## 1 Introduction  
Boiler tubes in thermal power plant facilities are exposed to high-temperature and high-pressure environments and operate under extreme conditions. The environmental conditions cause damage that interferes with the operation of the power plant facilities, such as corrosion or cracks. The flaws are mainly found in welds, owing to the effects of stress concentration caused by the material properties and geometric shapes of welds [1, 2]. In addition, fabrication flaws such as lack of fusion, incomplete penetration, porosity, and inclusions are generated during the construction phase, and they grow into defects that affect failure depending on the environmental conditions during operation [3]. To ensure the integrity of boiler facilities, nondestructive testing (NDT) suitable for the applied facilities should be performed. Recently, various NDT techniques for power generation facilities, including boiler facilities, have been developed and tested [4, 5]. With technological development, new NDT techniques emerge, and relevant institutionalized regulations or standards are established or revised for industrial applications of these techniques [6; 7]. The conventional NDT method used in power generation facilities is radiographic testing (RT), but recently, owing to the tightened regulations on the use of radiation, the application of RT has become more difficult and limited. Therefore, the development of alternative volumetric nondestructive evaluation methods to replace RT is imperative [8]. The volumetric inspection techniques for the replacement of RT include ultrasonic testing (UT) using a single element transducer and phased array ultrasonic testing (PAUT) using an array of elements transducers. [9]. In the PAUT technique, a focal law is applied to control the propagation angle and focal position by electronically applying time delays for each transducer [10]. In contrast to the conventional UT methods, the PAUT method is advantageous for saving digitalized images and allows high-speed inspection of flaws through rapid changes of the focusing position [11]. Owing to these advantages, research has been conducted for adopting the PAUT method in various industrial applications, e.g., research on the development of inspection systems, such as inspection devices and transducers, for applying the PAUT technique to nuclear power plant components [11]; research on the application of PAUT to the inspection of composite materials such as carbon fiber-reinforced polymer (CFRP) and glass fiber-reinforced polymer (GFRP), which have attracted considerable attention for a wide range of industrial applications [12; 13]; and research on optimizing the inspection conditions of PAUT for stainless steel weld inspection using simulations [14]. As indicated by the previous studies, the scope and range of applications of PAUT have been increasing. However, few empirical studies involving the application of the method to boiler tube welds in thermal power plant facilities have been conducted. Before in-service application of the method in industrial fields, reliability evaluation of the PAUT method through a round robin test (RRT) should be performed [15; 16]. The RRT is the most representative method for reliability assessment of NDT for power generation facilities such as in the case of the Programme for Inspection of Steel Components (PISC) project [17]. In this study, to verify the reliability of the PAUT technique, mock-ups with artificial flaws inserted into boiler tube welds of thermal power plant facilities were fabricated, and an RRT was performed. The mock-ups were fabricated with different base materials (carbon steel, austenitic stainless steel, dissimilar metal) and flaw types (planar, volumetric). The RRT was performed on the PAUT technique, and the results were compared with those of RT to examine the feasibility of PAUT.