Each main secondary system (CSP) on the EDF reactors is protected by seven safety valves installed on the main steam lines. Apart from their CSP protection function, these valves also constitute one of the limits of the third containment barrier.

Since the 1990s, cracks have been discovered on certain CSP protection valve nozzles. After carrying out a series of investigations – mainly on the Paluel site, which was the most seriously affected by this type of damage -– EDF proposed a maintenance strategy comprising in-service checks, installation modifications and a programme to repair the damaged valves.

These justifications were submitted to the experts in the SPN of the Central Committee for Pressure Vessels during the first quarter of 2005.

The SPN considered that the maintenance and in-service inspection strategy proposed by EDF for the nuclear power plant CSP valves, and in particular those at Paluel, was acceptable provided that a certain number of recommendations were taken into account.

The main recommendations, reiterated by the ASN, are as follows:

• maintaining a valve complete inspection interval of 7 years, as in the programmes currently in force;
• scheduling repair of damaged valves as rapidly as possible according to the availability of repair resources and the availability constraints associated with these devices.

The containments undergo inspections and tests with the aim of checking that they indeed meet the safety requirements and in particular that their mechanical performance is satisfactory and guarantees correct tightness when the pressure in the reactor building is higher than atmospheric pressure, which can be the case in certain types of accidents. These tests, particularly at the end of construction and then during the ten-yearly outages, include a pressure rise up to the inner containment design pressure.

The containments of the 900 MWe reactors consist of a single wall of pre-stressed concrete with an interior metal liner. Until now, the leak rates from these containments during the ten-yearly inspections were in conformity with the regulatory criteria. Their ageing was reviewed in 2005 as part of the 30-year periodic safety review, in particular with respect to leaktightness and mechanical strength for a further 10 years. This review brought to light no particular problem liable to compromise the length of the service life.

The containments of the 1300 MWe and 1450 MWe reactors comprise 2 concrete walls. A change in the leak rates from the inner wall of some of these containments, mainly under the combined effects of concrete creep and the loss of pre-stressing of certain cables, has been observed in recent years. Although account was taken of these phenomena at the design stage, they were sometimes underestimated. In an accident situation therefore, certain areas of the wall could find themselves under traction, a stress condition favourable to cracking and therefore leaks. To take account of this phenomenon, EDF has implemented a preventive repair programme aimed at restoring the tightness of the most heavily affected areas. On the basis of a recommendation of the Advisory Committee for nuclear reactors which met to discuss the subject in early 2002, the ASN gave EDF its approval of this strategy. This work is done at each ten-yearly outage and by the end of 2005, 18 of the 24 units had been dealt with. All the reactors concerned will have undergone the necessary maintenance work by 2011.