Assessment of Passive Cooling Capability of the Westinghouse Lead Fast Reactor Under Station Blackout Conditions

Abstract Westinghouse is developing its next generation of high-capacity nuclear power plants with the Westinghouse Lead-cooled Fast Reactor (LFR) (Ref. 1). The Westinghouse LFR is a pool-type 950 MWt (∼450 MWe Net) passively safe modular construction plant that contains all the primary system components within the reactor vessel, including the reactor coolant pumps (RCPs) and the primary heat exchangers (PHEs). Similar to the Westinghouse AP1000® Pressurized Water Reactor design, the Westinghouse LFR design incorporates passive cooling capability primarily through its Passive Heat Removal System (PHRS). During accidents, the PHRS allows decay heat to be transferred from the core to a pool of water surrounding the guard vessel, which transitions to long-term air cooling upon water depletion. All of this takes place without operator action or need for moving parts or power supply. Overall, when combined with natural circulation of liquid lead inside of the reactor vessel, this passive system fully relies on natural circulation, thermal radiation and boiling mechanisms to transport decay heat from the core to the atmosphere. In this study, complementary Computational Fluid Dynamics (CFD) and system code analyses were performed to assess the passive cooling ability of the Westinghouse LFR design during station blackout (SBO) conditions. The postulated scenario examined in this study is the longer-term phase of the SBO event, with the RCPs and PHEs assumed to be inactive and the water inventory in the pool exterior to the guard vessel fully depleted, thus relying purely on air cooling as the ultimate heat sink. The CFD analysis was conducted with Siemens STAR-CCM+ code using, as domain, the primary lead pool and with boundary conditions provided by the containment code GOTHIC. The CFD analyses provided high fidelity resolution of the three-dimensional natural circulation flow patterns formed throughout the reactor vessel. The results of the study not only provided confirmation of the passive cooling capability of the Westinghouse LFR design during an SBO event, but also provided granular input for vessel structural analysis activities accounting for thermal gradients developed during an SBO event.