Within the framework of a working program to characterize the effects of local stress-relieving heat treatment processes on welds on pressure equipment or piping, Framatome has, by means of a digital and experimental approach, been able to quantify the existence of possible remanent residual stresses induced by the use of this type of process.
The level of stresses induced depends on the geometry of the structures, the methods of performing the local treatment, as well as the properties of the materials in the associated temperature range.
There are no explicit elements present in the international codes and standards applicable to nuclear equipment that allow these stresses to be quantified and their level to be limited.
At this stage, Framatome has already completed the digital and experimental development work necessary to assess the residual stresses induced by local stress-relieving heat treatment processes on numerous configurations.
In-depth work has commenced to verify the integrity of the assemblies concerned in accordance with the methods available to date (methods which were not developed to deal specifically with the physical phenomenon concerned and which are strict as a result). To date, the verifications completed on the components for which they have been conducted have allowed the fitness for operation of the equipment already delivered or to be delivered in future by Framatome to be justified.
Framatome has also launched an experimental scientific and digital modeling program to take the physical phenomena which come into play into account, and thus to be able to estimate the impacts of these stresses more realistically.
Last but not least, the taking into account of this phenomenon allows the conditions for the performance of local stress-relieving heat treatment processes to be further improved. Processes which take these improvements into account will thus be able to be used from now on in upcoming operations to be carried out in plant or on site.
The process of sharing these new technical details with operators and regulators is already underway.
All of the knowledge acquired is being fed into operating experience feedback and into the approach being taken by Framatome to ensure the continuous improvement of its engineering and manufacturing methods.