Page 87 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

62 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Table 3.1.1 Some RIM fluids with a refractive index close to those of PMMA and glass
n
Fluid Hazard Fire Reactivity
Water 1.33 1 0 1
Glycerine 1.47 3 0 1
NaI solution 1.33–1.5 (60%) 2 0 1
Mineral oil 1.48 0 0 0
Benzyl alcohol 1.54 2 1 1
Tung oil 1.52
Methyl salicylate 1.526 2 1 1
Para-mycene 1.491 2 2 0

Notes: Water is added for comparison reasons. The numbers in the last columns indicate a value on a scale of 0 to 5.
Source: Data extracted from Hassan, Y.A., Dominguez-Ontiveros, E.E., 2008. Flow visualization in a pebble bed reactor
experiment using PIV and refractive index matching techniques. Nucl. Eng. Des. 238(11), 3080–3085.
the overlap region of two laser beams in LDA or the thickness of the laser sheet in PIV)
has a certain minimum size. Hence, an accurate determination of the flow field inside
a multirod bundle geometry with a sufficient development length at high Re numbers
becomes usually more attainable by using water (and finds a matching solid material)
than by applying matching fluids. Recently, Dominguez-Ontiveros and Hassan (2014)
performed rod bundle experiments with F2S in a relatively small experimental
setup with small subchannels and no development length. They used para-cymene
(1-methyl-4-(propan-2-yl)benzene) in combination with PMMA and determined
the velocity field inside the rod bundle with the help of PTV.

Matching solid with fluid (S2F)
For large flow systems, water is a great fluid to use because it is practically every-
where available and has, obviously, no health or environmental issues. Equipment
for water systems, such as pumps, piping, and sensors, are relatively cheap and easy
to get, provided that the pressure and temperature stays modest. For LM reactor
research, ambient conditions (1 bar, 20°C) are sufficient because water is mostly used
for measuring flow fields only.
There are not many solids with a similar refractive index as water. Table 3.1.2
shows a few possible materials. Besides the refractive index, the absorption coeffi-
cient, indicating the attenuation of the light per unit of length, is given as well. This
coefficient is defined by

IðdÞ αd
¼ 10 ,
Ið0Þ
where I(0) and I(d) denote the light intensity at position x ¼ 0 and x ¼ d, respectively,
(so d denotes the thickness of the material) and α is the absorption coefficient. Hence,
the larger the absorption coefficient, the less transparent the material becomes.
To the author’s knowledge, only fluorinated poly-ethylene propylene (FEP) has
been used in the literature for rod bundle geometries. Dominguez-Ontiveros and