- 85 - 3. Deployment of the improvements resulting from the periodic safety review: the 10-year in-service inspections provide an excellent opportunity to apply the modifications resulting from the periodic safety review. To determine the schedule for the 10-year inspections, EDF has to take into account the deadlines for the performance of hydrostatic tests set by the regulations for nuclear pressure equipment and the frequency of the periodic safety reviews provided for by the TSN Act. 4. Submission of the licensee's report on the conclusions of the safety review: on completion of the 10year in-service inspection, the licensee sends ASN a report on the conclusions of the safety review. In this report the licensee adopts its position regarding the conformity of its installation with the regulations and on the modifications made to remedy the observed anomalies or to improve the safety of the installation. The review report contains the elements provided for in article 24 of decree 2007-1557 of 2 November 2007, amended. 1.4 Use of probabilistic studies in the reactor safety assessment The demonstration of the safety of these installations is based firstly on a deterministic approach, by which the operator guarantees the resistance of the installation to reference accidents. This approach is supplemented by probabilistic safety assessments (PSA) based on a systematic examination of the accident scenarios to assess the probability of arriving at unacceptable consequences. They provide a global view of safety, integrating the resistance of the equipment and the behaviour of the operators. The PSAs help to determine whether the measures adopted by the licensee are satisfactory or not. They enable the safety problems relating to the design or operation of the reactors to be prioritized, and constitute a means of dialogue between the licensees and the administration. For the existing reactors, the PSAs are carried out and updated during the 10-year reviews. For the future reactors (case of the EPR), the PSAs are developed at the same time as the design becomes clearer so as to highlight situations involving multiple failures for which measures must be taken to reduce their frequency or limit the consequences. Two types of PSA are used in France: level-1 PSAs for identifying the sequences of events leading to fuel meltdown and to determine their probabilities; level-2 PSAs for assessing the probability of releases outside the containment (into the environment), according to their nature and scale. The level-1 and 2 PSAs are used in the periodic safety reviews to evaluate the frequency of core meltdown or release and, for PSA1, how it has evolved with respect to the evaluation made at the end of the preceding review, by integrating an analysis of the modifications of the system characteristics (equipment reliability for example) and operating practices. The identification of the main factors contributing to the total probability of core meltdown or the probabilities of releases reveals any weak points for which changes to the installation or its operation are considered advisable or indeed necessary. Classifying them in order of importance enables the priority improvements to be determined. If it is decided that modifications are necessary, the PSAs enable the advantages and drawbacks of the envisaged solutions to be measured or evaluated. The appropriateness of these modifications must be demonstrated by analysing their impact on the contributions to the probability of fuel meltdown. These studies take into account both the reactor operating and shutdown states. The table below defines the PSAs currently available and the main categories of initiating events considered per reactor series in France.
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