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

Sekioka, Yasushi*; Takeda, Seiji ; Kimura, Hideo

In safety assessment for geological disposal, release of radionuclides from vitrified waste is dependent on dissolution rate of glass matrix. It is assumed that the alteration and dissolution of glass in an engineered barrier system proceeds by a mechanism not related by the chemical affinity for dissolution reaction of glass matrix. Recent studies indicate that the glass dissolution rate is likely to decrease as time passed, but scientific basis on the dissolution process has not been necessarily understood. We estimate uncertainty of the glass dissolution rate represented by not only a model of constant dissolution rate 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/m/day. The glass dissolution rate by the decreasing model changes in the range of 10 to 10 g/m/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.