Demonstration test of underground cavern-type disposal facilities, fiscal 2010 status - 59180

A test to demonstrate practical construction technology for underground cavern-type disposal facilities is currently underway. Cavern-type disposal facilities are a radioactive waste repository excavated to a depth of 50 to 100 m below ground and constructed with an engineered barrier system (EBS) that is a combination of low-permeable bentonite material and low-diffusive cementitious material. The disposed materials are low-level radioactive waste with relatively high radioactivity, mainly generated from power reactor decommissioning, and certain transuranic wastes that are mainly generated from spent fuel reprocessing. The project started in fiscal 2005*, and since fiscal 2007 a full-scale mock-up of a disposal facility has been constructed in an actual sub-surface environment. The main objective of the demonstration test is to establish construction procedures and methods which ensure the required quality of an EBS on-site. Certain component parts of the facility had been constructed in an underground cavern by fiscal 2010, and tests so far have demonstrated both the practicability of the construction and the achievement of the required quality. This paper covers the project outline and the test results obtained by the construction of certain EBS components. The following results were obtained from the construction test of EBS in the test cavern: 1) The dry density of bentonite buffer at the lower layer constructed by vibratory compaction shows that 95% of core samples have densities within the target range. 2) The specified mix for the low-diffusion layer has uniform density and crack-control properties, and meets the requirements for diffusion performance. 3) The specified mix of the concrete pit has sufficient passing ability through congested reinforcement and meets the requirements of strength performance. 4) The dry density of the bentonite buffer at the lateral layer constructed by the spraying method shows that 65% of the core samples are within the target density range. 5) The specified mix of filler material has sufficient filling ability for narrow spaces and allows remote-operated filling work. 6) The dry density of the bentonite buffer at the lateral layer constructed by vibratory compaction shows that 100% of the core samples are within the target density range. Accordingly, the demonstration test has so far established a practicable construction methodology and confirmed satisfactory initial performance of the EBS built on-site. The following results have been obtained regarding construction methods. A) Construction procedure for the bentonite buffer by use of vibratory compaction and a spraying method were verified. This procedure has the ability to meet performance requirements for very low permeability. B) The use of SCM or SCC in a disposal facility leads to more labor-saving workability and less defective workmanship than does conventional vibrated concrete. (authors)