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...

The objective of the present scientific-research work was to develop a method and the technical means for efficient nondestructive monitoring of airtightne___~ and gas parameters of both fuel elements and fi~el assemblies of power reactors during the production process as well as the spent fuel under the conditions of a hot chamber, holding pond, or burial site.System for automatic measurement of pressure (SAMP) of hefium in fuel dements. The desked parameter is the gas pressure under the cladding. The crux of the thermophysical method which forms the basis of the pressure measurement is the production of a normalized thermal disturbance of the cladding in a local region of the gas collector of a fuel element, resulting in the appearance of natural convection and the accompanying thermodynamic processes in a closed volume of gas. Heat transfer between the heated cladding with the gas and the surrounding medium depends on the type, the physical properties, and the pressure of the gas. Information about the measured parameter can be obtained by recording the spatial distribution and time dependence of the temperature of the cladding. The information is interpreted using a calibrated characteristic obtained beforehand on standard samples fiUed with the same gas as the part being investigated but having parameters which are known with a high degree of accuracy in the required range. In so doing, however, it is necessary to overcome the effect of factors such as the variance of the geometric dimensions and thermophysieal properties of the cladding material, the instability of the parameters of the surrounding medium, the multicomponent composition of the gas being monitored, the presence of an oxide fdm and deposits (for the spent fuel elements) on the surface of the cladding, as well as the fuel and solid fission products in the fuel element.It is obvious that in the process of measurement the temperature field carries information not only about the parameter being measured but also, to a large extent, about the influencing factors indicated above. The sensor construction, the method of measurement, and the special data processing methods which we developed made it possible to separate information about the gas pressure with acceptable accuracy.The technical problems were solved with the aid of well-known approaches. These include the choice of materials and elements, automation of the measurement process, fabrication of standard samples, insertion of existing means of measurements into the production process, and metrological certification of these means.At the present time two modifications of a system for measuring the gas pressure in fuel elements, based on the method developed, are being used; this is due to the different areas of application. The first area is intended for exit monitorhag of the helium pressure in the fuel elements during the fuel element production process (SAMP).Until recently such monitoring was performed at fuel production plants by the method of selective perforation of the cladding of som...

Core components and fuel rods may melt partially or completely during a major accident at a nuclear power reactor, so one gets a multicomponent mixture of liquids and solids, whose physical properties are very different from those of the pure components [1]. There is little information on the viscosities of liquids in binary and more complicated systems, and this makes it necessary to examine and test methods of measuring the kinematic viscosity for fuel-composition liquids.Out of the set of methods [2, 3], that of [4] is a good one, as it has a reliable theoretical basis and requires comparatively simple apparatus. That method has been applied to the viscosities of liquids formed by various metallic and ceramic fuel materials at temperatures up to 3050~ [5].The method involves exciting and recording low-frequency freely damped torsional oscillations in a cylindrical crucible filled with the liquid. The oscillation parameters are determined by an optoelectronic method from the displacement of a beam reflected from a mirror in the support, and from which the characteristics are calculated. The apparatus for examining the viscosity of liquids and fuel composites up to 2400~ was similar in design to the viscometer described in [6]. A hightemperature vertical vacuum resistance oven type SShVl~-l.25/2.5-IZ was used with a maximum working temperature of 2500~ The system included an automatic one for recording the torsional oscillations.The period and the damping decrement were recorded with a special interface board attached directly to the highway in an IBM-compatible personal computer. The software was based on an algorithm for the characteristics [8, 9] and enabled one to perform the experiment with one or two photocells. The light source was a helium-neon laser and the detectors were photodiodes.The maximum temperature could be increased to 2600"C by a minor modification of the screening in the heating zone and the use of tungsten as material for the rod and supporting wire, as well as the use of a crucible having special coatings preventing interaction with the molten material. The crucible had already been tested at that temperature.The apparatus was tested on model materials, namely tin and lead. Test studies were made on the melting points of the material up to 1000~ and gave values for the kinematic viscosity in accordance with published ones within the error of measurement, which did not exceed + 5 %, and which indicates that the design is good and the apparatus is viable.Preliminary experiments were performed on the application limits and use of the viscometer before extensive use and costly studies on the temperature dependence of the viscosity for reactor fuel materials, and also because there are no similar data.The model material was 99.9% pure tin with added f'mely-divided anhydrous corundum powder (A1203) on the assumption that there would be no reaction between those components at the moderate working temperature. The specimens were prepared by a method providing a uniform distribution of the corundu...