CFD analyses of passive decay heat removal for the Very-Small, Long-LIfe, Modular (VSLLIM) reactor by natural circulation of ambient air

Abstract

The VSLLIM reactor, is developed at the University of New Mexico’s Institute for Space and Nuclear Power Studies to generate 1.0–10 MWth, for electricity generation and/or process heat for ∼92 and 5.9 full power years (FPY), respectively, without refueling. This walk-away and passive reactor is cooled by natural circulation of in-vessel liquid sodium during nominal operation and after shutdown, with the aid of in-vessel chimney (1–2 m tall) and a helically coiled tubes Na-Na heat exchanger (HEX) in the downcomer. This work investigates the passive removal of the decay heat generated in the reactor core after shutdown, following an unlikely mal-function of the in-vessel HEX, by natural circulation of ambient air along the guard vessel wall. The performed 3-D thermal-hydraulics and Computational Fluid Dynamics (CFD) analyses investigate the effects of using metal fins along the guard vessel wall and changing the width of the cold air intake duct on the decay heat removal rate and the time after shutdown for cooling the in-vessel liquid sodium to 400 K. Results demonstrate the effectiveness of natural circulation of ambient air, with and without metal fins, for passively and safely removing the decay heat generated in the reactor core after shutdown. Without metal fins, the heat removal rate is ∼29% (244 kWth) of the decay power, right after shutdown, but exceeds the decay power within 8 min after shutdown. Consequently, the average temperature of the in-vessel liquid sodium decreases from 682 K to 400 K within 120 h after shutdown. The metal fins increase the decay heat removal rate by 13.5%, prolonging to ∼100 h the time after shutdown for the average temperature of the in-vessel sodium to decrease to 400 K. Reducing the width of the cold air intake duct by 50%, and without metal fins, decreases by 35% the decay heat removal rate, increasing to ∼346 h the cooling time of the in-vessel sodium to 400 K.