Page 57 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

32 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

State of the art
At the local scale, several correlations for debris bed coolability have been developed
and validated, providing data on the effective exchange coefficient between the corium
and the surrounding liquid metal. These correlations have been integrated into single-
phase CFD simulations of the complete primary circuit in order to determine, for a
given design, the cooling paths from the recuperator to the decay heat exchangers: sim-
ulant fluid experiments have been used to validate these simulations (Kamide, 2016).
Development needs
It may be expected that further development is typically part of the national programs of
countries employing sodium fast reactors, like Japan, India, China, Russia, and France.

2.4.4 System thermal hydraulics

The category of system thermal hydraulics includes the following topics:
Normal operation:
One-dimensional code validation (see also Chapter 4)
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Challenge
System thermal-hydraulic codes are the reference tool for reactor transient analysis. Sev-
eral such codes have been developed and validated for LWRs (such as Reactor Excursion
and Leak Analysis Program (RELAP), TRACE, or CATHARE). In order to apply them
to liquid-metal applications, their physical laws and constitutive correlations must be
adapted to many specificities of liquid metals. In addition, the neutronic point kinetics
models integrated within these codes must often be modified in order to correctly predict
power evolution in liquid-metal-cooled reactors. Finally, liquid-metal reactor transients
often involve 3-D phenomena (in particular in pool-type designs) that are difficult to model
and validate at the system scale.
State of the art
Several system thermal-hydraulic codes are now capable of modeling single-phase liquid
metals, with a few codes even capable of predicting two-phase flows (such as CATHARE
and TRACE). Their predictive capabilities rely on validation databases for each component
(subassemblies, heat exchangers, and pumps), which are being extended as reactor designs
evolve.
Development needs
The existing datasets available for one-dimensional system code validation for LMFRs are
still limited compared with the dataset available for light-water reactors. Therefore, further
experiments will obviously contribute to improvements and further validation of first of all
one-dimensional system codes. But once such experiments are being designed, also possible
validation of subchannel codes, CFD, and/or coupled multiscale thermal-hydraulic codes
should be considered in order to use the experimental facility as efficient as possible.
Heat exchangers
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Challenge
Heat exchangers (i.e., steam generators, intermediate heat exchangers, and decay heat
removal heat exchangers) to be installed directly in the LMFR primary pool imply innova-
tive designs. For these reasons, heat exchangers are of overall importance and deserve accu-
rate studies and evaluations to be qualified. The main qualification studies regard the design
validation as well, unit isolation on demand, pressure drop characteristics, component
behavior in normal operation (e.g., forced, mixed, and natural convection), and operational