Masaru Toida

Design Options for HLW Repository Operation Technology: Part IV—Shotclay Technique for Seamless Construction of EBS

The shotclay method is construction method of the high density bentonite engineered barrier by spraying method. Using this method, the dry density of 1.6 Mg/m3 , which was considered impossible with the spray method, is achieved. In this study, the applicability of the shotclay method to HLW bentonite-engineered barriers was confirmed experimentally. In the tests, an actual scale vertical-type HLW bentonite-engineered barrier was constructed. This was a bentonite-engineered barrier with a diameter of 2.22 m and a height of 3.13 m. The material used was bentonite with 30% silica sand, and water content was adjusted by mixing chilled bentonite with powdered ice before thawing. Work progress was 11.2 m3 and the weight was 21.7 Mg. The dry density of the entire buffer was 1.62 Mg/m3 , and construction time was approximately 8 hours per unit. After the formworks were removed, the core and block of the actual scale HLW bentonite-engineered barrier were sampled to confirm homogeneity. As a result, homogeneity was confirmed, and no gaps were observed between the formwork and the buffer material and between the simulated waste and the buffer material. The applicability to HLW of the shotclay method has been confirmed through this examination.Copyright

Evaluation of a Method for Designing Sealing Plugs for HLW Geological Disposal Facilities, Taking Into Account the Heterogeneous Characteristics of the Geological Environment

In order to ensure that a repository for the geological disposal of HLW is isolated from the human environment, underground excavations, including pits and tunnels, must be properly sealed. Effective sealing requires that these excavations are backfilled, and that the Excavation Damage or Disturbed Zone (EDZ), which includes preferential flowpaths, must be intersected by sealing plugs. Methods for constructing a full-scale sealing plug and their influence on plug performance were evaluated and confirmed by a Tunnel Sealing Experiment (TSX). This experiment was carried out by an international partnership of the Japan Nuclear Cycle Development Institute (JNC) and Atomic Energy of Canada Limited (AECL). However certain specific roles of the sealing plugs at the scale of the whole repository were not studied. There remain issues to be clarified, notably the effectiveness of sealing plugs in a geological environment with heterogeneous characteristics and the resulting influences of the heterogeneities in performance assessment. Focusing on a geological environment with spatially heterogeneous characteristics, the authors have developed a method for designing the sealing plugs, based on a concept of “primarily design for closure, secondarily design for construction”. Though the proposed method for designing sealing plugs has presently been developed only at a conceptual level, it indicates the possibility of establishing a repository even in a strongly heterogeneous geological environment that may have been considered previously to be inappropriate for a repository.Copyright