Cfd Code Validation and Benchmarking against Bfbt Boiling Flow Experiment

The paper presents CFD code validation and benchmarking against the NUPEC BFBT (BWR FullSize Fine-Mesh Bundle Tests) data, performed within the OECD/NRC BFBT void distribution benchmark and as part of the NURESIM (European Platform for Nuclear Reactor Simulations) Integrated Project. Sub-cooled flow boiling occurs in many industrial applications and is characterized by large heat transfer coefficients until reaching the critical heat flux (CHF) condition. This phenomenon is a crucial design limitation factor in water cooled nuclear reactors, since it causes a rapid heater temperature excursion which potentially leads to heater melting and destruction. A common objective of NURESIM IP and BFBT Benchmark is to improve the use of two-phase CFD codes as a tool for understanding boiling flow processes, in order to subsequently help new fuel assembly design and to develop better CHF predictions. In this framework, numerical simulations of sub-cooled boiling in BWR fuel assembly were performed with NEPTUNE_CFD and ANSYS CFX codes. Sensitivity studies on problem parameters led to satisfactory overall void production, with some differences in the vapour distribution through the section probably due to the absence of spatial grids in the model. Further investigations should focus on non-drag forces modelling and grid refinement.