) and Np(VI) has been measured in three synthetic Na-K-Mg-Cl brines in the presence of CO2(g). Experiments were prepared from oversaturation by adding an excess of NpO-2 or NpO2+2 to the brines and allowing the neptunium solids to precipitate. Vessels were maintained in contact with fixed CO2(g) partial pressures at constant pH and 24±1 C. Dissolved Np(V) concentrations decreased several orders of magnitude within the first 100 days of the experiment, while dissolved Np(VI) concentrations decreased initially but then remained relatively constant for more than 400 days. The solid phases formed in all experiments were identified by X-ray powder diffraction as KNpO2CO3.xH2O(s). Steady state concentrations for Np(V) are similar to those observed for Pu(V) in the same brines under the same conditions, where Pu occurs predominantly as Pu(V). Similarly, steady state concentrations for Np(VI), which was not reduced over a two year period, compare well with measured Pu(VI) concentrations in the same brines before the Pu(VI) was reduced to Pu(V). (orig.)