Influence of Porosity on Deformation and Fracture of UO2

The effects of porosity on the deformation characteristics of sintered polycrystalline UO₂ were determined at 500°, 1250°, and 1600°C at a strain rate of ∼0.1 h⁻¹. At 500°C, fracture was brittle and transgranular and initiated from the large pores present after fabrication. An increase in porosity from 5 to 16 vol% caused a 79% reduction in strength. At 1250° and 1600°C, plastic deformation preceded fracture, and the effect of porosity was much more complex. At 1250°C, an increase in porosity increased the strength but lowered the ductility, and the fracture mode changed from intergranular to transgranular. In contrast, at 1600°C porosity had little effect on strength. This behavior was associated with the relative magnitudes of the stress necessary to extend a preexisting flaw and the yield stress and their influence on the brittle-to-ductile transition temperature, T_c.     

The System UO2-ZrO2

A phase diagram is proposed for the system UO₂-ZrO₂ on the basis of earlier work and the results of new studies in which the mixed oxides were melted in a solar furnace to avoid contamination. X-ray diffraction measurements were made on specimens melted in helium and on the same specimens after heating to 1350°C. in helium. The system shows a narrow two-phase region about the eutectic at high temperatures which increases in extent at lower temperatures. The high-temperature form of zirconia cannot be stabilized by the addition of UO₂.     

Radiation-Induced Creep of UO2

In-reactor creep tests were performed on stoichiometric UO₂ helices enriched with 1.82%²³⁵U at fission rates of 2×10¹² to 1.5×10¹³ fissions/cm³-s and outer-fiber shear stresses of 2890 and 2090 psi. Depending on the test procedure, the initial creep transient either decelerated continuously or was of the sigmoidal type; this difference is rationalized in terms of the radiation-produced substructure. For constant substructure, however, strain rate was linearly proportional to fission rate. Strain recovery was also observed when the specimens were unloaded and then irradiated. Postirradiation creep tests conducted at zero fission rate showed that fission events are required for either creep or strain recovery to occur. Incremental-temperature-change experiments and normalized strain-rate-vs-temperature plots verified that the deformation was athermal at T ≤200°C. Postirradiation examination showed that 0.01% strain was recovered after a specimen was fractured and irradiated, but no measurable swelling, changes in density, grain size, or stoichiometry had occurred. No evidence of microfracture was observed on ceramographic sections. A model for radiation-induced creep is proposed in which Frank loops and dislocations absorb interstitials produced locally by the fission spike and move with the aid of the localized thermal energy of the spike.     

Isostatic Hot-Pressing of UO2

Final-stage isostatic hot-pressing of nearly stoichiometric UO₂ was investigated. The rate of hot-pressing is linearly dependent on the pressure driving force F=P+(2γ/r)—p , where P and p are the external and internal pressures, respectively, γ the average surface energy, and r average pore radius. The apparent activation energy for hot-pressing agrees with that for U bulk diffusion. Grain growth during hot-pressing follows d∝tⁿ , where d is the grain diameter, t is time, and n =0.2. Grain size is also a function of temperature at constant density. The results indicate that Nabarro-Herring creep is the controlling mechanism of hot-pressing over the range of variables investigated, although the applicability of this model is questioned. The results also show that sintered UO₂ can entrap gas that can lead to swelling.     

Crack Healing in UO2

Isothermal annealing of thermally shocked UO₂ bars (O/M= 2.00 ±0.01) at 1600°, 1800°, and 2000°C caused crack healing, which was determined from recovery of room-temperature fracture strength. The activation energy for crack healing was ∼0.5 of that for volume diffusion, and healing occurred at the same rate as grain growth. This result has important implications with respect to crack healing in oxide fuels during in-reactor restructuring.     

