Impact of large forging macro-segregations on the reactor pressure vessel surveillance program

The monitoring of the reactor vessels depends to some extent on the use of the specimens contained in the irradiation capsules of the Pressure Vessel Surveillance Program (PVSP). One of the objectives of the PVSP is the validation of the formulas used to predict irradiation embrittlement. Certain results of the PVSP show atypical kinetics, which extreme cases correspond to over or under embrittlement. The assessment of the tested samples (methodical sampling) made it possible to explain these singularities by an azimuthal variability of the mechanical properties in the unirradiated state linked to the existence of metallurgical heterogeneities: macro-segregations. The presence and distribution of these macro-segregations in the forged shells were explained and quantified within the framework of a vast program of metallurgical description and characterization named 'Large Forging' program. Secondly, the presence of these macro-segregations raises the question of the representative nature of the PVSP. In fact, following the 'Large Forging' program, it appears that zones affected by positive segregation can locally be present at the internal skin of the core shells, the quarter-thickness of the acceptance ring, where the PVSP specimens come from, being for its part a zone with zero or negative segregation. An experimental action based on carrying out toughness tests in strongly and slightly segregated zones, located at the internal skin and at the quarter-thickness of a nozzle shell, made it possible to show that the representative nature of the PVSP is not affected by the presence of the macro-segregations. Finally, it appears that the existence of macro-segregations in the core shells may complicate the analysis of the PVSP results by the scattering it adds to the radiation induced embrittlement itself; but the PVSP representative nature is not challenged. (authors)