Characteristics Of Thermal Fatigue On Charging Line Of Nuclear Power Plant

Safety of Nuclear Power Plants(NPPs)had been and still is one of the major issues which is forbidding many interested consumers of the NPP energy to utilize it.Fatigue failure of components is a catastrophic failure and is a high safety risk for a NPP.Many components in NPP need to tolerate thermal constraints,internal pressure and thermal transients.These thermal transients being repeated again and again can lead to thermal fatigue of the component.It has significant effect on the degradation of the NPP components in long term.Studies of thermal fatigue on different NPP components such as elbows and mixing tees have been carried out before but the charging line in the chemical and volume control system(RCV)of the NPP seems to have been ignored for thermal fatigue analysis.Charging Line is the connection from RCV towards Reactor Coolant System(RCP).To enhance the safety of the charging line,thermal fatigue evaluation of piping system was performed using the Fluid Structure Interaction(FSI)analysis.Temperature distributions in the pipes were determined via thermal hydraulic analysis(CFD analysis)using ANSYS CFX and the results were applied to the structural model of the piping system to determine the thermal stresses(Finite Element Method)using ANSYS Transient Structural analysis.These stresses were further utilized to study the fatigue characteristics of the piping system using the Fatigue Tool of ANSYS Mechanical.Simulations of 200 s were carried out with a time step of 1 second.Turbulence in the CFD analysis was modelled using the SST k-omega model.Initial CFD results on the Charging Pipeline and the connected RCP Pipe revealed that the most critical area regarding the temperature fluctuation and hence the thermal stresses was the junction of the two pipelines.Hence a smaller model was built and CFD analysis performed on it.This analysis enabled us to clarify the role of turbulence with respect to the thermal loading of the structure.The results revealed that because of asymmetry of flow at the junction of two pipes,twisting began there.Temperature fluctuations obtained from this CFD analysis were mapped on the structural model in the FEM analysis using an ACT(Application Customization Toolkit)extension of ANSYS.The FEM results revealed that the maximum thermal stress and hence the maximum fatigue damage occurs near the side position from the symmetry line in the circumferential direction.The greatest variation of thermal stresses was seen on the RCP pipe downstream the mixing area towards the outlet.The minimum fatigue life occurs on left side or right side of the center of junction of the two pipelines where the maximum stress values were reported.This study is a quick estimate of fatigue characteristics of charging line and will provide valuable information for establishing a permanent methodology to help minimize thermal fatigue damage in NPP components.