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38 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Development needs
These guidelines should be updated where needed and complimented for the specific needs
of liquid-metal flows.

l Guidelines for multiscale simulations (see also Chapter 7)
Challenge
Although multiscale approaches are relatively easy to prototype and test out, their successful
implementation is often hindered by consistency, stability, and convergence issues. Com-
mon recommendations could significantly help to resolve these problems. In addition, most
developments tend to concentrate on a single experimental or reactor case. In this case,
guidelines could help to provide more generic methods.
State of the art
Not existing.
Development needs
Guidelines for multiscale coupled approaches need to be developed and continuously
updated. For the case of liquid metals, algorithms suitable for incompressible, single-phase
fluids (with possible free surfaces) should be defined.

l Guidelines for experiments (see also Chapter 3)
Challenge
Experiments on liquid-metal reactor thermal hydraulics are performed to increase the basic
understanding of fundamental phenomena and to qualify the performance of components or
systems and for the validation of models and their implementation in codes. Depending on
these objectives, different requirements have to be fulfilled. In this work, we mainly deal with
validation experiments for which the requirements on the description of the experimental
conditions and limitations and on the uncertainties in the experimental results are very high.
Moreover, thermal-hydraulic experiments with liquid metals are expensive to realize,
and instrumentation is challenging. Scaling of experiments and the use of optically transpar-
ent or low melting point simulants can provide a solution, if the scaling is performed properly
and depending on the phenomena at hand. Instrumentation that can be applied in a high-
temperature liquid-metal environment and provides the same level of resolution in space
and time and level of accuracy that are comparable with the established techniques for trans-
parent and/or low-temperature liquids should be developed.
State of the art
Not only in some European projects but also by OECD/NEA CSNI (2015) and in the ongoing
IAEA coordinated research project on sodium properties, general guidelines are produced
for the scaling, design, and execution of liquid-metal experiments. On the topic of validation
methodology and procedures, extensive literature exists. General guidelines for the design
and execution of validation experiments have been developed on the basis of this. Develop-
ments are made in the field of liquid-metal instrumentation for the characterization of
flow-field-based ultrasonic and (electro)magnetic techniques.
Development needs
The development and implementation of a systematic and practical framework for the appli-
cation of general guidelines to liquid-metal experiments that is accepted by all experimen-
talists would be very beneficial. The development on liquid-metal instrumentation needs to
be continued and amplified in order to increase the resolution and to determine the accuracy
and the long-term performance of the instruments.