Study on Applicability of Master Curve Methodology Using Miniature C(T) Specimen to a Reactor Pressure Vessel With Low Upper Shelf Energy

Miniature compact tension (Mini-C(T)) specimen can be an effective tool by utilizing together with Master Curve (MC) methodology for fracture toughness evaluation of irradiated reactor pressure vessel (RPV) steels. Recently, Mini-C(T) specimen has been incorporated into the Japanese standard test method related to MC methodology, JEAC4216-2015 and several studies were found focusing on applicability of Mini-C(T) specimen to irradiated RPV materials. However, there exist some other issues to be resolved considering application to irradiated materials. One of them is violation against the limitation criteria for ductile crack growth (DCG) specified in the standards. In general, upper shelf energy (USE) of RPV materials tends to decrease as well as shift in Charpy transition temperature due to neutron irradiation embrittlement. It may cause reduction in resistance of material against DCG and this leads to the problem peculiar to low USE materials such that the limitation for DCG might be dominant rather than that for KJclimit. Therefore, it is possible to fail to obtain valid KJc data even within valid temperature range of MC methodology, i.e. −50°C ≤ T-To ≤ 50°C, for low USE materials using Mini-C(T) specimens. In this study, the RPV steel with USE lower than 68J was made simulating reduction of USE due to neutron irradiation. Fracture toughness tests were performed using Mini-C(T) specimens as well as the standard 1T-C(T) specimens. Based on the test results, the validity for DCG limitation was also evaluated for each datum by post-test observation of fracture surface. Additionally, effectiveness of added side grooves and double thickness of specimen was examined as a countermeasure for Mini-C(T) specimen.

Upper Shelf Energy Prediction Model for Irradiated Reactor Vessel Steels

Current USA regulations in 10 CFR 50, Appendices G & H ensure adequate fracture toughness and provide for the monitoring of radiation embrittlement of the ferritic components of the reactor pressure vessel (RV). Regulatory Guide (RG) 1.99, Rev. 2 provides guidance on acceptable methods for predicting the effects of neutron irradiation in order to meet the requirements of Appendix G. Specifically, RG 1.99, Rev. 2 provides an embrittlement prediction model for Charpy transition temperature shift (TTS) and a prediction model for decreased Charpy upper shelf energy (USE). The prediction model for USE decrease has remained unchanged since introduction of RG 1.99 in 1975. The objective of this study is to present new USE prediction model(s) developed using an international light water reactor database similar to the effort behind the recently-updated ASTM E900-15 TTS prediction model. A database of ASME and similar specification USE decrease information was developed from USA and select international light water reactor surveillance capsule data, including the latest surveillance capsule fluence, irradiation temperature, material chemistry and other information. The USE database has more than 1,500 USE change measurements of irradiated RV steels. Several best estimation models to predict irradiated USE of materials were developed based on data fitting. Two types of best estimation models were investigated; one model type uses the ASTM E900-15 predicted TTS as a primary input parameter, while the other does not, so that a USE prediction could be made independently of the ASTM E900-15 TTS prediction. By using the ASTM E900-15 TTS as a primary input, the models of the first type implicitly considered the embrittlement mechanisms of matrix damage and copper rich precipitation. In the non-TTS models, the effect of copper was expressed by a hyperbolic tangent curve that has both an upper value and lower value in order to consider the effect of copper saturation. Associated standard deviations as a function of predicted USE were also established so that bounding predictions could be made. Bounding models from each type that conservatively predict irradiated USE by bounding at least 95% of the USE decrease data in the database were identified. These bounding models are estimated to have relatively low impact on the number of USA plants that are projected to have RV steels that drop below 50 ft-lbs (68 J) relative to RG 1.99, Rev. 2. Finally, a non-TTS model was selected as the recommended model, because it does not require calculation of TTS by ASTM E900-15 and thus is simpler to implement.

