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Chapter II Safety Assessment Methodology

2.1 The AEC Guidelines

In establishing a safety assessment methodology for H12, the AEC Guidelines have been

followed. These give specific guidance regarding safety indicators and their corresponding

timescales, namely:

The overall safety of the geological disposal system should be evaluated in terms ofradiation dose as a primary indicator and compared with criteria proposed by regulatory

bodies overseas;

Individual doses should be calculated based on the assumption that human activities donot change in the future, in order to illustrate the relative significance of the possible

consequences of geological disposal;

No time cut-off is defined. Consequence analyses should be continued until the maximumimpact on future generations arises;

Supplementary safety indicators should be used in order to compare the consequences ofgeological disposal with natural radiation levels, thereby minimizing uncertainties

associated with future human activities.

Furthermore, in order to achieve a transparent and comprehensive assessment, the AEC

Guidelines stress the importance of using systematic scenario development methodologies.

They classify assessment scenarios into two groups, namely:

Isolation failure scenarios, where the human environment is affected due to the physicalisolation of the waste being compromised;

Groundwater scenarios, where radionuclides are transported to the surface environment(biosphere) with flowing groundwater.

In addition to considering a wide range of scenarios, the AEC Guidelines emphasize the need

to take into account a variety of modeling options and alternative parameter values in order to

increase the robustness of the assessment results and thereby enhance confidence in them.

2.2 Development and treatment of safety assessment cases

The H12 safety assessment considers a range of disposal systems, including different possiblegeological and surface environments and repository designs. Furthermore, the effects of the

following sources of uncertainty on system performance are considered.

The long-term safety of a given geological disposal system cannot be assessed conclusively

due to the incompleteness of our knowledge about the system and its future behavior. These

uncertainties can be classified into the following types (OECD/NEA, 1991):

Scenario uncertainty; Model uncertainty; Data uncertainty.

In the H12 safety assessment, these types of uncertainty are treated in the following manner:

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Scenario uncertainty

Scenario uncertainty (see, for example, OECD/NEA, 1999) arises from limited

knowledge of:

The evolution of slow processes such as chemical interactions between theengineered barrier materials and groundwater and the transport of radionuclides inthe geosphere;

The timing and frequency of events that may affect the stability of the geologicalenvironment;

Future human activities.Scenario uncertainty is taken into account in the H12 safety assessment by defining a

number of calculation cases corresponding to a range of scenarios identified in the

process of scenario development. In order to reduce the risk of overlooking potentially

important scenarios, a systematic methodology based on the work of a number of

independent organizations has been applied. In particular, a comprehensive list of FEPs(Features, Events and Processes) has been developed by collating all the FEP lists

developed in earlier assessments and the potential impacts of these FEPs and their

interactions have been considered.

Model uncertainty

In some cases, two or more alternative conceptual models are able to explain the

observed behavior of phenomena equally well, but lead to significantly different

predictions when they are used to extrapolate the observations over time and/or space.

This is one source of model uncertainty. Model uncertainty can also arise from possibleerrors in formulating and simplifying mathematical equations and in programming

software.

In treating model uncertainty, it is important to recognize the hierarchy of models used

in an assessment. At the level of research models, the validity of alternative models can

be evaluated by comparing them with observations from laboratory experiments, field

tests and natural analogue studies. This has been done as part of international validation

projects (e.g. OECD/NEA and SKI, 1994). When two or more candidate research

models exist, a number of alternative calculation cases are defined so that their

significance can be evaluated. At the assessment model level, the approach used in H12

is to verify that simplifications of the corresponding research models leads toconservative results. In addition, calculation codes are verified against other programs

and, when available, analytical solutions, in order to minimize the possibility of

programming errors.

Data uncertainty

Data uncertainty arises from measurement errors, interpolation of spatially

heterogeneous geological properties (OECD/NEA, 1991) and extrapolation of results of

experiments and natural analogue studies over times and conditions relevant to the

assessment (Nagra, 1994a).

In selecting ranges of parameter values to be used in the present assessment, careful

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evaluations have been made of relevant data acquired by JNC and other organizations

and the resulting datasets have been reviewed by internal and external groups of experts.

