Development of a LOCA safety analysis evaluation model for the Westinghouse Small Modular Reactor

Abstract

The Westinghouse Small Modular Reactor (W-SMR) is an integral pressurized water reactor (PWR) with all major components located inside the reactor vessel, with a passive safety system that was derived from the AP1000® PWR passive safety design. The integral design of the reactor coolant system (RCS) effectively removes the postulated large break loss-of-coolant accident (LOCA). Therefore, small break LOCA (SBLOCA) is the most limiting design basis accident for the W-SMR. The LOCA safety analysis of the W-SMR requires an evaluation model (EM) that is adequate for a reactor design with passive safety systems. Development of the EM follows the Evaluation Model Development Assessment Process (EMDAP) by the United States Nuclear Regulatory Commission (US NRC). The first step in this process is the development of a SBLOCA Phenomena Identification and Ranking Table (PIRT), which identifies the figures of merit for the LOCA safety analysis and key thermal hydraulic phenomena pertinent to the passive safety design of the W-SMR. Next, an assessment matrix that consists of experiments from the AP600™ and AP1000 PWR development programs, University of California Berkeley (UCB) tube condensation tests, and the integral effects and separate effects tests for the W-SMR testing program is established to assess the evaluation model. The systems code used in the W-SMR LOCA evaluation model is the WCOBRA/TRAC-TF2 systems code, which has been extensively validated for previous LOCA applications. WCOBRA/TRAC-TF2 is being further developed to address the W-SMR EM PIRT. Among key thermal hydraulic phenomena, the assessment of the condensation on the containment wall and natural circulation/draining through the CMT are presented here as examples to demonstrate the W-SMR SBLOCA EM development, while all important phenomena are expected to be assessed using the WCOBRA/TRAC-TF2 systems code. Finally, the reference SBLOCA transient of W-SMR is presented to demonstrate the ability of the systems code to properly capture the key phenomena identified in the PIRT. It is expected that the development of an evaluation model for safety analysis of the Westinghouse SMR will facilitate its licensing and eventual commercialization.