Review of the modelling of radionuclide transport from U-series disequilibria and of its use in assessing the safe disposal of nuclear waste in crystalline rock

U-series disequilibia from crystalline rocks and U deposits have been used to estimate the timing of hydrogeological events for U and Ra mobility and to model the transport rates of U and Ra,both with some sucess. Unfortunately, activity ratio data alone are not unique to a given process or model, and it is shown how inappropriate choice of the model can have quite diverse consequences for interpretation of the data. Information from other sources is needed to help minimise these non-uniqueness problems but, as yet, the state of methodology is unadequate to assess completely the validity of the models. In particular, the rate of deposition of U in fractures seems highly dependent upon the availability of trivalent Fe and other low-temperature minerals whose rates of formation are largely unknown. This review reinforces the need to assess closely the suggestion that chemisorption is an effective retardant of fracture-borne borne repository source radionuclides.