Page 54 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Thermal-hydraulic challenges in liquid-metal-cooled reactors 29

State of the art
System codes are generally used for the analysis of RVACS performance and their effect on
the reactor cooling. Nowadays, this analysis is also possible with CFD codes (Wu et al.,
2015). The same is valid for the heat and mass transport in the cover gas where the
three-dimensional features of CFD codes present a large added value. Limited experimental
data are available for validation (Aithal et al., 2016).
Development needs
Validation and verification of the (turbulence models in the) numerical tools used based on
high-fidelity codes and experiments are mandatory.
l Chemistry control and coolant cover gas interaction
Challenge
Chemistry control of the coolant and cover gas is a critical issue of operating LMFRs. It
is essential to control the concentrations of impurities, because of the potential for acti-
vation and also because of the possible effect on corrosion, mass transfer, and scale
formation at heat transfer surfaces. Therefore, coolant chemistry control includes not
only oxygen but also pollution source term studies, mass transport, and filtering and
capturing techniques. In this framework, flow patterns in the pools are important for
mixing and chemistry control (e.g., to ensure oxygen control in lead fast reactors).
The development and validation of a multiphysics simulation tool that can take these
interactions into account is a necessary step. The assessment of the flow patterns is
one issue; the development and validation of practical methodologies and instrumenta-
tion for chemistry control is another issue.
State of the art
In Europe, where the design and R&D program is ongoing for LMFRs, most of the
efforts are focalized on the coolant chemistry control and purification during operation
(i.e., oxygen control, oxygen sensor reliability, coolant filtering, coolant purification,
and coolant cleaning from components) and cover gas control (i.e., radiotoxicity assess-
ment of different elements, migration flow path into cover gas, and removal and
gettering). This is addressed, for example, in the Italian national program (Tarantino,
2017) for ALFRED or supported by EU mainly for MYRRHA in European collabora-
tive projects.
Development needs
While the present activities are most related to the technology development (i.e.,
oxygen sensors and purification unit), both modeling and experiments are required
aiming at addressing mass transfer (i.e., oxygen for HLM and impurities for LMFRs)
in pool system. This topic is strictly related to pool thermal hydraulics and flow pattern
investigation, while modeling would consider also chemical interactions, impurity
sinks, and mass transfer.
l In-vessel fuel storage
Challenge
After removing fuel assemblies from the reactor core, they can be stored in an ex-vessel or
in-vessel storage. In the case of the in-vessel storage, the decay heat of the spent fuel in the
internal storage has to be dealt with by the reactor cooling system in all operational modes.
On one hand, the cooling of the fuel assemblies in the storage needs to be guaranteed at all
times. On the other hand, the influence of the additional heat source on the integral system
behavior needs to be assessed.