Intrinsically Safe Thermohydraulic Designs for SMRs: Design advantages and challenges.

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Abstract
Intrinsically safe design for nuclear power plants (NPPs) often refers to physics design where the fission chain reaction shuts down on its own because of the inherent physics characteristics which has been demonstrated in Triga and EBR-II reactors. Present day NPPs with two diverse automatic shutdown systems with negative power coefficient of reactivity are capable of shutting down the fission chain reaction whereas they do not have an Intrinsically Safe Thermohydraulic Design (ISTD) to remove the decay heat which caused the loss of plant accidents at Fukushima and TMI-2 (Three Mile Island-unit 2). Since SMRs have smaller site with reduced exclusion and emergency planning zones located in close proximity to population centres, the public domain starts practically from the site boundary. Hence they require ISTD to eliminate the risk of radioactivity release in public domain. The present paper describes several concepts for ISTD applicable to pressure tube type and vessel type LWR SMRs and liquid metal cooled MMRs (Micro Modular Reactors). The advantages and challenges of different ISTDs are also compared in the paper. Reported studies by the authors show that the use of two diverse direct fuel cooling systems in LWRs with one of them a low pressure passive system employing a liquid metal coolant can terminate all transients with a temperature below that causes significant metal-water reaction even with one cooling system being completely dry due to LOCA. However, use of two diverse direct fuel cooling systems require the use of annular fuel clusters or plate type fuel.