Page 399 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

362 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

and heat-transfer phenomena, without delving into the small-scale turbulent phenomena pro-
ducing these effects.
On a practical level, this separation of scales has also driven the wide diversity of
thermal-hydraulic codes currently used to model nuclear reactor thermal hydraulics:
l At the smallest scale, CFD codes with DNS capabilities are able to simulate microscopic
phenomena directly but are in practice mostly limited to the simulation of small domains
(smaller than a single-reactor subassembly). The maximum achievable Reynolds number
may also be limited.
l At a larger scale, LES and RANS CFD codes can be used to model larger domains with great
flexibility by using turbulence models to describe the overall effect of turbulent phenomena,
as long as it remains affordable to mesh the geometry of the domain under appropriate
meshing conditions.
l At the next level, subchannel and “coarse CFD” codes have been developed to model
domains where the complexity of local geometry features prevents their direct description;
this is the case for complete-core models, for which a detailed geometric meshing of the 10 5
wire wrappers would be prohibitive. In these codes, correlations are used to describe the
effects of unresolved geometric features.
l Finally, system codes have been developed to model the overall behavior of a complete reac-
tor for long transients, usually through a combination of 0-D, 1-D, and 3-D elements. At this
scale, many physical phenomena must be described by correlations.
Most of these codes have been the object of years to decades of development, verifi-
cation, and validation work. Thanks to these efforts, reference tools are usually avail-
able to study most thermal-hydraulic phenomena of interest, as long as these
phenomena interact in a simple way; however, difficulties can occur when, in a given
situation, complex interactions between phenomena become determinant.

7.1.2 Interactions between scales

Because of their intrinsic characteristics, liquid-metal reactors are unusually suscep-
tible to the type of complex interactions that are difficult to model using available
thermal-hydraulic tools. Most small- and intermediate-scale phenomena occurring
in SFRs and LFRs, such as turbulent friction and heat transfer, display a clear sepa-
ration of scales and can thus be described by simple models within an overall descrip-
tion of the reactor. However, some specific cases lead to more complex interactions
(Tenchine, 2010):
l Most SFR and LFR designs use a pool-type concept in which the primary circuit is contained
in a single, large vessel, with most components (core, exchangers, and pumps) connected by
large liquid-metal plena. The flow in these plena follows complex patterns: typically, an out-
let jet coming out from the core is sucked into the heat-exchanger inlets in the hot pool,
whereas, on the cold side, outlet jets exiting the heat-exchanger outlet are driven into the
primary pump inlets. Stratified layers form at the bottom of the hot pool and at the top of
the cold pool as a result of the imperfect insulation between these two plena (Fig. 7.1, left).
During postulated accident scenarios, such as a loss of forced flow or partial loss of heat sink,
these jets tend to exhibit complex dynamics as they transition from inertia- to buoyancy-
driven flow while interacting with the stratified layers in the pools. These phenomena