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Fuel element reliability is still a priority in the modern nuclear power industry. The manufacturers have programs for achieving defect-free operation of fuel [1, 2]. The quality control programs provide for, as a rule, quality control of the finished product at the factory and detailed examination of irradiated fuel assemblies for the purpose of revealing unsealed jackets.In countries where nuclear power is well developed, a quite wide class of methods and devices for comprehensive nondestructive examination of fuel assemblies and elements under conditions of both industrial production and the holding basin and shielding chambers has been developed over the last 15-20 yr. This made it possible to decrease the time required to implement large scientific-research programs for increasing the degree of fuel burnup and the reliability of fuel elements and to improve the construction [2][3][4][5][6].In practice, it is important to measure the parameters of the gaseous medium in the reactor fuel elements and to check their airtightness, especially to achieve deep burnup of fuel. The gaseous fission products released when the fuel is irradiated change the intensity of the heat transfer between the fuel and the fuel element jacket, and it changes the stressed state of the jacket, which must be taken into account when designing fuel elements and estimating their working characteristics, service life, and the possibility of long-term storage after use. Effective checking of the airtightness of individual fuel elements can reveal defective and damaged elements, and it makes it possible to reconstruct a sectional fuel assembly. Moreover, subsequent comprehensive investigation of the character and reasons for the unsealing is also important in practice and necessary to increase the reliability and service life of the reactor core.Scientists in leading research laboratories in the nuclear industry in highly developed countries have been studying these problems intently for several years. As a result, both in our country and abroad, a large number of methods and technical means for monitoring the parameters of the gas in the fuel elements and the airtighmess of the elements have been proposed and experimentally checked. For example, in our country a thermophysical method for measuring the pressure of a gas medium in the fuel elements of power reactors has been developed and is used successfully under real conditions [7, 8]. The method used for performing the pressure measurements is based on producing a normalized thermal disturbance of the jacket in a local region of the gas collector of the fuel element; this disturbance generates the appearance of natural convection and corresponding thermodynamic processes in a closed gas volume. Heat exchange between the heated jacket with the gas and the surrounding medium depends on the type, physical properties, and pressure of the gas. Information about the measured parameter can be obtained by recording the temperature field of the jacket in both space and time. The information is int...

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