The characteristic properties of hardened single-crystal alloys (Mo-Nb, W-Nb, W-Ta) and UO 2-x with open thermally stabilized porosity are presented. The combination of the indicated materials makes thermionic fuel elements very stable with a long service life. It is shown that the use of porous uranium dioxide under temperature conditions resulting in restructuring makes it possible to decrease the size of the columnar grains and increase the creep rate of the dioxide with optimal porosity and pore size. A model oxide fuel, which has been developed, with fission-product simulators for rapid determination under extra-reactor conditions of the properties of uranium with deep burnup and for performing accelerated reactor tests is described.Two distinguishing features of the fuel elements in thermionic nuclear power systems operating on thermal and fast neutrons is the high working temperature of the cladding (1500-1800°C) and the substantial yield of gaseous fission products from uranium dioxide. For this reason, the structural scheme of a ventilated fuel element is generally used. Then main service life limiting factor is the deformation of the cladding under the action of the swelling fuel kernel. Conceptually, the deformation is decreased by using strengthened cladding, fuel with a lower rate of swelling, and a high creep rate, all of which acting together redistributes the volume of the swelling fuel in the interior free volume of a fuel element. Strengthened single-crystal alloys based on molybdenum, tungsten, and a uranium dioxide modification that is optimized with respect to structure and composition have been developed to support this concept.Single Crystal Alloys Based on Molybdenum and Tungsten. To achieve and maintain high output electric parameters of the electricity-generating channel (EGC) during the service life, stringent, including also contradictory, requirements are imposed on the cladding material of a fuel element together with a high resistance to creep:• high vacuum work function;• compatibility with fuel and cesium vapor;• low diffusion penetration of the fuel components and fission products;• radiation resistance in a fast-neutron flux;• small cross section for the absorption of thermal neutrons. A compromise solution for satisfying these requirements was the development of bimetallic claddings with a strengthened substrate ~1 mm thick and an effective emission coating ~0.15 mm thick. Single crystal alloys Mo-Nb, W-Nb, and W-Ta have been developed for the substrate. The creep rate of these alloys is approximately 1000 times lower than the corresponding single-crystal metals (Fig. 1). In addition, the creep stage with high deformation rate, which is characteristic for metals at the nonsteady stage, is absent in these alloys [1]. The alloy Mo-Nb was used in the fuel element of a single-ele-
Models and computer codes, developed based on them, for simulating the swelling of uranium dioxide (BARS) and the stress-deformation state of a fuel element (SDS) under high-temperature irradiation are presented. It is shown that when developing a design for high-temperature fuel elements and validating their serviceability the quantitative indicator required for the swelling of uranium dioxide in the range ≥1400°C is the change in the external dimensions of the fuel caused by constant formation and growth of bubbles containing gaseous fission products during irradiation. The results of computational investigations using the models indicated are examined. These results eliminate the inconsistency of the data on the effect of the main operating parameters -the temperature and burnup -on the radiation characteristics and service life behavior of a fuel element. It is shown that the central channel in the fuel kernel and strengthening of the cladding improve the dimensional stability fuel elements.The cladding deformation caused by swelling of uranium dioxide is one of the main factors which limit the service life of high-temperature fuel elements of space nuclear power systems [1]. The computational-theoretical and experimental data on swelling under high-temperature irradiation are limited and contradictory [2]. According to some data, the swelling increases linearly with burnup, while according to other data the swelling saturates. Some works predict the existence of a maximum temperature dependence of swelling, while other works predict a sharp increase of swelling with temperature. These discrepancies could be due to the characteristic behavior of uranium dioxide due to the migration of bubbles of gaseous fission products and structural changes under high-temperature irradiation. The existing computational-theoretical models do not fully take into account the combined effect of these processes. The BARS statistical model of swelling (Bubble Analysis of Radiation Swelling of Fuel), which takes account of the characteristic behavior of bubbles of gaseous fission products and the change in the size and shape of uranium dioxide grains at high temperature, is proposed for analyzing the effect of the operational parameters on high-temperature swelling of uranium dioxide and to search for ways to increase the radiation resistance of oxide fuel.Reactor tests of samples [3] have made it possible to obtain data on free (not restrained by cladding) swelling of various modifications of uranium dioxide at high temperature and to develop a model of the deformation behavior of fuel elements SDS (Stress-Deformation State of Fuel Element). Fuel samples tested for density, oxygen content, grain size and shape, phase composition, and impurity content were used to obtain data on unrestrained swelling. This made it possible to determine the specific nature of the high-temperature radiation behavior of uranium dioxide and its modifications and to provide representative input data for the SDS codes. This factor and the possibility of...
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