1. 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)

2. 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

3. Introduction
EDF is currently operating a fleet of 58 Gen II PWRs (900, 1300 and 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

4. 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

5. 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

6. 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

7. 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)

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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:

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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

24. 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

25. 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

26. 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

27. 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 , 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

28. Thank you for your attention28