TAILORING REPOSITORY DESIGN AND ENGINEERING TO SPECIFIC SITE CONDITIONS

2.1. Key research and development challenges

in repository concept development during site selection

NUMO has a rather wide definition of the term ‘repository concept’, which includes not only the design of all surface and underground repository structures (tailored to a given siting environment) along with a description of how they would be constructed, operated and closed, but also an evaluation of operational and long term safety, ease of retrievability and the required level of monitoring, as well as an assessment of environmental impact and socioeconomic aspects [4].

When volunteers come forward, the initial requirement is to determine if construction of a repository is feasible at that site. Although locations that are inherently unsuitable with regard to long term safety will be screened out by exclusion criteria, sites may remain which would not be suitable for the very idealized designs presented in H12. One or more concepts thus need to be developed and evaluated in a rigorous and transparent manner — particularly if choices need to be made of which sites are to be carried forward to a charac-terization phase; this will inevitably be costly in terms of resources and

‘manpower’. As the stepwise programme proceeds, such concepts will be further developed to support decision making and eventually to form the basis of licensing applications.

The challenges can thus be identified as defining a process of repository concept development which is appropriate and accepted by all stakeholders and ensuring that the knowledge base required to support it is available in an accessible and timely manner.

NUMO has already [4] examined various top-down and bottom-up approaches to developing site-specific repository concepts. For a comparison of options and/or sites, a multi-attribute analysis (MAA) approach has been found to be useful. Indeed, as similar approaches are being increasingly used by partner organizations, this might be a good focus for collaboration. To date, however, the attributes included often involve surrogates and the scoring models used are extremely simplistic — in many cases effectively representing expert opinion. Further development of this methodology may thus be justified.

Development of the supporting knowledge base requires considerably more effort. Studies carried out over the last two decades have shown that, under the boundary conditions set by various national programmes, many different combinations of waste type/engineered structures and geological

ISHIKAWA et al.

settings can provide high levels of safety. Nevertheless, to make the task manageable, focus in Japan is being placed on the primary engineered barrier materials and the alternatives identified in the H12 project. Their behaviour and evolution with time is well supported by extensive laboratory studies, mechanistic modelling and natural analogues, at least on a generic basis.

Even restricting consideration to engineered barrier systems (EBSs) based on a steel overpack and a bentonite-based buffer/backfill, a very wide range of designs and layouts of the underground structures can be derived to respond to the geological environment encountered and the requirements of stakeholders [4]. Nevertheless, in order to build confidence in derived design concepts, the robustness of key safety functions of the EBS, such as physical containment by the overpack and retention of radionuclides in the bentonite buffer, have to be assured for specific siting environments, e.g. the saline groundwater expected at coastal sites.

An area which is currently less well defined is the practicality of construction of such an EBS under strict quality assurance controls in an operational repository environment, considering underground conditions of restricted space, humidity, emplacement rate, remote handling, operational safety, robustness to perturbations, etc. When viewed from such a perspective, there are clearly a number of aspects of the EBS designs that need to be revised or, at least, analysed in greater detail. These are mainly associated with the emplacement of the bentonite-based buffer, which plays many important roles in the associated safety case, e.g. colloid filter, hydraulic barrier, plastic mechanical buffer, chemical buffer, and providing for radionuclide sorption. To ensure that these roles are performed for relevant time periods, the buffer needs to be emplaced in a strictly quality assured manner and its mineralogical/

structural stability must not be degraded by other engineered barrier materials under the expected hydrogeological and thermal conditions.

Demonstration of buffer emplacement methods to meet defined quality levels (e.g. density, homogeneity), when implemented with appropriate remotely operated procedures, could be particularly challenging in the geological environment, where potential host rocks are likely to be rather wet.

The handling of highly compacted bentonite is known to be difficult under high humidity conditions and its entire practicality/quality assurance becomes questionable if significant liquid water is present. Nevertheless, there are certainly ways to engineer around this problem, such as the use of prefabri-cated EBS modules but, in the interim, it is being increasingly studied based on experience gained in full scale tests [5].

This is very much in line with recent international trends, which place increasing emphasis on site-specific tailoring of the rather simple concepts used originally for feasibility demonstration to improve operational practicality,

SESSION IIIa

robustness and safety. Apart from conventional laboratory studies, there seems to be much that could be gained from large scale, long term demonstration projects in underground test facilities which, in the past, have clearly illustrated the difference between a design that is possible to implement and one that is truly practical under the boundary conditions of a working repository.

