Analysis of 14C-bearing compounds released by the corrosion of irradiated steel using accelerator mass spectrometry

Abstract

The combination of ion chromatography (IC) with accelerator mass spectrometry (AMS) was developed to determine the speciation of ¹⁴C-(radiocarbon) bearing organic compounds in the femto to pico molar concentration range. The development of this compound-specific radiocarbon analysis (CSRA) of carboxylic acids is reported and the application of the method on a leaching solution from neutron-irradiated steel is demonstrated. The background and the dynamic range of the AMS-based method were quantified. On using ¹⁴C-labelled standards, the measurements demonstrate the repeatability of the analytical method and the reproducible recovery of the main target carboxylic acids (i.e., acetate, formate, malonate, and oxalate). The detection limit was determined to be in the mid fmol ¹⁴C per L level while the dynamic range of the analytical method covers three orders of magnitude from the low fmol to the mid pmol ¹⁴C per L level. Cross contamination was found to be negligible during IC fractionation and was accounted for during eluate processing and ¹⁴C detection by AMS. The ¹⁴C-bearing carboxylates released from an irradiated steel nut into an alkaline leaching solution were analysed using the CSRA-based analytical method with the aim to check the applicability of the approach and develop appropriate sample preparation. The concentrations of ¹⁴C-bearing formate and acetate, the main organic corrosion products, were at a low pmol ¹⁴C per L level for convenient dimensions of the alkaline leaching experiment which demonstrates that compound-specific ¹⁴C AMS is an extremely sensitive analytical method for analysing ¹⁴C-bearing compounds. The content of total organic ¹⁴C in solution (TO¹⁴C) determined by the direct measurement of an aliquot of the leaching solution agrees well with the sum of the ¹⁴C concentrations of the individual carboxylates within the uncertainty of the data. Furthermore, the TO¹⁴C content is in good agreement with the calculated value using the corrosion rate determined from the ⁶⁰Co release and the ¹⁴C inventory of the irradiated steel specimen.