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Development of the NUMO pre-selection, site-specific safety case

T. Fujiyama1, S. Suzuki1, A. Deguchi1, H. Umeki1

1Nuclear Waste Management Organization of Japan (NUMO), Tokyo, Japan

E-mail contact of main author: tfujiyama@numo.or.jp

Abstract. NUMO has developed a safety case for co-disposal of HLW and TRU waste to reflect current boundary conditions in Japan. In particular, this involves addressing public concerns in the wake of the Fukushima Dai-ichi nuclear power plant accident and a move by the Government to more strongly support moving forward with siting a geological repository, involving suggesting locations that are considered to be scientifically more suitable. This paper will provide a brief overview of this Safety Case, with a focus on advances from the old “H12 Report”, which is considered the first generic safety case in Japan. “The NUMO pre-selection, site-specific safety case” has been developed to provide a basic structure for subsequent safety cases that would be applied to any selected site, emphasising practical approaches and methodology, which will be applicable for the conditions/constraints during an actual siting process.

Key Words: Geological disposal, Safety case, Vitrified waste, TRU wastes.

1. Introduction

The “H12 Report” [1] published in 1999 by the Japan Nuclear Cycle Development Institute (now the Japan Atomic Energy Agency, “JAEA”) demonstrated the feasibility of safe and technically reliable geological disposal of high level waste (HLW), based on a generic study. On the basis of the H12 Report, “the Final Disposal Act” came into force and NUMO was established as the implementing body in 2000. Intermediate-level waste generated from reprocessing of spent nuclear fuel and mixed-oxide fuel fabrication (termed “TRU waste” in Japan) was also included in the inventory for disposal in 2007. NUMO has been developing key technologies required for the safe implementation of the geological disposal project since its establishment and initiated the siting process by open solicitation of volunteer municipalities in 2002. So far, however, no volunteer municipality has appeared and no candidate host rock type can be specified.

The Fukushima-Daiichi NPP accident in 2011 increased nationwide concerns about the feasibility and reliability of safe geological disposal in Japan. After re-thinking the implementation process, “the Basic Policy on the Final Disposal of Specified Radioactive Waste” was amended in 2015, so that the Government now leads the search for volunteer sites. This procedure involves nominating more suitable areas from a geo-scientific point of view to initiate discussions and cooperation with local municipalities, finally leading to acceptance of a site investigation, which will be carried out by NUMO.

Taking such changes in boundary conditions into account, it is important at this time for NUMO to integrate required technologies, including the latest R&D output, in order to confirm the feasibility and safety of geological disposal in Japan. Thus NUMO has developed the “NUMO pre-selection, site-specific safety case” which, with the site descriptive models (SDMs) recently developed, provides a more advanced site-specific basis than the generic safety case in the H12 Report. This has been developed in a manner that will allow it to

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provide the basic structure of future safety cases that would be applicable to any site that might arise from the site selection process.

2. Basic Strategy of the NUMO Safety Case

Despite the fact that there has not been a site or specific host rock identified, the NUMO Safety Case has developed detailed geological and hydrogeological models for potential host rock environments. Repository design and safety assessment have been thus performed for these geological models, thereby providing underpinning evidence to demonstrate the technical feasibility and the safety for the various types of Japanese geological environments. More background is provided in the companion paper by Suzuki et al. [2]

3. Site Characterisation and Synthesis into SDMs

NUMO needs to prepare reliable investigation and evaluation methodologies and an approach to synthesise their output in order to form the basis of selecting suitable repository sites. In suitable setting, the key safety functions (isolation and containment) of the host geological environment will persist for a long period of time. Advanced methodologies for precluding the potential impacts of natural disruptive events and processes are shown. Key concepts, technical knowledge bases, and basic methodology for geosynthesis of relevant information into representative spatial and temporal models of site evolution are also documented.

The illustrative site descriptive models (SDMs) are developed for subsequent repository design and safety assessment. Generic repository design and safety assessment were performed in the H12 Report for two illustrative geological settings, namely crystalline rock and sedimentary rock. However, geoscientific knowledge has expanded significantly since then due in particular to multidisciplinary research programmes; for example, JAEA’s underground research laboratory projects. It is thus very important to update the previous repository design and safety assessment on the basis of the current best understanding of Japanese geological environments in the NUMO Safety Case.

Following the categorization of all potential host rock environments, rock types are grouped by identifying key characteristics/properties relevant to geological disposal. As a result, three types of potential host rock environments, ‘Igneous rocks’, ‘Neogene sedimentary rocks’, and ‘Pre-Neogene sedimentary rocks’ are examined in the NUMO Safety Case.

FIG. 1. An example of the nested SDMs for Igneous rocks, including underground panel layout (bottom, left) and EBS of HLW (bottom, right)

1km

Active fault Active fault

Granite

Highly fractured (weathered) domainSedimentary overburden

100~200m100~200m

GW flow

Illustrative geological setting

3 km

3 km

Repository scale

Regional scale50 km ×50 km

Reserved area

Reserved area

Reserved area

Unpreferable area

Short travel time

Panel scale

Approx．800m

EBS + Rock

100m

100m

Near-field scale

Faults(Length > 1km)

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Realistic geological and hydrogeological models are developed in a stepwise manner for the three types of potential host rock environments: at scales of several kilometres (repository scale), for defining the location and layout of a repository and assessing groundwater flow through the potential host rock; then at several hundred metres (panel scale) and a hundred metres (near-field scale), for more precisely describing hydraulic properties. FIG. 1 shows an example of the geological and hydrogeological modelling for ‘Igneous rocks’ at nested scales. For geological modelling, key geological structures that control groundwater flow and have a major influence on solute transport, such as faults, fractures and sedimentary structures, are represented by a combination of deterministic and stochastic modelling approaches.

