Minoru Tomimatsu

Microstructural Characterization of RPV Materials Irradiated to High Fluences at High Flux

Understanding the embrittlement of reactor pressure vessel (RPV) steels at high fluence region is very important for the long term operation of nuclear power plants. In this study, extensive microstructural analyses were performed on the RPV steels irradiated to very high fluences beyond 1020n/cm2, E>1 MeVat high fluxes under the Pressurized Thermal Shock and Nuclear Power Plant Integrity Management projects in Japan. Three dimensional atom probe analyses were performed to characterize the solute atom cluster formation in these materials. The effects of fluence, flux, and chemical compositions on the characteristics of clusters were analyzed. The formation of dislocation loops was identified in the transmission electron microscopy analyses of high and low Cu steels, and the changes in loop size and number density with fluence were studied. P segregation on grain boundaries was also studied by surface analyses as well as grain boundary chemical analyses. We found that nonhardening embrittlement due to grain boundary fracture is not a major contributor to the embrittlement in these materials and irradiation conditions. The correlation of the microstructural changes and the Charpy transition temperature shifts was studied. The volume fraction of solute atom clusters has an excellent correlation with the transition temperature shifts. The Orowan model calculations of the contributions of dislocation loops to the transition temperature shifts show that in low Cu materials, dislocation loops may be a major contributor, but in Cu containing materials its contribution is weak. Root-sum-square of the contributions of solute atom clusters and dislocation loops seems to be a reasonable model to describe the total ΔRTNDT.

Reconstitution of Fracture Toughness Specimen for Surveillance Test

Fracture toughness is one of the most important material properties that influences the integrity of reactor pressure vessels (RPVs). Methods for reconstitution of fracture toughness specimen have been developed preliminarily in order to increase the number of fracture toughness data of irradiated RPV steels. The specimen adopted in this study was a 12.5mm thick compact specimen (½TCT). The size of insert material, which is removed from the assumed broken specimen, is 13 × 15 × 12.5mm 3 . Materials used were two kinds of unirradiated RPV steels. Welding procedures investigated were CO 2 laser beam welding(LBW) and electron beam welding(EBW). Several welding parameters, such as welding current, speed of travel, etc. were varied parametrically. Then optimum welding conditions of both weldings for reconstitution of CT specimen were investigated. It was found that fracture toughness values obtained from the reconstituted CT specimens of two steels are almost the same as those from original specimens in both the transition and the upper shelf temperature ranges. It turns out that the above methods can be applied for the reconstitution of irradiated fracture toughness specimen in hot cell.

Effect of Additional Irradiation at Different Fluxes on RPV Embrittlement

The effect of the neutron flux at high fluence on the microstructural and hardness changes of a reactor pressure vessel (RPV) steel was investigated. An accelerated test reactor irradiation of a RPV material, previously irradiated in commercial reactors, was carried out at the lowest possible neutron fluxes in order to obtain neutron fluences up to approximately 1×1020 n/cm2 (E>1MeV). State-of-the-art experimental techniques such as three-dimensional atom probe were applied to carry out advanced quantitative characterization of defect features in the materials. Results for the same material irradiated in both high and low flux conditions are compared. For neutron fluences above 6×1019 n/cm2 (E>1MeV) the difference in the neutron fluence dependence of the increase in hardness is not seen for any neutron flux condition. The volume fraction of solute atom clusters increases linearly with neutron fluence, and the influence of neutron flux is not significant. The component elements and the chemical composition of the solute atom clusters formed by the irradiation do not change regardless of the neutron fluence and flux. The square root of the volume fraction of the solute atom clusters is a good correlation with the increase in hardness.Copyright

Effect of Neutron Flux at High Fluence on Microstructural and Hardness Changes of RPV Steels

The effect of the neutron flux at high fluence on the microstructural and hardness changes of reactor pressure vessel (RPV) steels was investigated in succession to the previous study [1]. An accelerated test reactor irradiation of copper containing RPV materials, previously irradiated in commercial reactors, was carried out at the lowest possible neutron fluxes in order to obtain neutron fluences up to approximately 1×1020 n/cm2 (E>1MeV). State-of-the-art experimental techniques such as three-dimensional atom probe were applied to carry out advanced quantitative characterization of defect features in the materials. Results for the same materials irradiated in both high and low flux conditions are compared. For neutron fluences above 6×1019 n/cm2 (E>1MeV) the difference in the neutron fluence dependence of the increase in hardness is not seen for any neutron flux condition. The number densities and the diameters of solute atom clusters for the low flux irradiation materials tend to be lower and larger, respectively, than that for the high flux irradiation materials, while the volume fraction of solute atom clusters increases linearly with increasing neutron fluence, and the effect of neutron flux is not significant. The component elements and the chemical composition of the solute atom clusters formed by irradiation for the same material do not change regardless of the neutron fluence and flux. The square root of the volume fraction of the solute atom clusters provides a good correlation with the increase in hardness.Copyright

