Analysis of Resistance to Thermal Stress in Ceramic Oxide Nuclear Materials

Abstract

Uranium and uranium-related ceramic oxide fuel pellets are widely used in nuclear power reactors currently in operation for electricity generation. These oxide nuclear fuels developed so far and being used in the reactors have advantages over the metallic fuels; e.g., stable phases with high melting point, good compatibility with coolant and stability against neutron irradiation. Certain disadvantages of these materials are, however, the lower thermal conductivity than that of metallic fuels, which affects and, as a consequence, their thermo-physical and thermo-mechanical properties and their in-reactor performance. Currently, technologies are being developed for the improvement of such properties, especially when these oxide fuels are subjected to higher burn-ups. [1±3] Fuel behavior against thermal shock can be one of the important properties that can give estimations not only for the behavior of the fuels in service during reactor operation but also for the thermo-mechanical properties of the fuel materials, [4] which are affected by material characteristics such as density and microstructure. This paper describes the results of the analyses of the behavior of UO 2 -5 wt.-% CeO 2 , simulating (U-Pu)O 2 fuel, against thermal shock by measuring basic mechanical properties such as Vickers hardness H V , fracture toughness K IC , fracture strength r f and thermal shock resistance DT C for the different material densities before and after the thermal shock by quenching and microscopic observations. An attempt was COMMUNICATIONS 584

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“…Large pores associated with using coarse SiC power did not show any impairment and the maximum lower bound fracture strength and fracture toughness achieved were 201 MPa and 2.9 MPa m 1/2 respectively. These values are significantly higher than the reference MOX fuel and other IMF pellets (40-100 MPa and 0.7-2.5 MPa m 1/2 respectively) [34,35,37].…”

“…The highest hardness achieved before and after PIP cycle was 3.5 and 5.6 GPa, which are lower than the reference MOX fuel and other simulated IMF pellets (5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15) [34,35]. A decreased hardness of fuel pellets is likely to reduce the mechanical interactions between pellet and cladding during operation and is therefore considered advantageous.…”

“…Fracture toughness values of simulated MOX specimens measured by Lee et al [34] ranged from 0.7 to 1 MPa m 1/2 . For ZrO 2 based IMF pellets values from 1.5 to 1.7 MPa m 1/2 were reported by Lee et al [37].…”

“…For simulated MOX fuel, fracture strength values of around 40 to 50 MPa were reported [34] and a UO 2 pellets of 98.3% TD exhibited a fracture strength of 76 MPa [34]. For ZrO 2 based IMF, values ranging between 40 and 100 MPa were determined [37].…”

“…This suggests that the chemical binding strength (between grains) in conventionally sintered SiC pellets is stronger than the binding strength between the amorphous SiC phase and the crystalline b-SiC particles. Vickers hardness for simulated MOX pellets (UO 2 -5 wt.% CeO 2 ) has been studied by Lee et al [34]. In their study, the theoretical densities of the pellets ranged between 98.3% and 96.5%, and the [36].…”