Regional Basic Professional Training Course in Korea

❙ 615 ❙
9. Probabilistic Safety Analysis
The PSA-based event analysis consists of imagining and assessing degradation scenarios
that could lead to core damage starting from an observed plant situation.
Before making such analysis, it is necessary to select main events to analyze. Then, PSAs
are used to identify potential consequences and to assess the conditional probability of
core damage, knowing that the incident did occur. It provides an estimate of the
remaining protection against core damage during the event. When the result is above a
threshold, incident is called a precursor.
The objectives of the PSA-based event analysis
− Safety indicator: Quantitative results enable detection of precursor incidents and
make sure that relevant feedback actions are proposed.
− Prioritization of treatments: The analysis enables assessment of priorities in terms
of resource allocation as well as in terms of treatment (emergency of corrective
actions). Qualitative analysis helps to define the most appropriate actions.
− Enriching safety culture: The main results of the program should make operators
aware of the most important defence lines, as well as of the hazardous operational
situations or transients.
9.6.5.4. Maintenance optimization
PSAs allow ranking importance use (importance calculations) of SSCs. Two kinds of
figures of merit can be used:
Risk Achievement Worth (or Birnbaum factor) allows assessment of risk increase if
the SSC is totally unavailable. It assesses failure severity;
Risk Reduction Worth (or Fussel-Vesely factor) allows assessment of risk decrease
if the SSC is totally reliable or available. It assesses the failure contribution (part of
sequences for which SSC is required).