Crack initiation and growth simulation for CRDM nozzles by extended finite element method

The head penetration nozzles of control rod driving mechanisms (CRDMs) are susceptible components on primary water stress corrosion cracking (PWSCC) due to the dissimilar metal welds. The accurate integrity assessment of the CRDM head penetration nozzles is important for the safe operation of nuclear power plants. To resolve the integrity issue, conventional finite element methods, a cohesive zone model, and a virtual crack closure technique have been employed; however, there are still many uncertainties in accuracy and efficiency. In the present study, a specific Strain Rate Damage Model (SRDM) with stress and thermal dependent parameters was adopted to calculate crack initiation time. Also, a level set method, which defines the crack location based on the crack surface and vertical surface of crack tip, was considered to simulate arbitrary crack growth. By taking into account these two features, the eXtended Finite Element Method (XFEM) was implemented to simulate the PWSCC initiation and growth with a user subroutine code. Finally, the validity of the proposed method was evaluated by comparing the reference cracks that occurred in the CRDM head penetration nozzles.