Development of 231Pa AMS measurements to improve radiological dose assessment from uranium mining and milling

Limited information exists on the movement and partitioning of ²³⁵U series radionuclides following their release to the environment from activities associated with the nuclear fuel cycle. Only three isotopes in the series, ²³⁵U, ²³¹Pa and ²²⁷Ac, have half-lives greater than a month. The extent to which the radiologically-significant isotopes of the ²³⁵U series differentiate and fractionate in the environment, and then accumulate in biota, is largely controlled by the chemical properties of these three elements and by their biological uptake. This is particularly the case for biota with life cycles in the order of years. There is therefore potential for changes in the radiation dose received by humans and the environment during, for example, the operational and post-remediation phases of uranium mining practices, particularly where mine sites revert to traditional land uses such as hunting, fishing and camping. Natural activity concentrations of ²³⁵U are ∼20 times lower than for ²³⁸U. In an environmental context, however, the radiation dose that arises from subsequent members of the ²³⁵U decay chain will be determined by their abundance distribution and can exceed that from the ²³⁸U series. The half-lives of the protactinium isotopes allow only ²³¹Pa to accumulate independently of its parent radionuclide along the transport pathways and/or through preferential uptake by biota. In particular, ²³¹Pa could make a disproportionately high contribution to the dose if it is preferentially transported and/or taken up. The AMS technique is capable of assessing accumulation of ²³¹Pa in natural environmental samples. We report on a new, safer radiochemical extraction method to prepare the ²³³Pa isotopic tracer (one that does not use hydrofluoric acid), and on the development of ²³¹Pa AMS measurements at the Australian National University.