Itaru Muroya

Residual Stress Evaluation of Dissimilar Weld Joint Using Reactor Vessel Outlet Nozzle Mock-Up Model (Report-1)

In recent years, the occurrence of primary water stress corrosion cracking (PWSCC) in Alloy 600 weld regions of PWR plants has increased. In order to evaluate the crack propagation of PWSCC, it is required to estimate stress distribution including residual stress and operational stress through the wall thickness of the Alloy 600 weld region. In a national project in Japan for the purpose of establishing residual stress evaluation method, two test models were produced based on a reactor vessel outlet nozzle of Japanese PWR plants. One (Test model A) was produced using the same welding process applied in Japanese PWR plants in order to measure residual stress distribution of the Alloy 132 weld region. The other (Test model B) was produced using the same fabrication process in Japanese PWR plants in order to measure stress distribution change of the Alloy 132 weld region during fabrication process such as a hydrostatic test, welding a main coolant pipe to the stainless steel safe end. For Test model A, residual stress distribution was obtained using FE analysis, and was compared with the measured stress distribution. By comparing results, it was confirmed that the FE analysis result was in good agreement with the measurement result. For mock up test model B, the stress distribution of selected fabrication processes were measured using the Deep Hole Drilling (DHD) method. From these measurement results, it was found that the stress distribution in thickness direction at the center of the Alloy 132 weld line was changed largely during welding process of the safe end to the main coolant pipe.Copyright

Fracture toughness transition curve estimation from a notched round bar specimen using the local approach method

The local approach method for the brittle or transition region describes the fracture probability of specimens or structures using Weibull statistics. Many papers have discussed the characteristics of the Weibull parameter using notched tensile specimens and the applicability to fracture toughness scatter evaluation using CT specimens. However few papers have made clear whether the Weibull parameter of the Weibull stress is a material property or not. In this paper the distribution of Weibull stress in the brittle fracture region using notched round bar specimens and CT specimens were investigated and it was confirmed that both distributions agreed well. Furthermore the estimation method for the fracture toughness transition curve including its scatter from notched round bar tensile tests was proposed based on the relation between the Weibull stress and the Wallins fracture toughness transition curve. As a result, the estimated fracture toughness curve in the brittle and lower transition region from the notched round bar specimens coincided with the measured fracture toughness curve from CT specimens. This method will be applicable to fracture toughness curve estimation under plane strain conditions even if there is no possibility of obtaining thick enough CT specimens from a structure because of geometry or some other restrictions.

Measurement of Residual Stresses in the Dissimilar Metal Weld Joint of a Safe-End Nozzle Component

This paper presents results from a programme of residual stress measurements and modelling carried out on a Pressurised Water Reactor Safe-end Nozzle component. The full-scale Safe-end Nozzle component was manufactured to the same specifications as those typically found on Japanese Pressurised Water Reactors. The basic component consisted of a ferritic steel nozzle with a tapered outer diameter (OD) ranging from 883mm to 1192mm, an inner diameter (ID) of 735mm and a length of roughly 1080mm. A stainless steel ring (i.e. the safe-end) of 883mm OD, 735mm ID and length 100mm was attached to the ferritic steel nozzle using a double-V nickel base alloy (i.e. alloy 132) weld with buttering. Later on in manufacturing a stainless steel, main coolant piping section (883mm OD, 735mm ID and 500mm length) was then attached to the safe-end using a single-V stainless steel weld. The residual stresses generated through the centre-line of the double-V weld connecting the stainless steel safe-end to the ferritic steel nozzle were measured using the Deep-Hole Drilling (DHD) and inherent strain techniques. The residual stresses generated by welding were modelled using ABAQUS. Presented here are the DHD measurements from six locations circumferentially around the weld made at three different stages during the manufacture and testing of the component. The DHD measurements are compared against those measured on a similar component using the totally destructive inherent strain technique and those modelled using finite element analysis. Details of the FE modelling carried out for this project are to be presented in another paper at this conference (PVP 2009-77269). The measured and modelled results are also compared against the UK based BS7910 and R6 standards. It is shown that there is excellent agreement between the DHD, inherent strain and modelling results in the as-welded state, showing peak tensile stresses at the inner and outer weld cap surfaces, reducing into compression in the centre at the meeting of the double-V grooves. It is also shown with the DHD measurements that after attaching the main coolant piping, the peak tensile residual stresses present at the inner surface in the hoop and axial directions changed to become compressive. Furthermore, following hydrostatic and operating condition tests, the DHD measured residual stresses at the inner surface were shown to move towards tension again, with the axial residual stresses remaining slightly compressive, but the hoop residual stresses becoming slightly tensile. The residual stresses generated at the outer surface were relatively unchanged by the manufacturing and operating processes carried out.Copyright

