Performance Test of Waste Package System with Buffer Material

A high level waste (HLW) package system with buffer material was designed considering a long-term storage system of the packaged waste in a repository. For the γ-ray shielding, the buffer material of high density was prepared using bentonite powder and heavy sand of zircon. The density of the dry buffer material was about 3g/cm3. Heater test was carried out to study thermal characteristics of this buffer material and the waste package system. The buffer material was heated by an electric heater which was set at the center of the package to simulate the decay heat generation of HLW. The thermal conductivity of the buffer material was calculated from temperature distribution in the package. It was found that the measured thermal conductivity of the buffer material vary with the electric power of the heater and the radial distance from the heater surface. The thermal conductivity was in the range of 0.720-1.070W/m·K at the electric power of the heater of 0.3kW, and it was in the range of 0.685-1.410W/m·K at 0.7kW. This is attributed to the water migration in the buffer material, caused by the heat flux. The changes of the swelling pressure and the water uptake rate in the buffer material was measured to study swelling pressure, water uptake rate and water diffusion coefficient of the buffer material by immersing double distilled water from the outside of the package. Consequently the water diffusion coefficient in the buffer material was calculated to be 2.5×10-10mm2/s.