Experimental study of Dracs steady-state and transient performance

Direct Reactor Auxiliary Cooling System (DRACS), as a passive safety system, has been proposed for the Fluoride-salt-cooled High-Temperature Reactor (FHR) for decay heat removal. The DRACS completely relies on buoyancy as the driving force and uses the ambient air as its ultimate heat sink. It consists of three natural circulation/convection loops that are coupled through two heat exchangers, namely, the DRACS Heat Exchanger (DHX) and the Natural Draft Heat Exchanger (NDHX). The performance of the DRACS depends on not only that of each individual loop but also the couplings among them. To experimentally investigate the DRACS thermal performance, a Low-Temperature DRACS Test Facility (LTDF) has been built. The LTDF is down scaled from a 200-kW prototypic DRACS design developed at The Ohio State University, and uses water as the surrogate coolant. In this paper, the effects of the decay power level, loop flow resistance, and air inlet temperature on the DRACS long-Term steady-state performance are first examined. Next, two transient scenarios simulated in the LTDF are discussed. In the first scenario, startup of the DRACS system from a cold state is simulated with no initial primary coolant flow. In the second scenario, a reactor coolant pump trip process is studied, during which a flow reversal phenomenon in the DRACS primary loop occurs. In both scenarios, a modified LWR decay power curve is applied, and the results illustrate successful development of natural circulation flows as the transients approach their quasi steady states.