Search

A number of cracks due to primary water stress corrosion cracking (PWSCC) in pressurized water reactors and Ni-based alloy stress corrosion cracking (NiSCC) in boiling water reactors have been detected around Ni-based alloy welds. The causes of crack initiation and growth due to stress corrosion cracking include weld residual stress, operating stress, the materials, and the environment. We have developed the analysis code PASCAL-NP for calculating the failure probability and assessment of the structural integrity of cracked components on the basis of probabilistic fracture mechanics (PFM) considering PWSCC and NiSCC. This PFM analysis code has functions for calculating the incubation time of PWSCC and NiSCC crack initiation, evaluation of crack growth behavior considering certain crack location and orientation patterns, and evaluation of failure behavior near Ni-based alloy welds due to PWSCC and NiSCC in a probabilistic manner. Herein, actual plants affected by PWSCC have been analyzed using PASCAL-NP. Failure probabilities calculated by PASCAL-NP are in reasonable agreement with the detection data. Furthermore, useful knowledge related to leakage due to PWSCC was obtained through parametric studies using this code.

This study investigates the effects of high fluence neutron irradiation on the mechanical properties of two types of cladding materials fabricated using the submerged-arc welding and electroslag welding methods. The tensile tests, Charpy impact tests, and fracture toughness tests were conducted before and after the neutron irradiation with a fluence of 1 × 1024 n/m2 at 290 °C. With neutron irradiation, we could observe an increase in the yield strength and ultimate strength, and a decrease in the total elongation. All cladding materials exhibited ductile-to-brittle transition behavior during the Charpy impact tests. A reduction in the Charpy upper-shelf energy and an increase in the ductile-to-brittle transition temperature was observed with neutron irradiation. There was no obvious decrease in the elastic–plastic fracture toughness (JIc) of the cladding materials upon irradiation with high neutron fluence. The tearing modulus was found to decrease with neutron irradiation; the submerged-arc-welded cladding materials exhibited low JIc values at high temperatures.

In this study, the residual stresses generated within the overlay-welded cladding and base material of reactor pressure vessel (RPV) steel were measured for as-welded and postwelded heat-treated conditions using the sectioning and deep-hole-drilling (DHD) techniques. In addition, thermo–elastic–plastic creep analyses considering the phase transformation in the heat-affected zone using the finite element method (FEM) were performed to evaluate the weld residual stress produced by overlay-welding and postweld heat treatment (PWHT). By comparing the analytical results with the experimentally determined values, we found a good agreement for the residual stress distribution within the cladding and the base material. The tensile residual stress in the cladding is largely due to the difference in the thermal expansion of the cladding and the base material. It was also shown that considering phase transformation during welding was important for improving the accuracy of the weld residual stress analysis. Using the calculated residual stress distribution, we performed fracture mechanics analyses for a vessel model with a postulated flaw during pressurized thermal shock (PTS) events. The effect of the weld residual stress on the structural integrity of RPVs was evaluated through some case studies. The results indicated that consideration of the weld residual stress produced by overlay-welding and PWHT is important for assessing the structural integrity of RPVs.

Stress corrosion cracking (SCC) has been observed near the welded zones of pipes made of austenitic stainless steel type 316L. Residual stress is an important factor for SCC. In the joining processes of pipes, butt welding is conducted after surface machining. Residual stress is generated by both processes, and the residual stress distribution by surface machining is varied by the subsequent butt-welding process. In this study, numerical analysis of the residual stress distribution by butt welding after surface machining was performed by the finite element method. The SCC initiation time was estimated by the residual stress obtained at the inner surface. SCC growth analyses based on probability fracture mechanics were performed by using the SCC initiation time and the residual stress distribution. As a result, the residual stress distribution in the axial direction due to butt welding after surface machining has high tensile stress exceeding 1000 MPa at the inner surface. The effect of SCC initiation on leakage probability is not as significant as the effect of plastic strain on the crack growth rate. However, to perform crack growth analyses considering SCC initiation, evaluation of the residual stress due to surface machining and welding is important.Copyright © 2013 by ASME

