Page 388 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

(U)RANS pool thermal hydraulics 351

combination of in-pile section, scram rods, and control rods; (iii) the outer fuel assem-
blies; (iv) the inner dummies; and (v) the outer dummies. The last ring corresponding
to the core jacket is a solid component and was therefore subtracted from the flow
domain. The area directly below the core, composed of a grid with cones that hold
the fuel assemblies in place, makes the connection with the core restraint system.
As this intermediate ring is characterized by very little flow blockage from either axial
or radial directions, it is approximated as a nonporous zone.
Each of these porous zones is characterized by a certain porosity value γ, defined as
the ratio between the volume of fluid and the total volume of the media. From a numer-
ical point of view, these pressure losses due to friction are added to the standard equa-
tion as a momentum sink expressed as follows (Darcy-Forchheimer law taken from
Idelchik (2005)):

! μ ! ρ
! !

F i ¼ u i C 2 u u i ¼ —p porous (6.2.4.1)
α 2
The pressure drop across the entire core area, from the lower plenum up to the free
surface (including the radial losses through the barrel holes) is estimated to be around
2bar based on fuel assembly correlations. Due to the fuel assemblies’ hexagonal wrap-
per tubes, in reality, the flow in the porous media is blocked in the radial direction.
This condition is imposed numerically by assigning a very high value of the radial
resistance coefficients in the core region (as applied in OpenFOAM) or by defining
the lateral interface as an impermeable baffle (as applied in STAR-CCM+). The pres-
sure drop in normal operation is determined by the pressure drop in a fuel assembly.
These losses are estimated by using the Rehme (1973) correlation for a wire-wrapped
fuel bundle as a function of the local Reynolds number (noted as f(Re)):

L pin 2
ΔP ¼ f 0:5ρu (6.2.4.2)
D eq

In the above equation, u is the local flow velocity, L pin corresponds to the pin length,
D eq is the equivalent diameter, and f is the friction factor. The inner dummies are
modeled as fuel assembly rings since they have the same flow rate and pressure drop.
The axial resistance coefficient of the two other core rings and outer dummies was
kept constant and first approximated based on the desired mass flow-rate distribution.
Alternatively, in the case when all assemblies are modeled, the pressure loss from
the Rehme correlation was evaluated at nominal flow rate and at a low flow rate sig-
nificant for natural convection operation. The values obtained were used to curve-fit
(Eq. 6.2.4.1).
The heat in the core is modeled as a volumetric heat source of 100MW in total
and is limited to the inner and outer fuel assembly rings, where the nuclear reaction
is significant. The foreseen radial power distribution is approximated by

l a second-order polynomial giving a 2:1 ratio between the central and the outermost fuel
assembly power and normalized from direct code measurement for a total 100MW (as
applied in STAR-CCM+),