Sumio Masuda, Minoru Yamakawa, Hiroyuki Umeki, Kaname Miyahara and Morimasa Naito
Power Reactor and Nuclear Fuel Development Corporation
1-9-13 Akasaka, Minato-ku, Tokyo, 107 Japan
+81-3-3586-3311

The Power Reactor and Nuclear Fuel Development Corporation (PNC) has been active in developing an R&D programme for high-level radioactive waste (HLW) disposal in accordance with the overall HLW management programme defined by the Atomic Energy Commission (AEC) of Japan. The aim of the R&D activities at the current stage is to provide a scientific and technical basis for the geological disposal of HLW in Japan, which in turn promotes understanding of the safety concept not only in the scientific and technical community but also by the general public. As one of features of R&D programmes, its progress is documented at appropriate intervals, with a view to clearly determining the level of achievement of these programmes and promoting the understanding and acceptance of the geological disposal strategy by the general public. At a major milestone, PNC submitted a first progress report, referred to as H3, in September 1992. H3 summarised the results of R&D activities up to March 1992 and identified priority issues for further study. The second progress report, scheduled to be submitted around 2000, and should demonstrate more rigorously and transparently the feasibility of the specified disposal concept. It should also provide input for the siting and regulatory processes, which will be set in motion after the year 2000.

In Japan, as outlined in the overall HLW management programme defined by the Atomic Energy Commission (AEC) of Japan (1), the HLW separated from spent nuclear fuel at reprocessing plants will be immobilised in a glass matrix and stored for a period of 30 to 50 years to allow cooling; it will then be disposed of in a deep geological formation (geological disposal). Pursuant to the overall HLW management programme, an organisation with responsibility for implementing HLW disposal will be established around the year 2000. This will be followed by site selection and characterisation, demonstration of disposal technology, establishment of the necessary legal infrastructure, relevant licensing applications and repository construction, with the objective of starting repository operation by the 2030s and no later than the mid 2040s (Fig. 1).

The HLW disposal programme is currently in the research and development (R&D) phase and the Power Reactor and Nuclear Fuel Development Corporation (PNC) has been assigned as the leading organisation responsible for R&D activities. The R&D programme aims to provide a scientific and technical basis for the safe disposal of HLW but also to achieve a higher degree of understanding of geological disposal among scientists, decision-makers and the general public. Such broader understanding of geological disposal is critical to allow the implementing organisation to initiate its activities. A fundamental disposal concept applicable to a variety of geological settings is now being developed in such a way as to maximise flexibility in subsequent site selection process. At present, neither geological formation nor a site has been selected and no regulatory criteria have as yet been codified. The regulatory criteria will be formulated in parallel with the implementation, taking the results of R&D activities into account.

This paper introduces the key features of PNC's R&D programme and considers its likely future development in order to highlight aspects of common interest with other national HLW disposal programmes.

Each disposal programme among other countries has specific characteristics, such as geology, type of waste and regulatory framework. The Japanese approach to repository concept development has targeted neither a particular type of rock nor a particular area. Japan has complex geology and fairly active tectonic settings and hence has developed repository designs which combine deep disposal with a massive engineered barrier system (EBS). The major component of overall barrier performance of the disposal system is borne by the near-field, while the geosphere serves to reinforce and complement the performance of the EBS (2). This massive EBS was introduced to ensure long-term performance of the disposal system for a wide range of geological environments. The layout of the EBS involves axial, horizontal emplacement of vitrified waste, encapsulated in a thick steel overpack, in tunnels which are backfilled with highly compacted bentonite (Fig. 2).

Within the context of this defined disposal concept, it is important for R&D activities to focus on the natural system attributes which optimise the EBS performance. These attributes include relative tectonic stability, low groundwater flux, favourable geochemistry and a low risk of disruptive events. The safety concept assesses that disruptive events can be avoided by site selection. The geological environments identified as favourable for the disposal system then form the basis for repository design. Finally, the performance assessment is conducted for the reference repository design, including a massive EBS combined with a favourable geological environment, taking alternative futures evolutions of the system into account.

PNC's R&D programme (3) to realise Japanese disposal concept includes by three major areas: 1) studies of the geological environment, 2) repository design and engineering technology and 3) performance assessment. The coordination and integration of activities in these three areas are critical in order to maximise the output from the R&D programme (Fig. 3). In addition to these three studies, it is important to systematically pursue geoscientific studies, encouraging participation in relevant projects by researchers both from Japan and abroad. The results obtained from such studies will be utilised as a basis for R&D projects on geological disposal. Comprehensive information accumulated through geoscientific studies should be integrated to help in understanding more precisely the characteristics of the deep geological environment and in improving the reliability of performance assessment models for the geological disposal system.

