Flamanville EPR reactor: design anomaly on three main primary system nozzles
On 3 March 2021, EDF notified ASN of a significant event concerning a design anomaly on three nozzles1 of the main primary system of the EPR reactor under construction on the Flamanville site.
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 concerned 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 in 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 beginning of 2017, following anomalies detected on the welds of the VVP pipes, ASN asked EDF to examine the conformity of the main primary system pipes with the “break preclusion” baseline requirements. The subsequent ASN examination of the answers provided by EDF regarding these nozzles, at the end of 2020, revealed that the welds of these three nozzles did not comply with all the break preclusion requirements.
Before issuing a position statement, ASN asked EDF for its strategy to deal with this anomaly. ASN also asked EDF to identify its root causes and the reasons for its belated detection and to take corrective measures, notably to ensure that there are no other anomalies on the main primary system.
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1. The nozzle represents the part of a pipe which joins it to another pipe or container. The three nozzles concerned by the anomaly connect small diameter pipes (about 100 mm for the RCV letdown nozzle and for the two normal spray nozzles) to the main primary piping, with a larger diameter (about 780 mm).