Effects of thermal load and cooling condition on weld residual stress in a core shroud with numerical simulation

Stress corrosion cracking (SCC) in the heat affected zone is the primary damage form due to weld residual stress, corrosion and neutron irradiation environment in the core shroud of a boiling water reactor. The distribution of weld residual stress around a weld is necessary to be clarified to evaluate the structural integrity of core shroud for SCC. Moreover, studying the effects of welding parameters on residual stress on reducing the residual stress is very important to suppress the initiation and propagation of SCC.In this paper, we used a finite element method (FEM) to clarify the distribution of weld residual stress around the sixth horizontal weld (H6a) between the lower ring and the cylinder in the core shroud. The simulation results of axial stress were consistent with the experimental results at the inside and outside surfaces of the core shroud, respectively. The effects of thermal loads and cooling conditions were also investigated with the same model. We simulated the welding progress with water cooling on the inside and outside surfaces of the core shroud in order to study the influence of cooling conditions on the residual axial stress around the weld. The simulation results indicated that water cooling decreased the residual axial stress at the same side due to changing the temperature-affected fields. Moreover, with fixing the peak temperatures of weld passes, the simulation results of the distribution of residual axial stress by the thermal loads with different heating time were compared. The simulation results suggested that the heating time was expected to be longer and the heat flux to be smaller for reaching the small tension residual axial stress or even compression stress around the H6a weld.     

Probabilistic analysis of core shroud cracks

Traditional limit load analysis and fracture mechanics analysis have been applied to evaluate the integrity of the degraded nuclear power plant components. Although these methodologies are generally accepted by the regulatory authorities in the nuclear industry, conservatism introduced by the uncertainties of inspection, material property, crack geometry, applied loading, neutron environment, etc. is recognized to have great impact on the evaluation accuracy. A probabilistic analysis may overcome this shortcoming and reveal some additional insight to the problem. The purpose of the present study is to apply probabilistic methods to analyze the degraded core shroud, and to predict the quantitative risk of the cracked shroud. In the analysis, the loading condition, crack growth rate, material properties and existing defects are all considered random. A sample analytical result shows that, based on some previously observed data and under certain assumptions, the crack-through probability of the studied core shroud is in the order of 10⁻⁷ after 13 cycles of operation. The probability will increase considerably through operation cycles or operation years if no repair action is taken.     

Numerical weld modeling — a method for calculating weld-induced residual stresses

In the past, weld-induced residual stresses caused damage to numerous (power) plant parts, components and systems (Erve, M., Wesseling, U., Kilian, R., Hardt, R., Brümmer, G., Maier, V., Ilg, U., 1994. Cracking in Stabilized Austenitic Stainless Steel Piping of German Boiling Water Reactors — Characteristic Features and Root Causes. 20. MPA-Seminar 1994, vol. 2, paper 29, pp.29.1–29.21). In the case of BWR nuclear power plants, this damage can be caused by the mechanism of intergranular stress corrosion cracking in austenitic piping or the core shroud in the reactor pressure vessel and is triggered chiefly by weld-induced residual stresses. One solution of this problem that has been used in the past involves experimental measurements of residual stresses in conjunction with weld optimization testing. However, the experimental analysis of all relevant parameters is an extremely tedious process. Numerical simulation using the finite element method (FEM) not only supplements this method but, in view of modern computer capacities, is also an equally valid alternative in its own right. This paper will demonstrate that the technique developed for numerical simulation of the welding process has not only been properly verified and validated on austenitic pipe welds, but that it also permits making selective statements on improvements to the welding process. For instance, numerical simulation can provide information on the starting point of welding for every weld bead, the effect of interpass cooling as far as a possible sensitization of the heat affected zone (HAZ) is concerned, the effect of gap width on the resultant weld residual stresses, or the effect of the ‘last pass heat sink welding’ (welding of the final passes while simultaneously cooling the inner surface with water) producing compressive stresses in the root area of a circumferential weld in an austenitic pipe. The computer program (finite element residual stress analysis) was based on a commercially available code (Hibbitt, Karlsson, Sorensen, Inc, 1997. user's manual, version 5.6), and can be used as a 2-D or 3-D FEM analysis; depending on task definition it can provide a starting point for a fracture mechanics safety analysis with acceptable computing times.     

