Page 151 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Construction of experimental liquid-metal facilities 123

Differential pressure is measured with probes located at one edge of the hexagonal channel
l
and connected to the corresponding sensors following the construction. These results show
good reproducibility and good agreement with empirical correlations; see further details in
Pacio et al. (2016).

The local instrumentation in the backward-facing step test section, see Fig. 3.3.8,is as
follows:

l Several 0.5mm TCs are installed at fixed positions in the fluid channel. Small holes are
drilled in thin metal sheets, folded forming crosses that can accommodate many TCs with
a small impact on the flow (less than 1mm in width; see J€ ager (2016)).
l At five locations on one edge (two upstream of the step and three downstream), sensors can
be moved across the channel to obtain velocity and temperature profiles. Each sensor
includes a permanent magnetic probe for velocity measurement and a 0.25mm TC. Placing
a small magnet (2 2 1mm) between two electrodes, a voltage is induced by the LM flow,
proportional to the vector product of the velocity and magnetic field. Considering the char-
acteristic dimensions of the channel (90 40mm), the influence of the instrumentation on
the velocity and temperature profiles can be neglected.

3.3.4 Conclusions

In a context of growing interest in liquid-metal cooling technology, extensive activ-
ities in liquid-metal facilities are performed and planned for validating theoretical
models and closing the gap between theoretical works and experimental data. At
the Karlsruhe Institute of Technology, extensive experience with the design, construc-
tion, and operation of such facilities has been obtained for several decades, covering
different scientific issues of both sodium and heavy-metal (LBE and Pb) technology.
This work summarizes these experiences, focusing mostly on thermohydraulic
studies. It is structured distinguishing between the facility itself and test section for
specific experimental studies.
Experimental liquid-metal facilities can be compared, for example, with more
common water systems. In many aspects, they share some common characteristics.
For example, the same guidelines apply in both systems for the construction of vessels
and for the support infrastructure related to electrical and civil engineering. Moreover,
for high-temperature operation, thermal insulation and thermal expansion of the struc-
tures must be similarly considered for water and liquid metal. One further common
issue is the need for an exhaustive risk assessment and its associated control, moni-
toring, and alarm systems. Although this could be considered as a strictly operational
aspect, it must be accounted for in the design and construction stages.
In many other aspects, the special features of liquid metals impact the design and
construction of experimental facilities, with additional consideration not required for
water or other conventional fluids.
Among the physical properties, key roles are played by the melting point (freezing
must be avoided), density (large inertia and weight of the fluid, particularly for LBE
and Pb), and the characteristically large thermal conductivity (λ) and diffusivity (a),
particularly relevant for the construction of heated test sections.