Study on uncertainties of glass dissolution rate in safety assessment for a geological disposal of high level radioactive waste (Contract research)

In safety assessment for geological disposal of HLW glass, the release rate of radionuclides from the glass can be assumed to equal to the dissolution rate of the glass matrix. The latest Japanese assessment assumes that the glass matrix dissolution in an engineered barrier system proceeds under saturated condition at an extremely slow rate neglecting the effects of chemical affinity. Recent studies indicate that the glass dissolution rate is likely to decrease with time. However, scientific mechanism of the decrease in glass dissolution rate has not been evaluated sufficiently, and the sound understandings are required to be accounted for. We estimate uncertainty of the glass dissolution rate applied to not only a model of constant dissolution rate, which have been used for the safety assessment in Japan, but also a model of decreasing dissolution rate. In order to estimate the parameter uncertainty of the glass dissolution rate, we carried out the statistical analysis of the appropriate release rate data of boron for a hypothetical condition of disposal system, based on investigating the relation between published data of glass dissolution and those experimental conditions. As a result of the model of constant dissolution rate, the uncertainty of the glass dissolution rate is estimated to be a variance with about 3 orders and logarithmic mean of 0.004 g/m2/day. The glass dissolution rate by the rate decreasing model changes in the range of 10-5 to 10-4 g/m2/day by 1000 years and is lower than the minimum value estimated by the model of constant dissolution rate. The result indicates that the difference of two models may lead to large variance of the glass dissolution rate in safety assessment. (author)