Geological disposal is a globally common technology of HLW disposal for either vitrified HLW canisters or the spent nuclear fuel itself. Figure 24.4 shows the HLW disposal scheme employed in Japan, which incorporates the multi-barrier concept in the scheme. The first barrier is the vitrified HLW canister itself; the solubility of vitrified waste is very low and it is contained in a canister made of stainless steel. The second barrier is a thick package made of carbon steel, the third is a buffer made of bentonite and sand, and last, the multiply packaged waste is placed in stable host rock located deep underground.

Difficulty in securing the safety of HLW disposal comes from the requirement that risks associated with HLW disposal must be maintained below an acceptable level for a very long period, beyond 10,000 years. Whatever technical measures are taken, risks would remain. This is basically the same problem as the case of safety measures for severe accidents of nuclear power plants. The safety issue of HLW disposal, however, is more difficult because of the very long time period in which human intervention for maintaining safety cannot be expected.