Page 387 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

350 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

6.2.4.2.8 Approach for pump simulation

The flow field within the reactor is driven by the pumping system. There are two heat
exchanger/pump casings, each one consisting of two heat exchangers and one pump.
Concerning the pumps, modeling the flow through the rotor would require meshing
around the blades and modeling the dynamic behavior of these rotating parts. This
level of refinement is not required since we are only interested in respecting the nom-
inal flow conditions of the primary coolant loop. The presence of the rotor was there-
fore neglected in the CFD models.
There are two possible ways of representing the pumps: by having either an “open”
or a “closed” system. In the open system, the rotor is removed from the CFD domain.
The nominal mass flow rate can then be simply imposed at the pump outlet combined
with a pressure boundary at the pump inlet. However, for studying accident scenarios,
a closed loop is often needed to be able to follow the natural level change of the free
surfaces. In such a closed system, the flow motion is activated by a momentum source
applied to the rotor area and fixed to balance the desired mass flow rate.
The numerical setting of the pumps in three parts, namely, inlet, rotor area, and
outlet, allows a good level of versatility for the pump characterization. First, one
can set a swirl component to the flow. This was done initially by setting a carefully
calibrated rotational volumetric force in the rotor area. More recently, the “fan” setting
has been made available in STAR-CCM+, and the swirl can be simply enforced by
assigning a rotational component proportional to the vertical one while crossing
the pump outlet. Adding such a swirl component may destabilize the flow because
it increases a possible imbalance of the flow kinetic energy. The flow stabilization
can be reestablished in turn by setting a consistent inertial resistance coefficient at
the pump inlet that is now treated as a resisting baffle.
While under steady-state conditions, the mass flow rate is univocally determined
from the pump volumetric force; this is critically no more true during transients. Fur-
thermore, the pressure head from the different levels of the cold and hot free surfaces
also plays a decisive part. Ideally, one would need to couple the CFD code to a specific
pump model able to determine the pump head from its speed and the torque applied by
the fluid on the rotor blades. In practice, a better control on the mass flow rate would
be appreciated. With two settings available, the volumetric force and the inertial resis-
tance, besides the pump behavior under nominal condition, we should be able to have
also a correct behavior in one specific other configuration. By choosing this config-
uration to be the pump unpowered, we determine in fact the hydraulic resistance of
the pump.

6.2.4.2.9 Porous media approach in the core
The core in the OpenFOAM model has been modeled following the homogenization
approach with porous modeling. The different zones are separated according to the
structure layout, which is governing both porosity and pressure losses. Consequently,
the general core layout shown in Fig. 6.2.4.4 is composed, in the central core, of five
porous rings, representing from the center: (i) the inner fuel assemblies; (ii) the