Seismic analysis and fracture evaluation of stabilized core shroud

This paper deals with the seismic analysis and fracture evaluation of a stabilized core shroud in a boiling water reactor of nuclear power plant. To study the adequacy of original seismic loadings, the dynamic behaviors of core shrouds with cracks, without cracks and with stabilizers are analyzed. Seismic analysis of a lumped-mass model of reactor internals is then performed to obtain the seismic loadings around various weldments of the repaired core shroud. The interaction between the core internals and this repaired core shroud is thus taken into account in this study. Further, fracture analysis of the stabilized core shroud is performed to obtain the stress intensity factors along the crack front of horizontal welds based on these seismic loadings. The computed results show that the influence of existing cracks in the stabilized core shroud is insignificant on the overall structural integrity.     

Use of a new methodology for prediction of weld distortion and residual stresses using FE simulation applied to ITER vacuum vessel manufacture

The ITER VV Sectors have to be manufactured to extremely tight tolerances, without the production of a prototype to establish the welding distortions. It is thus necessary to develop a (computationally) successful methodology for modelling weld process in order to predict welding distortion and residual stresses. Further, in order to optimise the manufacture sequence and method, the model should be able to rapidly assess the effect of using different sequences. Once the optimal sequence has been decided, then the method has also to be able to accurately predict the final shape.     

Remaining life prediction of the core shroud due to stress corrosion cracking failure in BWRs using numerical simulations

Stress corrosion cracking (SCC) of the welded joints in a reactor core shroud is the primary result of the residual stresses caused by welding, corrosion and neutron irradiation in a boiling water reactor (BWR). Therefore, the evaluation of SCC propagation is important for the safe maintenance of the core shroud. This paper attempts to predict the remaining life of the core shroud due to SCC failures in BWR conditions via SCC propagation time calculations. First, a two-dimensional finite element method model containing H6a girth weld in the core shroud was constructed, and the weld processing was simulated to determine the weld's residual stress distribution. Second, using a basic weld residual stress field, the SCC propagation was simulated using a node release option and the stress redistribution was calculated. Combined with the J-integral method, the stress intensity factors were calculated at depths of 2, 3, 4, 8, 12, 16, 19, 22, 25 and 30 mm in the crack setting inside the core shroud; then, the SCC propagation rates were determined using the relation between the SCC propagation rate and the stress intensity factor. The calculations show that the core shroud could safely remain in service after 9.29 years even when a 1-mm-deep SCC has been detected.     

Allowing for the effects of residual stresses on creep in channel tubes

Conclusions The method enables one to calculate the creep strain in reactor structures with allowance for the relaxation of the residual stresses distributed over the wall thickness in the presence of radiation and temperature fields. The result for RBMK channel tubes is that the residual stresses have only a minor effect on the strain in prolonged operation. However, in short-time tests, this factor may appreciably distort the results and may even alter the sign of the strain. The internal stresses may also make themselves felt when one measures the strain due to radiation growth in zirconium-alloy specimens.Translated from Atomnaya Énergiya, Vol. 58, No. 1, pp. 13–17, January, 1985.     

Non-destructive detection and analysis of residual and loading stresses in thick-walled components

