Page 100 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

74 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Different thermal-hydraulic challenges can be addressed by using water modeling
of the upper plenum and lower plenum of an LMR:
In the upper plenum, the global velocity field and temperature distribution can be measured
in nominal condition but also in case of accidental scenarios. By measuring velocity profiles
and temperature profiles at different locations, it is therefore possible to identify the presence
of flow unsteadiness and thermal stratification. Water model tests can also be used to detect
free surface oscillation, which might induce thermal fatigue on the different structures sub-
merged in the above core plenum. The presence of vortex formation at the free surface, with
the possible gas entrapment, close to the inlet of the heat exchanger can be investigated. The
behavior of bubbles or particles escaping from the core of the reactor can be explored and
used to validate CFD simulations of accidental scenarios. Finally, the flow pattern in non-
symmetric situations can be measured.
In the lower plenum, a water model will provide important data on the dissipation of the swirl
created by the rotating pumps. The three-dimensional flow pattern can be measured and used
to validate or fine-tune CFD simulations. Stagnation regions of low velocity but high turbu-
lence can also be identified easily and the flow pattern in nonsymmetric situation can be
analyzed. Bubbles and particles (lighter or heavier than water) can also be injected to study
the flotation or decantation in case of an accident.

3.1.4.1 Scaling for pool-type experiments
The major scaling requirements for pool experiments in transparent fluids are the
following:
l The overall behavior in the prototype plant should be preserved.
l The major thermal-behavior phenomena should be reproduced.
l The scale of the water model must be sufficiently high to be able to represent the detailed
features on the reactor.
l The balance between buoyancy force and pressure losses must be preserved when studying
natural convection.
l The balance between heat generation and heat cooling must be preserved.
l The water model should be built at a reasonable cost.
Fig. 1.11B sketches the main elements of the typical primary HLM loop. In nominal
working conditions, the pumps drive the LBE through the core bundles, the barrel per-
forated walls, and finally through the heat exchangers, therefore realizing core heat
removal by forced convection. After a shut-down event, the core continues to produce
heat by spontaneous decay of radioisotopes. The decay heat is equal to 10% of the
nominal power. In the unlucky event of pump failure, the decay heat should be
removed passively (i.e., without the aid of moving or rotating parts) by natural con-
vection. Therefore, the thermal-hydraulic phenomena of interest for pool experiments
with transparent fluids are:
core heat removal in nominal operating conditions (forced convection); and
l
core heat removal through passive heat transfer (natural convection).
l
The interest is to simulate both heat transfer mechanisms with the same water model.
The nondimensional numbers (NDN) describing the phenomena can be derived
through a nondimensional analysis of the equations describing momentum and energy
transport for the primary loop.