High resolution ICP-OES analysis of neptunium-237 in samples from pyrochemical treatment of spent nuclear fuel

A straightforward and reliable ICP-OES procedure for 237Np analysis of samples potentially originating from spent nuclear fuel has been developed. In contrast to most other analytical techniques employed for the determination of 237Np in radioactive samples, 237Np was analyzed without prior separation from the residual elements present in the analyte solution. External calibration of the ICP-OES response was accomplished using a certified 237Np stock solution that was additionally characterized in-house using ICP-MS as well as γ-spectrometry. Thirteen prominent 237Np emission wavelengths were tested for their suitability in terms of sensitivity, peak shape, occurrence of spectral interferences and stability of spectral background. The investigated 237Np emission wavelengths revealed complex ICP-OES spectra with either broad peaks having widths of up to 30 pm or splitting of the 237Np signal into multiple peaks, i.e. multiplets. Compared to previous ICP-OES investigations, 237Np detection limits (LOD) were lowered by at least one order of magnitude in the current study. The most intensive 237Np emission line at λ = 382.92 nm providing the lowest LOD of 3.1 μg kg −1, however, was of limited practical use because severe spectral interference from Nd (a representative fission product with a significant content in spent fuel) at λ = 382.914 nm overlapped with the 237Np signal. Additional detailed interference studies helped identify potential complications during 237Np analysis caused by concomitant elements (Nd, Pu, Th, U) in the analyte solution. Ultimately, emission signals at λ = 410.84 nm, λ = 429.09 nm, and λ = 456.04 nm were used to analyze selected salt and deposit samples from pyrochemical separation experiments. ICP-OES Np results were cross-validated using sector field ICP-MS, largely confirming each other, and thus underpinning the validity of the developed ICP-OES procedure. The successful use of two independent analytical techniques for all analyses helped to prove the quality of the achieved results.