Review of fuel assembly and pool thermal hydraulics for fast reactors

Liquid metal cooled reactors are envisaged to play an important role in the future of nuclear energy production because of their possible efficient use of uranium and the possibility to reduce the volume and lifetime of nuclear waste. Thermal-hydraulics is recognized as a key scientific subject in the development of such reactors. Two important challenges for the design of liquid metal fast reactors (LMFRs) are fuel assembly and pool thermal hydraulics. The heart of every nuclear reactor is the core, where the nuclear chain reaction takes place. Heat is produced in the nuclear fuel and transported to the coolant. LMFR core designs consist of many fuel assemblies which in turn consist of a large number of fuel rods. Wire wraps are commonly envisaged as spacer design in LMFR fuel assemblies. For the design and safety analyses of such reactors, simulations of the heat transport within the core are essential. The flow exiting the core is made up of the outlets of many different fuel assemblies. The liquid metal in these assemblies may be heated up to different temperatures. This leads to temperature fluctuations on various above core structures. As these temperature fluctuations may lead to thermal fatigue damage of the structures, an accurate characterization of the liquid metal flow field in the above core region is very important. This paper will provide an overview of state-of-the-art evaluations of fuel assembly and pool thermal hydraulics for LMFRs. It will show the tight interaction required between experiments and advanced numerical simulations. Furthermore, it will highlight the latest worldwide developments using Computational Fluid Dynamics (CFD) simulation techniques with a special focus on the developments and achievements within NRG in the Netherlands. With respect to fuel assembly thermal hydraulics, the latest developments on simulation of fuel assemblies with wire wraps will be highlighted. As well defined experimental data is hard and/or expensive to obtain, detailed CFD with advanced turbulence modelling and large computational resources is used to create reliable reference data. Furthermore, simulations applying various turbulence models and different codes are inter-compared to gain confidence in the numerical results. With respect to pool thermal hydraulics, the latest developments using CFD with state-of-the-art numerical grid construction and turbulence modelling will be demonstrated. Although experimental data from water and sodium experiments is available in this case for specific designs, a proper validation for the CFD simulations is hard to achieve. Again, simulations by using different numerical codes, grids, and turbulence models are inter-compared to gain confidence in the numerical results.