Components and systems are designated to withstand loads. These loads are primarily mechanical ones, additionally thermal, chemical and other ones. The constructed part has to support these loads with safety margins. Additionally, peak values for stresses have to be avoided. During the last ten years the theoretical background as well as the numerical evaluation were developed and introduced in practice. Especially finite-element methods at least for loading stresses are the common aid of the designing engineer. The experimental verification of calculated stresses and stress distributions fails up to now. There is no experimental measurement of stress at all. For strain measurements - restricted to elastic strains and to the surface region, not exceeding some ten micrometres - only the X-ray technique allows static and dynamic measurements. Strain variations by changes of loads or stresses can be measured with strain gages, optical holographical interferometry and other methods - also restricted to the surface only. Nothing is known about the volume-distribution of strains and stresses, and for scientists and engineers it is a surprising phenomenon how practicians here rely on theories and calculations. During the last five years several approaches were developed as non-destructive methods for strain/stress-measurements not only on the surface but in the interior, too. The state of the art is described in this contribution covering ultrasonic as well as micromagnetic methods. Examples are given from research results and applications in practice. The most important literature differentiated referring to physics, review contributions and applications is listed in an appendix.     

Determination of global and local residual stresses in SOFC by X-ray diffraction

Solid Oxide Fuel Cell (SOFC) is a high-performance electrochemical device for energy conversion. A single cell is composed of five layers made of different ceramic materials: anode support, anode functional layer, electrolyte, cathode functional layer and cathode. The mechanical integrity of the cell is a major issue during its lifetime, especially for the electrolyte layer. Damage of the cells is mainly due to the high operating temperature, the “redox” behaviour of the anode and the brittleness of the involved materials. Since residual stresses are known to play a significant role in the damage evolution, it is important to determine them.For this purpose, residual stresses in an anode-supported planar SOFC were measured by X-ray diffraction. Firstly, macroscopic stresses in each phase of each layer were studied using the sin²ψ method on a laboratory X-ray goniometer at room temperature. This technique enables the calculation of residual stress of the material from the measurement of the crystal lattice deformation. The electrolyte has been found under bi-axial compressive stress of ?920 MPa. Secondly, X-ray measurements controlling depth penetration were made in the electrolyte using grazing incidence method. The results show that the stress is not homogenous in the layer. The first five micrometers of the electrolyte have been found less constrained (?750 MPa) than the complete layer, suggesting a gradient of deformation in the electrolyte from the interface with the Anode Functional Layer to the free surface. Finally, local stress measurements were made on the electrolyte layer by X-ray synchrotron radiation that allows high accuracy measurement on the (sub-) micrometer scale. Polychromatic and monochromatic beams are used to determine the complete strain tensor from grain to grain in the electrolyte. First results confirm the macroscopic stress trend of the electrolyte. These X-ray techniques at different scales will contribute to a better understanding of the residual stress in the electrolyte layer and thus to the involved damage mechanisms.     

Pressure wave propagation in an LWR downcomer and resultant load on the core shroud due to rupture of a primary coolant loop

Pressure differences and the resultant dynamic load act on the core shroud when pressure waves propagate in the downcomer of a light water reactor (LWR) pressure vessel after rupture of the primary pipe has occurred. An equivalent geometry, i.e. a diverging duct is used to solve by Euler and wave equation for acceleration and velocity of the fluid behind the wave front, that the two-dimensional, time-dependent pressure distribution, induced by the wave propagation, can be calculated. The assumptions lead to an approximate but conservative value of the resultant core shroud load.     

The effect of residual stresses on hydride orientation in a zirconium-2.5% niobium alloy

The distribution of residual stress in quadrants of Zr?2.5% Nb tube after flattening has been determined by measurement of the change in curvature during chemical thinning. The ‘zoning’ of hydride precipitate that occurs in such material is then explained in terms of the residual stresses and the elimination of such ‘zoning’ by a prior stress-relief anneal or plastic straining is examined.     

Core shroud replacement of Fukushima-Daiichi Unit #3

The core shroud replacement of a boiling water reactor (BWR) was successfully completed at Fukushima-Daiichi Unit #3 (1F3) of the Tokyo Electric Power Company (TEPCO) in Japan. The core shroud and other core internal components made of type 304 stainless steel (SS) were replaced with the ones made of low carbon type 316L SS to improve Intergranular Stress Corrosion Cracking (IGSCC) resistance. This project was the first application of the replacement procedure developed for the welded core shroud, and employed various advanced technologies. The outline of the core shroud replacement project and applied technologies are discussed in this paper.     

