Optimization of Height-to-Diameter Ratio for an Accelerator-Driven System

The height-to-diameter (H/D) ratio of a lead-bismuth eutectic (LBE)-cooled accelerator-driven system (ADS) has been evaluated in terms of the neutron multiplication, the coolant void worth, and the coolant velocity. For a model ADS, an optimization of the H/D ratio is performed with a Monte Carlo code both for the effective multiplication factor k_eff and for the multiplication of the external neutrons. In the optimization, ten cases of H/D values have been analyzed for a homogeneous fuel blanket. Also, the dependency of the optimal H/D ratio on the target/buffer is addressed. The Monte Carlo simulations show that the optimal H/D configuration of the ADS core is quite different for the two important measures, and a high H/D ratio can provide a significantly higher source multiplication than the traditional pancake core. Furthermore, various core analyses including depletion calculations are conducted for three selected heterogeneous cores with different H/D ratios, which are a small H/D value (pancake type), a medium H/D value, and a high H/D value, respectively. Void reactivity coefficients of the LBE coolant are evaluated and compared for the three designs to quantify the effects of the H/D ratio. Additionally, a thermal-hydraulic analysis has been performed to derive a maximum allowable core height subject to the LBE velocity limit due to its corrosion and erosion characteristics. It is shown that the practically optimal H/D ratio for source multiplication is tightly constrained by the maximum allowable LBE velocity, depending on the core design parameters.