Work progresses on stress-relieving heat treatment processes

In 2019, Framatome launched a program to consolidate knowledge about the performance characteristics of the local stress-relieving heat treatment (SRHT) processes used in its plants or on worksites on equipment supplied by Framatome.

The purpose of stress-relieving heat treatments is to relieve residual stresses from welding and obtain the appropriate mechanical characteristics. The technical manufacturing standard, the RCC-M code, sets the temperature range to be reached when performing this operation depending on the materials used.

Non-compliance with this temperature range was detected when applying certain local processes thus constituting a deviation from the standard.

Three deviations were opened in 2019 and 2020 for local SRHT processes implemented on welds of large equipment. The processes concerned are specific local processes, namely the use of muffles equipped with resistance heating elements, of ceramic panels and of cassette heaters.

In 2020, two other deviations have been opened concerning the SRHTs performed after welding on secondary piping. These deviations concern connections made during steam generator replacement operations on the one hand, and the assembly of the secondary system of the Flamanville 3 reactor on the other hand.

In consultation with the operators concerned, and within the framework of the regulations applicable to each component, these deviations are being processed by Framatome. To this end, Framatome is conducting detailed characterization studies, and is confirming the mechanical properties of the materials concerned (both in situations of under-SRHT as well as in situations of over-SRHT) for specific test programs.

As some components in operation are in particular involved, this program of verification by calculation and testing confirms, at this stage, that the mechanical integrity of the parts concerned is not called into question.

Framatome is keeping the operators, as well as the relevant Safety Authorities, regularly informed.