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The research activities of irradiation damage evaluation on reactor pressure vessel materials using NDE techniques in KoreaKeeOk Chang , ByoungChul Kim, SamLai Lee
Korea Atomic Energy Research Institute, 150 dukjindong, Yusunggu, Daejon, Korea.
Neutron radiation from the reactor core causes embrittlement of light water reactor pressure vessel beltline materials. Embrittlement refers to a decrease in the fracture toughness of reactor vessel material. Mechanical tests used to establish the level of radiation embrittlement of RPV material, such as Charpy and tensile tests or more sophisticated fracture toughness tests, are dependent on the intrinsic properties of the material, such as micro defects and precipitate size, concentration, and density. Nondestructive evaluation(NDE) techniques can be generally used to determine the intrinsic material properties. In this study, to decide the applicability of NDE techniques such as magnetic and ultrasonic techniques to detect radiation embrittlement in reactor pressure vessels have been investigated for use in detecting changes in the microstructure of pressure vessel steels of related materials in Korea. The results showed that a need exists for correlation not only between the microstructural changes and the NDE results, but also between the NDE results and the mechanical behavior or level of embrittlement in order to replace the conventional destructive techniques of tensile and Charpy testing of surveillance capsule specimens. In this paper, the research activities of radiation damage assessment on RPV surveillance materials using the various NDE techniques and corresponding results are reviewed.
KEYWORDS: Nondestructive Evaluation, Reactor Pressure Vessel(RPV), Irradiation Embrittlement, Magnetic Techniques, Ultrasonic Techniques
The high energy neutron radiation from the reactor core causes embrittlement of the RPV beltline materials. Radiation embrittlement of RPV materials is a concern because it causes a reduction in the capability of the material to withstand the presence of cracks and a reduction in the fracture toughness of material. Generally, low alloy ferritic materials show an increase in hardness and tensile properties and a decrease in ductility and fracture toughness during high-energy neutron irradiation. Therefore, the degree of radiation embrittlement in RPV material is determined periodically using approved models and appliable regulatory guidelines(USNRC 10CFR50,APP.G,1993). Reactor vessel surveillance programs(ASTM E185-82, 1982) provide further information about the condition of the vessel through mechanical testing of pressure vessel material samples removed from surveillance capsules. However, surveillance programs do not always provide enough accurate information to support decisions to end the life in advance, to continue operation until the license expiration, or to extend the life past the original design life using the annealing process because of the limitation of the number of specimens in surveillance capsule. Also, problems arise in surveillance programs if there is not enough inventoried material available. In addition, the effects of annealing and re-embrittlement on the vessel integrity have not been adequately addressed by the surveillance programs. Recently, to eliminate these problems, research activities on the development of new models for the radiation damage evaluation have been performing in many countries.
NDE has been used to inspect structural materials in industry for macroscopic defects, such as cracks, flaws, and pipe wall thinning as well as microscopic characteristics, such as grain size, effects of heat treatment, the presence of precipitates and residual stress. These intrinsic material properties affect the mechanical properties. Therefore, the level of embrittlement in a RPV material is determined by measuring mechanical properties using NDE techniques. General NDE techniques to assess the radiation embrittlement of RPV material are electrical, magnetic, electromagnetic, ultrasonic, and mechanical technique(Kim,B.C et al. 1997). In this study, to decide the applicability of an NDE technique to detect radiation embrittlement in reactor pressure vessels, magnetic and ultrasonic techniques have been investigated for use in detecting changes in microstructure of pressure vessel steels of related materials in Korea.
In the following sections magnetic and ultrasonic techniques as supplied to the study of embrittlement in RPV's will be reviewed.
Magnetic Hysteresis and Magnetic Barkhausen Effect
The changes in the magnetic parameter due to irradiation have recently been investigated within the efforts to find reliable NDE techniques in predicting, for example, fracture toughness of embrittled RPV steels (Govindaraju,M.R et al.1994) (Chi,S.H et al.1997). The magnetic hysteresis loop is determined by applying a magnetic field (H) to a material and measuring the resultant magnetic induction (B). The advantage of using a magnetic hysteresis measurement as an NDE technique is that a number of independent parameters are obtained from one measurement. Four such parameters, The remnant induction (Bg), The coercive field (Hc), The hysteresis loss (WH) and The magnetic permeability (m), may be used to determine the intrinsic properties of the material. The Barkhausen noise effect (BNE) as an another parameter is similar to the magnetic techniques. As a time varying magnetic field is applied to the material, the change in the magnetization is discontinuous, not smooth. These discontinuous changes are caused by the motions of magnetic domain walls when they break away from pinning sites. Because motions of the magnetic domain walls are dependent on the number, density and the nature of the pinning sites, such as grain boundaries, dislocations, grain size, stress, strain, and precipitates of a second phase, BNE is a good candidate for monitoring radiation embrittlement in RPV steels. In the present study, in view of the need for additional experimental data regarding the magnetic NDE of neutron irradiated reactor structural steels, neutron fluence effects on the magnetic parameter, i.e., Barkhausen noise amplitude (BNA), saturation magnetization (Ms), susceptibility (c), and coercivity (Hc), and the inter-relationships between the changes in the magnetic parameter and yield strength due to irradiation were investigated using the RPV's surveillance specimens with different neutron exposure levels in Korea nuclear power plant.
