Contribution of multiscale materials modelling for underwriting nuclear pressure vessel integrity

The continued safe operation of high integrity structures such as Magnox steel reactor pressure vessels is assured by using arguments which are usually based upon deterministic methodologies. These require models to provide the input parameters together with the necessary mechanistic understanding of the changes in mechanical or physical properties of the steels with service life. The use of modelling to provide key parameters for input to the fracture mechanics assessments is discussed, in particular, the application of statistical procedures to describe the required material's mechanical property data. However, there is a need to underwrite these data with an understanding of the underlying physical mechanisms. Here mechanistic models have been derived which span many orders of magnitude of length scale. At the atomic scale these provide an insight into the production of point defect damage in steels as a result of exposure to a neutron spectrum at various temperature and neutron doses. In addition, equilibrium and nonequilibrium segregation of alloying and impurity atom species to grain boundaries is modelled. Both are necessary for understanding the changes in the fracture mode and the mechanical properties for these steels after extended periods of service mode. At the microscale, models are adopted to develop insights into the fracture processes. The benefits of these models to the arguments developed within safety cases which underwrite safe continued operation of these reactor pressure vessels are described.