106 Lessons learned from the Seismic Post Fukushima Safety Evaluation of the French PWR. A licensee point of view I. Petre-Lazar*, P. Labbé*, J. Chantron*, H. Jadot*, S. Ravet*, N. Humbert* * EDF, Nuclear Engineering Division In response to the accident at the Fukushima Daiichi NPP, the French Safety Authority (ASN) required the licensee (EDF) to reassess the safety margins of the NPP with respect to extreme natural events leading to a severe accident. EDF performed this evaluation if the frame of the existing safety case basis and beyond design for the extreme conditions. The safety case based analysis proved that the French NPP are in compliance with the national and international standards rules and regulations. The periodic safety reviews conducted on a 10-year basis and the “standard” design proved to be an efficient way to improve the safety during the operation life. The safety margins assessment against extreme events is the novel part of the evaluation. One of the difficulties is related to the fact that no definition is yet stated for the “extreme” hazard level. The French seismic hazard assessment methods are deterministic by regulation. In some cases, EDF completed the evaluation by probabilistic methods which are better for the extrapolation of the “extreme” hazard level and the uncertainty evaluation. It is still an open question whether a target hazard level can be defined or an “all-inclusive” margin is a more practical approach. To what extent the extreme events should be consistent to the observed data and available knowledge? The engineering methods generally used for the Seismic Margins Assessment proved to be efficient tools for robustness evaluation. They are based on seismic walkdown and expert opinion completed by advanced numerical computations for the fragility. However, these methods require a different state of mind compared to the regular design methods that the majority of the engineers use every day. The main challenge for the industry is to have enough professionals able to perform these evaluations and the International Agency may contribute by editing the necessary technical documentation. The results of these analyses have been considered adequate by the French Safety Authority in the frame of the current safety case and as a prevention of the cliff-edge effects for a limited overshoot of the current baseline requirements. As a substantial safety improvement, EDF proposed to define a “hardened safety core” to minimize the radioactive releases of a potential severe accident. It is based on organization (Nuclear Rapid Response Force) and on a list of rugged new or existing structures & equipments required to mitigate the consequences of a severe accident in case of Site Black Out or Heat Sink Loss consequent to earthquake or flooding. The main challenge for the existing power plants is to deal to the complexity of the existing systems and interactions in order to identify this “core”. For the new reactors (like the EPR) a lot of the features that are required are already available by design.

Lessons learned from the Seismic Post Fukushima Safety Evaluation of theFrench PWR. A licensee point of view

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I. Petre-Lazar*, P. Labbé*, J. Chantron*, H. Jadot*, S. Ravet*, N. Humbert** EDF, Nuclear Engineering Division

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106

Lessons learned from the Seismic Post Fukushima Safety Evaluation of the French PWR. A licensee point of view

I. Petre-Lazar*, P. Labbé*, J. Chantron*, H. Jadot*, S. Ravet*, N. Humbert*

* EDF, Nuclear Engineering Division

In response to the accident at the Fukushima Daiichi NPP, the French Safety Authority (ASN) required the licensee (EDF) to reassess the safety margins of the NPP with respect to extreme natural events leading to a severe accident. EDF performed this evaluation if the frame of the existing safety case basis and beyond design for the extreme conditions.

The safety case based analysis proved that the French NPP are in compliance with the national and international standards rules and regulations. The periodic safety reviews conducted on a 10-year basis and the “standard” design proved to be an efficient way to improve the safety during the operation life.

The safety margins assessment against extreme events is the novel part of the evaluation. One of the difficulties is related to the fact that no definition is yet stated for the “extreme” hazard level. The French seismic hazard assessment methods are deterministic by regulation. In some cases, EDF completed the evaluation by probabilistic methods which are better for the extrapolation of the “extreme” hazard level and the uncertainty evaluation. It is still an open question whether a target hazard level can be defined or an “all-inclusive” margin is a more practical approach. To what extent the extreme events should be consistent to the observed data and available knowledge?

The engineering methods generally used for the Seismic Margins Assessment proved to be efficient tools for robustness evaluation. They are based on seismic walkdown and expert opinion completed by advanced numerical computations for the fragility. However, these methods require a different state of mind compared to the regular design methods that the majority of the engineers use every day. The main challenge for the industry is to have enough professionals able to perform these evaluations and the International Agency may contribute by editing the necessary technical documentation.

The results of these analyses have been considered adequate by the French Safety Authority in the frame of the current safety case and as a prevention of the cliff-edge effects for a limited overshoot of the current baseline requirements. As a substantial safety improvement, EDF proposed to define a “hardened safety core” to minimize the radioactive releases of a potential severe accident. It is based on organization (Nuclear Rapid Response Force) and on a list of rugged new or existing structures & equipments required to mitigate the consequences of a severe accident in case of Site Black Out or Heat Sink Loss consequent to earthquake or flooding. The main challenge for the existing power plants is to deal to the complexity of the existing systems and interactions in order to identify this “core”. For the new reactors (like the EPR) a lot of the features that are required are already available by design.