Chang and co-workers(Chang,K.O. et al.2000) performed the measurement of magnetic parameters on SA508 Cl.3 forging and weld surveillance specimens irradiated to 2.2x1019~3.8x1019ncm-2(E>1.0MeV), and observed that hysteresis loop appeared to turn clockwise further as the fluence increased as shown in Figure 1. BNA/BNE and susceptibility decreased with increasing the fluence levels in Figure 2. However, the coercivity increased with radiation. Park et al.(Park,D.K, et al.1996) studied the BNA effect of irradiation on SA508 Cl.3 RPV steel with low fluence levels (1.0x1011~1.0x1016ncm-2(E>1.0MeV)). BNA signal slightly decreased with up to x1014 ncm-2 fluence level. However, at the above fluence level the signal sharply decreased with increasing fluence. Sipahi et al.(Sipahi,L.B, et al.1994) and Gillemont et al.(Gillemont,F, et al.1993) also found a decrease in the noise peak level with irradiation (power reactor) in similar RPV steel.
|Fig 1: Comparison of hysteresis loops for unirradiated and irradiated forging and weld metals.|
Fig 2: Change of Barkhausen wave forms for unirradiated and irradiated forging and weld metals for two fluence levels (2.2x1019ncm-2, 3.8x1019ncm-2, E>1.0MeV).
The detection of a change in the BNA signal with irradiation is dependent on the size of the precipitates. If the precipitate size is much smaller than the size of a magnetic domain wall (-60 nm in iron), the precipitate does not affect the movement of the magnetic domains. Sipahi states that "due to the increased number of defect pinning centers which impede both the movement of dislocations and magnetic domain walls" the BNE should decrease with irradiation. It is not clear if the radiation environment is the reason for the difference in Barkhausen measurements.
In summary for magnetic NDE techniques, the magnetic hysteresis parameters showed changes with fluence, and the BNA/BNE results were sensitive to the size of the precipitates and to the dislocation density. However, more work is needed to determine how each microstructural change affects each parameter and to separate the contribution of each microstructural effect. Finally the results must be shown to correlate with the mechanical changes in the material.
Ultrasonic Velocity and Attenuation
Ultrasonic testing is one of various nondestructive testing techniques widely used to detect minor defects in structures such as RPV and piping, and measure mechanical properties such as elastic constants. Since an ultrasonic beam is a stress wave, evaluation of material properties is done using the fact that the velocity and energy vary when the wave passes through the material. The velocity and attenuation are dependent on density, grain size, dislocations and precipitates due to neutron irradiation to RPV. Therefore, evaluation to embrittlement becomes possible by achieving the interrelationship among attenuation, velocity and neutron fluence based on the fact that ultrasonic characteristics such as the Raleigh critical angle sensitive to the material surface condition depends on the material properties(Hunter,D.O,1967). The Rayleigh wave method consists of interrogating th material with a sound beam at an angle incident to the surface equal to the Rayleigh critical angle, the angle at which a Rayleigh (surface) wave is produced in the material, This critical angle is dependent on material properties such as density, ultrasonic attenuation, and sound velocities. A promising correlation was observed between the ultrasonic signal amplitude and the neutron fluence. However, up until now, research for evaluation of irradiation embrittlement using ultrasonics has not been sufficiently performed.
Lee, et al.(Lee,S.L, et al.1999, 2000) performed the measurement of ultrasonic parameters on SA508 Cl.3 forging and weld surveillance Charpy specimens irradiated to 2.2x1019~3.8x1019ncm-2(E>1.0MeV). The ultrasonic phase velocity increases as the fluence increases in the base metal, while such a consistent relationship was not shown in weld metal as shown in Figure 3. Such a phenomenon can be verified in tensile testing where the yield strength increases as the fluence increases in base metal, while the yield strength decreases as fluence increases in weld metal in Figure 4. This indicates that the weld metal might have metallurgical complexities and such a result is considered to show similar results for measuring the ultrasonic velocity.
|Fig 3: Relationship between ultrasonic velocity and frequency for base and weld metal.|
|Fig 4: Relationship between neutron fluence and mechanical stresses for base and weld metal.|
In summary, a parametric analysis by ultrasonics techniques may be useful for identifying the material state related to neutron irradiation embrittlement and can be considered useful for supplementing the current technology for the evaluation and prediction of reactor vessel integrity if further research to identify such an inconsistent phenomenon can be performed.
Nondestructive evaluation techniques for surveillance Charpy impact specimens in order to evaluate the radiation embrittlement of reactor pressure vessel materials in Korean nuclear power plant have been performed. The magnetic and ultrasonic techniques appear to be closely related to the mechanical properties of interest in the radiation embrittlement of pressure vessel steel and seem to be promising for monitoring embrittlement. Further study has to be continued in order to achieve more accurate and reliable results for the assessment of radiation embrittlement.
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