EFFECTS OF RESIDUAL STRESS BY WELD OVERLAY CLADDING AND PWHT ON THE STRUCTURAL INTEGRITY OF RPV DURING PTS

摘要

In order to assess the structural integrity of a reactor pressure vessel (RPV), it is assumed that a surface crack resides through the cladding at the inner surface of the vessel. It is, therefore, important to precisely evaluate stress intensity factor (SIF) under the residual stress field due to weld overlay cladding and post-weld heat treatment (PWHT). In this work, numerical simulation based on thermal-elastic-plastic-creep analysis using finite element method was performed to evaluate residual stress distribution near the cladding layer produced by weld overlay cladding and PWHT. The tensile residual stress of about 400 MPa occurs in the cladding at room temperature after the PWHT. The residual stress distributions under the normal operating conditions (system pressure and temperature) of RPV were also evaluated. The effect of residual stress and evaluation methods on SIF behavior for various crack size were studied under typical pressurized thermal shock (PTS) conditions such as small break loss of coolant accident (SBLOCA), main steam line break (MSLB) and large break loss of coolant accident (LBLOCA). It is clarified from comparison of this weld simulation with the other simple methods that SIF is affected by residual stress by weld overlay cladding and PWHT.