A feasibility study of a liquid shutdown system for a single-fluid molten salt fast reactor

The present study exposes a feasibility analysis for a nuclear reactor shutdown system by means of a liquid neutron absorber. The presented system has been designed for single-fluid Molten Salt Fast Reactors (MSFR) but can be easily extended to other Molten Salt Reactor (MSR) designs and even to liquid metals (LM) reactors. Most of the safety shutdown concepts for MSRs designs rely on their intrinsic negative temperature coefficient to make the reactor subcritical. In case of emergency, most MSRs concepts make use of the widely accepted drainage system by means of frozen plugs or other passively activated mechanisms that drive the fuel to an also passively cooled set of containers below the reactor. In this study, an alternative shutdown system is described, consisting of filling a set of pipes inside the reactor with a liquid neutron absorbing material. The system may be used as the main shutdown system, but in the present work it is studied as a complementary system to a central shutdown/control rod. A simplified single fluid external indirect cooling MSFR model with axial and central reflectors has been used to test the system. The only purpose of this model is to check the feasibility of the system and not to show an actual working system in a reactor design. Therefore, a pre-conceptual design is presented. Several representative analyses including neutronics, hydraulics and a mechanical assessment, have been carried out to evaluate the feasibility of the concept. This feasibility study aims to be a decided step towards a safe passive shutdown system for Generation IV (GenIV) reactors with special focus on MSRs.