Kyoung Soo Lee

Estimation of Residual Stress Distribution for Pressurizer Nozzle of Kori Nuclear Power Plant Considering Safe End

Abstract In nuclear power plants, ferritic low alloy steel nozzle was connected with austenitic stainless steel piping system through alloy 82/182 butt weld. Accurate estimation of residual stress for weldment is important in the sense that alloy 82/182 is susceptible to stress corrosion cracking. There are many results which predict residual stress distribution for alloy 82/182 weld between nozzle and pipe. However, nozzle and piping system usually connected through safe end which has short length. In this paper, residual stress distribution for pressurizer nozzle of Kori nuclear power plant was predicted using FE analysis, which consideded safe end. As a result, existing residual stress profile was redistributed and residual stress of inner surface was decreased specially. It means that safe end should be considered to reduce conservatism when estimating the piping system. 1. 서 론 최근 들어 V.C Summer, Ringhals 및 Tsuruga 2 등의 원전 이종 금속 용접부(DMW, Dissimilar Metal Weld)에서 (1,2)균열이 발생되었다. 균열의 위치는 상대적으로 운전 압력 및 운전 온도가 높은 원자로(reactor) 및 가압기(pressurizer)의 노즐 용접부로서 균열의 원인은 용접부 금속인 니켈 합금(Alloy 82/182) 재료의 일차수 응력부식균열(PWSCC)로 판명되었다. Alloy 82/182 재료는 응력 부식 균열에 민감하다고 알려져 있으며 주로 이종 금속 용접부 용접 재료로 사용되었다. 응력부식균열은 재료의 민감도, 수화학 환경 및 용접 잔류응력(welding residual stress)의 상호 작용에 의해 발생한다.

Sensitivity analyses of finite element method for estimating residual stress of dissimilar metal multi-pass weldment in nuclear power plant

Abstract In nuclear power plants, ferritic low alloy steel components were connected with austenitic stainless steel piping system through alloy 82/182 butt weld. There have been incidents recently where cracking has been observed in the dissimilar metal weld. Alloy 82/182 is susceptible to primary water stress corrosion cracking. Weld-induced residual stress is main factor for crack growth. Therefore exact estimation of residual stress is important for reliable operating. This paper presents residual stress computation performed by 6” safety & relief nozzle. Based on 2 dimensional and 3 dimensional finite element analyses, effect of welding variables on residual stress variation is estimated for sensitivity analysis. 1. 서 론 최근 V.C Summer 원전 등에서 이종금속용접부 (Dissimilar Metal Weld) 균열이 관찰되었다. (1~3) 균열은 Inconel 계열 용접 금속에서 발생하였으며 일차수응력부식균열(PWSCC)이 균열의 주요 발생 원인이었다. 일차수응력부식균열은 재료의 민감도, 용접부 인장 잔류 응력 및 사용 중 하중(In-service load), 부식 환경과 같은 3가지 인자의 상호작용에 의해 발생한다. (2,4,5)

Assessment of Round Robin Analyses Results on Welding Residual Stress Prediction in a Nuclear Power Plant Nozzle

This paper provides simulational round robin test results for welding residual stress prediction of safety/relief nozzle. To quantify the welding variables and define the recommendation for prediction and determination of welding residual stress, 6 partners in 5 institutes participated in round robin test. It is concluded that compressive axial and hoop residual stress occurs in dissimilar metal weld and pre-existing residual stress distribution in dissimilar metal weld was affected by similar metal weld due to short length of safe end. Although the reason for the deviation among the results was not pursued further, the effect of several key elements of FE analyses on welding residual stress was investigated in this paper.

Effects of Similar Metal Weld on Residual Stress in Dissimilar Metal Weld According to Safe End Length

Nozzle in nuclear power plant is connected to pipe using safe end. Dissimilar metal weld between nozzle and safe end is followed by similar metal weld between safe end and pipe. And thus residual stress in dissimilar metal weld can be affected by similar metal weld. Similar metal weld impose bending stress on dissimilar metal weld, which is according to the length of safe end. In this study, simple nozzle model which covers various radius to thickness ratios was proposed to quantify residual stress in dissimilar metal weld based on finite element analyses. As a result, short length of safe end was proved to be more effective to mitigate residual stress in dissimilar metal weld and critical effective length of safe end is provided according to the radius to thickness ratio.

