Page 50 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Thermal-hydraulic challenges in liquid-metal-cooled reactors 25

Development needs
The methods to assess the impact numerically have already been established. Validation is
the missing link. Up to now, focus has been on the impact of an internal blockage, rather than
on the formation. Both for experiments and for numerical analysis development of new
methods are required before validation and application can start. On the other hand, an (com-
plete) inlet blockage of a fuel assembly has only been studied numerically so far. Such an
analysis requires modeling of neighboring fuel assemblies and eventually of the complete
core. To achieve this, either the existing subchannel approaches can be used in combination
with information from more detailed CFD simulations or new CFD methodologies need to
be developed and validated including the assessment of the interwrapper heat transfer.
Interwrapper flow (see also Section 6.2.3)
l
Challenge
Interwrapper heat transfer is the heat transfer that takes place through the gap between adja-
cent fuel assemblies. Because of the high thermal conductivity of the coolant, this may play a
particularly significant role in LMFRs. This heat transfer is mainly important during the tran-
sition from nominal to off-normal situations, where it plays a 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 wrap-
per wall of the fuel assembly and secondly by transport of heat to adjacent fuel assemblies.
State of the art
Very few experiments for examining the heat transfer through the interwrapper region have
been performed with liquid metal as fluid. Kamide et al. (2001) performed experiments in the
Japanese sodium PLANDTL facility. In this facility, seven fuel assembly mock-ups repre-
sent 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,
covering steady-state situations and transients, contributed to a better understanding of the
role of the IWF in the safety assessments of LMFRs.
Development needs
The results from the PLANDTL facility were specific for the decay heat removal of one spe-
cific reactor system, and the data don’t allow for validation of CFD codes that are widely
used in design support and safety analysis of nuclear reactors nowadays. Therefore,
Doolaard et al. (2017) are designing a new experiment to analyze the interwrapper flow
cooling. Such data should also allow a first step toward validation of complete core models.

Severe accidents:
l Molten fuel relocation and refreezing
Challenge
Severe accidents are very-low-probability sequences with large radiological consequences.
In LMFRs, an in-vessel retention (IVR) strategy has been adopted so far, meaning that the
ultimate barrier to the radioactive confinement is the vessel. Under severe accident condi-
tions involving core degradation, the vessel integrity is normally challenged only by the
decay heat of the core debris, unless strong energetic excursions occur during the core deg-
radation phase. The hypothetical core disruptive accident (CDA) is a postulated scenario that
envelopes the consequences of all possible core degradation sequences. Mechanistic codes
are needed to represent the complex phenomena concurring in determining a realistic value
of energy released by the core during a CDA. Several pathways can be defined depending on
the strength of energetic excursion. Assuming that the vessel survives the energetic