Composition adjustment of low activation materials for shallow land burial

The three representative low activation materials for a fusion reactor are ferritic steel, V-alloy and SiC/SiC composite. The adjustment of the material composition of these materials to increase the fraction of shallow land burial in Japan was considered. In Japan, the fission waste having any single radionuclide exceeding the limiting concentration value, causing 100 μSv year−1 individual dose, determined by the Nuclear Safety Commission will not qualify as a low level waste (LLW), which could be disposed by shallow land burial. The limiting concentration values of radionuclides produced in fusion reactor were derived based on the methodology of the Nuclear Safety Commission. Radionuclide concentrations of the radwastes generated from the fusion power reactors using the three low activation materials based on the composition of existing materials were evaluated. Radwastes are classified into LLW and medium level waste (MLW), which is defined as the waste which does not qualify for LLW because one or more of the radionuclides exceeds the derived limiting concentration value. The weight fraction of MLW among the sum of LLW and MLW is found to be 10% for ferritic steel, 54% for V-alloy and 43% for SiC/SiC. The possibility of decreasing the MLW fraction by the material composition adjustment is considered. It is found that if Nb impurity content in V-alloy and N impurity content in SiC/SiC composite could be reduced, the MLW fraction can be significantly decreased. On the other hand, the content of the alloy component material (W), needs to be reduced to further decrease the MLW fraction in case of the ferritic steel F82H.