Assessment and testing of CTF for LOCA reflood conditions

The nuclear industry is actively moving toward longer fuel cycles to improve fuel utilization and the economics of the current fleet of light-water reactors' operation. However, increasing burnup poses an increased risk, as new phenomena such as fuel fragmentation, relocation, and dispersal (FFRD) must be addressed. The large-break loss-of-coolant accident (LBLOCA) is one of two design basis accidents (DBAs) that increases the risk of fuel failure and possibly FFRD for high-burnup conditions. To further investigate this risk, the Nuclear Energy Advanced Modeling and Simulation (NEAMS) program is developing multiphysics modeling and simulation tools that aim to answer questions about the impact of model fidelity on safety margin in high-burnup fuel for LBLOCA conditions. One of these tools, CTF, is capable of predicting subchannel pin-by-pin resolution thermal hydraulics in the core region. CTF includes a suite of models for prediction of liquid, vapor, and droplet distribution under normal and abnormal reactor operation conditions; however, these models have not been assessed under LOCA-relevant conditions.To prepare for using CTF for this application, a review of these models was performed, which includes sensitivity analysis and validation testing using a selected flooding experiment with blocked arrays (FEBA). A review of high-impact models was performed, and several model corrections were made. Several improvements to the numerical controls have also been implemented. This study shows that CTF is capable of modeling the FEBA Series II cases using both single-channel and pin-resolved meshing. It has been found that CTF tends to overpredict the quench front velocity and the droplet carryover rate using the LOCA models that have been assessed to date. Future activities will focus on investigating additional models not yet tested, expanding the validation tests, and performing in-core LOCA analysis on high-burnup fuel rods.