Page 367 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Core thermal hydraulics 331

Fig. 6.2.3.17 Flow blockage formation test simulations using the discrete particle model (VKI).

assembly. As the heated zone is only far downstream, this region is not affected
largely.
To analyze the effects of a blockage is only one side of the story. The other side is to
analyze how and why a blockage is formed allowing to design measures to prevent the
formation of such blockages. Apart from experimental work in which particles repre-
sentative of fuel after cladding failure will be injected in a transparent fuel assembly
mock-up, also simulations are being set up. Such simulations are hard to perform using
tradional CFD methods like Euler-Lagrangian particle tracking or Eulerian-Eulerian
multiple fluid-phase modeling. However, a novel technique based on Euler-
Lagrangian particle tracking called macroscopic particle modeling (Agrawal et al.,
2004) might be useful in this case. This model allows to track spheres larger than
the computational grid size through a computational domain. In the simulation, the
blockage of fluid volume by the large particles is taken into account, and the method
also allows interaction between multiple particles, in principle allowing flow blockage
through congestion of multiple large particles trying to squeeze through a narrow
region that might be the case in a wire-wrapped fuel assembly. Fig. 6.2.3.17 shows
a proof-of-principle test of this method in which a number of large particles are
released and tracked in a flow channel including a blockage leaving a flow-through
area just a bit larger than the particle diameter.

6.2.3.4.3 Inter-wrapper flow

Interwrapper heat transfer is the heat transfer that takes place through the gap between
adjacent fuel assemblies that in fast reactors typically have a casing that is also called
wrapper. Because of the high thermal conductivity of the coolant, this may play a sig-
nificant role in LMFRs. This heat transfer is mainly important during the transition
from nominal to accident situations, where it plays an important role in the passive
decay heat removal operation. The interwrapper flow contributes in that situation
in two ways to the reduction of the peak cladding temperature: firstly through direct
cooling through the wrapper wall of the fuel assembly and secondly by transport of
heat to the adjacent fuel assemblies. Very few experiments for examining the heat
transfer through the interwrapper region have been performed with liquid metal as
fluid. Kamide et al. (1998, 2001) performed experiments in the Japanese sodium
PLANDTL facility. In this facility, seven fuel assembly mock-ups represent a part
of the core of a fast reactor. The six outer fuel assemblies contain seven wire-wrapped
rods each, while the central one contains 37 wire-wrapped rods. The experiments, cov-
ering steady-state situations and transients, contributed to a better understanding of the
role of the interwrapper flow in the safety assessments of LMFRs. However, the