As part of the process of continuous improvement of the safety of its reactors, but also to improve the industrial performance of its production tool, EDF periodically makes changes to equipment and operating rules. These changes can be the normal result of the correction of conformity deviations, periodic safety reviews, or taking account of experience feedback, such as that arising from the 2003 heat wave.

The ASN has set up a process to approve these modifications, compatible with the reactor safety issues.

For equipment changes, the first part of this process aims to adapt the level of review to the relevance for safety, by classifying the changes in 3 groups according to safety criteria. Only modifications belonging to groups 1 and 2, which have the most pronounced safety impact, require prior ASN approval. The second part of the process specifies the nature of the information the ASN expects from the licensee by stipulating the content and transmission frequency of certain information documents.

In 2005, ASN approvals primarily concerned the balance of the equipment modifications implemented on the occasion of the second ten-yearly outages of the 1300 MWe reactors and the "commissioning completion package" lot for the 1450 MWe reactors.

Documentary changes are subject to prior approval by the ASN when they affect chapters III, VI, VII, IX and X of the RGE (see § 1.2.2). For these changes, the ASN asked EDF to draw up a preliminary note on the safety issues of the main changes to the operating rules.

Since 2004, this has in particular led to an improvement in the time taken to review operating rule changes.

Nuclear power plants, like all industrial installations, are subject to ageing. The role of the ASN is thus to ensure that EDF's general operating strategy takes account of all ageing-related phenomena, in order to guarantee a level of safety compatible with the regulations, throughout the plant's operating lifetime.

The nuclear power plants currently in operation were built in a very short space of time: 45 reactors, representing 50,000 MWe, or three quarters of the nuclear power plants, were commissioned between 1979 and 1990, with an additional 10,000 MWe between 1990 and 2000.

In December 2005:

• the average age of the thirty-four 900 MWe reactors was 24 years (between 18 and 29);
• the average age of the twenty 1300 MWe reactors was 18 years (between 13 and 21).

To understand the ageing of a nuclear power plant, other than simply the time that has elapsed since it was commissioned, a number of factors must be looked at.

The lifetime of non-replaceable items

In the design, a certain number of reactor components were designed on the basis of a predetermined operating period. These items are therefore subject to close supervision to ensure that their condition and the trends involved are as expected. This is in particular the case with the reactor vessel, sized to withstand the effects of embrittlement due to neutron irradiation of the core zone steel for a period of 40 years (equivalent to continuous operation for 32 years). The reactor vessel is checked by monitoring "control samples" of metal and appraising them at regular intervals.

Deterioration of replaceable items

These are phenomena such as wearing of mechanical parts, hardening and crazing of polymers, corrosion of metals, etc. The equipment requires particularly close attention during the design and manufacturing stages (particularly the choice of materials) along with a monitoring and preventive maintenance, repair or replacement programme as necessary. It must also be possible to demonstrate the feasibility of possible replacement.