Compressive Deformation of Polycrystalline UO2

The deformation of polycrystalline stoichiometric UO₂ in compression exhibits a low-temperature (<1200°C) behavior that is distinct from its high-temperature behavior. The data for both temperature regions fit either an Arrhenius equation, =ν exp [-Δ H _(τ)/ RT ], or the relation = A τⁿ/ T exp [−Δ H ₀/ RT ]. At low temperatures, the activation energy and volume, the shape of the yield-stress-temperature curve, and the grain size-strength relation suggest a Peierls mechanism as rate-controlling in the deformation process. At high temperatures (≳ 1300°C), a different dislocation mechanism becomes rate-controlling for coarse-grained material, whereas very fine-grained (1 μm) material exhibits Nabarro-Herring deformation.     

Densification in Irradiated UO2 Fuel

A densifcation algorithm for irradiated UO₂ fuel, modified to reflect a rapid densification rate from 0 to 10 MW.h/kg U, is presented. Included in a fuel performance code, the algorithm gives improved agreement between measured and calculated sheath strains.     

Low-Temperature Equilibria Among ZrO2, Tho2, and UO2

Studies made on low-hafnium-content ZrO₂, show that the monoclinic-tetragonal inversion temperature is 1170°C., and it is raised to approximately 1190°C. in the "natural" ZrO², which contains approximately 2% HfO₂. No explanation could be found for the knownmarked hysteresis during cooling, when the reverse polymorphic transformation takes dace at 1040°C. In the system ZrO₂-ThO₂ the monoclinic-tetragonal ZrO₂, inversion temperature is lowered to 1000°C., although the maximum solid solution extent of ZrO₂, in Thon and vice versa is approximately only 2% at this temperature. Below about 400°C. under hydrothermal conditions it was possible to prepare a continuous, although metastable series of solid solutions with the fluorite structurewith compositions varying from ThO₂, to nearly pure ZrO₂. Contrary to earlier work only 8 mole ZrO₂, dissolves in UO₂ and less than 4 mole of UO, in ZrO₂ at temperatures up to 13OO0C. A continuous series of solid solutions could be made between Th₂ and UO₂ at 13OO°C., and extensive defect fluorite solid solutions could be prepared between Tho₂ and U₃O₈; there is some evidence for exsolution into uranium-rich and thorium-rich members at low temperatures.     

Model for the Deformation of Hyperstoichiometric UO2

The results of deformation studies of stoichiometric and hyperstoichiometric uranium dioxide single crystals are briefly reviewed. The observed differences in slip system and dislocation substructure are rationalized in terms of a proposed model for the deformation of hyperstoichiometric single crystals.     

Deformation of UO2 at High Temperatures

The effects of temperature, strain rate, and grain size on the mechanical properties of UO₂ were investigated using the four-point bending technique. Strain rates were varied by two orders of magnitude, and test temperatures up to 1800°C were used. Data are presented on the ultimate tensile stress, yield stress, and plastic strain-to-fracture. Below the brittle-to-ductile transition temperature, T_c , the material fractured in a brittle manner, with no macroscopic plastic deformation. Between T_c and a second transition at a higher temperature, T_t , a small amount of plastic deformation was measured before fracture. Beyond T_t , the strength of UO₂ decreased continuously, and extensive plasticity was observed. This high-temperature plasticity was characterized by a thermally activated rate-controlling process; this behavior is consistent with observations of creep behavior under high stresses. The following phenomenological equations for the strain rate fit the data for the material with 8-μm grain size above T_t :
and
where σ_p and σ_88f are the proportional limit and steady-state flow stress, respectively, and temperature T is in °K.     

Deformation of Hyperstoichiometric UO2 Single Crystals

Hyperstoichiometric UO₂ single crystals having O/M ratios from 2.06 to 2.10 were deformed in compression. The active slip plane for deformation, as evaluated from slip traces, has an orientation between (112) and (111). The critical resolved shear stresses for hyperstoichiometric specimens are approximately the same as those for stoichiometric specimens; the dislocation structures, as revealed by transmission electron microscopy, however, are very different and suggest more rapid motion and/or climb of dislocations in the hyperstoichiometric samples. The response to strain rate changes during deformation also reflects the difference in deformation behavior of hyperstoichiometric and stoichiometric crystals.