Manufacturing deviation at Framatome stress-relieving heat treatment of welds
On 9th September 2019, EDF informed ASN of a deviation concerning the manufacture of nuclear pressure equipment installed in its reactors, and primarily steam generators. During the stress-relieving heat treatment (see box) applied to the welds made to assemble nuclear pressure equipment components, the required temperature range has not been respected over the entire area to treat.
Assembling components by welding creates mechanical stresses in the welded areas. These stresses, which are internal to the material, add to the stresses to which the equipment is subjected in service. To reduce these stresses, the manufacturer applies a stress-relieving heat treatment, which consists in heating the material for several hours to a temperature of several hundred degrees. This heating can be carried out on the complete part in a furnace if the size of the part so permits, or locally by using heating devices such as electrical heating elements. The treatment temperature and duration must be controlled in order firstly to remove the stresses resulting from the welding, and secondly not to alter the mechanical properties of the material.
Framatome, the manufacturer, discovered that the process implemented in its Saint-Marcel plant was not giving sufficient control over the uniformity of temperatures in the circumference of the treated welds. Eighteen steam generators manufactured by Framatome and installed on the in-service nuclear reactor fleet as from 2008 are concerned. These steam generators are installed on the reactors of Bugey 3, Fessenheim 2, Dampierre 4, Blayais 3, Blayais 4, Paluel 2 and are currently being installed on Gravelines 5. Some twenty items of equipment still in the manufacturing process or undergoing their conformity assessment, including the steam generators and pressurizers of the Flamanville EPR reactor, are also concerned.
This deviation in the stress-relieving heat treatment conditions can lead to changes in the metallurgical characteristics of the materials with respect to the hypotheses considered in the design files, or insufficient relief of the mechanical stresses induced by welding.
ASN carried out two inspections on September 18th with the aim of checking the extent of the equipment items concerned and the methods of addressing the deviation. The inspection follow-up letters are posted on the ASN web site.
EDF and Framatome have sent ASN justification documents showing that the integrity of the equipment was not called into question, and these documents have been examined by ASN with the technical support of IRSN. ASN has requested complementary information concerning a steam generator of the Fessenheim power plant reactor No. 2, on account of its particularities.
On the basis of these elements and their analysis, ASN considers that the reactors in question can continue to function as is.
ASN has asked EDF to take advantage of the scheduled outages of these reactors - which will be staggered over the period until summer 2020 - to characterise the welds concerned, in particular through non-destructive tests and thickness measurements, in order to confirm the hypotheses adopted by EDF.
EDF must also undertake a work programme on mock-ups to characterise in detail the operations performed during manufacture and the phenomena observed for the various equipment configurations. The conditions of these investigations and tests shall be monitored by ASN and its technical support organizations.
Lastly, ASN has asked the manufacturer and the licensee to analyse the causes of this situation, which results primarily from inappropriate qualification of the heat treatment process prior to its implementation.