The embrittlement of the reactor pressure vessel (RPV) due to extended operation can lead to difficulties in demonstrating safe operation beyond 40 years when using traditional assessment methods. Therefore, utilizing the beneficial WPS (Warm Pre-Stressing) effect in assessments is important for continued operation beyond 40 years of the RPV. The practise of utilizing the beneficial WPS effect in RPV assessments have been adopted already in several European countries. However, there are still some uncertainties about the limitations of the engineering methods that are being used. These uncertainties need to be addressed to ensure safe utilization of the WPS effect.
The WPS effect is the increase of the apparent brittle fracture toughness for a ferritic component when pre-loaded at a temperature in the ductile upper shelf region and then cooled to the brittle lower shelf region of the material fracture toughness transition curve.
The WPS effect can be attributed to four main mechanisms. These mechanisms have different impact, depending on the pre-load level and load path. All the mechanisms are related to plastic straining at pre-load. The engineering methods used today do not consider constraint and do not take into account the different impact of the mechanisms in relation to different load paths.
There is a need to evaluate thoroughly the importance of the four main mechanisms behind WPS for realistic situations that could be encountered in a RPV. This in order to understand the limitations and possibilities in the engineering methods used to assess the magnitude of the WPS effect.
Within this research project (Inspecta Technology AB (Sweden), Royal Institute of Technology (Sweden), SINTEF (Norway) and Swedish Radiation Safety Authority (Sweden)), the main mechanisms behind WPS and their importance relating to RPV assessments will be validated using both experiments and numerical methods. This project will try to answer the question of which of these mechanisms, or combination of, is the governing mechanism in situations that closely resemble those that can arise in a RPV. This is important to be able to assess the reliability and limitations of the engineering methods that are employed today in assessing the magnitude of the WPS effect. The results will also be used to formulate guidelines in utilizing the WPS effect in RPV assessments.