Validation and benchmarking of CTF for single- and two-phase flow

As part of the Consortium for Advanced Simulation of Light Water Reactors (CASL), the subchannel code CTF is being used for single- and two-phase flow analysis under light water reactor operating conditions. Accurate determination of flow distribution, pressure drop, and void content is crucial for prediction of margins to thermal crisis and neutronic feedback, and therefore more efficient plant performance. In preparation for the intended applications, CTF has been validated against experimental facilities comprising the General Electric (GE) 3×3 bundle, the BWR Full-size Fine-mesh Bundle Tests (BFBT), the RISO tube, and the PWR Subchannel and Bundle Tests (PSBT). Meanwhile, the licensed and industrially well-recognized subchannel code VIPRE-01 is used to generate a baseline set of simulations for the targeted tests, and solution parameters are compared to CTF results. The flow split verification problem and single-phase GE 3×3 result in essentially perfect agreement between CTF and VIPRE-01. For two-phase GE 3×3 cases, flow and quality discrepancies arise in the annular-mist flow regime, yet significant improvement is observed in CTF when void drift and two-phase turbulent mixing enhancement are considered. BFBT pressure drop benchmark shows close agreement between predicted and measured results in general, although considerable over-prediction of CTF is observed at relatively high void locations of the facility. This over-estimation tendency is confirmed by RISO cases. While overall statistics are satisfactory, the BFBT bubbly/slug/churn-turbulent flow void contents are markedly over-predicted by CTF, the latter being consistent with PSBT results. Such two-phase models in CTF as turbulent mixing, interfacial and wall friction, and subcooled boiling need further improvement using more mechanistic models and advanced calibration techniques.