The progress of R&D programmes is intended to be documented at appropriate intervals, with a view to clearly determining the level of achievement of these programmes and promoting the understanding and acceptance of the geological disposal strategy by the general public. So far, there are two major milestones for the documenting in PNC's R&D programme as described below.

As a first milestone of the documenting, PNC summarised the results of R&D activities up to March 1992, and submitted them as a first progress report (4), referred to as H3 in short. The primary objective of H3 is to increase the general understanding of how disposal system may perform upon a variety of Japanese geological environments and to define the direction of following work for the next milestone, namely a second progress report.

In July 1993, the Advisory Committee on Radioactive Waste Management (ACRWM) of the AEC summarised its views on H3 in a report entitled "On the Progress of Research and Development in Geological Disposal of High-Level Radioactive Waste" (hereinafter the ACRWM Report) (5). The ACRWM Report found that the H3 had shown that multibarrier systems can maintain their performance levels over very long time periods, provided the engineered barrier system (EBS) and the other repository components are designed and constructed appropriately for their geological environment. The technical feasibility of safe geological disposal in Japan has therefore been demonstrated in a generic sense. The ACRWM Report also pointed out that it is important to investigate the behaviour of the EBS in its geological environment, i.e. the near-field, in order to develop appropriate design and construction concepts.

The extensive research effort planned should provide reliable information for the relatively small area of the near-field geosphere surrounding the EBS and will certainly contribute to demonstrating the existence of a geological environment which is suitable for radioactive waste disposal. Comprehensive reports on performance assessment of geological disposal systems for vitrified high-level waste or spent fuel were published in countries such as Sweden (6), Finland (7), Switzerland (8) and Canada (9), at around the same time as H3 was published in Japan. All the reports acknowledge the importance of the near-field in a concept based on an EBS in an appropriate host rock formation, although the geology being considered varies from country to country, as does its weighting in the overall safety case. These studies also identify low radionuclide release rates and the retardation of radionuclide migration in the near-field as factors contributing to safety.

The geosphere, on the other hand, could be regarded as complementing near-field performance. From this point of view, it is important to investigate the capability of the geosphere to retard radionuclide transport, reducing radionuclide concentrations by dispersion, dilution and radioactive decay.

The geological environment is required to play a number of important roles. It must act as an effective physical barrier, isolating the HLW from the human environment for very long times, and must also provide the long-term stability which is critical to the performance of the multibarrier system, particularly near-field performance. The groundwater in the geological environment should maintain low flow rates and be chemically reducing over relevant timescales. These requirements can be met by selecting an appropriate host rock formation and it is therefore essential to conduct extensive research into identifying such rock formations in Japan.

Since its establishment in September 1995, the AEC's Advisory Committee on Nuclear Fuel Cycle Backend Policy (hereinafter "the Committee", successor to ACRWM) has been discussing ways in which future R&D programmes relating to geological disposal should be conducted. These discussions are based on the recognition that, given the urgency of the problem, it is essential to formulate concrete technical measures for geological disposal and to inform the general public clearly and transparently of these measures with a view to obtaining their understanding and acceptance of disposal projects.

The H3 provides a comprehensive evaluation of the technical relevance of geological disposal and demonstrates the feasibility of ensuring safe geological disposal in Japan. The second progress report (tentatively called "H12"), which will be founded on the basis of the achievements identified in H3 and is scheduled to be submitted around 2000, will further demonstrate the technical feasibility and reliability of the geological disposal concept and will provide key input for site selection and development of regulations. In this context, the Committee has been considering issues such as the approach to R&D to be performed by PNC in cooperation with other agencies and organisations, the way to identify technically important issues and how to evaluate research results objectively and transparently in the H12 report.

The Committee has summarised the results of its deliberations and issued the guidelines entitled "Guidelines on Research and Development Relating to Geological Disposal of High-level Radioactive Waste in Japan" (hereinafter the Guidelines) (10) in April 1997. The Guidelines requires the H12 report to demonstrate more clearly the feasibility of the specified disposal concept in Japan. This report should also provide a scientific and technical basis for both the siting procedure adopted by the implementing organisation and for development of the appropriate regulatory infrastructure. The generic assessment being carried out for the H12 report thus plays an important role in the overall HLW disposal programme by setting it in a clear perspective.