Effect of Preemptive Weld Overlay on Residual Stress Mitigation for Dissimilar Metal Weld of Nuclear Power Plant Pressurizer

Weld overlay is one of the residual stress mitigation methods which arrest crack initiation and crack growth. Therefore weld overlay can be applied to the region where cracking is likely to be. An overlay weld used in this manner is termed a preemptive weld overlay(PWOL). In pressurized water reactor(PWR) dissimilar metal weld is susceptible region for primary water stress corrosion cracking(PWSCC). In order to examine the effect of PWOL on residual stress mitigation, PWOL was applied to a specific dissimilar metal weld of Kori nuclear power plant by finite element analysis method. As a result, strong compressive residual stress was made in PWSCC susceptible region and PWOL was proved effective preemptive repair method for weldment.

Sensitivity Analysis of Finite Element Parameters for Estimating Residual Stress of J-Groove Weld in RPV CRDM Penetration Nozzle

In nuclear power plants, the reactor pressure vessel (RPV) upper head control rod drive mechanism (CRDM) penetration nozzles are fabricated using J-groove weld geometry. Recently, the incidences of cracking in Alloy 600 CRDM nozzles and their associated welds have increased significantly. The cracking mechanism has been attributed to primary water stress corrosion cracking (PWSCC), and it has been shown to be driven by welding residual stresses and operational stresses in the weld region. The weld-induced residual stress is the main factor contributing to crack growth. Therefore, an exact estimation of the residual stress is important for ensuring reliable operation. This study presents the residual stress computation performed for an RPV CRDM penetration nozzle in Korea. Based on two and three dimensional finite element analyses, the effect of welding variables on the residual stress variation is estimated for sensitivity analysis.

Evaluation for Weld Residual Stress and Operating Stress around Weld Region of the CRDM Nozzle in Reactor Vessel Upper Head

Primary water stress corrosion cracking (PWSCC) has been observed around the weld region of control rod drive mechanism (CRDM) nozzles in nuclear power plants overseas. The weld has a J-shaped groove and it connects the CRDM nozzle with the reactor vessel upper head (RVUH). It is a dissimilar metal weld (DMW), because the CRDM is made of alloy 600 and the RVUH is made of carbon steel. In this study, finite element analysis (FEA) was performed to estimate the stress condition around the weld region. Generally, it is known that a high tensile region is more susceptible to PWSCC. FEA was performed as for the condition of welding, hydrostatic test and normal operation successively to observe how the residual stress changes due to plant condition. The FEA results show that a high tensile stress region is formed around the weld starting point on the inner surface and around the weld stop point on the outer surface.

Environmental Fatigue and Fatigue Monitoring System in Korea

In accordance with the recommendation of USNRC and the U.S. license renewal experiences, the effect of reactor coolant environment on the fatigue life has to be considered for the continued operation of operating nuclear power plants as well as for the design of new plants in Korea. The reason is that it is very important to maintain the structural integrity and reliability of the nuclear power plants against the fatigue failure during operation. Fatigue monitoring system has been considered as a practical way to ensure safe operation of the nuclear power plants in terms of the fatigue. The fatigue monitoring system evaluates various plant conditions and their effects on the monitored location to give quantified value that indicates accumulated fatigue damage up to date. From this, the authors have developed a fatigue monitoring system, named NuFMS (Nuclear Fatigue Monitoring System) in web environment and has been being applied widely to Korean nuclear plants. In this paper, overall configuration and characteristics of the NuFMS are described in detail.Copyright

Evaluation of PWSCC at Dissimilar Metal Butt Welds in NPP

Primary water stress corrosion cracking (PWSCC) instances have been reported in the Alloy 600 reactor pressure vessel head penetration nozzle and the Alloy 82/182 dissimilar metal butt weld nozzle in several PWRs. Therefore, in-service inspection programs have been adopted worldwide to prevent failure at the weld region. If a PWSCC is observed at the dissimilar metal weld region during inspection, its structural integrity should be evaluated; however, this requires considerable time and effort, and this might lead to a decrease in the plant utilization coefficient. To prevent this, KHNP-CRI have established integrity assessment criteria and developed a computer program for the fast evaluation and judgment of PWSCC. In this paper, the results and current status of the same are presented. Through this study, criteria for the structural integrity evaluation of PWSCC have been established, and a computer program has been developed to realize technical means for the evaluation of PWSCC structural integrity.

Effect of Adjacent Safe End to Piping Weld and Preemptive Weld Overlay Repair on Residual Stresses on Nozzle to Safe End Weld

In this study, simplified nozzle geometry was proposed to quantify the effects of adjacent similar metal weld and weld overlay on residual stresses in dissimilar metal weld. Finite element analyses were conducted with various thickness ratios and safe end lengths and corresponding residual stresses were provided. According to the results, residual stresses in dissimilar metal weld were improved after adjacent similar metal welding. The effect of similar metal welding is more evident with shorter length of safe end. Thus, short length of safe end was recommended for new design of nozzle. Appropriate thickness of preemptive weld overlay reduces the conservative thickness recommended by ASME Code.Copyright