## 1 Introduction  
Non-destructive testing (NDT) is very valuable from a quality control perspective because it does not modify or destroy the component being tested. Additionally, it saves time and economic costs and improves reliability of equipment, parts, and materials. Equipment and facilities have standard design requirements and expected lifespans, and non-destructive testing is performed according to various requirements and standards for safety and reliability [1, 2]. Recently, thermography testing (TT) has been receiving a lot of attention as an advanced technology that can quickly inspect large areas in a non-destructive manner with no contact [3]. Furthermore, due to its powerful advantages, its application field is expanding as an advanced technology that can inspect microscopic mineral crystallization processes, including macroscopic property inspection [4]. TT complements the limits of common NDT technologies such as visual testing (VT), penetrant testing (PT), magnetic particle testing (MT), eddy current testing (ECT), radiographic testing (RT), and ultrasonic testing (UT), and can be applied as an alternative technology [5]. The nuclear reactor containment building is a concrete building where the core facility, the nuclear reactor, and the reactor coolant system are installed. The reactor containment building of the Korea standard nuclear power plant (KSNPP) is a structure with a hemispherical roof and is a symbol of the nuclear power plant. As shown in Figure 1, a nuclear power plant containment building includes several layers of containment structures. The reactor containment building is made of reinforced concrete walls about 1.2 m thick, and the interior is sealed with a 6 mm steel plate called a CLP [6]. The concrete structure and CLPs are multi-layered protective walls that protect the internal reactor from external attacks in case of an emergency, prevent external leakage and exposure of radioactive materials in the event of an accident inside the dome, and perform the final protective function. When CLP is constructed, various structures (concrete, rebar, etc.) and additives are included inside and outside the CLP. The back of the CLP is attached to concrete, and voids may be created in this process. Corrosion defects are created on the back of the CLP due to voids in the concrete, which develop into thinning defects [7; 8; 9]. Ultimately, it reduces the strength of CLPs and concrete containment buildings and can lead to serious accidents [10; 11]. Currently, non-destructive testing of the CLP of nuclear power plants in operation is mainly performed by examining the thickness of the CLP using local ultrasonic testing (UT) and visual inspection (VT) to check whether the CLP is penetrated or deformed [12]. However, the containment structure is very large and there are limitations due to the disadvantages of UT inspection (small inspection area, inspection result quality depending on operator proficiency, etc.). In addition, there is a method of estimating the concrete voids that cause CLP thinning through the striking sound of the CLP surface. Most techniques only apply inspection to the sidewall shell portion of the containment building, and only perform VT on the upper dome portion due to limited accessibility. These inspection techniques are mostly localized and have limitations in identifying CLP thinning or the shape of concrete voids. Recently, phased array ultrasonic testing (PAUT), acoustic resonance method (ARM), and CLP inspection techniques using automated scanners are being studied [6; 12]. However, relatively complex mechanisms and signal processing are required to determine the shape of the defect. Therefore, if it is possible to evaluate CLP thinning defects using infrared thermography, quick and useful inspection will be possible. Reviewing the existing literature, it can be seen that various thermographic testing studies that simulated thinning defects have been conducted [13; 14; 15; 16; 17; 18; 19; 20].