India is developing a 100kWth Compact High Temperature Reactor (CHTR) to facilitate demonstration of technologies for high temperature process heat applications. CHTR is being designed as thorium based TRISO fueled and beryllium oxide moderated prismatic block type vertical core cooled with lead-bismuth eutectic (LBE) under natural circulation for 1000°C outlet. The new concept of high temperature core requires multi-physics multi-scale modeling based tools for investigating the normal operational behavior as well as anticipated transients of CHTR. In view of that, 3D multi-physics code ARCH-TH is being indigenously developed and validated for coupled neutronics-thermal hydraulic benchmarks. The multi-group diffusion based neutron kinetics in code ARCH-TH is integrated with thermal-hydraulic (TH) capability considering 1D- radial heat conduction in multi-channel system. The 1D based hydraulic in the code is capable to simulate the natural circulation phenomena of coolant in coupled parallel multi-channel system. The steady state operation and case of unprotected transient of control rod withdrawal in CHTR have been simulated with ARCH-TH. The rising transient power in CHTR is being assumed to be arrested with temperature feedbacks in the core cooled with natural circulation of molten metal coolant LBE. The variation of several key parameters defining neutronics and thermal hydraulics of CHTR have been predicted and discussed in detail. It has been found that peak fuel and coolant temperatures are well within the safety limits during transient of inadvertent withdrawal of control rod in CHTR operating at full power during initial core life cycle
