Graduate Thesis Or Dissertation
 

Modeling of the thermal behavior of irradiated sphere-pac mixed carbide fuel

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/37720h17v

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  • Several fuels are being investigated for fast breeder reactor applications. One promising concept is sphere-pac mixed carbide fuel. As a part of the development of this fuel concept it is necessary to determine the behavior of the irradiated fuel for safe and reliable operation on a sound economic basis. It is with the thermal aspect of this behavior that the current investigation is concerned. With a view to predicting the overall thermal behavior of the fuel pin during irradiation, theoretical models for various phenomena have been developed so that the various interrelated thermal components can be determined. These components include the thermal conductivity of the fuel in its initial configuration and during restructuring, the restructuring boundary, the temperature distribution, the extent of restructuring due to sintering, grain growth and porosity redistribution, fuel swelling and gas release, and the behavior of the fill gas in the free volume of the fuel pin. A two-dimensional unit cell model is developed to evaluate the effective conductivity of the fuel in its initial configuration. By implementing descriptions of sintering mechanisms on this model, the effective conductivity of the fuel during restructuring is evaluated. The restructuring boundary at each axial section is determined based on a thermal conductivity criteria. Under certain conditions the initial fuel configuration in the inner fuel regions is lost and is replaced by a porous pellet-shaped region. In the outer regions, however, fuel restructuring is not completed. The radial temperature distribution is evaluated by finite differences at a discrete number of axial sections. The local heat source has previously been computed for each section. Allowance is made for axial asymetry as well as power histories. The porosity redistribution in the inner fuel regions which have completed restructuring is evaluated by using a model describing the conservation of pores migrating up the thermal gradient. Swelling and gas release are modeled allowing for bubble nucleation, bubble and pore growth and re-solution of gas atoms. Gas release occurs through diffusion of individual gas atoms in the fuel matrix and follows the subsequent buildup of contiguous intergranular bubbles. Allowance is made for changes in the fuel microstructure due to restructuring and thermally-activated grain growth. Following gas release, the temperature and pressure of the free gas are evaluated. The models describing the various thermal behavior components were embodied into the computer code SPECKLE-I to predict the thermal response of the irradiated fuel pin. Results computed from SPECKLE -I indicate that the effective fuel conductivity increases with temperature, fill gas pressure and with the extent of sintering occuring between the fuel spheres. As gas release occurs, the effective conductivity is altered by the fission gas and the pressure buildup in the free volume. The fuel temperature, initially high, decreases significantly due to restructuring and is affected by the compensating effects of gas release and pressure buildup. The extent of restructuring and porosity redistribution increases with the fuel power. Fuel swelling and gas release are essentially controlled by the fuel microstructure and porosity. The results indicate that the extent of fuel restructuring affects swelling and gas release. As gas release occurs, the pressure of the fill gas increases significantly, its temperature remaining almost constant. Results from SPECKLE-I were compared with several fuel irradiations. The agreement obtained between predicted and measured thermal components suggests that SPECKLE-I is a valuable first step for evaluating the thermal response of irradiated sphere-pac mixed carbide fuel. The performance of this code, however, can be improved by further model development and experimental studies of phenomena not fully characterized.
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