Advanced PEM concept for geological disposal of vitrified high-level radioactive waste in Japan: design optimization and technical basis

An advanced Prefabricated Engineered Barrier System Module (PEM) concept was developed to improve the engineering practicality of geological disposal of vitrified high-level radioactive waste in Japan while maintaining the safety functions required of the multi-barrier system. The concept was developed as a refinement of the previous horizontal PEM design examined in the NUMO safety case, with the primary aim of maintaining the essential post-closure safety functions of the engineered barrier system while reducing module weight, excavation requirements, and repository footprint. The revised concept combines a carbon-steel overpack design with reduced wall thickness, a 100% bentonite buffer with increased dry bulk density and reduced thickness, a perforated PEM container with a filter layer to promote more uniform resaturation, and a dead-end underground layout compatible with staged emplacement and prompt backfilling. The associated operational concept includes controlled surface assembly, remote underground transport, emplacement, and immediate backfilling. Compared with the previous horizontal PEM concept, the advanced PEM substantially improves handling feasibility, repository layout efficiency, and constructability, while reducing total module weight and excavation volume. Post-closure safety assessment, performed within the same overall framework as the NUMO safety case, indicates that the long-term safety performance of the advanced PEM is broadly comparable to that of the previous concept and that the calculated maximum doses for the basic scenario remain well below the dose reference level established with reference to international safety standards. These results indicate that the reduced engineered barrier dimensions remain within the robustness of the multi-barrier system and do not represent a practically significant reduction in safety margin. The advanced PEM concept therefore provides a technically credible repository option for geological disposal in Japan and a sound basis for future engineering development.