Modelling of Accident Sequences

The system fault trees models developed for the power PSA could, with appropriate changes (i.e. components and trains in maintenance), be used for SLP PSA. The logic of systems basically remains the same, but the conditional availabihty of components or systems may be significantly different. 

The success criteria are less stringent, primarily due to lower deeay heat levels. Thermal-Hydraulic analysis are recommended to determine the success criteria. Some of the suceess eriteria may be adopted from previous determiuistie analysis with some additional verification of the applieability of the assumptions. 

Determination of the sueeess eriteria should take into account the following issues  

In a standard SLP PSA, the event sequence modelling is usually performed using event trees. In this case, the event trees developed for power PSA may be modified for use in SLP PSA. The modification will typically include removal of some headings (i.e. reactor trip) and relaxation of the others due to lower decay heat levels. Some new headings may be added to reflect operator actions which may not be possible during power operation. 

Shutdown state also has some specific characteristics which are not modelled in the power PSA. Operation of the RHR system and related operator responses often requires the development of new sequence models. A longer time is available to operators to recover from initial failures. Possibilities to establish a non-conventional accident mitigation (as an example, suppl5dng water into the open reactor vessel via fire water system) require from the PSA analysts to consider options which have not been addressed in the power PSA. 

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The models you use to portray failures that lead to accidents, and the models you use to propagate their effects, are attempts to approximate reality. Models of accident sequences (although mathematically rigorous) cannot be demonstrated to be exact because you can never precisely identify all of the factors that contribute to an accident of interest. Likewise, most consequence models are at best correlations derived from limited experimental evidence. Even if the models are validated through field experiments for some specific situations, you can never validate them for all possibilities, and the question of model appropriateness will always exist. 

This section discuses various modelling issues which are of relevance for a full scope shutdown and low power PSA models. The modelling issues discussed include the definition of plant operating states, initiating events, screening analyses, modelling of accident sequences, dependency analysis, quantification of sequences and applications of SLP PSA. 

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