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Natural convection systems are a promising method to passively remove heat from reactor cavities during loss of forced flow accident scenarios. At Argonne National Laboratory (ANL), a highly instrumented Natural Convection Shutdown Heat Removal Test Facility (NSTF) was used to demonstrate the effectiveness of air-cooled natural convection systems. In previous work, RELAP5-3D simulations were performed on this facility with favorable comparisons to experiment for mass flow rate, pressure drop, air temperature increase, and air velocity. Both experimental and simulation efforts with this facility present a useful opportunity to perform a benchmark study with the System Analysis Module (SAM). SAM is an advanced thermal-hydraulic system code currently in development at ANL for advanced non–light water reactor safety analysis. [ABSTRACT FROM AUTHOR]

Recent design efforts have used the reactor cavity cooling system (RCCS) for passive decay heat removal in the Next Generation Nuclear Plant. Employing a series of riser tubes and cooling panels that line the containment walls, the RCCS can provide an ultimate heat sink for decay power removal from the system without the need for AC power. With vessel wall temperatures expected to reach 450°C, intuition suggests that radiation will be the dominant mode of heat transfer. However, the authors show that several factors can alter these modes; variations in cavity height, riser tube geometry, and vessel heat flux may have significant roles in the heat removal by the RCCS. The authors have constructed a one-quarter-scale water-cooled experimental facility at the University of Wisconsin-Madison that is based on available open literature of the General Atomics modular high-temperature gas-cooled reactor, with a three-riser tube and cooling panel test section representing a 5-deg slice of the full-scale design. Under prototypic heat flux conditions, a series of scoping tests with linear and asymmetrically skewed heating profiles were performed to investigate the split in flow distribution among the parallel channels. Numerical results, using RELAP5 models and FLUENT simulations, provide a comparison to experimental data sets and insight into the split among heat transfer modes present in the cavity. Application of these passive decay heat removal systems demands a pragmatic approach that can account for the irregularities and nonuniformities present in a real design. In areas of blocked views, such as near support structures and primary cooling pipes, convection can provide a mechanism to smooth the otherwise skewed radiative heat flux for heat transfer from the reactor pressure vessel walls to the cooling panels. Integral to the design of the RCCS, the cooling fins serve to protect the cavity wall while adding additional pathways for heat dissipation by conduction into the cooling tubes. [ABSTRACT FROM AUTHOR]