Page 369 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Core thermal hydraulics 333

scaled fuel bundle of MYRRHA taking into account a simplified model for the oxi-
dation kinetics of fuel cladding in LBE. The model was applied to investigate the
details of the velocity, temperature, and concentration boundary layer at the interface
of LBE and fuel cladding. The results showed that local oxygen concentrations can be
much lower than the expected bulk oxygen concentration (see, e.g., Fig. 6.2.3.19). In
general, a wire-wrapped fuel bundle is characterized by quasi-stagnant areas and
swirls. These regions are more inclined to corrosion due to high temperature and
low oxygen concentrations. Furthermore, various operating conditions were simulated
to assess the achievement of oxygen concentration levels required to maintain a stable
protective oxide layer at the steel/LBE interface. CFD modeling showed to be impor-
tant and necessary to bridge the gap on the information needed in the transition
between “basic” corrosion experiments and the application of the experimental data
in a realistic geometry.

6.2.3.6 Summary

In summary, the following main messages can be derived from this chapter with
respect to wire-wrapped fuel assembly core thermal-hydraulic modeling for LMFRs:

l Many experiments have been performed in the past (up to mid-80s) with increasing pin
numbers. However, these data can have limited use for validation of modern simulation tools
like CFD. Therefore, since 2000, a trend can be observed to reschedule experiments with
increasing pin numbers, and in addition, high-fidelity CFD is used to complement the
experimental data.
l Care should be taken when applying empirical correlations for liquid-metal flow and heat
transfer. From the experience of the author, application of the correlations of Rehme and
Cheng and Todreas can be recommended for fuel assembly pressure drop, and the correlation
of Kazimi can be recommended for heat transfer in wire-wrapped fuel assembly geometries
(if any).
l The influence of RANS turbulence models on the simulation of wire-wrapped fuel assem-
blies is limited.
l A fundamental change in bulk flow behavior is observed between 19-pin bundles and 37-pin
bundles. Within the 19-pin bundle, the bulk flow in the central subchannels still shows the
influence of the wrapper walls, whereas in the 37-pin bundle, the bulk flow behavior in the
central subchannels seems to be decoupled from the influence of the wrapper walls.
l The following accuracy levels can typically be obtained:
Up to 10% for velocity and mass flow rate in a well-resolved mesh compared with exper-
iments and high-fidelity simulations
Up to 15% for temperature and heat transfer in a well-resolved mesh compared with
experiments
Up to 15% for pressure drop in a reduced resolution mesh compared with experiments
Up to 20% for temperature for a reduced resolution mesh compared with a well-resolved
mesh (accuracy of velocity and mass flow rate is much better)
l Most of the current analyses are focused on geometries as-designed. Do not forget that a fuel
assembly will deform, even under normal operation conditions due to thermal loads and irra-
diation. The effects of such deformations on the cooling and peak temperatures are only ana-
lyzed in a few studies until now.