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

Abstract 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. [ABSTRACT FROM AUTHOR]