The steam generators are exchangers of heat between the water of the primary system and that of the secondary system. The exchange surface consists of a tube bundle comprising about 3000 to 6000 tubes, depending on the model. These tubes contain the primary system water and exchange heat while preventing any contact between the primary and secondary fluids.

The integrity of the steam generator tube bundle is a major factor in safety, as any deterioration of the tube bundle could lead to a leak from the primary system to the secondary. Furthermore, a break in one of the bundle tubes in an accident scenario would thus bypass the reactor containment, which is the third confinement barrier. These steam generator tubes are subject to a variety of deterioration phenomena: wear, corrosion, and so on.

The steam generators are covered by a specific in-service surveillance programme drawn up by EDF and revised every 3 years. The current version of this programme was reviewd by the ASN in 2003 and accepted by DGSNR decision n° 030472 on 1 December 2003 (available on the website www.asn.gouv.fr). Following the checks, those tubes which show excessive levels of damage are plugged to remove them from service.

Since the early 1990s, EDF has been conducting a replacement programme for steam generators with the most heavily damaged tube bundles. This programme will continue at the rate of one reactor a year. In 2005, the steam generators at Dampierre 2 were replaced and currently twelve of the first thirty-four 900 MWe reactors are still equipped with Inconel 600 alloy steam generator tube bundles which were not heat treated (600 MA), and which are the main victims of stress corrosion induced cracking (see point 3.5.2).

In addition to in-service surveillance, the steam generators undergo a hydrostatic test every 10 years: during the reactor ten-yearly outage (see point 3.2.3), the primary system undergoes an overall pressure test subjecting it to a pressure higher than its normal operating pressure. On the occasion of the second ten-yearly outages for the 900 MWe reactors, which began in 2002, major leaks were found on some of the steam generators most heavily affected by stress corrosion.

On the advice of the SPN of the Central Committee for Pressure Vessels (see chapter 2, point 2.1.5 b), the ASN asked EDF to take particular measures for inspection and maintenance of these steam generators. EDF proposed an early replacement programme scheduling replacement of the steam generators in the twelve 900 MWe reactors still equipped with Inconel 600 MA tube bundles no later than the third ten-yearly outage.

In 2006, EDF also launched a study and appraisal programme for the 900 and 1300 MWe reactors equipped with steam generators with heat treated Inconel 600 alloy (600 TT) tube bundles, to gain a clearer understanding of their performance during the hydrostatic test and determine how to avoid leaks during the tests.