Probabilistic Assessment of CANDU Reactor Core for Risk of Pressure Tube Failure due to Presence of In-Service Flaws

Technical requirements for analytical evaluation of in-service Zr-2.5Nb pressure tubes in CANDU(1) reactors are provided in the Canadian Standards Associate (CSA) N285.8. The evaluation must address all in-service degradation mechanisms including the presence of in-service flaws. Flaws found during in-service inspection of CANDU Zr-2.5Nb pressure tubes, including fuel bundle scratches, debris fretting flaws, fuel bundle bearing pad fretting flaws, dummy bundle bearing pad fretting flaws, erosion-shot flaws and crevice corrosion flaws, are volumetric and blunt in nature. These in-service flaws can become crack initiation sites during pressure tube operation and potentially lead to pressure tube failure. Any detected flaws that do not satisfy the criteria of acceptance as per CSA N285.4 must be analytically evaluated to justify continued operation of the pressure tube. Moreover, the risk of pressure tube failure due to presence of in-service flaws in the entire reactor core must be assessed. A review of assessment of the risk of pressure tube failure due to presence of in-service flaws in CANDU reactor core is provided in this paper. The review covers the technical requirements in the CSA N285.8 for evaluating degradation mechanisms related to flaws in the reactor core. Current Canadian industry practice of probabilistic assessment of reactor core for pressure tube failure due to presence of in-service flaws is described, including evaluation of flaws for crack initiation, subsequent crack growth to through-wall penetration, and pressure tube rupture due to unstable crack growth prior to safe shutdown of the reactor. Operating experience with the application of probabilistic assessment of reactor core for the risk of pressure tube failure due to presence of in-service flaws is also provided.

Verification of Probabilistic Fracture Mechanics Analysis Code PASCAL Through Benchmark Analyses With FAVOR

A probabilistic fracture mechanics (PFM) analysis code called PASCAL has been developed by the Japan Atomic Energy Agency to evaluate failure frequencies of Japanese reactor pressure vessels (RPVs) during pressurized thermal shock (PTS) events based on Japanese data and Japanese methods published for or provided in Japanese codes and standards. To verify this code, benchmark analyses were carried out using the FAVOR code, which was developed in the United States and has been utilized in nuclear regulation. The results of these analyses confirmed with great confidence the applicability of PASCAL to failure probability and frequency evaluation of Japanese RPVs. An outline of PASCAL, the benchmark analysis conditions and analysis results are reported in this paper.

Corrosion Behavior of X70 Pipeline Steel and Corrosion Rate Prediction Under the Combination of Corrosive Medium and Applied Pressure

Natural gas transmission pipeline is prone to internal corrosion due to the combination of corrosive impurities in the pipe (such as CO2, H2S and chlorides) and applied pressure of the pipeline, which seriously affects the safe operation of the pipeline. In this work, the corrosion behavior of a typical X70 pipeline steel was investigated by using potentiodynamic polarization and electrochemical impendence spectroscopy (EIS). The polarization and EIS data under different CO2 partial pressures (0–1 atm), H2S concentrations (0–150 ppm), chloride concentrations (0–3.5 wt%) and tensile stress (0–400 MPa) were obtained. The results show that corrosion rate increases with the increase of CO2 partial pressure and chloride concentration, respectively, while first increases and then decreases with the increase H2S concentrations. The corrosion rate is less affected by elastic tensile stress. In addition, a quantitative prediction model for corrosion rate of natural gas pipeline based on adaptive neuro-fuzzy inference system (ANFIS) was established by fitting the experimental data which maps the relationship between the key influencing factors (i.e. CO2 partial pressure, H2S concentration, chloride concentration and tensile stress) and the corrosion rate. The prediction results show that the relative percentage errors of the predicted and experimental values are relatively small. The prediction accuracy of the model satisfies the engineering application requirement.

Numerical Simulation of Local Post Weld Heat Treatment by Electric Heating Method of Pressure Equipment

The local post weld heat treatment by electric heating method is widely used to eliminate welding residual stress in processes of manufacture and maintenance of pressure equipment. The key point of local post weld heat treatment is to choose a reasonable heated band width and insulated band width. But the criterions to determine the minimum heated band width and insulated band width are different according to Chinese, European and American standards, which are GB/T 30583-2014, EN 13445-4: 2009 and AWS D10.10/D10.10M :1999, respectively. Taking the local post weld heat treatment for the circumferential butt weld between two thick cylinders both with a 115 mm thickness as an example, numerical simulation is used to compare the wall temperature distribution of the cylinders during the heat preservation stage when the heated band width and insulated band width are chose according to the above three standards, and the numerical simulation was verified by the tested temperature from one field experiment. The results show that the numerical calculation method can accurately predict the wall temperature of the cylinders during the local heat treatment, and the wall temperature of the surfaces on which the heaters are arranged according to the three standards all well meets the requirement of the heat treatment, but the wall temperature of the surfaces without the heaters cannot meet the temperature requirement. So double-side heating and double-side insulating are suggested to be adopted during local post weld heat treatment.

