Page 147 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

120 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

At the same time, the high thermal diffusivity (a) of liquid metals allows for an
in situ relative calibration. In an unheated test with sufficiently large flow rate, the
temperature is expected to be the same at all TC locations, and the measured differ-
ences can be interpreted as relative offsets. The minimum required flow rate, at which
the conditions can be considered isothermal for all practical purposes, can be obtained
through an estimation of the thermal losses at the outer insulated wall and an energy
balance. Repeating this operation at several temperature levels, an individual empir-
ical fit can be obtained for each TC: see, for example, Pacio et al. (2014) and Pacio
et al. (2016).
For other instruments, such as flow meters and differential pressure gauges, for
example, with current output (4–20mA), which can be scaled to a voltage signal in
any desired range, the considerations on the electric signals are less demanding. Nev-
ertheless, it is usually necessary to adjust the offsets with zero-flow tests at the process
temperature.

3.3.3.3 Some examples at KIT: Rod bundles (LBE),
backward-facing step (Na)

This section presents two test sections for liquid-metal thermohydraulic tests installed
at KIT, focusing on constructional aspects rather than on the scientific evaluation,
covered elsewhere. Both experiments include detailed measurement for the validation
of numerical simulations.
The first example is a hexagonal 19-rod bundle with wire spacers, representative of
the fuel-element geometry in LM fast reactors, installed in the THEADES LBE loop.
Within the European FP7 project SEARCH, the heat transfer and pressure drop are
studied in nominal conditions (Pacio et al., 2016), and blockage elements are included
to study postulated accidental scenarios in the ongoing MAXSIMA project. Some
detailed views of the bundle are shown in Fig. 3.3.7.
The second example is the backward-facing step test section, installed in the
KASOLA sodium loop. This geometry is selected as a benchmark for representing
sudden-expansion scenarios, as found, for example, at the core outlet, in the ongoing
European H2020 project SESAME ( J€ ager, 2016). Fig. 3.3.8 shows detailed views of
this arrangement.
Although these two test sections have different geometries, their construction pre-
sents some common characteristics, which can be identified representative of liquid-
metal experimental setup.
A relatively long developing length is installed upstream of the heated zone, in order to
l
obtain a reproducible, fully developed velocity profile. For the rod bundle, an axial distance
of 2.5 times the wire pitch (H) is considered and 55 hydraulic diameters for the backward-
facing step. In this last case, a flow straightener with 60% porosity is installed in order to
avoid secondary flows following the eccentric transition from a round to a rectangular
channel.
l The mechanical construction considers that the test section must be able to sustain the same
operating conditions as the rest of the facility. These considerations usually lead to a feed-
back between the mechanical and thermohydraulic design. For the rod bundle, it is desirable