ASN examines the conditions for repairing the defects detected in a through-wall penetration on the bottom of Gravelines 1’s reactor pressure vessel

Published on 20/12/2011 • 10:45 am

Information notice

View of vessel and TWP

During an inspection carried out during the third ten-yearly visit of Gravelines 1’s reactor, EDF detected defects in a through-wall penetration on the bottom of the reactor pressure vessel.

A through-wall penetration (TWP) on the bottom of the pressure vessel is a tube positioned at the bottom of a reactor vessel in a nuclear power station. The TWP is welded to the bottom of the vessel. Its role is to permit the introduction of instrumentation probes into the reactor core. 50 TWP are positioned in Gravelines 1’s reactor vessel. A leaking TWP would represent a breach in the reactor’s primary circuit.

Small cracks were detected on a TWP within the framework of a national inspection programme carried out during the ten-yearly visits of certain reactors. This inspection programme was carried out after the cracks had already been noted on other components using materials identical to those used for the TWP. This is particularly the case of the pressure vessel covers which have been replaced.

 This is the first time that a defect of this type has been discovered on a TWP in France. Similar damage has been noted in the United States on the South Texas reactor where two signs of leakage were discovered in 2003 and led to repair works being carried out.

Section of a TWP and location of defects (source : EDF)

EDF proposed a solution for the stoppage of the TWP. This is currently being examined by ASN and its IRSN technical support. Its introduction will be submitted to ASN for approval. The latter will ensure that the chosen solution will provide all guarantees concerning the reactor’s safety.

While awaiting repairs, Gravelines 1’s reactor remains shut down. The reactor vessel currently contains no nuclear fuel and thus presents no safety issues.

In addition, ASN has asked EDF to carry out an inspection of all TWP present on 900 MWe and 1,300 MWe reactors.

Date of last update : 08/06/2017