Page 150 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

122 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Fig. 3.3.8 Backward-facing step test section installed at KASOLA (Na).

to construct the hexagonal channel as thin-walled as possible in order to reduce azimuthal
thermal conduction. Thus, an external vessel filled with static LBE is installed to sustain the
pressure conditions of the facility, and only the differential pressure due to friction is
imposed on the hexagonal-channel wall.
l High heat-flux densities up to 1.0MWm 2 (representative of nominal reactor full-power
conditions) are imposed in order to obtain sufficiently large temperature gradients that
can be accurately measured. This specification is very challenging for the construction of
the electric heaters, which must be as compact as possible. In order to improve the perfor-
mance of electric insulation at elevated temperatures, it is desirable to operate at lower volt-
ages and higher currents, in a low-resistance design. This approach is limited by
manufacturing tolerances, and sufficient air cooling must be provided to all copper lines.
In both test sections, detailed instrumentation is installed for local measurements. Dif-
ferent approaches are followed, exploiting the geometric characteristics of each test
section:
Two types of local measurements are performed in the rod bundle test section; see
Fig. 3.3.7.

For measuring the local wall temperature at selected locations defined in three measuring
l
levels, TCs with a diameter of 0.5mm are installed at grooves of the same width and depth,
machined in the cladding of the heater rods. Additionally, TCs with a reduced tip (0.25mm),
supported by the wire spacers, are placed at the center of selected subchannels, which must
be determined accurately. The signal cables are laid carefully for not disturbing the flow
velocity profile and collected at connections at the top of the vessel. Extension cables with
the same metallic pair as the TCs transfer the signals to an electronic ice point and data acqui-
sition system, as described earlier.