Reliability and reliability analysis concepts are increasingly gaining importance with the increase in product complexity and end-user requirements. This is particularly noticeable in high-risk facilities, such as nuclear power plants, where reliability is a crucial aspect not only in preserving the plant's integrity but also preventing the prolongation of the outage phase of power plants that can occur due to in-service inspection device failures during the inspection of the plant parts. For this reason, aim of this paper was to analyze the reliability of a device for ultrasonic testing of reactor pressure vessel welds and propose required design changes that will increase the reliability of components and subsystems where an increased risk of failure has been identified. In order to get acquainted with the issue of reliability engineering, terms such as failure, risk, reliability, availability, maintainability, and quality were addressed, after which the methodology for conducting reliability analysis was defined. Within the methodology, great emphasis was placed on the correct and thorough implementation of functional analysis and the selection and application of different methods for reliability analysis. An overview of the characteristics, advantages, and disadvantages of each of the methods is given in order to select the most appropriate methods for the respective case and to ensure correct application and better results. In the next step, an FMEA analysis was performed on said in-service inspection device, currently being developed by the INETEC company. First, the device itself, its principle of operation and interaction with other subsystems were described, after which the main and auxiliary functions of the device were collected through the functional decomposition process. Based on the decomposition, the FMEA table was created in which all failure modes, causes and effects for said functions are defined. In defining them, as well as quantifying their severity and occurrence, the company's previous experience and data from suppliers and manufacturers helped. FMEA analysis identified 1067 minor and medium risks, 141 high and 29 critical risks in the system. The high risks are largely due to the interruption of air and energy flows through the system, as well as the possible penetration of water inside the device. Malfunctions and errors that can occur in the system, are most often the result of inadequate input control of parts, incorrect calculation, inadequate construction, as well as errors in software, instrument, control box or due to operator handling. For all risks, suggestions for their mitigation have been proposed, while for those critical, appropriate design changes have been made. At the very end, a critical review of the use of these methods is given as well as future work suggestions.