The solubility of some of the radiologically important actinide elements have been determined in a water whose chemistry was representative of that believed to exist in potential repositories in the U.K. The solubilities of the actinides have been determined as a function of the pH of the water. The results of these experiments have been compared with the results of predictions made using the PHREEQE geochemical modelling code in order to test and validate the data base used in the model. In the light of these comparisons, suggestions are made for alternative values for the thermodynamic data and for further studies.

This paper reports the results of some recent experimental studies of the solubility and sorption behaviour of lead-210 and carbon-14 under cementitious near-field conditions.These studies have shown that under these conditions carbon-14 will have a maximum solubility limit of 10−4 M and that the distribution ratio, RD, will increase with increasing carbon-14 concentrations from 10−9 to 10−7 M. Not all of the carbon in the cement is available for exchange with carbon in the pore water. Differences in values of RD are observed between the two cement grout types studied, SRPC and OPC/BFS. Lead has been shown to have a maximum solubility limit of about 10−3 M at high pH. Good agreement is obtained between these measurements and thermodynamic modelling using the PHREEQE code. No differences were observed between lead solubilities under reducing or oxidising conditions at high pH values using the same phase separation techniques. Lead is particularly sensitive to the phase separation techniques employed. A factor of up to 250 difference is observed between 25000 and 30000 molecular weight cut-off filters. The values of RD for lead increase with decreasing lead concentrations and the values of RD for 10−3 M solutions are observed to be 500 mlg−1 for SRPC and 1300 mlg−1 for OPC/BFS.

The solubility and sorption of radium and tin have been studied for a cementitious near-field environment.The solubility of radium was observed to be greater than 10-7 M, even with the addition of iM sulphate to the cement equilibrated water. The values of RD for radium varied from 102 to 103 ml g-1 for the cement types and initial radium concentrations studied.The solubility of tin was observed to be about 10-4M for a range of cement equilibrated waters above tin oxides and metallic tin. The solubility of tin was observed to increase with pH. Tin solubilities were not affected by the redox conditions.Tin was present in a range of particle sizes in the sorption studies. A value of RD of 104 ml g-1 was observed for tin species less than 0.45nm.