In the event of a reactor incident or accident, the operation teams have specific operating documents at their disposal, designed to enable them to keep the reactor in or return it to a stable condition.

Incident and accident operation today uses the states-based approach (APE). The APE consists in applying operating strategies which are drafted according to the identified physical state of the nuclear steam supply system, regardless of the events that led to this state. Should the state deteriorate, a permanent diagnosis enables the procedure or sequence in progress to be aborted and a more appropriate procedure or sequence to be applied.

The operating documents used in incident and accident situations are drafted on the basis of incident and accident operating rules which constitute chapter VI of the general operating rules. These rules, which describe operating strategies in the event of an incident or accident, must be approved by the ASN.

During 2005, the ASN continued to review changes to the operating rules proposed by EDF and in particular approved:

• the creation of new reactor operating rules to deal with fire situations, called "licensee fire action sheets" (FAIOp), for the 1300 MWe, P'4 and CP0 plant series. Production of the FAIOp operating rules is part of the fire action plan that EDF was committed to implementing on the 900 and 1300 MWe reactors before the end of 2006;
• a modification to the operating rules in the event of a black-out, to make it easier to restart the reactors and reconnect them to the national electricity transport grid;
• evolution of the N4 series rules, in particular comprising the creation of APE rules for states when the reactor is not closed, in place of the "event-based" rules currently applied.

Generally speaking, the documents submitted by EDF to the ASN for approval are of high quality, even if progress is still needed with respect to the traceability of the origin and the end-purpose of the modifications submitted for approval.

Regular inspections are held on the subject of incident and accident operation. These inspections in particular review the management of incident and accident operation documents (transcription of reference national documents into local documents, reproduction, distribution, etc.), management of specific equipment used in accident operation conditions, and training of operation staff. The inspections performed in 2005 highlighted no major issues. Overall, the ASN considers that the sites have satisfactorily assimilated incident and accident operation rules (transcription into local documents, distribution and training of staff).

Reactor operation in severe accident situations

If the reactor cannot be brought to a stable condition after an incident or accident and the scenario resulting from a series of failures leads to core damage (core melt), the reactor is said to be entering a severe accident situation.

For this type of very hypothetical situation various steps are taken to enable the licensees, supported by the emergency teams, to manage reactor operation and ensure containment of radioactive materials in order to minimise the consequences of the accident. The emergency teams may in particular use the serious accident response guide (GIAG). EDF revised this guide in 2005, primarily to take account of installation of the hydrogen recombiners. The new versions are currently being reviewed by the ASN and its technical support organisation.

In December 2004 and March 2005, for the periodic safety reviews included in the third ten-yearly outages of the 900 MWe reactors, the ASN consulted the Advisory Committee for nuclear reactors concerning the modifications to be made to these reactors in order to improve consideration of the risks associated with serious accidents.

Following this consultation, the ASN in particular asked EDF to install a device to detect corium (mixture of molten fuel and core structure) in the reactor pit on all 900 MWe reactors. The ASN also asked EDF to review the possibility of installing instrumentation for a real-time assessment of the evolving risk of hydrogen explosion in the containment, to help provide data on the progress of the accident.

EDF has also made a commitment to assessing the risks linked to a steam explosion in the event of a vessel puncture in a vessel pit already flooded and, on the occasion of the third ten-yearly outages of the 900 MWe reactors, to install a reliable device for depressurising the primary system, even if electrical power is completely lost.

Finally, and following an ASN request to optimise definition of all serious accident safety requirements, EDF proposed a draft "serious accidents" reference system. This was reviewed by the Advisory Committee for nuclear reactors in 2005. It will need to be revised, particularly to take account of the conclusions of the 900 MWe reactors periodic safety review associated with the third ten-yearly outages, as well as of long-term accident management.