Arrangement study on heat-generating packages in concentrated emplacement of vitrified high-level waste after minor actinide recovery

Recovering minor actinides (MA) suppresses the long-term decay heat of vitrified high-level radioactive waste (HLW). It allows emplacement geometries that are far more compact than the vertical in-hole (H12V) concept in the Japanese safety case. This study asks how such a vault should be laid out internally. Four-canister waste packages are alternated with inert concrete blocks in a checkerboard pattern within a structural frame, and six design parameters are examined in a limited case-based screening against a 100°C buffer criterion using transient conduction analysis in Abaqus, with verification of the heat-conduction implementation against the NUMO vertical-emplacement reference case. The disposal area per canister, the unit disposal area, was 10.4, 6.5 and 4.1 m2/canister for surface storage periods of 50, 80 and 120 years, that is, 1/4.3 to 1/11 of the H12V reference (1/3.4 to 1/10 for the package-based loadings of 12, 24 and 36 canisters). These ratios relative to the H12V reference combine the effect of MA recovery and that of concentrated emplacement. Within the cases examined, the storage period had the largest effect: the package arrangement changed the area by less than 10% and never improved it, which justifies representing the vault interior as a single homogenized heat-generating region. Since only about 17% of the vault is occupied by heat-generating packages, reducing the tunnel cross-section is a more efficient next step than diluting the heat with inert blocks.