Page 53 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

28 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

refined meshes and high-order turbulence modeling (e.g., LES) to capture the details of
temperature gradients and instabilities at the interface. These are available nowadays; there
application is however still limited to small domains.
Development needs
Development and validation of CFD codes able to capture the stratification interface, and its
dynamics remain necessary. Experimental campaigns on small-scale setups and in integral
facilities are indispensable and need sufficient resolution in instrumentation for this purpose.

l Above core structure (see also Section 6.2.4)
Challenge
The above core structure is an important component from a thermal-hydraulic point of view
as it influences the flow not only in the region above the core outlet but also in the whole
upper plenum of the reactor (Tenchine, 2010). For example, it can enhance the mixing of
nonisothermal jets exiting the fuel assemblies in the core and in this way reduces thermal
striping and stratification in the upper plenum. One of the main components in the upper
pool is the above core structure. The above core structure also supports the instrumentation
that is used for monitoring the temperature at the core outlet during transiens and to detect
eventual blockage in the fuel assemblies. It is thus important to know precisely not only
the velocity and temperature field in the above core structure but also how these are related
to the temperatures in the upper plenum and in the core outlet region.
State of the art
Limited, mostly design-specific experimental and numerical studies are available for this
structure.
Development needs
The optimization of the above core structure design is a difficult process with multiple
parameters and multiple objectives. The use of specific optimization tools in CFD
(e.g., porous media optimization) might be explored for this purpose (Borrvall and
Petersson, 2003). Obviously, this should include an experimental validation program.
l Vessel cooling
Challenge
In scenarios where the normal heat removal systems are lost, the impact of auxiliary cooling
systems such as reactor vessel auxiliary cooling system (RVACS) becomes essential.
RVACS generally use natural circulation to provide decay heat removal to the atmosphere.
Typical configurations use air tubes or air channels around the safety vessel. Heat transfer
occurs from the reactor vessel through the safety vessel to the RVACS by a combination
of conduction, (natural) convection, and radiation processes. It is important to characterize
the performance of these systems in case of accidents. In normal operation conditions, the
impact of the RVACS system on the vessel temperatures is important for the assessment of
thermal stresses in the vessel. In a similar way, the heat transfer from the liquid-metal pool
through the cover gas is a combination of natural convection and radiation processes that
needs to be characterized properly in order to determine the heat load on the reactor cover.
Penetrations in the vessel cover and structures in the cover gas plenum complicate the cel-
lular convection that can lead to large circumferential temperature variations in the cover and
structures. The natural convection of the cover gas might also lead to the transport of coolant
vapor or aerosols from the coolant-free surface to colder regions in the cover gas plenum,
where they deposit and can lead to difficulties in moving components (see, e.g., Velusamy
et al., 2010).