The Advanced High Temperature Reactor (AHTR) is one of the advanced rector concepts that have been proposed for Gen IV reactors. The AHTR combines four main proven nuclear technologies, namely, the liquid salt of molten salt reactors, the coated particle fuel (TRISO particle) of high-temperature gas-cooled reactors, the pool configuration and passive safety system of sodium-cooled fast reactors, and the Brayton power cycle technology. The Direct Reactor Auxiliary Cooling System (DRACS) is a passive heat removal system that has been proposed for AHTR. The DRACS features three coupled natural circulation/convection loops relying completely on buoyancy as the driving force. In the DRACS, two heat exchangers, namely, the DRACS Heat Exchanger (DHX) and the Natural Draft Heat Exchanger (NDHX) are used to couple these natural circulation/convection loops. In addition, a fluidic diode is employed to restrict parasitic flow during normal operation of the reactor and to activate the DRACS in accidents.
While the DRACS concept has been proposed, there are no actual prototypic DRACS systems for AHTRs built and tested in the literature. In this report, a detailed modular design of the DRACS for a 20-MWth FHR is first developed. As a starting point, the DRACS is designed to remove 1% of the nominal power, i.e., the decay power being 200 kW. FLiBe with high enrichment in Li-7, and FLiNaK have been selected as the primary and secondary salts, respectively. A 16-MWth pebble bed core proposed by University of California at Berkeley (UCB) is adopted in the design. Shell-and-tube heat exchangers, based on Delaware Method have been designed for the DHX and NDHX. A vortex diode that has been tested with water is adopted in the present design. Finally, pipes with inner diameter of 15 cm are selected for both the primary and secondary loops. The final DRACS design features a total height less than 13 m.
Following the prototypic DRACS design is the detailed scaling analysis for the DRACS, which will provide guidance for the design of scaled-down DRACS test facilities. Based on the Boussinesq assumption and one-dimensional formulation, the governing equations, i.e., the continuity, integral momentum, and energy equations are non-dimensionalized by introducing appropriate dimensionless parameters, including the dimensionless length, temperature, velocity, etc. The key dimensionless numbers, i.e., the Richardson, friction, Stanton, time ratio, Biot, and heat source numbers that characterize the DRACS system, are obtained from the non-dimensional governing equations. Based on the dimensionless numbers and non-dimensional governing equations, similarity laws are proposed. In addition, a scaling methodology has also been developed, which consists of the core scaling and loop scaling. Due to the importance of the core heat transfer in establishing the DRACS steady state, core scaling is started with, from which the convection time ratio is obtained. The loop scaling is accomplished by utilizing the convection time ratio obtained from the core scaling, and by making two assumptions which can be on the power and loop height. The consistence between the core and the loop scaling is examined through the reference volume ratio which can be obtained from both scaling processes. The scaling methodology and similarity laws have been applied to obtain a scaled-down low-temperature DRACS test facility (LTDF), and a scaled-down high-temperature DRACS test facility (HTDF).