Case Study of Shakedown Evaluation of a Shell With Nozzle Based on Elastic-Plastic Analysis

In the past, shakedown evaluation was usually based on the elastic method that the sum of the primary and secondary stress should be limited to 3Sm or the simplified elastic-plastic analysis method. The elastic method is just an approximate analysis, and the rigorous evaluation of shakedown normally requires an elastic-plastic analysis. In this paper, using an elastic perfectly plastic material model, the shakedown analysis was performed by a series of elastic-plastic analyses. Taking a shell with a nozzle subjected to parameterized temperature loads as an example, the impact of temperature change on the shakedown load was discussed and the shakedown loads of this structure at different temperature change rates were also obtained. This study can provide helpful references for engineering design.

Effect of Mechanical Model on Limit Load Analysis of High Pressure Heater Tubesheet

Tubesheet is the main part of high pressure heater, which is very thick based on chinese code GB151 for the design of heat exchangers. Increased tubesheet with large thermal stress are not conducive to manufacture, heat transmission and detection. The stress and structure of tubesheet are so complex that the time costs too large during the analysis design, and stress classification exists uncertainty. Limit load method contributes to tubesheet lightweight. 3-D finite element model used for analysis design should be simplified reasonably. In this paper, the effect of mechanical model on limit load analysis of high pressure heater tubesheet conforming to the design-by-analysis code is researched. It is found that the tubesheet could pass the plastic collapse assessment, and the thickness of tubesheet could be decreased. The difference between the equivalent sold tubesheet model and the whole tubesheet model exists during plastic collapse assessment. Though the local stress distribution is different, the limit load results occurred plastic collapse by the equivalent sold tubesheet model is close to that by the whole tubesheet model. The limit load occurred plastic collapse is influenced by max circular diameter of tube layout little. The reason is attributed to original tubesheet owning enough rigidity related to thickness, and high stress appeares on the inner wall of jointing of tubesheet with head. The equivalent sold tubesheet model could be used for primary evaluation of limit load, and the whole tubesheet model is suited for partial analysis. The results provide some reference for the design-by-analysis of high pressure heater tubesheet.

Numerical Simulation of the Four-Roll Bending Process for 2.25Cr-1Mo-0.25V Thick-Plate at Elevated Temperature

In this paper, the four-roll plate bending process of 2.25Cr-1Mo-0.25V steel at elevated temperature is investigated by numerical simulation. This 3-D simulation is finished by using the elastic-plastic dynamic explicit finite element method (FEM) under the ANSYS/LS-DYNA environment. The strain softening behavior of 2.25Cr-1Mo-0.25V steel at elevated temperature is presented and discussed. The stress-strain relationship of the steel plate is modeled using a piecewise linear material model, with the stress-strain curve obtained through tensile tests. The plate bending process with a plate thickness of 150 mm is investigated. The amount and position of maximum plastic deformation are analyzed. The present study provides an important basis for the optimization of bending parameters and further investigation of the effect of high-temperature deformation on the resistance to hydrogen attack of 2.25Cr-1Mo-0.25V steel.

Current Status of the ASME Materials Properties Database Development

The ASME Materials Properties Database has been under development in the past few years to support the ASME Codes and Standards under the supervision of the Boiler and Pressure Vessel Code Committee on Materials. With the guidance of its Working Group on Materials Database, the project has completed the Phase I development for the Data File Warehouse that offers a depository for various files containing ASME Code Week records, materials test data from codification inquiries, and information associated with code rules development. While the database is in operation, the development has continued into Phase II to create a relational Digital Database that offers customized and relational schemas with advanced software functionalities and tools facilitating digital data processing and management. This paper discusses the current status of the project including its development management, database components and features, business operation, and future growth. Some issues and prospective resolutions for meeting the needs and requirements from Codes and Standards are also discussed.

Load and Resistance Factor Design of Dual Criterion Against Gross Plastic Deformation and Collapse

ASME VIII-2 code [1] and EN 13445 standard [2] give the design criterion against plastic collapse and gross plastic deformation respectively. Duan et al. [3] propose a new criterion combining the advantages of both. This paper introduces two characteristics of the new criterion, and points out that it is a dual criterion against gross plastic deformation and collapse. The Load and Resistance Factor Design form of the new criterion and an application example are given.

Impact of Undetected Fabrication Flaws on LBB Risk

On-going assessments of the impact of active degradation mechanisms in US nuclear power plants previously approved for leak before break (LBB) relief are being performed with, among other methods, the extremely low probability of rupture (xLPR) code being developed under a memorandum of understanding between the US Nuclear Regulatory Commission (US NRC) and the Electric Power Research Institute (EPRI) [1]. This code finished with internal acceptance testing in July of 2016 and is undergoing sensitivity and understanding analyses and studies in preparation for its general release. One of the key components of the analysis tool is the integration of inspection methods for damage and the impact of leak detection on the risk of a pipe rupture before the pipe is detected to be leaking. While it is not impossible to detect a crack or defect that is less than 10% of the pipe wall thickness current ASME code does not give credit for inspections identifying a crack of this size. This study investigates the impact of combining the probabilistic analysis results from xLPR with a pre-existing flaw to determine the change, if any, to the risk. Fabrication flaws were found to have a statistically significant impact on the risk of rupture before leak detection.