In order to demonstrate the technical reliability and safety of HLW disposal, it is required for the H12 to show that the safety framework provided by a multibarrier system constructed in a suitable host formation is capable of functioning as designed, given the characteristics of the geological environment in Japan. The R&D requirements for the H12 can therefore be defined as follows:

• The characteristics of the geological environment that are important for geological disposal must be specified and it must also be shown that suitable rock formations with these characteristics exist in Japan. Information on the near-field properties required to ensure long-term safety and feasibility of HLW disposal should be compiled via appropriate investigations.
• The design criteria for the EBS and other repository components must be specified. The technical feasibility of meeting these criteria must also be demonstrated.
• The performance of the disposal system under the specific conditions of the geological environment in Japan must be evaluated with higher reliability.

Obtaining public understanding and confidence is an essential component of performing R&D relating to geological disposal. A considerable investment of manpower and economic resources over an extended period is also necessary for such an activity. Close cooperation among the research organisations involved in R&D activities is required for real progress to be made. To this end, PNC, which acts as the core organisation for R&D, the Japanese Atomic Energy Research Institute (JAERI), the Geological Survey of Japan (GSJ), the National Research Institute for Earth Science and Disaster Prevention, the Central Research Institute of the Electric Power Industry (CRIEPI), universities and private sector agencies should cooperate in R&D programmes, with support from power companies, to make maximum use of available expertise. It is important that a technical foundation for geological disposal in Japan be established through these joint efforts. PNC and other organisations involved have been working in close cooperation to conduct R&D work in an effective and efficient manner. From this point of view, the "Geological Disposal R&D Coordination Conference" has been organised with the aim of promoting the sharing of results among different organisations and strengthening mutual cooperation.

The issues identified and prioritised by the H3 assessment have been considered in the Guidelines and addressed in ongoing R&D activities for the H12. Those activities critical to making a more robust safety case for given disposal concept are briefly summarised below. For model and database development, special attention is paid to achieving a balance between robustness and realism.

The ACRWM Report notes that basic data on the characteristics of the geological environment in Japan have been collected and compiled in H3, but that more reliable and comprehensive data still require to be obtained. The Guidelines requires the H12 will therefore collect and compile more extensive data based on the results of geoscientific studies. The Guidelines will also define the requirements on the geological environment which are critical for geological disposal. These are very important both in terms of input for performance assessment studies and defining R&D for repository design and engineering technology.

H3 set the framework for R&D activities relating to design and construction of the disposal facility and fabrication of the EBS components, but that technological development should be continued. Therefore, the H12 report will specify design criteria for a robust EBS, taking into consideration advances in relevant technologies. In this respect, the geosphere surrounding the EBS and changes in the near-field environment brought about by the construction of the EBS must be considered, based on data provided by the studies of the geological environment. Design criteria must be established considering the geological environment of the entire disposal facility. Specific design criteria for the EBS and the disposal facility should be input to performance assessment studies using a flexible approach.

It is also imperative to demonstrate the feasibility of constructing the EBS and the disposal facility in such a way as to satisfy the design criteria using existing technology. An economic analysis will also be necessary once such design criteria have been established.

The Guidelines states that H3 has successfully demonstrated the feasibility and safety of the concept of geological disposal of HLW in Japan, identified a suitable methodology for evaluating the performance of the multibarrier system and established a foundation for constructing the models which will be used to analyse system performance.

The H12 report will determine the scenarios to be studied in the performance assessment in a more comprehensive and transparent way and will refine the analytical models to be used on the basis of information on the geological environment and system design criteria provided by the other two R&D areas mentioned above. The report will evaluate near-field performance using an improved database. Research on the performance of the geosphere, in terms of retarding radionuclide migration and reducing radionuclide concentrations by dispersion and dilution, should proceed in order to demonstrate the role of the geosphere in complementing the performance of the near-field. In this connection, attention should focus on the main groundwater flowpaths between the near-field and the biosphere. Case studies on radionuclide migration in the biosphere should be performed to calculate radiation doses from any releases and assess the level of safety of the entire multibarrier system.

An evaluation of the reliable performance of the disposal system for the geological environment in Japan, focused on the near-field, requires, in particular, validated models of near-field performance and quality assurance of near-field performance assessment databases.

Geoscientific studies, which form the basis of R&D relating to geological disposal, focus on 1) studies of the characteristics of the geological environment, 2) the development of investigation techniques to be used for these studies and on 3) the long-term stability of the geological environment. These studies cover various fields of earth science, including deep rock and groundwater properties and natural phenomena such as seismic and volcanic activity and they therefore make a significant contribution to scientific research on the deep underground environment of Japan. As mentioned above, the results of these studies will provide basic information to the above studies: studies of the geological environment, repository design and engineering technology and performance assessment.

