ASN reports on controls imposed on nuclear reactors’ vessels in France

In France, ASN controls manufactoring and in service inspection of key operating pressure equipments of nuclear reactors, including the reactor pressure vessel. A specific monitoring of these equipments’ construction has been applied since 1974. For all components of French reactors’ vessels, a number of controls aiming at detecting defects in forged pieces were carried out during production.

Moreover, ultrasonic inspection of highly irradiated areas of operating French reactors’ vessels are conducted every ten years.

During inspections conducted in July 2012 in Belgium on the Doel 3 nuclear reactor’s vessel, many indications that "could be assimilated to potential cracks" were detected.

Checks carried out in France show that no French vessel presents defects corresponding of those concerned in Doel 3.

In France, two types of defects of different nature have been detected . They have been identified, are monitored and are subject to information from the ASN:

• Defects under vessels’ cladding

The vessels are covered on their inner side with a layer of stainless steel to protect the water of the primary circuit.

Controls specifically aimed at detecting undercladding defects have been carried out since 1991. These controls include the entire highly irradiated area of the vessel and extend significantly beyond the monitoring of the only welds. 37 defects have been detected on the core's shells¹of the French reactors’ vessels, including 20 on Tricastin No. 1 reactor’s vessel. These manufacturing defects are periodically checked and do not evolve while operating.

 Defects under vessels’ cladding are subjected to a special justification which demonstrates their harmlessness. This justification is periodically reviewed by ASN to take account the vessels’ aging as well as the evolution of knowledge and of available information. The last review of this justification has been the subject of a position of the ASN in September 2010, published on its website.

The undercladding defects in cladding identified in France are isolated and well-defined. They do not affect the quality of metal over a large area and the reasons for their occurrence are known. As such, they are not comparable to the defects detected on Doel 3’s vessel, where several thousands of clustered defects have been observed.

• Defects detected in a weld of vessel's bottom mounted instrumentation penetration² at Gravelines 1

In September 2011, a defect was observed on a bottom mounted instrumentation penetration at Gravelines’ reactor n°1. It led ASN to request the extension of controls in this
 area of the vessels.

The detected defect was located in the weld of the penetration, which is made of an alloy that can be sensitive to stress corrosion cracking under certain conditions. EDF demonstrated the harmlessness of this defect in the short term and decommissioned the affected penetration while setting up a monitoring system dedicated to the vessel’s bottom. ASN also requested that EDF propose a solution to repair the defect’s final elimination.

The defect detected at Gravelines, given its location, its type and its isolated nature, is not comparable to the defects found at Doel 3.

ASN underscores that in France, the actions to be taken in case of defects’ detection follow strict rules, specified in particular in the Ministerial Order of November 10, 1999, on the monitoring of the operation of pressurized water nuclear reactors’ main primary and secondary systems.

“Cracks” defects, in particular, must be repaired if they are not appropriately justified. The justification must demonstrate the absence of any evolution of the defect in any situation, including in case of an accident, taking into account the margins imposed by regulation and ensuring that each step of calculations is based on penalizing hypothesis.

ASN enforces the compliance with these rules before any reactor involved can be restarted.

Press Contact: Evangelia Petit, Press Officer, tel.: 0140198661 - evangelia.petit@asn.fr

1. The core’s shells, by the number of 2 or 3 in French vessels, are forged rings approximately 20 cm thick, with a 4-meter diameter, which are welded together and make up the body of the vessel, in the part where it harbours the reactors core.

2. A penetration of vessel bottom is a tube going through the vessel’s bottom to allow the insertion of instrumentation probes in the reactor’s core.