Page 97 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Rod bundle and pool-type experiments in water serving liquid metal reactors 71

reduction of the flow velocity. The question is how these wire wraps, which are
located between the fuel rods, influence the flow field and, consequently, heat transfer
and core pressure drop. Table 6.2.3.1 provides a nice overview of the different exper-
iments that have been performed to tackle this question. Phenomena, such as mixing,
friction factors, cladding temperatures, have been extensively measured during the
past decades. Quite recently, detailed measurements of the flow field by using PIV
and RIM have been performed in a bundle geometry using water and wire wraps.
Unfortunately, the exact location of the wire wraps has not been recorded so that com-
parison with computational fluid dynamics (CFD) was not possible (Sato et al., 2009).
Recent CFD calculations by, for example, Shams et al. (2015) reveal a very complex
flow where the main flow follows the helical path of the wire wraps. The effect of the
low Pr number is to make the temperature field look much less complex than the
velocity field due to the high thermal conductivity of the LM.

3.1.3.6 Lateral mixing

As mentioned previously, temperature fields in LM cooled fuel bundles are impossi-
ble to mimic in a water-based facility because the Pr number (Pr 0.01) is much
smaller than the one for water (Pr 1 10). Nevertheless, a number of experiments
have been performed in the past, and the applied techniques will be briefly mentioned
for completeness here. Recently, Lee et al. (2015) determined lateral mixing in a
water-based facility by using a so-called wire-mesh sensor. By injecting salt as a rep-
resentative scalar for energy, one could measure the radial spreading of the salt at a
certain height in the bundle geometry. Originally, this technique has been developed
by Prasser et al. (1998). Yl€ onen et al. (2011), Bulk et al. (2013), and Buskermolen
(2014) have applied this technique to bundle geometries. The disadvantages of this
technique are: (i) the sensor influences the flow as it is placed in the flow itself (called
an intrusive technique) and (ii) the resolution is limited, because the spatial density of
wire crossings cannot be too large.
These disadvantages can be overcome by using a nonintrusive technique. Recently,
Wang et al. (2016) applied LIF in combination with the dye tracer Rhodamine B.
Together with an RIM technique, they could measure lateral mixing by converting
the fluorescence intensity (indicated by gray values) to concentrations. Unfortunately,
Wang et al. do not indicate the accuracy of this conversion, or, in other words, the
accuracy of the measured concentration field. Fig. 3.1.10 shows the result of one
of their measurements.

3.1.4 Pool-type experiments

Thermal-hydraulic analysis is a key point for the design and safety of reactors. The use
of system codes or CFD tools can address a lot of the different challenges and the capa-
bilities of CFD codes to simulate multiphysics phenomena continuously increase but
nevertheless, the use of water modeling for the study of the thermal-hydraulic behav-
ior of the primary circuit remains a very valuable tool. Furthermore, one-dimensional
system codes are not always well suited for the analysis of pool-type reactors. Also,