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POSIVA Report 2018-2

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Name:

Plan for Uncertainty Assessment in the Safety Case for the Operating Licence Application

Writer:

Nummi, O.

Language:

English

Page count:

56

ISBN:

978-951-652-269-5

Summary:

Management of uncertainties is an essential part in the compilation of a safety case for nuclear waste disposal. Posiva's strategy for management of uncertainties can be described by the following activities: identify, avoid, reduce and assess. This report focuses on the uncertainty assessment and presents a plan for such an assessment for the safety case for the operating licence application.

The plan is based on earlier safety case work, on the regulatory guidance and international guidance and experiences. The main points in the plan are:

  • Scenario and model uncertainties will be mainly assessed qualitatively by systematically estimating the impact of identified uncertainties. In addition, alternative model assumptions can be used to assess the impact of identified model uncertainties.
  • Parameter value uncertainties in the performance assessment (analysis of the evolution of disposal system, but not radionuclide transport) models will be mainly assessed deterministically.
  • Parameter value uncertainties will be assessed in all the scenarios leading to radionuclide releases both deterministically and probabilistically in analysing radionuclide transport and calculating the doses. Probabilistic analysis consists of uncertainty and sensitivity analyses and based on the outcomes of the probabilistic analysis, deterministic calculation cases will be used to highlight the impact of specific uncertainties.
  • Uncertainties in the parameter values are presented as probability distributions. A systematic methodology is planned for determining the probability distributions. Possible dependencies between the parameter values are estimated separately.
Uncertainty is almost always understood to be a combination of epistemic and aleatory uncertainties. Consequently, the resulting uncertainty estimate is understood as a combination of subjective estimation and supporting data. The resulting uncertainty estimate, including the impact of all identified uncertainties, helps to determine the confidence in the results. The 95th percentile of the quantitative uncertainty estimate, obtained using probabilistic uncertainty assessment, will be compared against the regulatory criteria in conjunction with the deterministic modelling results.

Keywords:

Management of uncertainties, uncertainty assessment, deterministic analysis, probabilistic analysis, safety case

File(s):

POSIVA 2018-02_web (pdf) (1.9 MB)


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