This paper presents results of residual stress measurements and modelling within the cladding and J-groove weld of Control Rod Drive (CRD) specimens in the as-welded and Post Weld Heat Treated (PWHT) states. Knowledge of the residual stresses present in CRD nozzles is critical when modelling the fracture mechanics of failures of nuclear power plant components to dictate inspections intervals and optimise plant downtime. The specimens comprised of ferritic steel blocks with 309L stainless steel cladding and a single J-groove weld attaching the 304 stainless steel nozzles. Multiple measurements were made through the thickness of the specimens in order to give biaxial residual stress profiles through all the different fusion boundaries. The results show the effect of PWHT in reducing residual stresses both in the weld and ferritic material. The beneficial use of measurements is highlighted to provide confidence in the modelled results and prevent over conservatism in integrity calculations, costing unnecessary time and money.

Residual stresses were measured in cladded steel specimens using deep hole drilling (DHD) and block removal and surface layering (BRSL) techniques. The samples consisted of a A533B steel substrate and cladded with Type 304 stainless steel using two different welding techniques; electro-slag (ESW) and submerged welding (SAW). Two SAW samples were created; one with a single layer of weld and a second with a double layer of welding. Only a single weld layer of ESW was used on another sample. All three samples were subjected to post-weld heat treatment prior to measurement. The measured residual stress distributions revealed (as expected) tensile stresses in the clad. However, the DHD method measured compressive stresses in the substrate adjacent to the clad for the single layer ESW and SAW welds. In contrast, the BRSL method found that the residual stresses in the substrate were close to zero or approximately tensile. The measurements are compared with results obtained from finite element (FE) simulations of the welding and PWHT treatment. The predicted tensile residual stresses in the clad were found to be larger than the measurements while in the substrate the FE analysis did not predict the measured compressive stresses.Copyright © 2010 by ASME

Austenitic stainless steel is cladded on the inner surface of ferritic low alloy steel of reactor pressure vessels (RPVs) for protecting the vessel walls against the corrosion. After the manufacturing process of the RPVs including weld-overlay cladding and post-weld heat treatments (PWHT), the residual stress still remain in such dissimilar welds. The residual stresses generated within the cladding and base material were measured as-welded and PWHT conditions using the sectioning and deep-hole-drilling (DHD) techniques. Thermal-elastic-plastic-creep analyses considering the phase transformation in heat affected zone using finite element method were also performed to evaluate the weld residual stress produced by weld overlay cladding and PWHT. By comparing analytical results with those measured ones, it was shown that there was a good agreement of residual stress distribution within the cladding and base material. Tensile residual stress in cladding is mostly due to the difference between the thermal expansions of cladding and base materials. It was also shown that taking the phase transformation during welding into account is important to improve the accuracy of weld residual stress analysis. Using the calculated residual stress distribution, fracture mechanics analysis for a postulated flaw during pressurized thermal shock (PTS) events have been performed. The effect of weld residual stress on the structural integrity of RPV was evaluated through some case studies. The result indicates that consideration of weld residual stress produced by weld-overlay cladding and PWHT is important for assessing the structural integrity of RPVs.Copyright © 2010 by ASME

A number of cracks due to primary water stress corrosion cracking (PWSCC) in PWR and Ni-based alloys stress corrosion cracking (NiSCC) in BWR have been observed near Ni-based alloy welds. One of the causes of initiation and growth due to SCC is high tensile residual stress as well as operating stress. In this study, an analysis code, PASCAL-NP, for the PWSCC/NiSCC growth at the dissimilar metal welds based on probabilistic fracture mechanics (PFM) was developed. This PFM analysis code has a function of SCC growth calculation for some patterns of crack locations and orientations in a probabilistic manner. This code can also evaluate the failure probability of Ni-based alloy welds due to PWSCC/NiSCC. Using this code and results from welding simulations, case studies on PWSCC growth have been performed focusing on the location and orientation of PWSCC. Effects of the weld residual stress and scatter of PWSCC growth rate on the crack penetration such as leakage are shown in comparison with deterministic analyses.

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.Copyright © 2007 by ASME