Assessment of the long-term safety of geological disposal needs to be based on a clear understanding of uncertainty and the role of performance assessment is to support this undestanding. For this purpose, it is important to identify and evaluate uncertainties associated with understanding the geological environment, which often exhibits significant heterogeneity at various scales. This evaluation must be done at every stage of the site characterization so that insights, thus gained, can be used to guide designing effective measures to reduce the uncertainty. In many applications, however, only a limited number of most plausible model options and data-sets are sought after and this prevents one from recognizing uncertainty explicitly. In the current study we propose an alternative approach where all the possible options in the models and data-sets that cannot be excluded in the light of the evidence available is identified. This approach enables uncertainties associated with the understanding at a given stage of the site characterization to be made explicitly. This, in turn, supports the design of the following stage to reduce the uncertainties efficiently. In this paper a methodology for analyzing the uncertainty in a heterogeneous geological environment is discussed. The methodology has also been tested in Tono area, to demonstrate its applicability.

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  The safety assessment of the H12 report pointed out that groundwater flow in the excavation disturbed zone (EDZ) is one of the important processes affecting system performance. This determination was made by using an EDZ model, which conservatively ignores the retardation effect in the EDZ. This study focuses on nuclide migration in the EDZ. The variation in the conceptual models and the mathematical models for the EDZ (zero concentration boundary model, mixing model, porous medium model and fracture model) were defined. To examine the effect this model variation has on safety assessment, nuclide migration analyses were performed using each model. These simulations confirmed that zero concentration boundary model gives the most conservative results and that the conservatism of the fracture model changes depending on the outer boundary conditions. Furthermore, the EDZ models were qualitatively classified ranging from simple to complex and from realistic to conservative. The direction of model development for the EDZ is also discussed.

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  Gas production in a geological repository has potential hazard, as overpressurisation and enhanced release of radionuclides. Amongst data needed for assessment of gas impact, gas migration properties of engineered barriers, focused on clayey and cementitious material, was evaluated in this report. Gas injection experiments of saturated bentonite sand mixture, mortar and cement paste were carried out. In the experiments, gas entry phenomenon and gas outflow rate were observed for these materials. Based on the experimental results, two-phase flow parameters were evaluated quantitatively.
  A conventional continuum two-phase flow model, which is only practically used multidimensional multi-phase flow model, was applied to fit the experimental results. The simulation results have been in good agreement with the gas entry time and the outflow flux of gas and water observed in the experiments.
  It was confirmed that application of the continuum two-phase flow model to gas migration in cementitious materials provides sufficient degree of accuracy for assessment of repository performance. But, for sand bentonite mixture, further extension of basic two-phase flow model is needed especially for effect of stress field. Furthermore, gas migration property of other barrier materials, including rocks, by long-term gas injection test, clarification of influence of chemical environment and large-scale gas injection test is needed for multi-barrier assessment tool development and their verification.

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  One of key issues in assessing a safety of a deep geological disposal system for high-level nuclear waste is to evaluate the effects of model uncertainties and parameter value uncertainties on the results of the radionuclide migration analysis. In general, models and parameter values used for the analysis in geosphere are obtained from in-situ tests data, that have great uncertainty because of a limited number of tests conducted in a heterogeneous rock formation. Therefore, it is necessary to evaluate the uncertainties of model and parameter value obtained from in-situ tests quantitatively.
  From this point of view, we investigate uncertainties due to model and parameter values obtained from inversion analysis of in-situ tracer test. This report describes the inversion analysis of Nagra MI tracer test and discusses the model uncertainties due to the number of migration pathways between injection well and extraction well and parameter value uncertainties due to data fluctuation of tracer breakthrough curve. From the results, it was found that the results of inversion analysis showed good agreement with the results of the in-situ tracer test, that the parameter value uncertainties due to data fluctuation were much smaller than the model uncertainties due to the number of migration pathways, and that the information criteria could be useful for choosing the best model among several prospective models.

