Dissolution of ThO2: study of dissolution process with initial 229Th spike

The sparingly soluble ThO2 dissolution behaviour was studied under conditions that are expected to prevail in bedrock after the closure of the spent nuclear fuel disposal facility. The objective was to investigate the characteristics of initial dissolution of crystalline 232ThO2 by adding 229Th tracer to the aqueous phase in the beginning of the 534 days experiment.The evolution of 232Th concentration and 229Th/232Th ratio were followed by sector field ICP-MS (inductively coupled plasma mass spectrometer). Selected samples were measured also by alpha spectrometry to compare the results from the used analysis techniques and they gave comparable results. In the early stage of dissolution there was no significant control by chemical equilibrium and the dissolution process seemed to be controlled by the stability of surfaces.