Characterization of nuclear reactor pressure vessel (RPV) surrogate specimens with ultrasound

Abstract

This research determines the sensitivity of nonlinear ultrasound (NLU) to the changes in microstructure of heat treated Fe-1.0% Cu and Fe-0.1% Cu binary alloys. The Fe-Cu material investigated in this research serves as a surrogate material to simulate the evolution of radiation damage that occurs in reactor pressure vessels (RPVs) of light water reactors (LWRs). The formation of the Cu-precipitates in RPV steel is one of the main factors leading to radiation embrittlement. With RPVs seeing more neutron irradiation than originally anticipated, developing nondestructive evaluation (NDE) techniques capable of evaluating the integrity of these structures is highly desirable. This research investigates the sensitivity of the NLU technique, second harmonic generation (SHG), to the changes in the surrogate material microstructure. It is experimentally observed that as the surrogate material is heat treated the nucleated Cu-precipitates grow up to a few nanometers and then coarsens. For the experimental procedure used in this research, ultrasonic longitudinal waves are propagated through the thickness of the specimens where higher harmonics are generated. The nonlinearity parameter, β, can then be calculated by relating the amplitude of the fundamental frequency to the amplitude of the second harmonic wave. Additionally, a theoretical model was applied to relate the change in β to the change in the Cu-precipitate radius. As will be seen in the results, there is a signification increase in β for the Fe-1.0% Cu specimens in contrast to there being little change in β for the Fe-0.1% Cu specimens. The significant increase in β demonstrates the sensitivity of SHG to these Cu-precipitates.