ASN inspects weld repairs on the reactor vessel head for the Flamanville EPR
In November 2010, AREVA NP detected defects in several nozzle penetration* welds during its regulatory manufacturing inspections of the Flamanville 3 EPR reactor vessel head.
Purpose and location of EPR vessel head nozzles
Nuclear power plant reactor vessel heads are penetrated by nozzles which are designed to allow control rod drive mechanisms and core monitoring instruments through the vessel head into the reactor vessel. It is the control rod drive mechanism that controls the nuclear reaction inside the reactor core. The EPR vessel head totals more than a hundred such penetrations.
Under normal operating conditions, the pressure inside a nuclear reactor is very high, reaching a value of 155 bar. Each component passing through the vessel head is welded to the inner surface. Welding involves depositing several layers of metal and each weld undergoes regular inspection both during and after the operation using various methods.
ASN asked AREVA NP to propose corrective action as these nozzles are important for ensuring reactor vessel pressure resistance.
In order to restore the required quality level, AREVA NP proposed reworking all vessel head nozzle welds. In January 2011, this solution was successfully tried out on two welds. The existing welds were completely removed by grinding and replaced with new welds using a procedure that meets quality requirements.
The same month, ASN carried out two inspections, one on AREVA NP, the other on APAVE GROUPE, an ASN-approved organization that the Authority had asked to check compliance with regulatory technical requirements . The inspections set out to examine:
- the steps taken by AREVA NP to correct the problem,
- the supervision methods employed by APAVE GROUPE regarding these operations.
After examining the data submitted by AREVA NP in February and March 2011, ASN gave the company the go-ahead for initial weld repair work. The Authority will then ensure that adequate measures have been taken by AREVA NP before authorising the next stages of the process.
ASN will perform a compliance check on the vessel head of the Flamanville 3 EPR at the end of the manufacturing process.
 Nuclear pressure equipment undergoes an assessment to ensure that it meets the technical requirements set out in regulations. This assessment is performed by ASN, which also calls on independent bodies that it approves and regularly inspects. Equipment cannot be commissioned until ASN has declared that it meets regulatory requirements and, in particular, that no unacceptable defects have been observed.