The thermal modeling of a composite fuel consisting of continuous second phase in a ceramic (uranium oxide) matrix has been carried out with aid of detailed examination of the microstructure of the composite and the interface structure。 BeO and SiC were considered as second phase dispersed in UO2 matrix by weight from 0–15% to enhance the thermal conductivity。 It is found that with 10% SiC, the thermal conductivity increases from 5。8 to 9。8 W/m-K at 500 K。 A finite element analysis computer program ANSYS was used to create composite fuel geometries with set boundary conditions to produce accurate thermal conductivity predictions。 The results were compared to analytical calculations as previously performed with the series geometry to verify the validity of using ANSYS in producing accurate thermally enhanced nuclear fuel models。 Good agreement was found between experimental measured thermal conductivity for BeO-UO2 matrix and the model predictions。84892

INTRODUCTION The most common fuel material UO2 used in fission nuclear reactor has low thermal conductivity, though it has many other desirable characteristics。 The low thermal conductivity of UO2 leads to a variety of fuel performance problems, such as pel- let cracking, fuel relocation, and further degradation in thermal conductivity。 Composite fuel designs capable of providing im- proved thermal performance are of great interest in advanced reactor designs, where high efficiency and long fuel cycles are desired。 The dispersion fuel consisting of uranium dioxide and mixed plutonium-uranium dioxide in a stainless steel matrix has been studied in the advanced test reactor in rod and plate geome- tries [1, 2]。 A composite fuel for nuclear reactor consisting  of a ceramic fuel matrix containing metal fibers uniformly aligned throughout the matrix has been suggested in the past [3]。

The overall objective of this study was the development of oxide fuels with a continuous high conductivity phase that is stable at high temperatures and to neutron and fission frag- ment damage, is compatible with oxide fuels, exhibits minimum neutronic effects, and provides the maximum improvement  in

Address correspondence to Dr。 Shripad T。 Revankar, School of Nuclear Engineering, 400 Central Drive, Purdue University, West Lafayette, IN 47907, USA。 E-mail: shripad@ecn。purdue。edu

thermal conductivity。 Improved thermal conductivity has many advantages, including lower stored energy in the fuel; reduced radiation-induced degradation like fuel cracking, swelling, and gas release; and possible power up rating。 In addition, prop- erly chosen stable compounds can yield a waste form of higher stability。 SiC and BeO are two materials that demonstrate com- patibility with UO2 and have a high thermal conductivity [4, 5]。 The high conductivity compounds can be formed either by an impregnation process, such as polymer impregnation and py- rolysis (PIP), that permits, for example, silicon carbide (SiC) to be formed within sintered fuels of reasonably high density but with large open porosity; or by dispersed whiskers of SiC, which is simpler to fabricate, though it may have discontinuous SiC phase。

In the present study two fuel types were examined, UO2/BeO and UO2/SiC。 The goal of this study was to determine the effect of the high conductivity phase (SiC, BeO) on the thermal con- ductivity of the UO2/BeO and UO2/SiC composite fuels in terms of volumetric fraction and micro-structure configuration。 The thermal conductivity of SiC and BeO are about one order mag- nitude higher than the UO2。 The literature shows a wide range of the SiC thermal conductivity data depending on the method of preparation。 In the UO2/SiC, two types of fuel composite were considered: UO2/PIP-SiC and UO2/SiC whisker compos- ites。 The UO2/PIP-SiC has a continuous highly conductive but

porous SiC phase, while the UO2/SiC whisker composite has  Table 1    UO2-BeO pellet   data