| Summary: | Three aspects of the mechanical behaviour of fast reactor fuel have been studied, two being theoretical studies of deformation during operation and the third an experimental investigation of creep in a particular fuel. a. Thermal stress in fuel pellets A computer program was constructed to calculate, by a difference approximation, thermal stress in finite cylinders for a given temperature distribution. This was used to describe the effect of length to diameter ratio, central hole diameter and heat flow conditions, on the stresses and surface displacements of fast reactor fuel pellets. Estimates of the likely crack behaviour of pellets were made and it was concluded that cracking would prevent pellet shape changes from producing clad ridges. However, under certain conditions ridging could be produced by temperature dependent swelling shape change. b. Restraint of the swelling fuel by its clad Mathematical techniques and computer programs were developed to treat this problem in the radial direction for transient and steady state conditions, The programs were then used to derive the basic characteristics of fuel pin behaviour, to determine the effect of changes in variables, and to compare oxide and carbide fuels. c. Compressive creep of zone refined uranium monocarbide Single crystals and polycrystalline specimens of carbon rich and metal rich UC were deformed in a vacuum in a compressive creep furnace, in the 1000-1300°C range. Metallography and Laue back-reflection patterns were used to aid the interpretation of the creep results. A slip system of {111} type was found to be consistent with the results. Comparison with the single crystal results of Bentie et al, (1963) indicated that a single creep mechanism, probably controlled by uranium diffusion, was operating for carbon rich UC in the temperature range 1000-2000°C. Creep in metal rich UC is more complex, being dependent on grain size and metal solubility.
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