Low alloy steel piping test for fracture criteria of leak before break

A total of seven pipe fracture tests were performed to provide the data for establishing fracture criteria of leak before break for the low alloy steel pipe, which is expected to be applied to reactor coolant piping and feedwater piping in advanced PWRs in Japan. Test pipes were 6-inch and 8-inch diameter pipes made of SFVQ1A or STPA24 low alloy steel. A circumferential through-wall crack was introduced at the center of a pipe, and four-point bending load was applied without internal pressure. Stable crack extensions were observed in all of the experiments. The net-section criteria (NSC), R6 method option 2 and option 3 were used to estimate the maximum applied load. The predicted values by three kinds of evaluation methods were compared with the experimental loads. Most of the predicted maximum loads agreed well with the experimental maximum loads within 20% difference. The NSC gave the most accurate prediction but also gave unconservative results in some test cases. The predicted maximum loads by R6 option 2 were conservative in all of the test cases. From the viewpoint of conservativeness R6 method can be used for evaluation of the low alloy steel pipe fracture. Therefore, the leak before break (LBB) concept could be applied to the protective design standard against pipe break for the material.

Fracture Evaluation of Ni-Based Alloy Weld Joint of Cylindrical Model Subjected to 4-Point Bending or Inner Pressure

For the purpose of establishing fracture evaluation method of nickel based alloy weld of nuclear power plants, fracture tests using pipe models (8B and 14B for bending, 12B for inner pressure) with an alloy 132 weld joint have been performed at room temperature and high temperature (325°C). The predicted loads calculated by limit load evaluation method using the measured and code regulated flow stresses were compared with the maximum test loads. And the predicted bending loads of the pipes at 325°C (8B and 14B) and at room temperature (8B) with the initial surface crack whose depth is 75% of the pipe thickness were in good agreement with the maximum test loads. Also the predicted inner pressure of the pipe at room temperature (12B) agreed with the measured maximum pressure. Only for one case of the 14B pipe subjected to the bending load at room temperature, the predicted load by limit load evaluation method has 20% unconservative difference from the measured data, on the other hand, the predicted load by J-T analysis made this difference smaller and conservative.Copyright

Structural Evaluation for Repaired J-Weld Portion of Reactor Vessel Head Penetration

Leakage was found in a Reactor Vessel (RV) Head Penetration of Ohi unit 3 of the Kansai Electric Power Co., Inc. in May 4, 2004. Non-destructive examinations identified flaws in a J-weld portion of the Head Penetration. The J-weld portion was repaired by using Embedded Flaw Repair Technique [1] that performs welding of 52 weld metal on the J-weld surface remaining the flaws. In order to show the structural integrity of the J-weld portion, a fracture mechanics evaluation was performed in accordance with the Rules on Fitness-for-Service for Nuclear Power Plants of the JSME Codes for Nuclear Power Generation Facilities, JSME S NA1-2002 [2] (hereafter, the JSME Fitness-for-Service Rules) and literatures related. The flaw was characterized as both case of an embedded flaw and a surface flaw and KI for each flaw was directly calculated by using FE analysis. Fatigue crack growth analysis using KI calculated showed the amount of the crack growth was quite small. The fracture mechanics evaluation followed confirmed that the result satisfied the criteria. This paper explains the method and results for evaluation of the structural integrity of the J-weld portion.Copyright

