Page 152 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

124 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Especially for sodium and other alkali metals, the chemical interaction with com-
mon substances, such as water and air, represents a safety concern, and the construc-
tion must include proper measures (e.g., use of a cover gas and sump tank) to reduce
the operational risk.
The compatibility of liquid metals with solid materials is a broad, ongoing research
topic. Based on the current state of the art, the range of materials that can be selected
for structures (pipes and vessels) and other surfaces in contact with the fluid (e.g., sea-
ling and instrumentation) is limited (less severe for sodium than LBE). Technical solu-
tions based on controlling the oxygen concentration are developed for improving the
performance of stainless steels within the temperature range expected in liquid-metal
reactors. For higher temperature, solutions based on ceramics, refractory metals, or
glass (e.g., as coating) are most suitable.
Focusing on thermohydraulic facilities, some guidelines regarding the main com-
ponents (pumps and heat exchangers) and instrumentation (flow rate and differential
pressure) are given in Section 3.3.2. Nevertheless, it should be noted that most of the
design and construction details depend on the operating parameters (temperature, flow
rate, pressure head, and thermal power), and for laboratory purposes, sometimes, effi-
ciency can be sacrificed in favor of simplicity, for example, using an electromagnetic
pump. Representing the experiences at KIT, the THEADES (LBE) and KASOLA
(sodium) loop facilities are described as examples.
Some considerations are discussed for the construction of heated test sections for
thermohydraulic studies. In this aspect, the specific features of liquid metals imply the
need for high-accuracy measurements and compact construction for imposing high
heat-flux densities. Special emphasis is placed on obtaining reproducible operating
conditions and an in situ calibration of the instrumentation, particularly for tempera-
ture measurements. Two examples are described, a rod bundle with wire spacers
installed in THEADES (LBE) and a backward-facing step test section studied in
KASOLA (Na). Naturally, some constructional aspects are characteristic of the geom-
etry under investigation and cannot necessarily be extrapolated to other systems. In
that respect, this work provides an overview and some examples of typical challenges
faced in the construction of liquid-metal facilities and test sections.

References

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Buchenau, D., Eckert, S., Gerbeth, G., Stieglitz, R., Dierckx, M., 2011. Measurement technique
developments for LBE flows. J. Nucl. Mater. 415, 396–403.
Eckert, S., et al., 2017. Liquid metal flow meters. In: VKI Lecture Series: Thermohydraulics and
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