Review of the upgraded severe accident mitigation strategies for the generation II PWRs in France foreseen in the framework of plant life extension International Conference on Topical Issues in Nuclear Installation Safety: Safety Demonstration of Advanced Water Cooled Nuclear Power Plants 6-9 June 2017, Vienna, Austria R. COZERET & E. RAIMOND (IRSN, France)

Summary Introduction French Gen II PWRs existing backfittings for severe accident Reinforced Gen II PWRs safety objectives Review of modifications to avoid reactor concrete basemat melt-through by the molten core after vessel failure Review of modifications to remove the decay heat from the containment without opening the emergency containment venting system Review of modifications to reduce iodine release in case of filtered containment venting during a core melt accident Conclusion

Introduction EDF is currently operating a fleet of 58 Gen II PWRs (900, 1300 and 1450 MWe series) built between 1977 et 1999 Periodic Safety Reviews (PSRs) are conducted every 10 years These reactors were not designed to face a core melt accident and several reinforcements have been discussed and progressively implemented by EDF to allow for the management of a severe accident (SA) since the 90’s In 2009, EDF presented to the French Safety Authority (ASN) a Plant Life Extension (PLE) program, in order to give a possibility to extend the Gen II PWRs operation duration beyond 40 years

Introduction The EDF PLE program includes an ageing program but also some reinforcements to reduce the gap with the safety objectives of the new reactors like the Gen III EPR This program has been reviewed since 2010 by IRSN for ASN in parallel with the Complementary Safety Assessments (CSA) developed after the Fukushima Daï-ichi accident Some IRSN statements after a first-step review (achieved in 2016) of the EDF upgraded severe accident mitigation strategies for the Gen II PWRs in the framework of PLE and CSA are presented hereafter

Summary Introduction French Gen II PWRs existing backfittings for severe accident Reinforced Gen II PWRs safety objectives Review of modifications to avoid reactor concrete basemat melt-through by the molten core after vessel failure Review of modifications to remove the decay heat from the containment without opening the emergency containment venting system Review of modifications to reduce iodine release in case of filtered containment venting during a core melt accident Conclusion

Gen II PWRs (initial design) All French PWRs have been designed with large dry containment: 900 MWe series: single containment with liner 1300 & 1450 MWe series: double containment Connection between reactor cavity and upper part of containment Design P: ~5 bar Fixed vessel insulator No draining of water out of the cavity pit (filled by spray) No core catcher Diversity of water injection possibilities

Examples of PWRs existing backfittings for Severe Accident Hydrogen PARs Examples of PWRs existing backfittings for Severe Accident EFCVS (sand filter) Primary circuit safety valves improvement for SA Containment pressure instrumentation Material access penetration reinforcement H2 detection instrumentation Vessel rupture detection Reinforcement of electrical supply of the containment isolation system Reinforcement of the annulus ventilation system (1300 and 1450 MWe reactors) Re-injection of contaminated water from auxiliary buildings to the reactor building Basemat reinforcement (Fessenheim reactors 1 & 2)

French Gen II PWRs existing backfittings for severe accident These backfittings were decided and designed taking into account: Research on severe accidents Deterministic safety studies Level 2 Probabilistic Safety Assessment (L2 PSA) The verification that the equipments can withstand to SA conditions Even if these reinforcements bring very substantial risk reduction, it remains some important gaps with the solutions developed for Gen III reactors like EPR, especially for the long term accident management strategies For IRSN, these limitations, regarding the progress in the knowledge on SA progression, justify to continue efforts in Gen II PWRs reinforcement for SA

Summary Introduction French Gen II PWRs existing backfittings for severe accident Reinforced Gen II PWRs safety objectives Review of modifications to avoid reactor concrete basemat melt-through by the molten core after vessel failure Review of modifications to remove the decay heat from the containment without opening the emergency containment venting system Review of modifications to reduce iodine release in case of filtered containment venting during a core melt accident Conclusion

Reinforced Gen II PWRs safety objectives ASN stated to EDF that the safety objectives of the Gen III reactors (Flamanville EPR for instance) should be used as a reference for all studies undertaken in the framework of PLE For EPR, the general objective is “to achieve a significant reduction of potential radioactive releases due to all conceivable accidents, including core melt accidents” : No necessity of protective measures for people living in the vicinity of the damaged plant for accident situations without core melt Accident situations with core melt would lead to large early releases (high pressure core melt accident for instance) have to be practically eliminated For low pressure core melt accident situations: no permanent relocation, no need for emergency evacuation outside the immediate vicinity of the plant, limited sheltering, no long term restrictions in consumption of food

Reinforced Gen II PWRs safety objectives The second objective (no large early release) has been largely addressed by the existing backfittings summarized before For the third objective (low pressure core melt accident), two remaining issues have to be considered to extend the design conditions: The long term stabilization of the corium in case of vessel failure which cannot be demonstrated for all conditions The EFCVS decontamination factor (for gaseous iodine) which is not sufficient to limit need for emergency protective measures of population at the immediate vicinity of the plant

Reinforced Gen II PWRs safety objectives EDF, in accordance with its initial PLE program, the post-Fukushima lessons and the ASN requests, has included three important upgrades in its PLE program: A strategy to allow corium stabilization without concrete basemat melt-through A strategy to remove heat from the containment without venting A strategy to reduce the iodine release in case of containment venting

