Evaluation of a Dual-Cooled Annular Fuel Heat Split and Temperature Distribution

The heat flux and fuel temperature of an annular fuel rod was analyzed using a newly developed program, DUO_THERM. The dual-cooled annular fuel rod has both inner and outer coolant channels. The heat fluxes to inner and outer claddings can change during irradiation because of the variation in the gap conductances of the inner and outer sides. The heat flux is a very important design factor affecting fuel integrity and safety. However, in an annular fuel design, the prediction of heat flux is very complicated because it is closely connected with size changes of two gaps, which are different from each other during irradiation. A burnup increase and power change can cause pellet and cladding radial deformation and eventually change the gap width. Because an annular fuel has two gaps at both sides and their heat resistances are very large, an imbalance of the gap width can lead to heat flux asymmetry. Therefore, a pellet and cladding dimensional change including densification, swelling, creep, and thermal expansion must be considered in the calculation of heat flux and temperature. By using the DUO_THERM program, the changes in heat flux and temperature during an operation were evaluated with respect to a reference design of an annular fuel. The results of the evaluation showed that the heat flux of an annular fuel was greatly affected by the inner and outer gap conductance changes. The heat flux of the inner cladding was maximum at zero burnup, but that of the outer cladding was maximum at the burnup of the outer gap closing. In addition, the maximum fuel temperature location was changed by gap conductance asymmetry.