Considering operation beyond 40 years of the reactor pressure vessel (RPV) in a nuclear power plant, long term operation (LTO), irradiation induced embrittlement shifts the temperature range for the transition between brittle and ductile failure to higher temperatures. This can lead to difficulties in demonstrating safe operation when using traditional assessment methods for certain loading conditions. One such loading condition is a pressurized thermal chock (PTS) transient. This is a typical case where the warm pre-stressing effect (WPS effect) could be utilized in the analysis to show enough margin to fracture to ensure safe operation. To explain the WPS effect let’s consider a structure with a crack like defect. The structure is loaded in tension at a temperature corresponding to the ductile upper shelf region of the material and unloaded either completely or partially. The structure is then cooled to the brittle lower shelf region of the material fracture toughness transition curve and, when reloaded, fracture occurs at a higher load than what is expected. This phenomenon is called the warm pre-stressing effect. There are still gaps in the knowledge about the WPS effect and its use for real components during realistic loading transients. One of these gaps is what the margin to fracture is during the cooling for a load transient. Theoretically a component would not fracture if the load is held constant during the cooling. But there is no knowledge of what the margin to fracture is. Published experiments show that it seems to be a significant scatter in the level of margin during the cooling. This justifies the need to evaluate the margins to fracture during a loading transient. This can be done by use of a numerical model that fully accounts the load history and changes in temperature. Such a model does not exist today. This research project aims to answer what the margin and probability of fracture is during the cooling part of a typical PTS transient in a RPV. For this to be possible a non-local probabilistic model for cleavage fracture that accounts for effects of load history and changes in temperature will be developed. To be able to develop such a model a large experimental program will be conducted where the material is characterized at several temperatures from the transition region to the lower shelf region. Tests for validation of the developed model will also be conducted. The developed model will be applied to evaluate the probability of fracture for a RPV subjected to a PTS transient. Thus, reaching the goal and answering what the margin to fracture is during cooling part of a PTS transient.