safety assessment methodology
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overall methodologyTRANSCRIPT
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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
100 300
Sv y-1
Evaluation of confidence
Results of foreign safetyassessments
Supplementary safetyindicators
Natural analogues
Total system performance
analysis
Combination of
uncertainties andvariationsK
eyfactors