Neptunium and Plutonium Solubilities in a Yucca Mountain Groundwater

Abstract

Solubilities of neptunium and plutonium were studied in J-13 groundwater (ionic strength of about 3.7 mmol; total dissolved carbonate of 2.8 mmol) from the proposed Yucca Mountain Nuclear Waste Repository site, Nevada, at three different temperatures (25, 60, and 90 °C) and pH values (6.0, 7.0, and 8.5). Experiments were performed from both over- and undersaturation at defined CO2 partial pressures. The solubility of 237Np from oversaturation ranged from a high of (9.40 ± 1.22) × 10-4 M at pH 6.0 and 60 °C to a low of (5.50 ± 1.97) × 10-6 M at pH 8.5 and 90 °C. The analytical results of solubility experiments from undersaturation (temperatures of 25 and 90 °C and pH values 6, 7, and 8.5) converged on these values. The 239/240Pu solubilities ranged from (4.70 ± 1.13) × 10-8 M at pH 6.0 and 25 °C to (3.62 ± 1.14) × 10-9 M at pH 8.5 and 90 °C. In general, both neptunium and plutonium solubilities decreased with increasing pH and temperature. Greenish-brown crystalline Np2O5·xH2O was identified as the solubil...