Capability of Matrix Phased Array UT for the RV Head Mockups

PWR reactor vessel (RV) head has a number of penetration nozzles, including nozzles with thermal sleeve for control rod drive mechanisms (CRDM) and without thermal sleeve. These nozzles are typically made of nickel-based Alloy 600 or 690 and welded to the inside of the RPV head. Cracking has been reported in Alloy 600 nozzles and the J-weld regions. These nozzles are normally inspected by eddy current testing (ECT) and ultrasonic testing (UT). In the case of nozzle with thermal sleeve, blade probes with TOFD and/or zero degree UT sensors are applied without removing thermal sleeve. On the other hand, bore-type probes with time of flight diffraction (TOFD) and/or zero degree UT sensors are applied for the inspection of nozzle without thermal sleeve [1,2]. However these probes are not enough to inspect weld region because of attenuation of ultrasonic wave in weld. Phased array UT technique has been utilized to improve the capability of UT inspection for the components with complex configuration and coarse-grained materials. Phased array probe consists of many elements that are able to change beam angle and focal depth without optical lens or special wedge by electrical scanning. Recently, these probes are applied to various components which are difficult to inspect using conventional UT probes because of complex geometry and/or coarse-grained materials. Although it is known that UT inspection of Alloy 600/82/182 weld is difficult because of attenuation of ultrasonic wave due to the varied microstructures, it is expected to increase the detectability of cracks in the region if appropriate focal point is created in weld region. In our previous study [3], blade phased array probe that can insert into the small gap between the thermal sleeve and CRDM penetration nozzles was developed and a series of laboratory tests showed that phased array technique can improved the detectability in weld region of CRDM penetration. In this study, bore-type matrix phased array UT probe, can insert into nozzle with the dimension of RV head penetration nozzle, was developed by optimizing various design parameters. The capability of the matrix phased array UT probe is demonstrated by using RV head mockups with various signal sources in the weld region.