Pwr effect on crack initiation under equi-biaxial loading
Résumé
The lifetime extension of the nuclear power stations is considered as an energy challenge worldwide. That is why, the risk analysis and the study of various effects of different factors that could potentially represent a hazard to a safe long term operation are necessary. These structures, often of great dimensions, are subjected during their life to complex loading combining varying mechanical loads, multiaxial, with non-zero mean values associated with temperature fluctuations and also PWR environment.
Historically, the methodology for fatigue design of the Pressurized Water Reactor components (PWR) (ASME BPV III, RCC-M, JSME, EN-13445-3, Chopra O.K. and Shack W.J. (2007)) is based on the use of design curves established from tests carried out in air at 20°C on smooth specimens by integrating safety coefficients that cover, among other parameters, the dispersion of tests associated with the effects of structures.
Based on more recent fatigue data (including tests at 300°C in air and PWR environment, etc…), some international codes (RCC-M, ASME and others) have introduced a modification of the austenitic stainless steels fatigue curve combined with a calculation of an environmental penalty factor, namely Fen, which has to be multiplied by the usual fatigue usage factor.
The 304L and the 316L stainless steel are used for the manufacturing of the pressurized water reactors (PWR). Many components of this type of reactors are subjected to a multiaxial thermo-mechanical cycling. Therefore, the multiaxial fatigue assisted by environment may be considered as one of the main possible degradation mechanisms affecting the life of the PWR components.
Unfortunately, experimental data on this issue are rare. In order to obtain fatigue strength data under structural loading, biaxial test means with and without PWR environment were developed at LISN in collaboration with EDF and AREVA. Two kinds of fatigue device have been developed. Within the same specimen geometry, structural loads can be applied in varying only the PWR environment.
The first device (FABIME2) is devoted to study the effect of biaxiality and mean strain/stress on the fatigue life. A second and new device based on FABIME2 is for the study of the impact of the environmental effect. With these new experimental results, the possible PWR environment effect on the fatigue life of stainless austenitic steels will be discussed. These tests allow quantifying accurately the effect of PWR environment on semi-structure specimen. This device combines the structural effect like equi-biaxiality and mean strain and the environmental penalty effect with the use of PWR environment during the fatigue tests.