Swedish nuclear power plants (NPP) were originally designed and analyzed with an assumed operating period of approximately 40 years. As Swedish NPPs reach their original technical design life the safety of the additional operating time has to be verified. With increased operating time, the shift in the fracture toughness curve of the reactor pressure vessel (RPV) must be accounted for in structural integrity assessments.
This project investigates the mechanisms that contribute to the apparent increase in fracture toughness of components with an assumed crack like defect, i.e Warm Pre-Stressing (WPS). WPS is a benificial effect that increases the fracture toughness of a component when certain conditions are met, e.g. the component is first preloaded in the ductile region after which it is cooled to the brittle fracture region. Upon reloading, fracture is expected to occur at higher loads than for virgin, unprestressed material.
The research was initiated by SSM and is relevant considering the aging reactor fleet in Sweden. The project is expected to provide answers to the applicability of the engineering methods currently applied to analyse the WPS effects. The project aims to provide insights for improved analytical approaches.
An extensive fracture mechanic testing programme, where the effects of the contributing mechanisms were singled out and studied one-by-one showing that
- the most influential mechanism behind the WPS effect is the change in yield strength due to the lowering of the temperature,
- as long as the load is constant or monotonous declining during cooling, the actual load path can be considered path independent,
- the deactivation of initiation sites due to the preloading is an active and significant mechanism, and
- the load-cool-fracture (LCF) load path is more beneficial than the load-unload-cool-fracture (LUCF). Hence, the change of yield strength mechanism is found to be the most beneficial mechanism.
SSM intend to consider the results from all three projects when revising the section on WPS in SSM 2018:20 and as such use it as a scientific base for guidance and regulatory review.
Need for further research
This is the first of three projects researching WPS that SSM have planned. This first project investigated the mechanisms that contribute to the WPS effect, both numerically and experimentally. The next project will develop a non-local probabilistic model for cleavage fracture that accounts for effects of load history and changes in temperature. This project will also investigate the effect of load history on brittle fracture.
The third project shall experimentally evaluate the margins to fracture associated with WPS. The analytical model developed in the previous project shall also be validated.
Since defects are usually found in welds, the interaction between weld residual stresses and WPS should be investigated. This is, however, outside the scope of the above mentioned three projects.