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 B-10 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. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.