A number of alternative calculation cases have been defined to cover the feasible ranges

of parameter values to illustrate the impact of parameter uncertainty. In practice,

conservative values which lead to greater consequences are often selected, where

appropriate, to keep the number of calculation cases manageable.

The number of cases for quantitative analysis or qualitative consideration that can potentially

be defined if all combinations of geological and surface environments and repository designs

and uncertainties are to be considered is very large. There are two different approaches to

dealing with this situation, namely (for quantitative analysis):

the use of stochastic (Monte-Carlo) simulations; deterministic analyses for a suitably defined subset of potential cases.

Each approach has advantages and disadvantages. In H12, the latter approach is adopted,

since it is well suited to providing a transparent assessment of the sensitivity of the system touncertainties and to variations in the geological and surface environments and repository

designs.

The assessment consists of the following steps:

Development and application of a systematic methodology to ensure that relevant features,events and processes (FEPs) are fully taken into account in developing scenarios for the

assessment;

Definition of a Base Scenario (one of a number of groundwater scenarios) and, within thisscenario, definition and quantitative analysis of a Reference Case and alternative cases;

Definition of a range of isolation failure scenarios and alternative ("perturbation")groundwater scenarios for either quantitative or qualitative analysis2-1);

Identification of key phenomena and uncertainties from the results of these analyses; Overall assessment of system performance in a range of geological and surface

environments, particularly with respect to the feasibility of safe geological disposal in

Japan.

By establishing a Base Scenario and Reference Case, around which sensitivity analyses are

performed, the cases that are considered are reduced to a number that is manageable using a

deterministic approach.

The following calculation cases are considered within the Base Scenario:

The Reference Case; Alternative geological environment cases and alternative design cases to address various

geological disposal systems;

Model and data uncertainty cases.Model and data uncertainty cases are identified based on an evaluation of their potential

2-1) The possibility of occurrence of many of the isolation failure scenarios can be reduced significantly by

appropriate site selection and repository design. Nevertheless, it is appropriate, in some cases, to endeavor toquantify or bound their possible consequences, as well as the probabilities of occurrence of the events that

initiate them.

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significance. In addition, some of the model uncertainty cases include FEPs that could be

relevant but are not considered in the Reference Case.

In addition, a number of calculation cases are considered for the perturbation scenarios.

Isolation failure scenarios are treated either qualitatively or by less formal "what if?"

calculations.

Figure 2-1 outlines the procedure for evaluating barrier performance and the system safety in

H12.

Figure2-1 Procedure for evaluating barrier performance and the system safety

2.3 Modeling strategy

The near-field, consisting of the EBS and a limited volume of the surrounding host rock, are

the elements of a repository system that tend to be characterized by least uncertainty. In

H12, this has led to the development and application of relatively realistic near-field datasets

and models, although with moderately conservative assumptions made where there is

uncertainty2-2)

. In the case of the surrounding geosphere, greater uncertainties associated, forexample, with the characterization of large-scale heterogeneity from a limited number of in-

situ measurements, lead to a more conservative modeling strategy for this part of the system.

Emphasis on the near-field is considered appropriate given the complex geological structure

of Japan, especially at the stage prior to site selection.

A different approach is taken for the biosphere assessment. No attempt is made to model the

evolution of the surface environment and the lifestyles of future generations, due to

uncertainties that are largely irreducible. Rather, certain sets of assumptions are made about

2-2) In the case of the near-field host rock, hypothetical datasets have been defined for generic geological

environments, with a level of conservatism that matches the confidence level established through in-situ

measurements and experiments at a number of field test sites.

Evaluation of Barrier Performance Evaluation of System Safety

Reference Case Sensitivity analysis

Construct/compile

appropriate

Conceptual modelsMathematical modelsDataThe Reference Case

provides a baseline for

the derivation and

assessment of alternativecases

Uncertainties inscenarios, models and

dataVariations in the

geological

environment anddesign

Safety

standards

Safety criteria

proposed by

foreign

regulatory

bodies

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Sv y-1

Evaluation of confidence

Results of foreign safetyassessments

Supplementary safetyindicators

Natural analogues

Total system performance

analysis

Combination of

uncertainties andvariationsK

eyfactors