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Delineation of a dynamic event tree (DET) is a critical point in the evaluation of a given PSA. The designer's task is very complex. The regulation task is to check the validity of a given proposal.

Methodology & Codes

The ISA methodology is a systematic approach to verify a number of conditions characterized by risk damage limits imposed by the sequence. It can be seen as a dynamic extension of the PSA, based on the automatic generation of Dynamic Event Trees (DETs). Operator actions are also conditioned by dynamic events (alarms), but its execution is occurring at a random time within a time window after the event conditioning. Such transitions are included in the method through SIMPROC.

Implementing ISA Methodology, requires a set of computational tools shown schematically in Figure 1, which enables the automatic generation DET.



The Event Scheduler was initially conducted by DYLAM, a system developed at the JRC Ispra code for this purpose. ISA Methodology is currently implemented by means of Dendros , improving overall system modularity and parallelizing the sequences of the DET. (Via the Parallel Virtual Machine PVM)

Driver Simulation

Until Ekergy Software SCAIS began developing two codes developed internally by the CSN is used for simulation of the core;

  • TRETA for PWR plants; and
  • TIZONA for BWR.

Subsequently BABIECA was developed and allows simulations of non-nuclear technologies. Besides simulation capabilities, BABIECA allows the use of external simulation modules when necessary.

All these codes are used in order to verify integrity of Event Tree once a given sequence is identified as conflicting from the viewpoint of safety. Plant models used coupled to SCAIS currently are MAAP (both PWR and BWR) and TRACE. Due to the large number of simulations using the methodology, MAAP has become the code base for our applications. Subsequently, sequences of interest can be simulated with TRACE to study the problem with a greater level of detail.

Emergency procedures and human reliability analysis

Two systems have been considered for modeling procedures. A prototype called HOI was used for cases in which the generation of DET was fully automatic. Subsequently COPMA-II, a computerized procedures developed in the Halden reactor project to support operator, was adapted for the simulation with SCAIS resulting SIMPROC. Human factors and Human Reliability Analysis HRA models were not included in the ISA system used in CSN yet. Although a proposal was made to expand their use and add  HRA.

Probability calculations engine

The minimum set of fails used in the PSA is not feasible to calculate probability at runtime (because it is too heavy calculation). Algorithms based on alternative representation of fault trees using Binary Decision Diagrams (BDDs) and have been surveyed to be implemented in SCAIS. Currently, RiskSpectrum and CAFTA (The main programs used by industry for PSA) are being studied to transform their outputs the standard MEF-PSA [Epstein] format and add your results directly as headers SCAIS.

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