Page 49 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

24 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Control rod behavior
l
Challenge
The high density of heavy liquid-metal coolants allows buoyancy to be the passive driving
force for the emergency insertion of the safety rods during scram, in contrast to the normal
gravity-driven operation. This needs to be demonstrated.
State of the art
While numerous experience and operational feedback has been gathered on safety rod devel-
opment and operation in liquid sodium, as documented in (IAEA, 1995), the operation of a
buoyant system within a heavy liquid metal is rather different from standard systems. In view
of this, hydrodynamic experiments have been performed on a one-to-one scale mockup of a
buoyancy-driven safety rod in lead-bismuth eutectic providing proof of principle (Kennedy
et al., 2017). These have been complemented with a numerical program. For the realistic
simulation of the rod insertion, a fully explicit two-way coupling between fluid and moving
rod is implemented in CFD. The overset mesh methodology implemented in the code
allowed for the entire rod displacement to be simulated.
Development needs
As the current experience in this topic is design-specific, new experiments and simulations
will be required for other designs. Obviously, they can build upon the existing experience.
For the existing and new designs, off-design conditions need to be analyzed.

Off-normal operation:
l Flow blockages (impact and formation) (see also Section 6.2.3)
Challenge
A flow blockage accident in a fuel assembly of a nuclear reactor can consist of a partial or total
obstruction of the flow area. This leads in general to a degradation of the heat transfer poten-
tially causing a temperature peak in the fuel cladding that can eventually lead to failure of the
cladding and a fuel melt. A partial inlet blockage may be dangerous for the integrity of the fuel
assembly. An internal blockage can be even more dangerous, and it is not easy to detect.
State of the art
Complete or partial blockages of fuel assemblies have been studied since long. Kayser et al.
(1994) discuss the main conclusions of the French Scarabee program, while Van Tichelen
(2012) provides an elaborate overview of international activities in this field. She concludes
that most studies in the past focused on planar blockages, mainly relevant for grid-spaced
fuel bundles. The number of experiments related to the more likely long and porous block-
ages in wire-wrapped fuel bundles is limited. Furthermore, although the flow in the wake
downstream of a blockage is significantly disturbed, the effect of the blockage remains lim-
ited to the blocked channels and no propagation to other subchannels seems to occur in a
wire-wrapped configuration. Obviously, local temperatures increase because of the reduced
flow. In this case, blockage size and flow rate are important parameters. She also concludes
that the position of a blockage has an impact. A blocked side subchannel has a different
impact than a blocked central subchannel. Finally, only large blockages will lead to a reduc-
tion in flow rates or an exit temperature increase that is detectable at the fuel assembly out-
lets, while at the other hand, only these large blockages will lead to blockage propagation.
The numerical assessment in recent years profits from the availability of modern simulation
methods like CFD that allows much more detailed analysis. CFD simulations nowadays
cover the full range of blockages in wire-wrapped fuel assemblies and fuel assemblies with
grid spacers (Di Piazza et al., 2014).