Page 60 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Thermal-hydraulic challenges in liquid-metal-cooled reactors 35

State of the art
As one-dimensional or subchannel codes have limited possibilities in modeling three-
dimensional effects typically occurring in LMFRs, development and validation of state-
of-the-art multiscale coupled thermal hydraulics with appropriate experimental data are
necessary. In this case, system codes, subchannel codes, and/or CFD codes might be
coupled to arrive at the desired level of accuracy while maintaining reasonable
computational costs.
Development needs
In order to successfully implement a multiscale simulation involving two or more codes, one
must first develop a coupling strategy to ensure that all codes compute part of a consistent
overall state of the reactor. This coupling strategy usually defines data exchanges between
the codes and a common time scheme. It may rely on deep intervention within one or more of
the codes, in which case additional developments may be needed. Finally, this strategy
should be generic enough to apply both to validation experiments and to the reactor case,
so as to allow for the definition of a validation database.

l Thermal hydraulics coupled with neutronics
Challenge
The transport of neutrons depends on several parameters, including fuel and coolant
temperature. Temperatures are calculated by thermal-hydraulic codes and used as input for
neutronic codes to calculate a power distribution. The power distribution, in its turn, serves
asinputtothermal-hydrauliccodes.Hence,thermal-hydrauliccodesandneutroniccodesneed
to be coupled for transient reactor calculations. For this purpose, mostly 1-D thermal-
hydrauliccodeshavebeenused,limitingtheaccuracyandspatialresolutionofthepredictions.
State of the art
In order to increase the accuracy and the degree of spatial resolution of core design studies,
today, coupled 3-D neutronic (deterministic and Monte Carlo) and 3-D thermal-hydraulic
(CFD and subchannel) codes are being developed worldwide.
Development needs
Development of efficient coupling methodologies for transient analysis needs to be contin-
ued. Apart from the development of coupled codes, verification and validation of these codes
with appropriate experimental data are required.

l Sodium-water and sodium-air interaction
Challenge
The assessment of sodium-water interaction plays a primary role in the design of SFR steam
generators. In comparison with a conservative approach assuming total reaction, finer
models capable of computing the reaction kinetics are expected to provide large gains. Sim-
ilarly, the assessment of sodium fire risks could be improved by better modeling the sodium-
air reaction kinetics.
State of the art
The interaction of sodium with either water or air is typically part of the national programs of
countries employing sodium fast reactors, like Japan, India, China, Russia, and France.
Development needs
Most of the efforts nowadays are focussed on obtaining new experimental data to update and
further validate legacy numerical codes or to develop and validate new codes, for example,
based on CFD.