NuScale is a light water-cooled Small Modular Reactor (SMR) that features an integral reactor pressure vessel design and relies on natural circulation for primary coolant flow under all operating conditions. This research investigates the reactor's thermal-hydraulic behavior during a critical Loss of Coolant Accident caused by the rupture of the Chemical and Volume Control System (CVCS) discharge line inside the containment vessel. To conduct this safety analysis, a detailed three-dimensional model of the NuScale Power Module 250 MWth (NPM-20 design) was developed using the TRACE system code. This model accurately incorporates all relevant safety systems, including the passive Emergency Core Cooling System (ECCS), the decay heat removal system, and the ultimate heat sink. The study analyzes various accident scenarios based on the LOCA event tree from the NuScale Design Certification Application (DCA). These scenarios include successful ECCS actuation, cases of partial ECCS failure, and sequences where the ECCS fails but the CVCS is successfully actuated to provide a backup cooling path. The results confirm that core integrity is maintained not only with full ECCS functionality but also in challenging scenarios where the ECCS suffers partial failure, but the CVCS can inject coolant in time to prevent core damage. A major focus of the study was determining the available time for the main control room crew to manually intervene via the CVCS. For the M5 cladding, the crew has a significant margin to actuate the CVCS after partial ECCS failure, ensuring sufficient time for recovery actions. To further assess safety enhancements, the analysis was extended to investigate the impact of Accident Tolerant Fuels (ATF) materials, specifically Cr-coated Zry, FeCrAl, and SiC claddings. The results of these advanced fuel cases were used to determine how these materials affect the time margin to fuel damage and the available time window for manual crew action, thereby quantifying the potential safety benefits offered by ATF technologies. • LOCA sequences with CVCS actuation and Zry or Cr-coated Zry are limited by ECR criterion. • LOCA sequences with CVCS actuation and FeCrAl and SiC are limited by PCT criterion. • The time available for CVCS actuation increases more in SiC than in FeCrAl and Cr-coated Zry.
Martinez-Gonzalez et al. (Thu,) studied this question.