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.
plan is based on earlier safety case work, on the regulatory guidance and
international guidance and experiences. The main points in the plan are:
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.
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
value uncertainties in the performance assessment (analysis of the evolution of disposal system,
but not radionuclide transport) models will be mainly assessed deterministically.
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.
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
Management of uncertainties, uncertainty assessment, deterministic analysis, probabilistic analysis, safety case