Thermal modeling of the ceramic composite fuel for light water reactors

Full text of publication follows: Composite fuel designs capable of providing improved thermal performance are of great interest in advanced reactor designs where high efficiency and long fuel cycles are desired. Thermal modeling of the composite fuel consisting of continuous second phase in a ceramic (uranium oxide) matrix has been carried out with detailed examination of the microstructure of the composite and the interface. Assuming that constituent phases are arranged as slabs, upper and lower bounds for the thermal conductivity of the composite are derived analytically. Bounding calculations on the thermal conductivity of the composite were performed for SiC dispersed in the UO2 matrix. It is found that with 10% SiC, the thermal conductivity increases from 5.8 to 9.8 W/m deg. K at 500 K, or an increase of 69% was observed in UO2 matrix. The finite element analysis computer program ANSYS was used to create composite fuel geometries with set boundary conditions to produce accurate thermal conductivity predictions. A model developed also accounts for SiC-matrix interface resistance and the addition of coatings or interaction barriers. The first set of calculations using the code was to model simple series and parallel fuel slab geometries, and then advance to inter-connected parallel pathways. The analytical calculations were compared with the ANSYS results. The geometry of the model was set up as a 1 cm long by 400 micron wide rectangle. This rectangle was then divided into one hundred sections with the first ninety percent of a single section being UO2 and the remaining ten percent consisting of SiC. The model was then meshed using triangular type elements. The boundary conditions were set with the sides of the rectangle being adiabatic and having an assigned temperature at the end of the rectangle. A heat flux was then applied to one end of the model producing a temperature gradient. The effective thermal conductivity was then calculated using the geometry, boundary conditions, and results from ANSYS model. The thermal conductivity of the series geometry was compared to hand calculations that used the electric-heat analog. A results showed that the two calculations agree, demonstrating that the ANSYS model performs as expected. Therefore, with this initial comparison completed, modeling of a parallel and inter-connected parallel geometry was performed. The results were compared to hand calculations as previously performed with the series geometry to verify the validity of using ANSYS in producing accurate thermally enhanced nuclear fuel models. Experimental measured thermal conductivity was compared with the model predictions for commercial two-phase composites. (authors)