Extensive resources and the experienced, multidisciplinary teams are required for such projects, and they can, therefore, often be implemented efficiently as multinational collaborations. Such projects can also play a valuable role in communicating design concepts to non-technical audiences.

In this context, a further international trend is the increasing general acceptance of the idea that enhanced retrievability/reversibility may need to be built into repository designs, to increase acceptance but also to allow flexibility by keeping options open for future societies to be able to make use of possible technical advances in waste management and materials technologies. There has been little research on the extent to which such enhanced retrieval provisions

— such as delaying the placement of repository isolation barriers — could have negative impacts on safety. Again, long term demonstration experiments in situ could be useful.

2.2. Special challenges associated with implementation

As the stepwise process moves closer to identification of the final repository site, the details of implementation will need to be more clearly specified. Even though this is expected to be three decades from now, in Japan, it is recognized that some of the requirements for implementation have long lead times and hence need to be considered now. These requirements can be subdivided into structures and ‘manpower’.

Initially, repository concept development focuses on the primary engineered barriers, even though a number of other repository structures may have barrier roles, e.g. tunnel liners, borehole caps, backfilling materials, plugs and seals for tunnels, ramps and shafts. Particularly when considering the safety and practicality of construction and operation, such features may play critical roles. As yet, however, there has been relatively little detailed study of the performance of such structures and their possible long term interactions with each other (and the primary EBS). For example, managing groundwater inflow might involve the use of high quality tunnel (or borehole) liners. Indeed, the use of some form of liner may be required for mechanical stability/operational safety, as increasingly recognized internationally, even in programmes focusing on strong, hard rock. Designs of such liners tend to be focused on the use of concrete, which raises questions with regard to the long term degradation of bentonite. In fact, similar concerns arise from all uses of cementitious materials

ISHIKAWA et al.

in repositories — including floors, plugs, seals, grouts, etc. As noted elsewhere [6], there are several possible approaches to solving (or avoiding) this problem but, as yet, they have not been assessed in a rigorous manner.

Current concepts envisage that the main emplacement operations in a repository will involve some kind of remote operation, although this has not yet been shown to be feasible with existing technology. The special difficulties of handling radioactive materials underground and the need to be able to recover from any perturbations which might arise during the decades of operation lead to a requirement for robustness which, realistically, will require several cycles of iterative design and testing; as cycle lengths could be around 5–10 years, the need to initiate work now is clear.

Implementation will allow considerable potential for optimization and some areas where design improvements are possible have already been identified, e.g. prefabricating the main components of the EBS, placing several vitrified waste packages in a single overpack. These conceptual options do, however, need considerable study to bring understanding up to the level of more conventional approaches and to clarify any consequences for post-closure safety. For example, optimization resulting in higher emplacement densities leads, inevitably, to higher thermal loading and a potentially significant increase in both the maximum temperatures within the EBS and the duration of the thermal transient which could, in turn, have a large impact on kinetically controlled chemical interactions. The technical background needed to carry out a rigorous cost–benefit analysis within an optimization study would require a considerable extension of present-day knowledge.

In terms of ‘manpower’, it is clear that an HLW repository project will require significant numbers of widely experienced staff, particularly at the time around the initiation of operations. Recruitment of highly qualified staff is recognized to be a problem throughout the nuclear industry and hence an active programme of ‘manpower’ development is needed. An HLW repository will be a ‘first of its kind’ facility in Japan and, even if a few repositories are operational by this time in other countries, the extent to which experience can be directly transferred will be limited. The multidisciplinary experience needed cannot be gained in conventional projects and, hence, this is seen to be a key role of underground research laboratories (URLs). This is an area with great potential for international collaboration, especially when there are clear links between URL experiments, laboratory experiments, process modelling and data gathering [7]. As previously noted, URL experiments are valuable in their own right for demonstrating the functioning of repository components under realistic conditions and for developing, testing and demonstrating engineering standards and quality assurance methods; this complements their training role.

SESSION IIIa

In accordance with the new framework specified by the Atomic Energy Commission of Japan in 2000 [8], JNC continues to be responsible for the research and development activities supporting the HLW programme. Given the importance of in situ work as identified above, a particular feature of JNC research and development activities in this implementation phase is to promote two purpose-built generic URL projects — one at Mizunami in crystalline rock and the other at Horonobe in sedimentary rock [9]. Given the degree of inter-national interest in large scale demonstration projects, this is an area where Japan, with its strong engineering base, could take a lead and attract other national programmes into shared, long term, high profile tests and demonstra-tions of repository engineering technology.

3. INTEGRATION OF TECHNICAL UNDERSTANDING