France’s Autorité de Sûreté Nucléaire (ASN – Nuclear Safety Authority) said on 16 March that EDF notified it of a significant event relating to a design deviation concerning three taps of the main primary circuit of the EPR reactor under construction at unit 3 of the Flamanville NPP.
The main primary circuit contains the water which cools the reactor core and transfers the energy resulting from the nuclear reaction to the steam generators. This circuit is made up of four piping loops which are installed in the reactor building. Several auxiliary circuits are connected to the main primary circuit, through nozzles. The most important nozzles are integrated by forging during the manufacture of the main piping. Small diameter taps (less than 150 mm) are connected to the main primary circuit piping by means of a connection welded to the main piping called "set-in". Three of these taps are affected by the deviation, ANS noted.
In 2006, in order to make it easier to check the “set-in” weld on the main piping, EDF and Framatome changed the design of these three nozzles by widening the diameter of this weld. They did not then identify that the size of the breach to be taken into account in the event of failure of this weld then became greater than that considered in the safety studies.
This design deviation was identified by EDF and Framatome in 2013, after the pipes concerned had been manufactured. In 2014, EDF decided to deal with this by extending to these welds the failure exclusion process applied to the main primary piping. This approach implemented by EDF on certain piping of the Flamanville EPR reactor consists of strengthening design, manufacturing and in-service monitoring requirements, in order to ensure that equipment failure is highly improbable, and thus will not have to study all the consequences of a rupture in the safety demonstration of the installation.
At the start of 2017, following the deviations detected on the welds of the VVP pipes, ASN asked EDF to examine the conformity of the main primary pipes with the so-called “rupture exclusion” reference. In this context, the examination carried out by ASN of the last elements of response provided by EDF on these taps at the end of 2020, showed that the welds of these three taps did not meet all the requirements of the process.
ASN has asked EDF to indicate its strategy for dealing with this and to identify the root causes of this deviation and the reasons for its late detection. EDF was also asked to implement the corrective actions which should in particular make it possible to ensure that there are no other deviations on the main primary circuit.
Image courtesy of Autorité de Sûreté Nucléaire
