Radioelement Behaviour in a Cementitious Environment

The Nirex Safety Assessment Research Programme (NSARP) comprises scientific research to support the safety assessment of a deep geological repository for low- and intermediate-level radioactive wastes. The NSARP addresses several areas of research, one of which is a continuing investigation of the behaviour of radioelements in a cementitious environment. This paper draws together recent experimental measurements of solubility and sorption under such conditions, along with the results of interpretive thermochemical and sorption modelling. Under conditions similar to those expected in the repository, the predicted solubility of selenium is about 3xl0 −8 M. Experimental measurements confirm that at low redox potential the solubility of selenium is substantially less than that previously observed under an air atmosphere, and is closer to the predictions of the thermochemical modelling. The solubility of sodium stannate has been studied after heating solutions at 50, 80 and 200°C. Crystalline tin solids were formed at all temperatures. For solutions heated at 200°C the solids were identified as cassiterite. The solubilities of all of the crystalline solids were 1x10 −6 M to 1x10 −9 M which are at least five orders of magnitude lower than that of the original sodium stannate. Distribution ratios (Rd) between 1x10 3 and 5x10 3 ml g -1 have been measured for the sorption of tetravalent technetium on to a cementitious material. Preliminary results from an in-diffusion experiment involving americium and an intact cement sample are in good agreement (at 1sx10 4 ml g -1 ) with Rd values determined from previously reported batch sorption experiments.