Figure 2 The PIP-SiC thermal conductivity curves with different densities generated from the 88% dense PIP-SiC thermal conductivity curve。

Figure 3 SiC phase (dark) within the open porosity of UO2 produced by polymer impregnation and pyrolysis。

In a separate experimental work [18], specimens of UO2-SiC composites were prepared using modified PIP process。 An exam- ple of the SiC phase in UO2 is shown in Figure 3。 The interface between the SiC and UO2 shows very little gap, indicating that the bonding is good between two phases。 In modeling this com- posite, the thermal resistance between the two can be neglected。 These specimens’ microstructures were examined in devel- oping ANSYS models。 The ANSYS finite element computer program was then used to model the composite fuel, with ten percent of its volume (the open porosity in the UO2) being as- signed to the SiC phase。 This volume percentage is one example of the current composite fuel being produced。 The thermal con- ductivity curve [6] for the UO2 phase was calculated at 97% TD, which is very close to the measured closed porosity in the UO2 in our specimens。 The thermal conductivity for composite

fuel is shown in Figure 4。

Figure 4 The calculated thermal conductivities of the UO2/PIP-SiC fuel with 10 vol% PIP-SiC and varying densities versus 95% dense UO2。

The composite thermal conductivity is calculated to be 33% greater than the typical UO2 fuel even when using the mea- sured conductivity of the SiC, which is lower than our previ- ously assumed values。 From the composite thermal conductiv- ity curves in Figure 4, the fuel temperature profiles were then calculated for the composite fuel using the following worst case assumptions:

•The fuel pellet was assigned a linear heat generation rate (LHGR) of 17。52 kW/m, for the hot channel in a PWR。 This LHGR was then multiplied by a 1。7 peaking factor, a thermal power limit。

•The temperature at the outer radius of the fuel pellet at the hot spot was calculated to be 576◦C and was kept constant to

demonstrate the effect of the high conductivity phase under equal power generation conditions。 This was accomplished using PWR data including the average moderator temperature of 312◦C, the fuel’s LHGR, and the use of the Dittus-Boetler equation [19] to determine the heat transfer coefficient of the fuel rod。

These heat conduction calculations account for the linear heat generation in a fuel rod that is conducted through the fuel, gap, and cladding to the nearly saturated water coolant。 At the outside of the cladding, heat is transferred away from the fuel rod by forced convection, which is quantified by the heat transfer coefficient。 The heat transfer coefficient is de- rived from the Nusselt number using the Dittus-Boelter equa- tion, which is based on core flow and water property data。 The heat conduction calculation provides a temperature pro- file, and, more specifically, a temperature difference, as shown in Figure 5。 The results in Figure 5 show that there is approxi- mately a 250◦C difference in centerline fuel temperatures be- tween the 70%TD SiC composite and normal 95%TD UO2 fuel。

Figure 5 Radial temperature profile in pressurized water reactor fuel rod with UO2/SiC composite fuel。

FEM THERMAL MODEL OF UO2/SIC WHISKER COMPOSITE

A thermal modeling study was performed to examine the difference between a continuous but porous SiC phase and one composed of dispersed high density SiC whiskers。 Using high- density SiC whiskers dispersed in UO2 may be, in   principle, a simpler processing method, and may localize the reactions between SiC and UO2 at high temperatures。

The initial study was to compare the enhancement in con- ductivity with the same volume percentages of the SiC phase, the only differences being either continuous SiC but porous SiC from an impregnation process or discontinuous high-density SiC whiskers。 We also chose the whisker form because we were pre- viously able to sinter 6 vol % of these whiskers in alumina powders to 95%TD。