Page 145 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

118 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

While ALIP does not have moving parts, the centrifugal pump must be sealed, and this
task is simpler at lower hydrostatic pressures (for LBE, 1m height represents roughly
1bar). In both cases, the pump and most valves are placed on the cold leg, downstream
of the air cooler.
Furthermore, these facilities deviate from a simpler rectangular loop construction
because their design considers the study of additional test section geometries, other
than channels with upward flow. These design choices greatly affect the layout of
the facility, for example, with additional auxiliary lines, valves, and instrumentation,
as well as the test port itself. In particular, at the top of THEADES, free-surface exper-
iments were performed, for study of spallation targets in accelerator-driven system
such as MYRRHA. In KASOLA, an additional test port is envisaged for hosting a slab
pool simulator.

3.3.3 Thermo-hydraulic test sections

Compared, for example, with water at the same temperature, the most relevant specific
feature of liquid metals for the construction and operation of a thermohydraulic test
section is their 20–150 times larger thermal conductivity (λ) and 50–500 times larger
diffusivity (a); see Table 3.1. This means that for a given geometry and imposed ther-
mal power, the temperature gradients are much smaller and more difficult to measure.
In order to obtain measurable temperature differences, thermohydraulic test sec-
tions with liquid metals are constructed to operate at high heat-flux densities.
A more compact construction involves additional challenges related to, for example,
manufacturing tolerances, sealing, and electric insulation. A compromise solution is
often necessary, stretching the limits of manufacturing and instrumentation capabil-
ities. At KIT, test sections with power densities up to 1.0MWm 2 have been
constructed.
Since relative temperature differences are involved in liquid-metal thermo-
hydraulic experiments, some considerations on the facility infrastructure, described
in the section below, must be taken into account in order to obtain the necessary accu-
racy and repeatability.

3.3.3.1 Reproducible operating conditions
Considering the required accuracy, the following considerations must be taken into
account:
Similarly as with other fluids, reproducible velocity and temperature profiles in the measur-
l
ing section are necessary for accurate measurement. This means that a sufficiently long flow
developing length after valves and fittings must be installed upstream of the test section. For
the special case of consecutive off-plane bends, a flow straightener can be used to eliminate
secondary flows and reduced the necessary developing length.
l Usually, adiabatic thermal boundary conditions are imposed to the outer wall. As perfect
insulation is not possible, it is important that the thermal losses are reproducible. To this