Estimating J-R Curve From CVN Upper Shelf Energy and its Application

Accuracy in predictions of burst pressures for cracks in pipelines has significant impact on the pipeline integrity management decisions. One of the fracture mechanics models used for failure pressure prediction is API 579 Level 3 FAD ductile tearing instability analysis that requires J-R curves, i.e., crack resistance curves, for the assessment. However, J-R curves are usually unavailable for most pipelines. To overcome this technical barrier, efforts have been made to estimate the J-R curve indirectly from commonly available toughness data, such as the Charpy V-notched Impact Energy CVN values, by correlating the upper-shelf CVN value (energy) to the ductile fracture resistance J-R curve. In this paper, the theoretical background and studies made by various researchers on this topic are reviewed. Attempts made by the present study to establish correlations between CVN and J-R curves for linepipe materials are then presented. Application of this CVN-JR correlation to API 579 Level 3 FAD tearing instability assessment for failure pressure predictions is demonstrated with examples. The accuracy of the correlation is analyzed and reported.

Brittle Fracture Arrest in High Charpy Energy Steels: Comparisons With Some Existing Data

Current line-pipe steels have significantly higher Charpy upper-shelf energy than older steels. Many newer line-pipe steels have Charpy upper-shelf energy in the 300 to 500J range, while older line-pipe steels (pre-1970) had values between 30 and 60J. With this increased Charpy energy comes two different and important aspects of how to predict the brittle fracture arrestability for these new line-pipe steels. The first aspect of concern is that the very high Charpy energy in modern line-pipe steels frequently produces invalid results in the standard pressed-notch DWTT specimen. Various modified DWTT specimens have been used in an attempt to address the deficiencies seen in the PN-DWTT procedure. In examining fracture surfaces of various modified DWTT samples, it has been found that using the steady-state fracture regions with similitude to pipe burst test (regions with constant shear lips) rather than the entire API fracture area, results collapse to one shear area versus temperature curve for all the various DWTT specimens tested. Results for several different materials will be shown. The difficulty with this fracture surface evaluation is that frequently the standard pressed-notch DWTT only gives valid transitional fracture data up to about 20-percent shear area, and then suddenly goes to 100-percent shear area. The second aspect is that with the much higher Charpy energy, the pipe does not need as much shear area to arrest a brittle fracture. Some analyses of past pipe burst tests have been recently shown and some additional cases will be presented. This new brittle fracture arrest criterion means that one does not necessarily have to specify 85-percent shear area in the DWTT all the time, but the shear area needed for brittle fracture arrest depends on the pipeline design conditions (diameter, hoop stress) and the Charpy upper-shelf energy of the steel. Sensitivity studies and examples will be shown.

Research Progress on Irradiation Effects of RPV Materials

Reactor pressure vessel (RPV) is an important safety component which holds reactor core and keep on high temperature and high pressure coolant. The irradiated region of RPV is subjected to neutron damage so that the fracture toughness decrease and RTNDT of materials with neutron fluence increase. According to fracture toughness requirements based on the rule, RPV beltline materials must have Charpy upper-shelf energy of no less than 102J initially and must maintain upper-shelf energy throughout the life of the vessel of no less than 68J. Chinese manufactures (China First Heavy Industries, China National Erzhong Group Co. and Dongfang Electric Corporation, Shanghai Electric Corporation) have supplied A508-3 steel forging and fabricated RPV for many constructing NPP in China. Nuclear Power Institute of China (NPIC) has being put into practice a program about the irradiation tests and irradiation brettling research of Chinese A508-3 steel from these manufactures. This paper introduces the program and progressing briefly. Specimen, irradiation test parameters, irradiation facility and post irradiation mechanical tests are also described.

Understanding Equivalent Margins Analysis for Required Upper Shelf Energy

Equivalent Margins Analysis (EMA) involves the calculation of an alternative minimum reactor pressure vessel (RPV) upper shelf energy (USE) when the projected value falls below current limits codified in Title 10, Code of Federal Regulations, Part 50 (10 CFR 50), Appendix G. One set of calculation methodologies for performing the analysis are provided in the Nuclear Regulatory Commission’s (NRC’s) Regulatory Guide (RG) 1.161 and American Society of Mechanical Engineers Boiler & Pressure Vessel Code (ASME Code), Section XI, nonmandatory Appendix K. Careful application of fracture mechanics principles is necessary in order to properly carry out the evaluation. This is particularly the case for demonstrating compliance with the ductile crack growth stability criterion. This paper discusses robust implementation of EMA calculations and identifies recommended changes to RG 1.161 and ASME Code, Section XI, nonmandatory Appendix K.