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Emergency Decay Heat Removal by Reactor Vessel Auxiliary Cooling System from an Accelerator-Driven System

Abstract The passive emergency decay heat removal during severe cooling accidents in Pb/Bi-cooled 80- and 250-MW(thermal) accelerator-driven system (ADS) designs was investigated with the computational fluid dynamics code STAR-CD. For the 80-MW(thermal) design, the calculations show that no structural problems occur as long as the accelerator proton beam is switched off immediately after accident initiation. A highly unlikely delay of beam stop by 30 min after a combined loss-of-heat-sink and loss-of-flow accident would lead to increased reactor vessel temperatures, which do not cause creep failure. By using a melt-rupture disk on the vacuum pipe of the accelerator proton beam to interrupt the beam at elevated temperatures in a passive manner, the grace time before beam stop is necessary is increased from 30 min to 6 h. An emergency decay heat removal design, which would prevent radioactive release to the atmosphere even more reliably than the Power Reactor Inherently Safe Module (PRISM) design, was also investigated. For an ADS of 250-MW(thermal) power with the same vessel as the 80-MW(thermal) ADS examined, the maximum wall temperature reaches 745 K after an immediate beam stop. This does not cause any structural problems either. The grace time until a beam stop becomes necessary for the 250-MW(thermal) system was found to be ~12 min. To reduce elevated vessel temperatures more rapidly after a beam stop, alternative cooling methods were investigated, for example, filling the gap between the reactor and the guard vessel with liquid metal and the simultaneous use of water spray cooling on the outside of the guard vessel. This decreases the coolant temperatures already within minutes after switching off the proton beam. The use of chimneys on the reactor vessel auxiliary cooling system, which increase the airflow rate lowers the maximum reactor vessel wall temperature only by ~20 K. It can be concluded that the critical parameter for the emergency cooling of an ADS is the time delay in switching off the accelerator after accident initiation.