Impact of high porosity on thermal transport in UO 2 nuclear fuel
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During the advance of the nuclear fission reaction, fission products accumulate and form pores (gas bubbles) that decrease the thermal conductivity of the nuclear fuel, potentially leading to overheating of the fuel element. To investigate this important phenomenon, a finite-element method is used to simulate the effect of 3-dimensional (3D) distributions of pores on the thermal transport in a nuclear fuel element consisting of uranium oxide (UO2) nuclear fuel pellet and Zircaloy cladding. Spherical pores ranging in size from 70 to 172 lm are introduced to create up to 30 vol% total porosity. The simulations demonstrate that the centerline temperature increases with the total porosity and the increase is nonlinear. The results also show that the centerline temperature, at fixed total porosity, weakly depends on the pore size distribution. This method can provide useful information regarding the effect of high porosity levels that may occur in off-normal operation conditions.
I. INTRODUCTION
Thermal transport in the fuel elements is a phenomenon of critical importance for reactor operations. The performance of nuclear fuels in general and UO2 fuels in particular strongly depends on the operating temperature. Making accurate measurement and developing predictive models of the fuel thermal conductivity are essential components of fuel performance assessment. Extending the operation limit of nuclear fuels also calls for accurate fuel thermal conductivity evaluations to predict microstructure evolution, fission gas release, and fuel swelling at high burnup levels. Significant research was conducted to address this need since the 1960s, and several review articles summarize the available experimental data and the effects of various factors on the oxide fuel thermal conductivity.1–4 Among these factors, the most critical are fuel porosity and fuel stoichiometry. Porosity is introduced to the oxide fuel either by fabrication (sintering) or during irradiation of the fuel in the reactor via fission products and gas bubble formation. Fuel porosity varies in both size and morphology and tends to be distributed heterogeneously in the fuel pellet. A correction factor is often used to characterize the effect of porosity on thermal conductivity of oxide fuels.5,6 In this approach, the effective thermal conductivity of the fuel is given by: k ¼ k0 f
;
ð1Þ
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.142 2308
J. Mater. Res., Vol. 28, No. 17, Sep 14, 2013
http://journals.cambridge.org
Downloaded: 14 Mar 2015
where k is the effective thermal conductivity of the porous fuel, k0 is the thermal conductivity of the fully dense fuel, and f is the porosity correction factor. Several models have been proposed for the correction factor in Eq. (1), emerging from early theoretical studies of porous materials. For example, the modified Leob model accounts for the size, shape, spatial distribution, and the emissivity of the pores.7 The Maxwell–Eucken model, inspired by Maxwell’s work on the electri
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