Regulator OKs fix to Flamanville EPR nozzle welds

21 October 2021

France's nuclear safety regulator, the Autorité de Sûreté Nucléaire (ASN), has approved EDF's proposed solution for rectifying a design anomaly on three nozzles in the main primary system of the Flamanville EPR reactor. EDF's proposal to install a retaining collar around each of the concerned connections is not expected to delay the planned startup of the unit in late-2022.

The location of the nozzle affected by the anomaly (Image: ASN)

In March this year, EDF notified ASN of a design anomaly on three nozzles of the Flamanville EPR's main primary system. The concerned nozzles were produced in 2011 based on a 2006 design file.

The main primary system contains water used to cool the reactor core and transfer energy from the nuclear reaction to the steam generators. This system consists of four piping loops installed in the reactor building. Several auxiliary systems are connected to the main primary system, via nozzles. The largest nozzles are integral forgings made during manufacture of the main pipe. The small diameter nozzles (less than 150 mm) are connected to the main primary system piping by a "set-in" weld to the main pipe. Three of these nozzles are affected by the reported anomaly.

In 2006, to make it easier to inspect the "set-in" weld on the main pipe, EDF and Framatome changed the design of these three nozzles, increasing the diameter of this weld. At the time, they did not identify the fact that the break size to be considered in the event of rupture of this weld now exceeded that considered in the safety studies.

This design anomaly was identified by EDF and Framatome in 2013, once the pipes concerned had been manufactured. In 2014, EDF decided to process this anomaly by extending to these welds the break preclusion approach applied to the main primary system pipes. This approach implemented by EDF on certain pipes of the Flamanville EPR reactor consists of reinforcing the design, manufacturing and in-service monitoring requirements, in order to ensure the highly improbable nature of any break in this equipment, "thus obviating the need to study all the consequences of a break in the installation's safety case".

At the request of ASN, EDF on 18 June submitted its strategy to deal with this anomaly. The company has opted for a so-called "rupture exclusion" approach, which consists of installing a retaining collar around each of the three connections. This collar aims, in the event of rupture of the weld around the nozzle, to limit the size of the resulting breach. The consequences of this breach would then be covered by current reactor safety studies.

In an 8 October letter to EDF, ASN said it "has no objection in principle to the solution you have adopted". ASN will give its final decision when EDF has provided the expected information concerning the demonstration of the device's efficiency, the design, manufacturing and operating requirements applicable to the collars and the quality of the installation welds around which these clamps will be mounted.

Construction work began in December 2007 on the 1650 MWe unit at the Flamanville site in Normandy. The EPR reactor was originally expected to start commercial operation in 2013. The loading of fuel into the Flamanville EPR is now scheduled for the end of 2022.

The implementation of EDF's solution is "compatible with the objective of authorising the commissioning and loading of the reactor at the end of 2022," an EDF spokesperson was quoted as saying by Le Figaro.

Researched and written by World Nuclear News