# Quantification of the Effective Detectable Period for Concrete Voids of CLP by Lock-In Thermography
## 6 Conclusions and Future Works  
This study is a case study on the LIT thermography inspection technique for the concrete voids on the back of the CLP of the nuclear power plant containment building. First, design/manufacture CLP mockup specimens containing four sizes of concrete voids. In the first step, a FEM model identical to the shape of the specimen was built, and thermography was replicated and analyzed through LIT simulation, considering four cases. As a second step, the LIT test under four conditions was run on the mockup specimen, and the obtained thermography was analyzed to compare/analyze the difference between the simulation and test results. As a result, the following conclusions were obtained:  
1. Simulation through FEM can simulate thermography testing for LIT flaw detection conditions, and through simulated thermography analysis, EDP that can detect concrete voids can be confirmed and the 'comparative analysis point' can be derived and utilized;
2. In the case of the simulation, it was expected that the detection of concrete voids on the back of the CLP would be smooth with the LIT inspection technology, as it showed high detection and resolution in all cases;  
Figure 17: Analysis of LIT experiments results in Test 3 condition (2 kW, 0.01 Hz): (**a**) temperature image and distribute (#925 frame, 148 s); (**b**) amplitude image and distribute (#725 step, 116–141–166–191 s); and (**c**) phase image and distribute (#725 step, 116–141–166–191 s).  
3. In the actual test, detection and resolution were confirmed only in the phase image due to the influence of the hot spot and uneven heat energy transfer by the halogen lamp, but the temperature and amplitude images were not sufficient. Therefore, it is judged that the phase image is effective in overcoming the effect of uneven heat addition;
4. The proportionality between the concrete void size and the SNR was found to be significantly influenced by the void area, but the effect of the depth was minimal.  
In the future, it is expected that concrete voids can be inspected more effectively if stronger energy is applied evenly to the CLP surface without a hot spot in the inspection condition of the concrete voids on the CLP backside. Therefore, we will consider device and software solutions to solve the problem of non-uniform heat addition, analyze the effect of improvement through problem solving, and report the results.  
Conceptualization, C.K.; methodology, C.K., Y.C. and W.K.; software, C.K., S.K. and Y.C.; validation, C.K. and Y.C.; formal analysis, C.K. and Y.C.; investigation, C.K. and S.K.; resources, C.K. and W.K.; data curation, Y.C.; writing--original draft preparation, C.K., S.K. and Y.C.; writing--review and editing, O.K. and W.K.; visualization, C.K. and S.K.; supervision, O.K.; project administration, C.K.; funding acquisition, C.K. and W.K. All authors have read and agreed to the published version of the manuscript.  
This work was supported by a project for Industry-University-Research Collaboration R&D funded by the Korea Ministry of SMEs and Startups in 2023 (RS-2023-00224854).  
Not applicable.  
This study received administrative and technical support from enesG, and we are grateful for all support. All authors consent to acknowledgments.  
The authors declare no conflict of interest.