NEUTRONIC AND THERMAL HYDRAULIC DESIGNS OF ANNULAR FUEL FOR HIGH POWER DENSITY BWRS

Abstract

As a promising new fuel for high power density light water reactors, the feasibility of using annular fuel for BWR services is explored from both thermal hydraulic and neutronic points of view. Keeping the bundle size similar to conventional GE 8×8 solid fuel bundles, annular fuel bundles of 5×5 and 6×6 lattices, that have increased thermal output potential, are explored. The large annular fuel rods allow both external and internal cooling, which increases the fuel surface to volume ratio and significantly reduces the fuel temperature. A methodology is developed and VIPRE code calculations are performed to select the best annular fuel bundle design on the basis of its Critical Power Ratio (CPR) performance. Within the limits applied to the reference solid fuel, the CPR margin in the 5x5 and 6x6 annular fuel bundles is traded for an increase in power density. It is found that the power density increase with annular fuel in BWRs may be limited to 23%. This is smaller than possible for PWRs due to the difference in the mechanisms that control the critical thermal conditions of the two reactors. The neutronic aspects of annular fuel in BWRs, including the reactivity history, power distribution, and burnup characteristics, are investigated. Results are compared to the conventional BWR solid fuel bundle for the same power density and total energy. In general switching to annular fuel implies smaller neutronic differences than in the PWR case. The local peaking factors are found to be similar or slightly better than those of the solid fuel bundle. To maintain the same fuel cycle length, the burnup needs to be increased even for the same bundle energy output, due to reduced fuel loading. These results are based on two dimensional bundle models using the CASMO code, whose validity has been checked using MCNP models. In summary, the annular fuel could be a profitable alternative to the solid fuel due to neutronic and thermal advantages. Further study of the trends observed in this report are needed to increase their certainty.