Simulation of Stress Corrosion Cracking in In-Core Monitor Housing of Nuclear Power Plant

Abstract

In the in-core monitor (ICM) housing of a reactor pressure vessel (RPV), residual stress has been widely reported to cause stress corrosion cracking (SCC) damage in the weld heat-affected zone. For this reason, it is important to evaluate the crack growth conservatively, and with high confidence to demonstrate fitness for service. This paper presents crack growth simulations in an ICM housing, which is welded at two different angles to the RPV. One weld angle is at the bottom of the RPV, and the welding area of the ICM housing is axisymmetric. The other angle is at the curved position of the RPV, and the weld area of the ICM housing is asymmetric. In these weld areas, crack growth behavior is estimated by superposed-finite element method (S-FEM), which allows generation of a global finite model and a detailed local mesh representing the crack independently. In the axisymmetric weld area, axial, slant and circumferential surface cracks are assumed at two locations where the residual stress fields are different from each other: one is isotropic and the other is circumferential. It is shown that crack growth behaviors are different under different residual stress fields. The results of S-FEM are compared with those of the influence function method (IFM), which assumes that an elliptical crack shape exists in a plate. It is shown that the IFM result is conservative compared to that of S-FEM. Next, an axial surface crack is assumed at the uphill, downhill, and midhill asymmetric weld areas. The midhill crack growth behavior is different from the uphill and downhill behaviors. Finally, two surface cracks are simulated in the asymmetric weld area and two initial crack arrangements are assumed. These results show the differences of the crack interaction and the crack growth process.