dc.description.abstract
The reflood phase of a Loss-of-Coolant Accident (LOCA) in a nuclear power plant (NPP) is critical for safety, as it is during this phase that the peak cladding temperature (PCT) is reached. The reflood process involves rewetting the reactor core after a dry-out period, continuing until long-term cooling is achieved. NPPs must demonstrate, using thermal-hydraulic codes such as RELAP5 and TRACE, that the PCT remains within the safety limits defined by regulatory authorities. More accurate predictions of this parameter can provide additional safety margins. Therefore, a thorough understanding of the complex phenomena occurring during the reflood phase is essential. Experimental programs offer valuable data, serving as benchmarks for participants to perform simulations, evaluate codes, and develop new models and correlations. The Rod Bundle Heat Transfer (RBHT) project conducted a series of reflood experiments under specific conditions using the advanced NRC/PSURBHT facility. Aspart of this project, UPC developed a RELAP5 model that performed well but faced limitations in predicting the phenomena in the upper regions of the rod bundle. This master’s thesis aims to assess TRACE’s ability to accurately simulate the evolution of the reflood phase, with a particular focus on the complex post-CHF regimes present in these upper regions above the quench front. A TRACE model was developed, leveraging features such as the spacer grid model and the droplet field. The simulated results were compared with both experimental data and predictions from the RELAP5 model. A sensitivity analysis was also conducted to evaluate the impact of some parameters and physical models on the simulation outcomes, such as PCT and quench time. The results demonstrated that TRACE performs well in high reflood rate scenarios. However, limitations were observed, particularly for tests involving low flooding rates or variable flow conditions, where the model struggled to capture the heat transfer mode transitions. The sensitivity analysis revealed that the heat transfer during dispersed flow film boiling and the quench temperature significantly impacted PCT, quench times, and liquid and vapor mass balance. While TRACE shows promise in simulating certain phenomena, such as heat transfer enhancement due to the spacer grids, further development offer opportunities for improving its current capabilities.
