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 syntheti c Na-K-Mg-Cl brines in the presence of CO2(g). Experiments were prepar ed from oversaturation by adding an excess of NpO2+ or NpO22+ to the b rines and allowing the neptunium solids to precipitate. Vessels were m aintained in contact with fixed CO2(g) partial pressures at constant p H and 24+/-1 degrees C. Dissolved Np(V) concentrations decreased sever al orders of magnitude within the first 100 days of the experiment, wh ile dissolved Np(VI) concentrations decreased initially but then remai ned relatively constant for more than 400 days. The solid phases forme d in all experiments were identified by X-ray powder diffraction as KN pO2CO3 . xH(2)O(s). Steady state concentrations for Np(V) are similar to those observed for Pu(V) in the same brines under the same conditio ns, where Pu occurs predominantly as Pu(V). Similarly, steady state co ncentrations for Np(VI), which was not reduced over a two year period, compare well with measured Pu(VI) concentrations in the same brines b efore the Pu(VI) was reduced to Pu(V).