The first real-scale production of bentonite buffer blocks for a high-level waste repository in Korea: An empirical study of key production parameters and challenges

Adopting real-scale monolithic bentonite blocks as a buffer construction material in a high-level waste repository is a preferable approach for ensuring the buffer's target dry density, a requirement difficult to meet with smaller-scale blocks. The objective of this first-in-Korea case study by the Korea Atomic Energy Research Institute (KAERI) was to establish a foundational block manufacturing concept. A four-stage procedure utilizing an upward demolding technique was established, resolving catastrophic failures. A subsequent parametric study identified the key variables controlling block quality. Block quality was significantly influenced by press operation (holding and confinement time) and raw material conditions (water content and montmorillonite content), while loading speed was negligible. The optimal conditions for achieving integrity—including a 30-minute holding and confinement time and a bentonite source with 16% water content and lower montmorillonite content (67.8 wt% vs. 76.7 wt%)—resulted in a bulk dry density of 1.630 g∙cm −3 and a defect area fraction below 0.2%. While the target bulk dry density was achieved, a localized low-density region was observed in the central lower section; the preliminary model, though accurately predicting the overall density distribution trend, lacks precision at the core, necessitating ongoing refinement. Furthermore, a critical trade-off was identified between manufacturability and the requirement for higher-montmorillonite content, as the bentonite source yielding superior integrity was less preferable for long-term buffer performance. Lastly, post-processing steps, including carving and block transportation, were explored. This foundational study successfully demonstrates a viable production pathway and offers empirical insights for a real-scale demonstration project planned for 2027.