4. Repository Design

Design methodologies should be developed so as to maintain flexibility for the range of potential geological conditions encountered in Japan. In the NUMO Safety Case, alternative repository concepts are presented, which are applicable for a wide range of potential geological conditions. The designed repositories should be technically feasible to construct and fulfil the safety functions required to isolate and contain radionuclides.

The design requirements and specifications of the engineered barrier system (EBS), disposal tunnel, panel layout and sealing system (tunnel back-filling and plugs) have been defined. The methodology is demonstrated by carrying out a full repository design study, tailored to the SDMs of three types of potential host rock. The engineering feasibility of construction, operation and closure of the repository is evaluated based on techniques demonstrated in domestic or overseas underground laboratories and related R&D facilities. The diagram in FIG. 1 (bottom, left) illustrates an example of an underground layout tailored to the geological and hydrogeological model for Igneous rocks. The single level emplacement panel is applicable in this case, avoiding faults with lengths greater than 1 km (the minimum length that can be identified by surface-based investigation), and avoiding any less preferable areas where calculated groundwater travel times are relatively short. Optimal operational processes and material flow logistics, ventilation and water drainage systems for the underground facility are also considered while determining the layout. Such site-specific design demonstrations show progress in practicality of design methodologies.

5. Safety assessment

During the siting stages, both pre- and post- closure safety will proceed in an iterative manner and the resulting output will support decisions made at the end of each stage from the perspective on safety. The required safety assessment technology for scenario development, modelling, database development, etc. will be maintained at the state-of-the-art.

For pre-closure safety, it is needed to demonstrate the feasibility of radiological protection for local residents and workers during repository operation. Learning from the Fukushima-Daiichi NPP accident, the regulatory guidelines for nuclear facilities have been revised, but those for geological disposal have not been discussed in detail so far. In developing methodology for operational safety assessment of geological disposal, relevant guidelines for other nuclear facilities, such as those for vitrified waste storage, are considered in the NUMO Safety Case. An the first stage, radiological safety is highlighted, focusing on activities relevant to HLW handling and transport, based on specific repository designs and defined procedures of operation.

For post-closure safety, it is needed to develop an assessment approach and methodologies which can be applied to specific sites and the repository design concepts tailored to them. In

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the NUMO Safety Case, procedures and methodologies to assess long-term safety are demonstrated. A risk-informed approach is introduced, based on international guidelines as well as recent national discussions on safety regulations. Scenarios are developed and classified with consideration of their probability of occurrence and target doses are defined as illustrated TABLE I. Referring to the guidelines of international organizations on assessment timescales, dose calculations are carried out for up to one million years after closure.

TABLE I: SCENARIO CLASSIFICATION AND TARGET DOSE

Scenario classification Definition Target dose

Likely Scenario

The scenario is used to assess the performance of the geological disposal system based on best understanding of the probable evolution, as a reference for the optimisation of protection.

10 μSvy-1

Less-likely scenarioThe scenario is used to assess the safety of the geological disposal system in view of uncertainties in scientific knowledge supporting likely scenarios.

0.3 mSvy-1

Very unlikely scenario Possible scenarios with extremely low likelihood. 1-20 mSvy-1

Human intrusion scenario

The scenario is used to check whether the geological disposal system is robust with assumption of the human intrusion after loss of institutional control.

Residents:1-20 mSvy-1 Intruder:20-100 mSv per event

A hybrid methodology of scenario development is introduced, which combines a more conventional, bottom-up, FEP-based approach and a top-down method based on safety functions, appropriate to a risk-informed assessment. Safety assessment is being conducted by applying a approach and methodology of realistic radionuclide transport modelling, as needed to allow comparison of sites and also possible repository concepts that could be tailored to them. This advanced modelling includes more realistically representing the geometry of all components of the engineered barriers (essential for distinguishing between different repository design options) and realistically representing the 3-dimensional geometry of the geosphere, with particular emphasis on the solute transport characteristics of all relevant formations (shown in FIG.1, bottom right). The estimated doses of scenarios in different categories are smaller than the assumed criteria in the NUMO Safety Case. This provides a basis for more comprehensive demonstration of post-closure safety at this stage.

The outline of assessment pre- and post- closure safety in the NUMO Safety Case is presented in the companion paper. [2]

6. Conclusions and a look to the future

The safety case developed by NUMO is inherently limited by the lack of an actual site to focus on, but the SDM-based approach provides critical experience in integrating the activities of site characterisation and engineering design teams, focused by the fundamental requirement to robustly assure safety. This will prove invaluable in the next phase when parallel characterisation of potential sites may occur.

[1] Japan Nuclear Cycle Development Institute, H12: Project to establish the scientific and technical basis for HLW disposal in Japan, JNC-TN1410-2000-001～004, (2000).

[2] S. Suzuki, et al., “Assessment of pre- and post-closure safety in the NUMO safety case for a geological repository”, International Conference on the Safety of Radioactive Waste Management, IAEA, Vienna (2016) (in press).