Steady-state stresses in concrete structures subjected to sustained and cyclically varying temperatures

The state of stress in non-uniformly heated concrete structures is significantly influenced by creep. When temperatures are sustained or vary in a cyclic manner over long periods, the long term state of stress may be deduced by direct calculation and without knowledge of the previous history of stress. The long-term stress states which correspond to cyclic variations of temperature are termed steady-state-cyclic stresses to distinguish them from the steady-state stresses which relate to sustained temperatures. Theory and numerical examples are presented and it is concluded that a power dissipation formulation of the theory is appropriate for general analyses for which compatibility criteria cannot be specified in advance. Stress redistribution is less severe under cyclic conditions of temperature than when temperatures are long sustained.     

Numerical simulation of residual stresses induced by laser beam welding in a SUS316 stainless steel pipe with considering initial residual stress influences

With the development of computer hardware and software, numerical simulation technology has been widely used to predict welding temperature field, residual stresses and distortion. However, till now the influences of initial stresses induced by the manufacturing process before welding on the welding-induced residual stresses are rarely investigated experimentally and numerically. In the present work, we have developed a computational approach based on thermal elastic plastic FEM to clarify how the initial stresses due to heat treatment affect the welding-induced residual stresses in an austenitic stainless steel pipe. A heat treatment process, which is similar to solution heat treatment, is employed to produce initial stresses in the pipe before welding. After the heat treatment, the laser beam welding is used to perform a girth weld in the middle of the pipe. Through comparing the residual stress distributions after heat treatment and laser beam welding, we have investigated the influence of the initial residual stresses on the welding-induced residual stresses. The numerical results suggest that the initial residual stresses prior to welding have significant effects on the residual stresses after welding in the pipe model.     

CLAM钢基体表面Al2O3功能梯度涂层残余应力模拟分析

在ITER中国液态锂铅测试包层模块设计中拟采用Al2O3作为防氚渗透耐蚀绝缘涂层,为了防止涂层与CLAM钢基体间由于材料热膨胀不匹配而开裂失效,采用热应力缓和型功能梯度涂层。利用有限元分析软件ANSYS研究了Al2O3/CLAM钢系功能梯度涂层的残余应力分布。模拟结果表明,当成分分布指数p=0.8、层数N=6、梯度层厚度为H=0.6 mm时,应力缓和效果最优。此分析结果可为该涂层制备工艺的优化提供理论指导。     

Thermo-mechanical FE-analysis of residual stresses and stress redistribution in butt welding of a copper canister for spent nuclear fuel

The transient and residual temperature, stress and strain field present during electron beam welding of a plane copper end to a canister for spent nuclear fuel is calculated by the use of FEM. The subsequent stress redistribution is calculated up to 10,000 years. The canister consists of two concentric cylinders, an inner steel cylinder containing the spent nuclear fuel and an outer copper cylinder. It was found that the maximum plastic strain (plastic+creep) accumulated in the (possibly brittle) heat affected zone is ≈7%, which seems to be well below the reported ductility for the copper used.     

Improvement in accuracy of the measurements of residual stresses due to circumferential welds in thin-walled pipe using Rayleigh wave method

To achieve an acceptable safety in many industrial applications such as nuclear power plants and power generation, it is extremely important to gain an understanding of the magnitudes and distributions of the residual stresses in a pipe formed by joining two sections with a girth butt weld. Most of the methods for high-accuracy measurement of residual stress are destructive. These destructive measurement methods cannot be applied to engineering systems and structures during actual operation. In this paper, we present a method based on the measurement of ultrasonic Rayleigh wave velocity variations versus the stress state for nondestructive evaluation of residual stress in dissimilar pipe welded joint. We show some residual stress profile obtained by this method. These are then compared with other profiles determined using a semi-destructive technique (hole-drilling) that makes it possible to check our results. According to the results, we also present a new method for adjusting the ultrasonic measurements to improve the agreement with the results obtained from other techniques.