In order to tackle the above key technical issues, experiments on both a laboratory and engineering scale and in-situ tests using shafts and drifts in existing underground facilities are essential and being conducted.

A set of carefully designed, relatively large-scale, non-radioactive experiments, referred to as the "ENTRY Project", has been initiated for fundamental data acquisition and model development and validation (11) (12). The basic concept of the ENTRY Project is to integrate a number of large-scale experimental studies focusing on various aspects of the safety case for HLW disposal.

In addition, based on the experience in the ENTRY project, construction of a new radioactive facility (QUALITY) for radionuclide migration experiments are planned. Major purpose of this project is to acquire data on radionuclide migration behaviours such as glass dissolution and nuclide solubility under simulated geological condition in controlled glove boxes.

Because most of the geoscientific information collected on a nationwide basis in Japan originates from diverse sources and different academic disciplines, it is of a varying degree of quality and applicability within the framework of geoscientific studies which provide basic information to R&D activities of geological disposal. Some type of yardstick was considered necessary to evaluate these data so that their optimum application for the purpose of generic assessment could be assured. For this purpose, the Tono and Kamaishi mines were chosen to host experimental programmes as their geological features are favourable for relevant investigations. In addition, the readily available infrastructure and a wealth of background information on these two sites were important prerequisites for their selection (13).

In addition, PNC is currently planning to construct a deep underground research laboratory in Mizunami City (MIU: Mizunami URL) in Gifu Prefecture (14). This will focus mainly on crystalline rocks down to a depth of about 1,000 metres. As stated in the national programme, the main objective of the facility is to carry out a wide range of geoscientific studies over the next 20 years in a realistic and undisturbed geological environment, at depths of up to 1,000 metres. There is a clear separation between plans for such an underground research laboratory and siting studies for a waste repository. In addition to these geoscientific studies, this facility will also be utilised for other projects, including a recently established national seismic observation and research programme, in which PNC will participate. Requirements as to the quality of the R&D activities are high and PNC's intention is that the facility should become an international centre of excellence in relevant scientific areas. Meanwhile, to further geoscientific studies in sedimentary rocks, a deep URL programme is being planned, which will be constructed by PNC in Horonobe Town in Hokkaido. This plan will be promoted with the cooperation of the local communities and the Hokkaido Government.

Figure 4 shows the location of both these PNC's R&D facilities for geological disposal and field test sites for geoscientific studies including URLs being in operation or planning in order to tackle the above mentioned key issues.

PNC has been active in promoting international cooperation in connection with the Japanese HLW disposal programme, based on both bilateral and multilateral approaches (15). Both types of collaboration are extremely useful; in particular, bilateral collaboration has the advantage of providing opportunities for in-depth discussion in mutual areas of interest. By way of contrast, multilateral collaboration provides an international arena for broader discussion and corroboration of output from individual R&D programmes. Depending on the issues to be tackled, appropriate forms of collaboration have been integrated into PNCs strategy, which should lead to improved cost/benefit from such studies.

The lessons learnt from collaboration have proved very valuable and are used directly in PNC's programme to improve its structure by identifying particular areas of strength and weakness and also to generally enhance its credibility. The format of collaboration has also been extensively developed: it has been found that national resources can be utilised more effectively by sharing them with appropriate partners.

PNC is required to submit the H12 report, including an integrated performance assessment, to the government by the year 2000. The major purpose of the report is to demonstrate more rigorously and transparently the technical feasibility of HLW disposal in Japan. This will involve making a safety case with emphasis on near-field performance. R&D work will focus on development of detailed and realistic near-field models and on improving understanding of key processes and corresponding databases, taking into account wide range of geological conditions. The current structure of R&D activities will continue to be appropriate for this next milestone.

In PNC's R&D programme for HLW disposal so far, three major areas; 1) studies of the geological environment, 2) repository design and engineering technology and 3) performance assessment, have been conducted taking into account comprehensive information accumulated through geoscientific studies. To promote R&D activities more effectively, a closer relationship between performance assessors, geoscientists and engineers is essential. The integration of laboratory and field experiments is also critical for efficient development of models and databases. Because the R&D programme for HLW disposal will continue over a long time period, the preparation and maintenance of key infrastructures such as underground research facilities are very important. From this point of view, international collaboration is also very valuable, ensuring that resources are utilised more effectively by sharing them with appropriate partners.

It is planned in the national programme that the implementing organisation for HLW disposal will be established around the year 2000 to initiate the siting phase. The future R&D programme will thus contribute to activities of both implementing organisation and regulatory body. PNC's projects such as ENTRY and QUALITY are expected to play key roles in the future R&D.

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