The vessel is one of the essential components of a PWR. This component is 14 m. high, 4 m. in diameter and 20 cm. thick. It houses the reactor core and its instrumentation and in normal operation is completely filled with water, bringing its weight to 300 t. It can withstand a pressure of 155 bars at a temperature of 300 °C.

Regular and precise monitoring of the state of the reactor vessel is essential for the following two reasons:

• vessel replacement is not envisaged, for reasons of technical feasibility and economics;
• rupture of the vessel is an excluded accident, so its consequences are not included in the reactor safety evaluation. Validating this assumption however means that appropriate design, manufacturing and operating measures be taken.

In normal operation, the vessel gradually deteriorates as the neutron radiation from the reactor's fissile core embrittles the vessel metal. This embrittlement makes the vessel particularly sensitive to pressurised thermal shocks or to sudden pressure surges when cold. The presence of a crack would then be potentially damaging.

To prevent against all risks of this type, the following measures were taken as of commissioning of the first EDF reactors:

• a program to monitor the effects of irradiation: capsules containing test specimens made of the same metal as the reactor vessel were placed inside the reactor, near the core. Some of these capsules are regularly extracted and subjected to mechanical testing. The results of these tests give a good picture of how the vessel metal is ageing, and in fact even give advance "early warning" as the capsules are situated close to the core and receive more neutrons than the actual vessel itself;
• periodic ultrasonic testing: this check is used to monitor any defects located under the vessel's inner stainless steel lining.

The ASN reviewed the vessel files forwarded by EDF on the occasion of the second ten-yearly outages. It considers that a 30-year lifespan for the 900 MWe reactor vessels has been demonstrated.

The ASN however thinks that EDF must still demonstrate the life of its vessels beyond 30 years. To do this, EDF provided answers to the questions asked following the session of the SPN of the Central Committee for Pressure Vessels, held in 1999. These answers are given in a summary file which is currently being examined and which was reviewed by the SPN's experts during its 18 October and 13 December 2005 sessions.

Following this examination and in the light of the results of the checks carried out during the reactors' third ten-yearly outage, the ASN will define its position regarding the vessel operating conditions beyond 30 years.