Investigation of flow instabilities in Natural Circulation NuScale type Reactors

As new advanced reactors, Small Modular Reactors (SMRs) provide enhanced safety and convenient operations with ability to be used at remote places and serve as small power hubs. SMRs usually equip with enhanced passive safety systems driven by natural circulation flow. Hence, a fundamental understanding of natural circulation flow instabilities is inevitable for the design of the passive safety systems. Wang et al. [1] did some experimental studies on natural circulation flow instabilities with a facility designed by scaling down from a PWR-type SMR (NuScale Power Reactor [2]). In their study, flow instabilities were observed during the blowdown process. In the current study, the same test facility is used and the discussion will be focused on the effect of break size (k-factor) at the top of the steam dome with a large compressible volume of the containment on the flow instabilities. It is found that larger break size (i.e. smaller k-factor) will lead to faster release of the accumulated pressure in the steam dome to the containment and hence two kinds of flashing processes can be observed along the riser section. However, for smaller break size (i.e. larger k-factor), flashing-induced instability is suppressed while the density wave oscillation is induced.