  Formation of large volume fractions of Mn-Ni-Si precipitates (MNSPs) causes
excess irradiation embrittlement of reactor pressure vessel (RPV) steels at
high, extended-life fluences. Thus, a new and unique, semi-empirical cluster
dynamics model was developed to study the evolution of MNSPs in low- Cu RPV
steels. The model is based on CALPHAD thermodynamics and radiation enhanced
diffusion ki- netics. The thermodynamics dictates the compositional and
temperature dependence of the free energy reductions that drive precipitation.
The model treats both homogeneous and heterogeneous nucleation, where the
latter occurs on cascade damage, like dislocation loops. The model has only
four adjustable parameters that were fit to an atom probe tomography (APT)
database. The model predictions are in semi-quantitative agreement with
systematic Mn, Ni and Si composition variations in alloys character- ized by
APT, including a sensitivity to local tip-to-tip variations even in the same
steel. The model predicts that heterogeneous nucleation plays a critical role
in MNSP formation in lower alloy Ni contents. Single variable assessments of
compositional effects show that Ni plays a dominant role, while even small
variations in irradiation temperature can have a large effect on theMNSP
evolution. Within typical RPV steel ranges, Mn and Si have smaller effects. The
delayed but then rapid growth of MNSPs to large volume fractions at high
fluence is well predicted by the model. For purposes of illustration, the
effect of MNSPs on transition temperature shifts are presented based on
well-established microstructure-prop- erty and property-property models.
