# Electromagnetic Testing of Rod Cluster Control Assemblies in Pressured-Water Reactor Power Plants
## 1 Introduction  
Rod cluster control assemblies (RCCAs) are used in pressurized-water reactors (PWRs) to control fast reactivity changes and stop the reactor when an accident happens. The RCCA usually consists of 16 (14\(\times\)14 type), 20 (16\(\times\)16 type), or 24 (17\(\times\)17 type) control rods. Each control rod includes an absorber consisting of Ag (80%)-In (15%)-Cd (5%) in the core and covered with a cladding tube made of stainless steel (STS340) to prevent corrosion and cracking [1]. Each RCCA is moved upward and downward in the reactor core by a control rod drive mechanism. There are eight guide cards located at the upper side of the reactor core to support the RCCA, which are visible after the nuclear fuel is withdrawn during operation of the nuclear reactor. During operation, wear can occur on the cladding tube due to the up/down movement of the RCCA and vibration of control rods against the guide cards, the vibration of the guide cards themselves, and vibration against the inner walls of the guide tubes on the fuel assemblies [1,2]. The swelling of the absorber can lead to an increase in diameter and result in the appearance of a crack in the cladding tube [1,2]. Additionally, irradiation assisted stress corrosion cracking usually occurs in the cladding tubes and end tips of the control rods [3]. Therefore, it is necessary to inspect and evaluate the defects to ensure the safety and integrity of the PWRs. Several nondestructive testing systems have been developed and applied for inspection of control rods such as encircling bobbin probes [3, 4, 5] and multi-array pancake coil probe systems [6]. The encircling bobbin probes use an absolute- or differential-type bobbin coil as a sensor for measuring the rate changes of the total magnetic flux due to the presence of a defect on the control rod. Thus, the spatial resolution is just one circumferential circle, which is low. The encircling bobbin probes are also limited to detecting circumferential cracks [3; 4]. The multi-array pancake coil probes use multiple pancake coils arrayed in a probe for improvement of spatial resolution in the circumferential direction. However, the multi-array pancake coil probes still have low spatial resolution due to the large size of the pancake coils. For instance, a probe consisting of an 8\(\times\)1 pancake coil array has been developed [6]. In addition to this, the sensing coil measures the rate of change of magnetic flux over time. The excitation source is a sinusoidal wave which produces the sinusoidal magnetic flux by the exciting coil. According to [PERSON]'s law, the output of the sensing coil is a voltage which is proportional to the rate of the magnetic flux over time. In addition, the output voltage is proportional to the frequency of excitation. Thus, the output voltage is low (low sensitivity) at a low frequency of excitation. Therefore, the previous systems are limited in their detectability and quantitative evaluation of defect sizes. Recently, magnetic sensors have been used for directly sensing the magnetic field due to the presence of defects. Numerous Hall sensor and Giant magnetoresistance (GMR) sensor elements are arrayed in the circumferential direction of the probes for inspection of defects at the inner or outer surface of tubes [7; 8; 9; 10; 11]. The developed system has efficient inspection capabilities and enables quantitative evaluation of defects. In this paper, we propose an integrated system comprising 24 encircling Hall sensor array (EHaS) probes that enable the inspection of a 17\(\times\)17 type RCCA in a single scan. Each EHaS has 16 Hall sensor elements arrayed around an 11.2 mm cylindrical supporter at an interval of 22.5\({}^{\circ}\) (2.2 mm). The experiment was performed on real RCCA control rods composed of an AgInCd absorber and a STS304 cladding tube. Artificial short-circumferential grooves (SCGs), sliding wears (SWs), full-circumferential cracks (FCCs), and partial-circumferential cracks (PCCs) on the cladding tubes were tested for verification of the proposed system.