Evaluation of the fracture resistance of nuclear reactor pressure vessel (RPV) regarding the risk of brittle fracture is a key point in the structural integrity assessment of the component (RPV). Such approach is codified in French RSE-M code, based on a very conservative methodology. With respect to long term operation, an improvement of the present methodology is necessary and in progress to reduce this conservatism.

One possible significant improvement is the inclusion of the warm pre-stress (WPS) concept in the assessment. After a short description of the WPS concept, the process engaged in France to allow inclusion of WPS in the integrity assessment is presented.

In a first step, experimental and numerical studies have been conducted in France by EDF, CEA and AREVA (also including international collaborations and projects) to demonstrate and validate the beneficial effect of WPS on the brittle fracture resistance of RPV steels. A large panel of experimental results and data is now available obtained on small, medium and large scale specimens on representative RPV steels (including highly irradiated RPV materials). These data have been included in a specific WPS experimental database.

Main experiments have been interpreted by refined computations, based on elastic plastic analyses and local approach to cleavage fracture.

In a second step, a new criterion (ACE criterion) has been proposed by French organizations (AREVA, CEA and EDF) for an easy simplified evaluation of warm pre-stress effect on the brittle fracture resistance of RPV steels. Accuracy and conservatism of the criterion is verified by comparison to experimental data results and numerical analyses.

Finally, implementation of the WPS effect in the French RSE-M code (for in service assessment) is in progress, based on the ACE criterion.

The present paper summarizes all these steps leading to codification of WPS in RSE-M code.

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