TRACE Analysis of a Loss of Alternating-Current Power Without Rod Insertion for the NuScale Power Module—II: Sensitivity to Varying Initial Temperature

In a companion paper, the U.S. Nuclear Regulatory Commission (NRC) staff has described analyses performed using the TRAC/RELAP Advanced Computational Engine (TRACE) code to study the transient system response of the NuScale power module to a postulated beyond-design-basis loss of alternating-current (LOAC) power transient where the module protection system completely fails to insert the control rods. The subject paper studies the sensitivity of the event progression and consequences to variation in the initial reactor coolant system (RCS) temperature. These studies were performed by varying the effective steam generator heat transfer surface area between 100% and 50% of the nominal area. The results of the NRC staff analyses show that at increased initial temperatures, it is possible for the NuScale primary side to remain critical for an extended period of time, leading to a sustained loss of primary-side inventory through pressure relief until the natural circulation flow pattern in the RCS becomes broken. After the flow loop is broken, reactor power decreases significantly, and the primary figures of merit important to safety are met with substantial margin.