Analysis of trace neptunium in the vicinity of underground nuclear tests at the Nevada National Security Site

摘要

A high sensitivity analytical method for ~(237)Np analysis was developed and applied to groundwater samples from the Nevada National Security Site (NNSS) using short-lived ~(239)Np as a yield tracer and HR magnetic sector ICP-MS. The ~(237)Np concentrations in the vicinity of the Almendro, Cambric, Dalhart, Cheshire, and Chancellor underground nuclear test locations range from ＜4 × 10~(-4) to 2.6 mBq/L (6 × 10~(-17)-4.2 × 10~(-13) mol/L). All measured ~(237)Np concentrations are well below the drinking water maximum contaminant level for alpha emitters identified by the U.S. EPA (560 mBq/L). Nevertheless, ~(237)Np remains an important indicator for radionuclide transport rates at the NNSS. Retardation factor ratios were used to compare the mobility of ~(237)Np to that of other radionuclides. The results suggest that ~(237)Np is less mobile than tritium and other non-sorbing radionuclides (~(14)C, ~(36)Cl, ~(99)Tc and ~(129)I) as expected. Surprisingly, ~(237)Np and plutonium (~(239,240)Pu) retardation factors are very similar. It is possible that Np(Ⅳ) exists under mildly reducing groundwater conditions and exhibits a retardation behavior that is comparable to Pu(Ⅳ). Independent of the underlying process, ~(237)Np is migrating downgradient from NNSS underground nuclear tests at very low but measureable concentrations.