Morimasa Naito

Public Acceptance as a Driver for Repository Design

The Japanese geological disposal programme for radioactive waste is based on a volunteering approach to siting, which places particular emphasis on the need for public acceptance. This, as established in law, emphasises the development of a repository project as a partnership with local communities and involves stakeholders in important decisions associated with key milestones in the selection of repository sites and subsequent construction, operation, and closure. To date, however, repository concept development has proceeded in a more traditional manner, focusing particularly on ease of developing a post-closure safety case. In the current project, we have attempted to go further by assessing what requirements stakeholders would place on a repository and assessing how these could be used to rethink repository designs so that they meet the desires of the public without compromising critical operational or long-term safety.

Development of Superfine Spherical Silica Grout as an Alternative Grouting Material for the Geological Disposal of Long-Lived Radioactive Waste

As an alternative grouting material for the geological repository of long-lived radioactive waste, the “S uperf ine S pherical silica G rout” (SFSG) material is developed using a fine spherical silica and a fine calcium hydroxide. The developed SFSG material takes an advantage of its smaller particle size distribution (max. ∼1 micron or less) than those of the cementitious materials, and also provides a low alkaline environment so as to reduce unfavorable effects on the long-term performance of geological disposal system. The SFSG is a mixture of the “super fine silica powder”, the superfine calcium hydroxide and additives such as superplasticizer. Presently, the mixture being investigated for grouting materials is focused on water/binder ratio (W/B) of 1.2. Some preliminary laboratory experiments were carried out to characterize its fundamental properties from the viewpoint of practical use for geological disposal, which is required to be equivalent with the conventional cementitious materials in terms of penetrability, strength, pH performance and workability. From a series of experiments, it was concluded that SFSG is expected to become an alternative grouting material for a geological repository.Copyright

Evaluation of Mechanical Effects of the Fault Movement on the Engineered Barrier System

Experiments and numerical analyses have been conducted to investigate the consequences of an active fault crosscutting the engineered barrier system (EBS) of a high-level radioactive waste (HLW) repository. Experiments were performed using laboratory simulation test equipment at a scale of 1:20, which can simulate the chosen shear displacements of 80% and 140% of the buffer thickness, and the chosen shear rates of 0.1 m/s, 0.05 m/s and 0.01 m/s. The experiments have so far demonstrated that the metal overpack will be rotated, but not breached, due to the plasticity of the surrounding bentonite buffer. The total pressure on the bentonite buffer increases with an increase in the shear rate. Numerical analyses were carried out using a finite element method assuming the constitutive model of modified Cam-clay, which was used to assess elasto-plastic behavior of the bentonite buffer.Copyright