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  Characteristics of safety assessment for geological disposal of radioactive waste are as follows.
      ・Considering long-term safety that far exceeds our experience and history
      ・Treating spatial heterogeneity of natural strata.
  International experiences accumulated until recently have significantly improved our knowledge and confidence. However, our knowledge is not necessarily complete in the present, prediction of phenomena in disposal system inheres uncertainties associated with scenarios, models, and parameters. From the view point of safety assessment, it is important that the disposal system has enough safety, even though considering above uncertainties. Therefore it is necessary to identify the realistic scenario and the scenarios including uncertainties that should be considered. On the other hand, for rational design, it is essential to identify the range of parameters that conducts favorable scenario and factors that have influence on bifurcation of scenarios.
  This paper presents a hybrid simulation system that consists of cellular automata, large-scale numerical analysis, and neural networks to simulate evolution of the near field after closure of the repository efficiently. This system was applied to a test case. As a result, several qualitatively different scenarios were identified by cellular automata, and for a number of representative cases were solved by large-scale numerical analysis using PC clusters. In addition, a comprehensive parametric study using neural networks, which “learned” mappings obtained by accurate numerical simulations. Therefore, it is possible that near-field performance is confirmed efficiently, and information about bifurcation of scenarios is presented. This hybrid simulation system could be applied to define the specifications for engineered barrier system rationally and robustiously.

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  A review is given of the performance assessment of high-level radioactive waste (HLW) glass in geological disposal in an aspect of its validity. In the last few decades much progress has been made in understanding elementary reactions of HLW glass dissolution in contact with aqueous solutions and of associated radionuclide release. On the basis of the present knowledge, a conservative assessment of the glass performance can be made in geological disposal. For confirming its validity, however, more progress is required to be made in understanding reaction mechanism of the glass dissolution for the long-term of disposal period under the specific disposal conditions for each country. In addition to sound understanding of scientific basis for the glass dissolution described above, it is also essential to develop the glass performance model taking the scientific basis into account. Progress in these fundamental studies on the glass performance is expected to contribute to a valid assessment of the total system performance of disposal, which can lead to the reasonable and economical disposal. Studies on the glass performance at the present in Japan are less advanced than those conducted from various points of view in France and the United States. Further works are required for the valid assessment of the glass performance in Japan.

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  The choice of geological disposal of high level radioactive waste is based on science. So, public understanding of science (PUS) becomes important issue in public acceptance (PA). Considering PUS, there are two problems. One is the literacy to understand scientific information and the other is the paradigm of the public on which the knowledge formed in the public depends heavily. In this research, survey of awareness and attitude to geological disposal on the postgraduate students was conducted. They have been studying civil & rock engineering, so they belong to ‘the group' that acquires high education, culture and faculty. The results of questionnaires show that the awareness of danger is affected strongly by given information even in this group, but they become thoughtful and prudent in their opinion & decision-making as increasing information.

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  The Japan Atomic Energy Research Institute (JAERI) decided to adopt an advanced volume reduction program for low-level radioactive wastes. In this program, inorganic wastes are converted to stable glassy products suitable for disposal by a plasma melting system in the Waste Volume Reduction Facilities (WVRF). High melting point wastes such as refractories are excluded from the plasma melting treatment in the WVRF, and wastes difficult to handle such as asbestos are also excluded. However, it is desirable to apply the plasma melting treatment to these wastes for stabilization and volume reduction from the viewpoint of disposal.
  In this paper, plasma melting test of crucibles, ceramic filter elements, asbestos, and simulated fly ashes were carried out as a part of technical support for WVRF. The plasma melting treatment was applicable for crucibles and asbestos because homogeneous and glassy products were obtained by controlling of waste and loading condition. It was found that SiC in ceramic filter elements was volatile with a plasma torch with inert gas, and adding reducer was ineffective against stabilizing volatile metals such as Zn, Pb in a solidified product in the melting test of simulated fly ash.

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  Regarding closure technology of underground facilities on geological disposal of the high-level radioactive waste, closure performance of a disposal facility should be assessed from view points of both engineering technology and performance assessment, not only depending on individual technology. Thus, based on closure concept described in H12 report, closure performance of the backfilled repository focused on radionuclides migration from a repository to a major fracture zone was discussed.
  We aimed at the case of via a connecting tunnel and a minor fracture zone as a potential of migration path other than host rock. Current knowledge on performance of the closure elements as backfill and water stopping plug, hydraulic properties of excavation damaged zone and concrete lining was summarized. Then, a fault tree was analyzed and a closure scenario was discussed for its closure performance.
  The assessment was concluded that the scenario of existence of the critical path via a connecting tunnel and a minor fracture zone has low potential because the scenario will be prevented by function of closure elements and by low potentiality to meet all necessary conditions simultaneously, and that the scenario of the critical path via host rock in H12 report is relevant.

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