In the building process of this level 1 probabilistic safety assessment, a first phase consisted in making a preliminary model that took only into account families of initiating events that were defined for the design of the decay heat removal dedicated loops, namely the loss of coolant accidents (representative of medium pressure situations) and loss of off-site power/station black-out transients (representative of high pressure situations). Owing to the results obtained with this preliminary L1PSA model, it emerged that an increased reliability of the DHR function in high pressure conditions (i.e. characterized by IEs not associated to the loss of integrity of the helium pressure boundary) is suitable to reduce the overall core damage frequency. The track was therefore chosen to require the use of normal loops as first line of provision of the DHR function, possibly including components or particular operating modes related to the secondary and tertiary circuits. In addition, this final L1PSA model is characterized by success criteria based on transient calculations performed with the CATHARE2 code and to a perimeter extended to all representative internal IEs at full operating power.
This paper presents the building process and the main results related to these two successive L1PSA models. Finally, useful insights are translated into GFR design improvements that are leading to an overall CDF at full operating power that satisfies nowadays with the probabilistic target defined for 3rd generation reactors, being at least the objective for 4th generation reactors.