Page 180 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

152 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

which is able to detect the conductivity distribution in one cross section of a pipe
(Ma et al., 2005). The latter one can be used to measure the flow rate or even resolve
the spatial distribution of the flow.
Available flow-rate sensors are eddy-current flowmeters like the phase-shift flow-
meter (Priede et al., 2011), which measures the phase shift between the excitation and
the receiving coils, or the Lorentz force velocimetry (LFV) (Thess et al., 2006), which
measures the mechanical force on a permanent magnet close to the channel. The cal-
ibration of those flowmeters becomes difficult if the temperature of the flowing metal
is changing, because the measured quantity does depend not only on the flow rate but
also on the temperature-dependent electric conductivity of the liquid. A new technique
called transient eddy-current flowmetering (TEC-FM) offers a way to strongly reduce
the influence of the electric conductivity of the melt on the measurement results
(Forbriger and Stefani, 2015). The basic idea is the creation of an eddy-current system
within the liquid metal that is moving with the same velocity as the liquid metal. By
tracking its movement, the velocity can be directly measured. Fig. 3.5.5A shows a
schematic sketch of the external version of a TEC-FM sensor that can be mounted
at the wall of a fluid container or a pipe. The emitter coil imprints an eddy-current
ring into the medium by switching on or off a constant current. The position of the
magnetic pole of this ring is tracked by measuring the voltages in the three detection
coils. The averaged flow velocity can be directly obtained from the movement of the
magnetic pole.
A new immersed version of the TEC-FM and of the phase-shift sensor is now avail-
able where the sensor coils are placed in a stainless steel thimble preventing a direct
contact with the liquid metal. This allows the measurement of the flow rate around the
sensor, and the sensor can be placed, for instance, in the reactor vessel above a fuel
assembly to detect local blockages (Krauter and Stefani, 2017). Fig. 3.5.5B shows a
schematic sketch of an immersed TEC-FM sensor that consists of two excitation coils
and two detection coils. Both immersed sensors were tested in GaInSn and sodium.
Additionally, the immersed phase-shift sensor was also tested in LBE.
A spatially resolved velocity measurement is allowed by the contactless inductive
flow tomography (CIFT) (Stefani et al., 2004), which enables the reconstruction of the
global three-dimensional flow field in liquid metals by measuring the flow-induced

Detector coils

Excitation coil

R R E
E 1 1 2 2
Eddy current
(A) (B)

Fig. 3.5.5 Schematic sketch of the external TEC-FM sensor (A) with one excitation coil and
three detection coils and the immersed version (B) with two excitation and detection coils.