Development and Verifications of Fast Reactor Fuel Design Code CEPTAR

Annular fuel is very beneficial for fast reactors because of its availability for both high power and high burnup. Most of the annular pellets irradiated up to high burnup showed central-hole shrinkage due to deformation and restructuring during irradiation. This shrinkage has a great influence on power-to-melt, which is a main factor in deciding the maximum power in the fuel design. To predict precisely the central-hole shrinkage during irradiation, the CEPTAR code was developed and verified by using the results of various experiments. In this code, the central-hole diameter is decided in accordance with the law of conservation of mass by using the radial profile of fuel density computed with the void migration model, and its deformation caused by the thermal expansion, swelling, and creep is computed by stress-strain analysis using the approximation of plane strain. Furthermore, this code can also estimate the effect of joint oxide gain (JOG) observed in a gap between the cladding and the fuel pellet with high burnup, which tends to decrease the fuel swelling and to improve the gap conductance due to deposition of solid fission product to the JOG layer. In this paper, an outline of the CEPTAR code and the results of verification are presented.