system, on the basis of the results obtained in the first two Supporting Reports.

The safety assessment methodology consistent with the AEC Guidelines consists of:

• Systematic development of scenarios concerning the evolution of the geological disposalsystem and the behavior of radionuclides;

• Development of realistic models and datasets with estimates of uncertainties due tospatial heterogeneity and non-trivial time dependence;

• Construction of a conservative safety case that bounds the range of uncertainty inpredictions of the long-term performance of the geological disposal system.

1.2 Features of the safety assessment and general methodology

Safety assessments of geological disposal consider timescales that surpass those relevant tonormal social or technological activities. In addition, assessments need to consider largespatial domains of rock whose properties are heterogeneous. These large spatial andtemporal scales, and the uncertainties associated with them, are key characteristics of safetyassessments of geological disposal. It is thus impossible to apply conventional engineeringapproaches, where an entire system is constructed and utilized in such a way as todemonstrate system safety (OECD/NEA, 1983). This is a problem specific to the safetyassessment of geological disposal. The general methodology for safety assessment, which hasbeen established in response to this problem, is illustrated in Figure 1-2 (OECD/NEA, 1991).

Figure 1-2 General methodology for safety assessment

Firstly, scenarios of the possible future behavior of the system are constructed by consideringfeatures of the disposal system and various events and processes which may affect theevolution of the system. Next, models are developed that represent the evolution of thesystem and are used for predictive analysis of system performance. The results of this analysisare compared with guidelines and standards1-1) in order to evaluate the safety of the system.Consideration must also be given to whether all potential scenarios have been defined without

1-1) For example, many countries use dose and/or risk to establish safety standards, and proposals for

supplementary safety indicators are also under discussion (e.g. IAEA, 1994).

ConsequenceAnalysis

ConsequenceAnalysis

SafetyStandards

- Properties of modeledsystem components

- Initial/boundaryconditions of models

- Properties of modeledsystem components

- Initial/boundaryconditions of models

ScenarioAnalysis

ScenarioAnalysis

Model Development

Model Development

- Features - Events - Processes

- Features - Events - Processes

- Physical/chemicaldescription ofphenomena withinscenarios

- Physical/chemicaldescription ofphenomena withinscenarios

omission or duplication and whether individual calculation codes are fully verified1-2) andmodels and data adequately validated1-3) for the purposes of the assessment.

Although performance assessment (PA) is a well-developed methodology, additional researchand development efforts are still ongoing in this field. For example, in the 1990s, theOECD/NEA published several consensus reports by international experts in this field statingthat methods for long-term safety assessment of geological disposal systems had reached analmost satisfactory level (e.g. OECD/NEA, 1991; OECD/NEA, 1997a). In this report,Japan’s geological disposal system is examined specifically in the light of this series ofactivities.

1.3 Objectives and scope of the H12 safety assessment

As described in Section 1.1, the main goal of the current stage of R&D is to establish theconcept of geological disposal in Japan in a generic context prior to site selection anddecisions on other key factors such as safety criteria. Accordingly, a range of geologicalenvironments, repository and EBS designs and safety measures are considered in the H12safety assessment.

The objectives of the H12 safety assessment are:

• To establish a reliable safety assessment methodology that is applicable to a variety ofJapanese geological environments and a range of repository designs;

• To assess the feasibility of safe geological disposal in Japan;• To provide a technical basis for site selection and development of safety criteria.

1.4 Structure of Supporting Report 3

This report consists of nine chapters. Chapter II summarizes the AEC Guidelines and safetyassessment procedures. In particular, the approach to treating a variety of alternative disposalsystems is discussed, together with consideration of the various types of assessmentuncertainties. In Chapter III, the geological disposal system defined in Supporting Reports 1and 2 is revisited briefly to make the content of the current report self-contained. Chapters IVto VII summarize key stages of the safety assessment, namely scenario development (ChapterIV), analysis of the Reference Case (Chapter V), sensitivity analysis for the EBS, geosphereand biosphere (Chapter VI), and total system performance analysis (Chapter VII). In ChapterVIII, the level of confidence associated with the assessment is reviewed from a number ofperspectives. Finally, Chapter IX presents a summary and the conclusions of this report.

1-2) Verification: ensuring that calculations associated with a particular model provide the correct results for the

assumptions made (IAEA, 1993).1-3) Validation: demonstrating, through comparison with observations, that a model adequately represents the

system to be evaluated (IAEA, 1993).