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Section 4.4.6, Severe accident analysis

Severe accident analysis

The applicant shall demonstrate that a severe accident analysis has been performed in accordance with the requirements of:

·       REGDOC‑2.3.2, Accident Management [23]

·       REGDOC‑2.4.1, Deterministic Safety Analysis [24]

·       REGDOC‑2.4.2, Probabilistic Safety Assessment (PSA) for Nuclear Power Plants [25]

The format and content of the beyond-design-basis accident (BDBA) analyses should be consistent with the presentation of the analyses for anticipated operational occurrences and design-basis events. In addition, the application should:

·       state the objective and/or the specific acceptance criteria for the BDBA analysis

·       include a discussion of the additional postulated failures in the accident scenario, including the reasons for their selection

·       summarize the key results of the analyses with specific acceptance criteria, and state how the acceptance criteria are met

The application should provide detailed information concerning the analysis to identify accidents that can lead to significant core damage, and/or offsite releases of radioactive material (severe accidents). In addition, the information submitted should describe the evaluation that has been carried out on the capability of complementary reactor facility design features to meet the design criteria, in accordance with REGDOC‑2.5.2, Design of Reactor Facilities: Nuclear Power Plants [9].

The application should demonstrate the capability of the design to mitigate certain BDBAs. The applicant should explain the choice of the BDBAs to be analyzed and justified, indicating whether the choice was made on the basis of a PSA or according to another fault analysis that identifies potential vulnerabilities of the reactor facility. Additionally, the applicant should describe, explain and justify the approach taken.

BDBA events are typically sequences involving more than one failure (unless they are taken into account in the DBAs at the design stage), such as reactor facility blackout, design-basis events with degraded performance of a safety system, and sequences that lead to containment bypass and/or confinement bypass. The application should describe how the analysis:

·       uses best-estimate models and assumptions

·       takes credit for realistic system action and performance beyond original intended functions, including systems not important to safety

·       takes credit for realistic operator actions

Where this is not possible, reasonably conservative assumptions should be made. These assumptions should consider uncertainties in the understanding of the physical processes being modelled.

The application should include an explanation of the analysis performed for severe accident sequences, including (as applicable) hydrogen fire, steam explosion and molten fuel/coolant interaction, and a description of the results of the most relevant severe accident analyses used in the development of the accident management programs and emergency preparedness planning for the reactor facility.

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