237Np analytical method using 239Np tracers and application to a contaminated nuclear disposal facility

Abstract

Environmental ²³⁷Np analyses are challenged by low ²³⁷Np concentrations and lack of an available yield tracer; we report a rapid, inexpensive ²³⁷Np analytical approach employing the short lived ²³⁹Np (t_1/2 = 2.3 days) as a chemical yield tracer followed by ²³⁷Np quantification using inductively coupled plasma-mass spectrometry. ²³⁹Np tracer is obtained via separation from a ²⁴³Am stock solution and standardized using gamma spectrometry immediately prior to sample processing. Rapid digestions using a commercial, 900 W "Walmart" microwave and Parr microwave vessels result in 99.8 ± 0.1% digestion yields, while chromatographic separations enable Np/U separation factors on the order of 10⁶ and total Np yields of 95 ± 4% (2σ). Application of this method to legacy soil samples surrounding a radioactive disposal facility (the Subsurface Disposal Area at Idaho National Laboratory) reveal the presence of low level ²³⁷Np contamination within 600 m of this site, with maximum ²³⁷Np concentrations on the order of 10³ times greater than nuclear weapons testing fallout levels.

Keywords: Np-237; Np-239; Subsurface disposal area.