Page 86 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Rod bundle and pool-type experiments in water serving liquid metal reactors 61

Here, n 1 is the refractive index of a material indicated by the subscript 1 and θ the
angle of incidence with respect to the normal of the interface between those materials.
Refractive indices are dependent on temperature and vary per frequency of the elec-
tromagnetic radiation under consideration. As long as the interfaces are flat, refraction
of the light can be predicted and corrections to the equipment can be made accord-
ingly. When interfaces are skew, curved, and relatively thick; however, such predic-
tions can result in very inaccurate results. The skewer/more curved/thicker the
interface is, the more the exiting light becomes sensitive to uncertainties in, for exam-
ple, the refractive index and geometry of that interface.
One solution is to make the blocking walls (such as the rods in a rod bundle geom-
etry) both transparent and very thin so that the path of the light is hardly affected. For
most applications, however, bending and/or fluttering is undesired so that no material
is available which is both thin and stiff/strong at the same time. A much more practical
solution to this problem is to match the refractive indices of the fluid and the solid
transparent walls, that is, to make the latter optically “disappear” in the former. There
are actually two ways of doing so:
l by matching the refractive index of the solid with the index of a typically “easy-to-use” liq-
uid, such as water; and
l by matching the refractive index of the liquid to a specific transparent solid material.
Both approaches depend on the situation to be investigated and will be briefly
described in the following paragraphs.

Matching fluid with solid (F2S)
The advantage of this method is that one can use common materials that are very trans-
parent, such as different types of glass or PMMA (poly-methyl-methacrylate or per-
spex or plexiglass). Glass is very transparent and cheap, although some types of glass
may be rather expensive such as quartz or sapphire. The brittleness can be a challenge
during the construction phase and in cases where connections are required between
metal and glass (stresses). A great advantage of glass is its chemical inertness to many
fluids, which can be a handy feature when selecting a matching fluid. On the other
hand, polymers, such as PMMA, are easy to handle but are more sensitive to some
chemical compounds, such as alcohols. The refractive index of glass and PMMA dif-
fer too much from the one of water (n PMMA ¼ 1.493, n glass ¼ 1.520 and n water ¼ 1.334
at 23°C for green light with a wave length of λ ¼ 532 nm). For this reason, another
liquid has to be used. An extended overview of the applications of different refractive
index matching (RIM) fluids found in the literature is given by Hassan and
Dominguez-Ontiveros (2008). A few of those fluids matching the refractive index
of PMMA and glass can be found in Table 3.1.1.
A disadvantage of F2S is that such fluids are more difficult and/or expensive to use
in large-volume facilities than plain water. Fluids can be volatile, flammable or harm-
ful to human beings so that additional measures are required and volumes have to be
limited to feasible measures. Moreover, spatial accuracy of measurements becomes
better by using large facilities as the probing size (e.g., of tips of thermocouples,