Comparison of water phase diffusion experiments in laboratory and in situ conditions

Abstract

In some countries the spent nuclear fuel produced by nuclear power plants will be deposited in crystalline granitic rock formations. In Finland, a repository for the spent nuclear fuel is being built at Olkiluoto. The safety assessment of the repository requires a careful determination of the transport properties of the bedrock.The porosity of the bedrock and the effective diffusion coefficients and distribution coefficients of different radionuclides for the bedrock are used as the main parameters in the safety assessment calculations. It has been questioned whether the parameters determined using laboratory experiments can be used to estimate the parameters in the in situ conditions.In this study, laboratory and in situ water phase diffusion experiments (WPDEs) were performed to resolve the issue.In the experiments, the transport of tritiated water (HTO), 36Cl, and 22Na was studied using similar experimental setups. Mathematical models were constructed and solved to determine the transport parameters from the measured breakthrough curves. On average, the in situ WPDEs resulted in 20(±6)% smaller porosities and 32(±10)% smaller effective diffusion coefficients for HTO and 36Cl than the laboratory WPDEs. It was also found that in veined gneiss, the most dominant rock type of the Olkiluoto bedrock, anion exclusion reduced the retention parameters of 36Cl compared with those of HTO. Furthermore, the distribution coefficient of22Na for veined gneiss was about one order of magnitude smaller in the in situ conditions than in previous laboratory batch sorption experiments. The effects of the results on the safety assessment were evaluated and discussed.