The effect of irradiation on BWR core shroud cracking

A multi-scale model was developed to estimate the effect of radiation hardening on stress corrosion cracking (SCC) in boiling water reactor (BWR) core shroud welds. The model combines the point defect cluster (PDC) model with Ford-Andresen's slip-dissolution model to evaluate the changes in the crack propagation rate resulting from radiation hardening. To evaluate the relative contribution of neutron and gamma irradiation to the material damage, we developed the displacement cross section for gamma ray and calculated both the displacements per atom (dpa) and the freely migrating defect (FMD) production. While the displacements produced by gamma radiation are essentially 100% FMD, of the total displacements produced by neutrons only about 2∼4% are FMD. To evaluate the irradiated material weldability we also calculate helium production from both one-step and two-step thermal neutron reactions with nickel using ENDF/B-VI cross section data.; The increase in yield strength of irradiated stainless steels under normal BWR operating conditions is estimated using the PDC model. In the core shroud region, the contribution of gamma ray to the hardening is not significant although the FMD production from gamma ray represents fully 10∼40% of the total FMD production. The amount of radiation hardening varies with the location of the core shroud, that is, higher dpa levels lead to more hardening.; To calculate the crack propagation rate in the core shroud weld region, we determined the crack tip strain rate which is proportional to the yield strength of material and a stress intensity factor under constant loading. Based on linear elastic fracture mechanics, the stress intensity factor is calculated with the weld residual stress and the model is used to predict the crack growth rates of Susquehanna BWR core shroud. The comparison of the results with crack measurements made at Susquehanna units I and II shows good agreement. The model calculations show that radiation hardening significantly increases the stress-corrosion crack growth rate.