Master Curve Approach for Some Japanese Reactor Pressure Vessel Steels

The Master Curve Approach for assessing fracture toughness of reactor pressure vessel (RPV) steels has been accepted throughout the world. The Master Curve Approach using fracture toughness data obtained from RPV steels in Japan has been investigated in order to incorporate this approach into the Japanese Electric Association (JEA) Code 4206, “Method of Verification Tests of the Fracture Toughness for Nuclear Power Plant Components”. This paper presents the applicability of the Master Curve Approach for Japanese RPV steels.Copyright

Overview of the Japanese Code of Surveillance Test Program for Reactor Vessels

The Japan Electric Association Code, JEAC 4201, “Method of Surveillance Tests for Structural Materials of Nuclear Reactors” was originally published in 1970 so as to design a surveillance program for monitoring radiation induced changes in mechanical properties of beltline materials in light-water moderated nuclear power reactor vessels and to evaluate the test results. Recently in 2004 the code was revised, and a new method for predicting the decrease in upper shelf Charpy impact energy (USE) of beltline materials was incorporated based on the results of the research performed as a national project by the Japan Power Engineering and Inspection Corporation. Japanese surveillance tests program, including materials selections, type of specimens, number of capsules, withdrawal schedule, and evaluation of the results, is overviewed. Furthermore, methods for predicting the decrease in USE and reference temperature shift for beltline materials are also presented.Copyright

Overview of the Revised JEAC4201-2007, Japanese Code of Surveillance Tests for Reactor Vessel Materials

The Japan Electric Association Code, JEAC 4201, “Method of Surveillance Tests for Structural Materials of Nuclear Reactors” was revised in 2007. Major revised points of JEAC4201-2007 are: 1) A new irradiation embrittlement correlation method, 2) Reconstitution of surveillance specimen, 3) Long-term surveillance program. An overview of the revised JEAC 4201-2007 is presented in this paper.Copyright

Microstrucutural Characterization of RPV Steels Irradiated to High Fluences

Neutron radiation embrittlement of reactor pressure vessel (RPV) steels is one of critical issues for the structural integrity assessment of the RPVs. Especially, the embrittlement at high fluences is of great interest for the long term operation of light water reactors because information on the mechanical property changes as well as embrittlement mechanisms is limited at high fluences. In this study, microstructural analyses were conducted on the RPV steels irradiated to high fluences in order to confirm the applicability of the trend curve at high fluence region. Steels investigated are five base metals and a weld metal with their copper content ranging from 0.02 to 0.25 wt. %. These steels were irradiated in the material test reactors to fluence up to 1.3 × 1020 n/cm2 , E > 1MeV, at temperature of about 290 °C. After irradiation, transmission electron microscope (TEM) observations were performed to characterize the nano-meter scale microstructural changes due to irradiation. Formation of dislocation loops was observed. Number density and diameter of dislocation loop was investigated. Effects of chemical composition of steel and fluence on dislocation loop formation are discussed.Copyright

Overview of the JEA Code of Fracture Toughness Requirements for Nuclear Power Plant Components

The Japan Electric Association Code, JEAC 4206, “Method of Verification Tests of the Fracture Toughness for Nuclear Power Plant Components” was originally published in 1973 in order to prescribe test methods, fracture toughness requirements and acceptance standards for materials used in nuclear power plant components. The code was recently revised so as to incorporate a new method to evaluate the structural integrity of reactor vessels with upper shelf Charpy impact energy (USE) less than 68J, based on the results of the researches performed as national projects by the Japan Power Engineering and Inspection Corporation and so on. In this paper, some contents of the code, which are applicable for reactor vessels, such as J integral based integrity evaluation method for reactor vessels with low USE including predicting J resistance curves (J-R curves) by using USE and temperature, and methods to evaluate integrity against pressurized thermal shock events and to determine pressure temperature limits, are overviewed.Copyright

Development of Prediction Equations on Charpy Upper Shelf Energy for Japanese RPV Steels

It is well known that as the embrittlement due to neutron irradiation on reactor pressure vessel (RPV) steels, there is the tendency of the reduction of Charpy absorbed energy at upper shelf region (USE), in addition to the shift of ductile-brittle transition temperature. Concerning to the regulation of the upper shelf region, no method is provided to evaluate integrity for RPV steels with USE of less than 68J in Japanese codes. Under the circumstance, the reduction tendency of USE using simulated Japanese RPV steels, irradiated by fast neutron up to 1024 n/m2 in OECD Halden test reactor, was investigated to establish the basis of the USE prediction after 60 years plant operation for the integrity assessment of the RPVs. The USE prediction equations have been developed through the regression analyses of the test reactor data combined with Japanese surveillance test data. This research was entrusted by the Ministry of Economy, Trade and Industry in Japan.Copyright