Prolonging the service life of nuclear power plants and more stringent safety requirements are increasing the need for miniature mechanical tests such as the Small Punch Test (SPT). This test was initially developed in the early 1980s at MIT and further developments with more extensive use were achieved by EPRI with a continuous program between 1990 and 1999. The present investigation is first based on a finite element simulation benchmark between EPRI and CEA. This provided a reciprocal validation of the numerical simulation of this test for an A533 B steel at room temperature. The loading mode progressively evolves from bending towards biaxial membrane loading of the specimen. Strain energy density was also computed as the failure criterion and the maximum value observed at experimental crack initiation is about 116 MJ/m3. A parametric finite element study was undertaken in order to evaluate the influence of factors such as specimen thickness, friction, yield strength, strain hardening coefficient, etc. Starting from this parametric study, an automatic routine was built which estimates the elastic-plastic Ramberg-Osgood stress strain relation. The need for standardization of this test will be underlined.

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This paper presents a systematic review of the behavior of phosphorus (P), highlighting the implications of P segregation to grain boundaries under neutron irradiation. The review focuses on Mn-Mo-Ni steels employed in US pressurized water reactors (PWRs), and other PWRs worldwide. Segregation of P to grain boundaries in reactor pressure vessel (RPV) steels can occur during fabrication (especially during the slow cooling stage of a post-weld heat treatment), and as a result of in-service exposure to high operating temperature and irradiation. This segregation of P to grain boundaries can promote a change in the brittle fracture mode from transgranular (TGF) to intergranular (IGF), and a degradation in the mechanical properties. In US RPV steels, most data are on thermal aging of the heat-affected zone (HAZ). Studies in coarse-grained HAZ have shown that the embrittlement arising from segregation of P to grain boundaries is approximately linearly related to the proportion of the brittle fracture that is IGF, and/or the P concentration at the grain boundary. Data are sparse on the effect of irradiation at 288°C on P segregation, and on the contribution of IGF to the total shift in the 41J transition temperature, T41J. In general, the bulk P content appears to be less than about 0.028 wt% P, with base metals having lower levels than weldments. In addition, the consequences of vessel annealing are considered at temperatures around 475°C. It is certain that the annealing treatment will have the consequence of reducing the irradiation hardening, but may significantly increase the grain boundary phosphorus coverage and the likelihood of IGF.

Surveillance materials consisting of a SA-508 Class 2 forging, a Mn–Mo–Ni Linde 80 submerged-arc weld, and an SA-533, Grade B, Class 1, correlation monitor material were thermally aged on a commercial reactor pressure vessel. The materials were exposed to a thermal environment of 260°C for 209000h. This temperature is below the range (minimum of 370°C) where the effects of long-term thermal aging are typically considered relevant. Charpy impact, master curve transition temperature, upper-shelf fracture toughness, and tensile testing were conducted to evaluate the long-term thermal aging changes in material properties. Small changes in the impact properties were observed for all the materials, but were generally within the 95 % confidence bounds for typical Charpy data. Upper-shelf energy also showed small variations, but a general decrease for all materials was not seen. Fracture toughness testing at the upper shelf indicated that the upper-shelf toughness had increased, however the data is scattered. Master curve T0 testing in the transition region showed little change in the forging and plate results; however an improvement in the transition temperature of the weld metal was measured.

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