Isolation of ²³⁶U and ^239,240Pu from seawater samples and its determination by Accelerator Mass Spectrometry

In this work we present and evaluate a radiochemical procedure optimised for the analysis of ²³⁶U and ^239,240Pu in seawater samples by Accelerator Mass Spectrometry (AMS). The method is based on Fe(OH)₃ co-precipitation of actinides and uses TEVA® and UTEVA® extraction chromatography resins in a simplified way for the final U and Pu purification. In order to improve the performance of the method, the radiochemical yields are analysed in 1 to 10L seawater volumes using alpha spectrometry (AS) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Robust 80% plutonium recoveries are obtained; however, it is found that Fe(III) concentration in the precipitation solution and sample volume are the two critical and correlated parameters influencing the initial uranium extraction through Fe(OH)₃ co-precipitation. Therefore, we propose an expression that optimises the sample volume and Fe(III) amounts according to both the ²³⁶U and ^239,240Pu concentrations in the samples and the performance parameters of the AMS facility. The method is validated for the current setup of the 1MV AMS system (CNA, Sevilla, Spain), where He gas is used as a stripper, by analysing a set of intercomparison seawater samples, together with the Laboratory of Ion Beam Physics (ETH, Zürich, Switzerland).