Analysis of unprotected loss of heat sink accident in compact high temperature reactor cooled under natural circulation of LBE coolant

摘要

India is developing 100kW(th) Compact High Temperature Reactor (CHTR) to demonstrate the technology enabling high-temperature process heat applications such as hydrogen production. The reactor core is envisaged with 233U-Th based TRISO coated fuel particles and beryllium oxide moderator in prismatic fuel assemblies for high-temperature operation. The core heat is designed to be removed by lead-bismuth eutectic (LBE) coolant under natural circulation in vertical channels of CHTR at nominal power. The heat generated in the primary circuit is to be transferred to the secondary side using high-temperature sodium heat pipes. The design and safety of the prototype core necessitate comprehensive, integrated neutronics-thermal hydraulics analyses of anticipated transients. In this context, the Unprotected Loss of Heat Sink Accident (ULOHSA) at full power during the beginning of the core life cycle is investigated with indigenous 3D multi-physics code ARCH-TH. The key parameters defining neutronics and thermal-hydraulics of the core have been assessed in the steady-state as well as during the transient. The findings, along with the model of the simulation and the code validation are discussed in this paper. The analyses indicate that the core is being shut down passively due to reactivity feedbacks and the loss of heat sink leads to reverse flow of the coolant in the low powered channels during the transient. However, the coolant and fuel peak temperatures are observed to stay well below the leak-tightness limit of TRISO particles and boiling point of LBE, even considering the failure of shutdown systems. The modifications in the design of CHTR are also investigated to pacify the condition of flow reversal during such transients.