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3.2 WP2.2 Testing Approaches for Treating Uncertainties
WP2.2 consists of five tasks:
2.2.A Parameter Uncertainty. Research in the area of parameter uncertainties is focused on developing
practical recommendations for reliable and defensible derivation of PDFs for key
parameters used in PA calculations. This includes testing the limitations and (dis)advantages
of methods such as statistical analysis, the Bayesian approach, expert judgement, and hybrid
methods.
2.2.B Model Uncertainty. Work in the area of conceptual model uncertainties includes calculations
to assess the consequences of using alternative representations of key processes, including
radionuclide retardation, gas migration and groundwater flow, in a PA system
model. This work is nearly completed and reports are available on the treatment of uncertainties
associated with modelling the gas pathway [3] and hydro-geochemical impacts on
uranium transport through an engineered barrier system [4].
2.2.C Scenario Uncertainty. This task is considering the extent to which the probabilities of different
types of scenarios can be evaluated, methods for amalgamating consequence results into
risk estimates and the associated limitations (e.g., related to statistical convergence and scenario
termination events), the extent to which it is reasonable to account for the uncertain
occurrence of FEPs in “normal evolution” scenarios, and the definition and utility of analysing
“less likely” and “what if” scenarios.
2.2.D Spatial Variability. This task is evaluating approaches to treating uncertainties in PA calculations
that arise from the spatial variability of facies, materials, and material properties inherent
in the geosphere.
2.2.E Probabilistic Safety Assessment. This task is developing and evaluating an integrated, fully
probabilistic safety assessment approach incorporating scenario, model and parameter uncertainty.
3.3 WP2.3 Synthesis and Integration of RTC-2 Activities
WP2.3 is a synthesis task pulling together research on the drivers for treatment of uncertainty under
WP2.1 and the testing and development of approaches to treatment of uncertainty under WP2.2 to
develop the guidance initially prepared under WP1.2. The main deliverable from the work package
will be a report at the end of the project giving guidance to users on approaches for the treatment of
uncertainty in PA and safety case development and containing illustrative examples from work undertaken
in RTDC-2, as well as from the WP1.2 review. In addition, the guidance will include approaches
to the prioritisation and screening of uncertainties, and consideration of the importance of
sound treatment of uncertainties to the safety case and its communication. The report will provide a
key international reference point for performance assessors and safety case developers.
4. Regulatory Evaluation of Uncertainty
4.1 Introduction
Under Task 2.1.A, the Swedish regulators, with assistance from Galson Sciences Ltd (GSL), organised
and hosted a workshop to elicit views on managing uncertainties in a safety case for a geological
disposal facility mainly from a regulatory perspective. The workshop was held in Stockholm on
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