Analytical study of the long-term mechanical deformation of the buffer on HLW disposal

Tanai, Kenji ; Kikuchi, Hirohito

An analysis was made focusing on an overpack sinking phenomena due to long-term mechanical behaviour of buffer material and volume expansion process due to corrosion products of the overpack within the surrounding buffer. The analysis was aiming to evaluate how such phenomena influence on the colloid filtration and self-sealing functions of the buffer material, which were key safety functions of the EBS. The displacement due to the overpack sinking was calculated under the salinity water condition. The sinking of the overpack and/or deformation of the buffer may cause air gaps in the upper part of the buffer. In this situation, the density of the buffer is still required to maintain above 0.80 Mg m, which is the lower limit of effective clay density necessary for the colloid filtration function. It would be important for the buffer emplaced above the overpack to have a sufficient density to swell and fill the generated air gaps. However, the volumetric swelling ratio was used to determine the increase in the volume of the buffer after swelling compared to the original volume, and to check whether the obtained value is sufficient for the buffer to fill the generated air gaps. The displacement due to the overpack sinking was calculated as 33 mm in 100,000 years. The buffer density after the swelling should be 1.27 Mg m. Thus, it can be concluded that the buffer should maintain its colloid filtration function even after 100,000 years.