Optimum design of shim rod assembly absorber bundle for Lead Bismuth Eutectic (LBE) cooled reactor

• The basic parts of the absorber bundle in shim rod for LBE-cooled reactor were analyzed.
• The reactivity perturbation introduced by the various cladding and inner wrapper thicknesses were calculated.
• The amount of boron carbide (B4C) to be installed in absorber bundle was calculated.
• The B4C burn up was calculated and the reaction gas release was analyzed.
• The air gap thickness and gas plenum volume were calculated.

The amount of boron carbide (B4C) installed in shim rod assembly bundle which covers the shutdown margin (SDM) demands has been preliminary analyzed by Super Monte Carlo Calculation Program for Nuclear and Radiation Process (SuperMC), which was developed by FDS Team. Based on absorbent material amount and shim rod assembly dimension limits, the optimal number and arrangement of absorbent rods have been determined. Calculation on the nuclear reaction gas production and the 10B burn-up has been performed. Following the first step, the burn-up dependence diametral swelling of boron carbide pellets and the release of gaseous product within absorbent rods have been analyzed. The air gap and the gas plenum were designed to avoid the absorber-cladding mechanical interaction (ACMI) and hold the released gaseous reaction gases simultaneously. It is calculated that the optimized design of the bundles is set as follows: the absorbent rods number is 19, the cladding thickness is 0.6 mm, the boron carbide pellets radius is 11.84 mm, the air gap thickness is 0.4 mm and the gas plenum length is 1010 mm.