Page 380 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

(U)RANS pool thermal hydraulics 343

considered for this thought experiment aims to reproduce the primary coolant flow and
heat transfer patterns.
Due to the low velocities considered and the nature of the cooling liquid, that is, a
liquid metal, the flow field can be considered as incompressible. This assumption
removes the direct link between the temperature, the density, and the pressure. The
primary coolant of the reactor is lead-bismuth eutectic, LBE, a heavy liquid metal with
low Prandtl number. Its material properties are highly dependent on temperature. As
such, known correlations could be taken from the LBE handbook (OECD/
NEA, 2007).
On this basis, we can consider to solve the incompressible Reynolds-averaged
Navier-Stokes equations, complemented with the energy equation. In many applica-
tions, the incompressible thermal equation is decoupled from the momentum, but in
the MYRRHA reactor case, due to the high temperature variation, the buoyancy
effects are expected to contribute significantly to the flow field. Two different ways
to take this phenomenology into account, effectively rebuilding the coupling between
velocity and temperature will be discussed later. Conjugate heat transfer plays a sig-
nificant role due to the high temperature gradients, so its effect might need to be con-
sidered as well. A volume of gas is lying above the LBE. It is an inert gas, argon,
chosen to moderate oxidation at the free LBE surface. The representation of the free
LBE surface depends on the conditions we wish to simulate. If the free surface is
changing shape and/or position, its tracking is inevitable.

6.2.4.2 MYRRHA operating condition case as the
quintessence of the pool modeling application

The authors’ experience in pool modeling has been acquired by struggling through
building a CFD model of the MYRRHA reactor entire primary coolant loop for oper-
ating conditions. Due to the practical nature of the problem, the procedure to obtain
two different numerical representations of the MYRRHA reactor will be introduced in
this section to show the very different approaches taken.

6.2.4.2.1 Diversification and redundancy strategy
The numerical modeling of MYRRHA in general and of its primary coolant loop in
particular has been performed following the modern safety-related “diversification
and redundancy strategy.” The primary loop of the reactor has been numerically
modeled with system codes (RELAP), coarse-mesh codes (SIMMER), CFD codes,
and a mix of these three kinds. In this chapter, the CFD modeling will be presented,
showing two different ways of tackling this challenging problem:
l CRS4, with a large specific experience of heavy-liquid-metal (HLM) modeling with CFD,
using the commercial software STAR-CCM+
l VKI, with a wide experience in CFD modeling, both with a commercial tool (Fluent) and
with an open-source platform (OpenFOAM)