Methodology and confirmatory analysis for density wave oscillation in NuScale US460 helical coil steam generators

In the current work, a TRAC-RELAP Advanced Computational Engine (TRACE) model was developed to study density wave oscillations (DWOs) in helical coil steam generator (HCSG) coils of the NuScale US460 design. The NuScale US460 design comprises an integral pressurized water reactor vessel that utilizes natural circulation to provide normal core flow. Heat is removed from the reactor pressure vessel by a pair of intertwined HCSGs. Secondary flow boils and becomes superheated inside the coils of the HCSGs. Under certain conditions, the flow in these coils may become unstable due to the density wave instability mechanism. The regulatory purpose is to evaluate the thermal and hydrodynamic loads of postulated instability to determine if these loads pose a challenge to the integrity of the coils. The Evaluation Model Development and Assessment Process (EMDAP) guided the model development effort. A Phenomena Identification and Ranking Table (PIRT) was used to determine knowledge and assessment gaps relative to key phenomena. Incremental TRACE assessment was performed to support the applicability of TRACE to analyze DWOs in helical coils. The authors considered the possibility of DWOs leading to condensation induced water hammer (CIWH) and developed an approach for conservatively estimating CIWH overpressure loads using TRACE. TRACE calculations were performed over the startup range of the NuScale US460 design where unstable DWOs could occur. TRACE predicted that certain operating conditions during startup would lead to unstable DWOs in the HCSG coils. The most severe instabilities were further evaluated to determine the safety significance. The results show that CIWH is unlikely to occur and that thermal fatigue considerations are manageable.