Summary Introduction French Gen II PWRs existing backfittings for severe accident Reinforced Gen II PWRs safety objectives Review of modifications to avoid reactor concrete basemat melt-through by the molten core after vessel failure Review of modifications to remove the decay heat from the containment without opening the emergency containment venting system Review of modifications to reduce iodine release in case of filtered containment venting during a core melt accident Conclusion

Review of modifications to avoid reactor concrete basemat melt-through by the molten core after vessel failure To limit the risk of reactor basemat melt-through by molten core after vessel failure, EDF intend to do the following modifications:

Review of modifications to avoid reactor concrete basemat melt-through by the molten core after vessel failure 1- corium pouring in the dry reactor pit after vessel failure

Review of modifications to avoid reactor concrete basemat melt-through by the molten core after vessel failure 2- corium spreading in the dry reactor pit and a adjacent area

Review of modifications to avoid reactor concrete basemat melt-through by the molten core after vessel failure 3- corium flooding by the sumps water thanks to passively activated triggers

Review of modifications to avoid reactor concrete basemat melt-through by the molten core after vessel failure After the review of the principle of these modifications, IRSN has highlighted that : This strategy reduce significantly the possibility of containment failure by steam explosion in a flooded vessel cavity and is a good compromise between efficiency and feasibility for the ex-vessel corium stabilization The size of the spreading area, depending of the reactor, has to be further discussed in accordance with appropriate safety criteria (e.g. thickness of basemat erosion or integrity of the steel liner for the 900 MWe reactors) Some design features have still to be defined (e.g. passive trigger system to activate corium flooding by water from the containment floor) New instrumentation is needed for the water level in the containment

Summary Introduction French Gen II PWRs existing backfittings for severe accident Reinforced Gen II PWRs safety objectives Review of modifications to avoid reactor concrete basemat melt-through by the molten core after vessel failure Review of modifications to remove the decay heat from the containment without opening the emergency containment venting system Review of modifications to reduce iodine release in case of filtered containment venting during a core melt accident Conclusion

Review of modifications to remove the decay heat from the containment without opening the EFCVS To remove the decay heat from the containment without opening the EFCVS, EDF intends to implement a disposal composed of: A pump qualified to extreme external hazards conditions and SA situations An injection line in the primary coolant circuit and another feeding the sumps A suction line connected to the safety injection tank and another pumping in the sumps A heat exchanger Actuators enabling the disposal activation from the control room A cooling mobile circuit (heat sink) which has to be lined by the rescue team

Review of modifications to remove the decay heat from the containment without opening the EFCVS After the review of the principle of this modification, IRSN has concluded that : The new disposal intended by EDF is satisfactory in principle and should enable to remove the decay heat from the containment An appropriate instrumentation is important to avoid any excessive pressure or temperature when activating the new circuit The possibility of circuit leakage during operation in SA conditions has to be considered with specific provisions (leakage detection, contaminated liquid management, reliable isolation valves…) Rescue team activation criteria are important to install the mobile ultimate heat sink in due time (to avoid containment over-pressurization) with margins

Summary Introduction French Gen II PWRs existing backfittings for severe accident Reinforced Gen II PWRs safety objectives Review of modifications to avoid reactor concrete basemat melt-through by the molten core after vessel failure Review of modifications to remove the decay heat from the containment without opening the emergency containment venting system Review of modifications to reduce iodine release in case of filtered containment venting during a core melt accident Conclusion

Review of modifications to reduce the iodine release in case of filtered containment venting during a core melt accident The presently installed venting system is able to trap aerosol fission products but it is not efficient to trap gaseous fission products such as molecular or organic iodine (I2 and ICH3) or noble gas (Kr and Xe) in-containment metallic pre-filter out-containment sand filter

Review of modifications to reduce the iodine release in case of filtered containment venting during a core melt accident In case of a core melt accident on the 900 MWe PWR series, silver released from the control rods and deposited into the sumps water would enable iodine stabilization in the sumps But the 1300 and 1450 MWe PWRs control rods include a limited quantity of silver so this stabilization process would be limited On these PWRs series, EDF is installing some sodium tetraborate (borax) baskets on the sumps floor to passively alkalize the sumps water and consequently trap iodine in the liquid phase

Review of modifications to reduce the iodine release in case of filtered containment venting during a core melt accident After the review of this modification, IRSN has conclude that: Implementation of borax baskets on 1300 and 1450 MWe PWRs will allow trapping a large part of the iodine in the reactor building sumps for accidents where no sodium hydroxide injection could have been operated by the containment spray system The gaseous iodine releases coming from the upper part of the containment would not be fully affected by the iodine stabilization in the reactor building sumps and have to be considered There is still an interest to examine, for all French Gen II PWRs, the possibility to upgrade the existing EFCVS to reduce the gaseous iodine releases

Summary Introduction French Gen II PWRs existing backfittings for severe accident Reinforced Gen II PWRs safety objectives Review of modifications to avoid reactor concrete basemat melt-through by the molten core after vessel failure Review of modifications to remove the decay heat from the containment without opening the emergency containment venting system Review of modifications to reduce iodine release in case of filtered containment venting during a core melt accident Conclusion

Conclusion For IRSN, the principles of the modifications planned by EDF in the framework of PLE (taking into account the Post-Fukushima action plan) will give more credit in reaching a stable reactor state in SA situation During the coming period, EDF will detail studies supporting the demonstration of the efficiency of these upgrades. In 2017-2018, IRSN will carefully examine these detail studies, especially for the corium coolability during molten core concrete interaction These modifications are a continuation of the design extention of the Gen II PWRs with the aim to reduce the gap with the safety level of Gen III reactors

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