Modeling Passive Safety in the Westinghouse Small Modular Reactor: Assessment of Wall Condensation and CMT Natural Circulation

Passive safety design that utilizes gravity, natural circulation, heat sink and stored potential energy for reactor safety functions is being increasingly adopted in advanced reactors, especially in the small modular reactor (SMR) designs. The passive safety design of the Westinghouse SMR is described in details and compared with the AP1000® passive safety design. The natural circulation loops and heat transfer mechanism in a postulated Westinghouse SMR loss of coolant accident (LOCA) are discussed. The key thermal hydraulic phenomena pertinent to the passive safety design of the Westinghouse SMR have been identified in the small break LOCA Phenomena Identification and Rank Table (PIRT). Among the identified phenomena, condensation on the containment wall and natural circulation in core makeup tank (CMT) loop are highly ranked. Those passive safety phenomena are expected to be assessed using the WCOBRA/TRAC-TF2 LOCA thermal hydraulic code, which will provide the design basis LOCA analysis in the SMR design control documentation. In this paper, the progress on the assessing two key phenomena in passive safety of Westinghouse SMR is reported. The preliminary assessments against UCB tube condensation tests and Westinghouse core makeup tank tests reveals the capability of WCOBRA/TRAC-TF2 code to reasonably predict the condensation on the containment wall and natural circulation in the core makeup tank (CMT) loop.