The high temperature deformation of uranium dioxide fuel material

• Main Author: Reynolds, G. L.
• Published: Swansea University 1978
• Subjects: 669
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.638647

The creep deformation of metals and non-metals is reviewed and the various models . discussed. The different modes of deformation are considered separately. Theories of creep due to dislocation movement are presented and experimental investigation into this type of creep considered. The diffusional creep of metals and ceramics is reviewed and its inhibition by second phase particles and interfacial processes are discussed. Creep fracture is considered and models of failure are reviewed in the light of the many papers on this wide subject. The chemical and defect structure of uranium dioxide is briefly described. The thermal creep of uranium dioxide is reviewed and the effect of environmental conditions on the creep properties considered. Results of compression creep tests carried out on stoichiometric uranium dioxide are presented. Above a transition stress of approximately 70 MN/m2 the rates vary strongly with stress in accord with theories of creep based on dislocation models, the value of the stress index being '6. The temperature dependence of the creep rate at high stress results in an activation energy for the process of 5.2 x 105 J/mole K. In the high stress region it is shown that the fine grain material has the greatest creep strength. This strengthening is considered in terms of the path length which dislocations move before meeting the barrier of a grain boundary. It is shown that at high temperature this effect is reduced. The threshold stress, below which diffusion creep becomes rate controlling, is shown to be affected by deviation from stoichiometry. It is considered that the excess oxygen can promote slip on additional slip systems. Creep fracture processes in stoichiometric and non-stoichiometric uranium dioxide are studied in some detail. At high strain rates fracture occurs by the propogation of triple point cracks which extend over large distances along grain boundaries. At lower strain rates the process involves the growth of rounded cavities on boundaries parallel to the compression axis. The growth of these is considered to result from vacancy condensation. The driving force for this process is considered to be the generated tensile components of the applied stress. It is shown that non-stoichiometric and large grained material is more susceptible to creep damage. This result is attributed to the increase in grain boundary off-sets in large grained material and an increase in the ease of sliding in the non-stoichiometric material. The effect of small additions of Nb205 on the creep properties of uranium dioxide fuel material has been examined. It is shown that the creep rate of uranium dioxide is markedly affected by these additions. The effect is to increase the effective diffusivities by several orders of magnitude over that of undoped material. It is considered that the dopant additions affect diffusional creep by the modification of the defect structure. It is shown that some agreement exists between the experimental observations and diffusivities calculated using existing models of the defect structure of hyperstoichiometric uranium dioxide. The anelastic creep behaviour of uranium dioxide in the form of springs, has been investigated. The transient strain is considered to result from the bowing of pinned dislocations. Removal of small amounts of the applied stress results in some cases in a period of negative creep. The experimental strain/time curves are shown to agree with curves computed from a consideration of the distribution of dislocation links within the material. A deformation map showing the boundaries of the various deformation mechanisms has been constructed using experimentally determined constitutive equations. This is a result of experimental work carried out in one laboratory (B.N.L.) on identical material.