Investigation on Fatigue Curve Against Cyclic Loads of an Earthquake for Piping Components

In Japan, seismic assessments considered aging phenomena have been conducted as part of activities for aging measures of nuclear power plants. In the activities, it is found that FAC (Flow Accelerated Corrosion) of piping components made of carbon steel impact on seismic integrity because of decreasing a cross section of piping. Therefore, many destructive tests of piping have been conducted in Japan and the other countries.At first, factors on which did not focus in the past destructive test have been extracted in this investigation. displacement controlled cyclic tests of piping components which focused on extracted factors have been conducted additionally in order to provide enough test data for past destructive test data of piping components. Moreover, fatigue curve for practical evaluation of piping components made of carbon steel has been settled after obtained test data were put in order including past destructive test data of piping components from a point of fatigue.Copyright

Sensitivity Analysis of Residual Stress Simulation of Dissimilar Metal Joint of Safe End Nozzle and Key Issues for Standard Procedure to Maintenance Rules

For evaluation of PWSCC crack extension behavior detailed residual stress evaluation such a stress distribution on the evaluation points’ thickness is needed. In order to establish the residual stress evaluation procedure, Japan Nuclear Energy Safety Organization (JNES) had a seven years’ project from 2001 to 2007. In this project several sensitivity analyses were performed for understanding key points for the numerical simulation of residual stress. And also residual stress measurements were carried out for validation of the simulation methods. In this paper focused on the safe end nozzles, the results of the sensitivity analyses and the measurement result of the residual stress of the U-groove weld joint were introduced. Based on these result, the key issues of a guideline of a standard residual stress analysis procedure was proposed.Copyright

Propose of Simplified Stress Intensity Factor Equation for SCC Extension of the Pipe Welds

Alloy 600 weld joints have potential for primary water stress corrosion cracks (PWSCC). At the present time it has been understood that PWSCC generates and propagates in the Alloy 600 base metal and the Alloy 600 weld metal and there has been no observation of cracking the stainless and the low alloy steel. For the life time evaluation of the pipes or components the crack extension analysis is required. To perform the axial crack extension analysis the stress intensity database or estimation equation corresponding to the extension crack shape is needed. From the PWSCC extension nature mentioned above, stress intensity factors of the conventional handbooks are not suitable because most of them assume a semi-elliptical crack and the maximum aspect ratio crack depth/crack half length is one (The evaluation in this paper had been performed before API 579-1/ASME FFS was published). Normally, with the advance of crack extension in the thickness direction at the weld joint, the crack aspect ratio exceeds one and the K-value of the conventional handbook can not be applied. Even if those equations are applied, the result would be overestimated. In this paper, considering characteristics of PWSCC’s extension behavior in the welding material, the axial crack was modeled in the FE model as a rectangular shape and the stress intensity factors at the deepest point were calculated with change of crack depth. From the database of the stress intensity factors, the simplified equation of stress intensity factor with parameter of radius/thickness and thickness/weld width was proposed.Copyright

J Resistance Curve Estimation of Inhomogeneous CT Specimen

In order to obtain the fracture toughness curve of inhomogeneous CT specimens, a simplified J-R curve estimation method has been proposed. To verify the applicability of this method, the fracture toughness test and the finite element analysis has been conducted. In overmatching case (mismatch ratio M = 2.2), the conventional ASTM standard’s J-R curve exceeded the J-R curve from the FE analysis in the plane strain condition by over 20%. On the other hand, the simplified J-R curve was located between J-R curves from the FE analyses in plane strain and plane stress condition. In undermatching case (M = 0.5), experimental J-R curves with and without the inhomogeneity effect were almost same and the conventional standard is applicable.Copyright