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Lifetime extension of French 900 MWe NPPs: French TSO main conclusions regarding long term sump performance during a severe accident

Abstract : In the framework of the lifetime extension of the French 900 MWe NPPs beyond 40 operating years, EDF has included, in case of severe accident, a strategy to remove the decay heat from the containment without opening the emergency containment filtered venting system. The new dedicated circuit uses an injection line with a heat exchanger connected to the cold leg of the primary coolant circuit and another feeding the sump of the reactor building and a pump qualified to extreme external hazards conditions and to severe accident situations. After the drainage of the RWST (Refueling Water Storage Tank), water is taken from the containment sump in the lower part of the nuclear reactor building using one of the filtering system implemented at the bottom of the containment. Consequently this filtering system was previously used during the LOCA phase, before reaching the severe accident phase. A cooling mobile device (ultimate heat sink) is lined on the heat exchanger by the EDF rescue team “FARN” (Nuclear Rapid Response Force). One of the major issues, common to the LOCA issue, is the clogging of the filtering system due to physical and chemical conditions which can lead to an inadequate net positive suction head (NPSH) margin for the pump during the severe accident and can affect the mechanical integrity of the strainers. Furthermore, despite the filtering system, a part of the debris bypassing the strainers is transported through the dedicated circuit: a second major issue is to ensure that these debris do not damage the pump, degrade the heat exchanger performance or clog other equipment of the circuit like valves and diaphragms. Compared to the debris released in the sump in case of a LOCA, additional debris created during a severe accident come from the core degradation, the erosion of the concrete basemat by the corium and paints damaged in the containment due to high irradiation process. EDF presented its safety demonstration based on studies and experimental program on strainer clogging. IRSN conducted the safety review of EDF safety case and performed simultaneously one severe accident test on the Viktoria loop in Slovakia. The paper will present the French background and IRSN conclusions (dated the end of March 2019) on sumps performance issue in case of severe accident for the 4th periodic safety review of 900MWe NPPs. Some open issues will be discussed.
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https://hal.archives-ouvertes.fr/hal-03106515
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Submitted on : Monday, January 11, 2021 - 9:02:17 PM
Last modification on : Friday, February 5, 2021 - 10:28:06 AM
Long-term archiving on: : Monday, April 12, 2021 - 7:27:47 PM

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Gerard Cenerino, Laurent Cantrel, Vojtech Soltesz. Lifetime extension of French 900 MWe NPPs: French TSO main conclusions regarding long term sump performance during a severe accident. OECD/NEA Workshop on Reactor Core and Containment Cooling Systems Long Term Management and Reliability, OECD/NEA Workshop Levice, Sep 2021, LEVICE, Slovakia. ⟨hal-03106515⟩

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