Page 47 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

22 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

formed by chemical reactions in heavy liquid metals, where breakup/coalescence, particle-
to-fluid mass transfer, and particle size distribution are required information.
Development needs
The validity of the Euler-Lagrange approach for small particles needs to be demonstrated
through experiments. For large particles, additional development and validation (e.g., on
particle shape) will be necessary. The CFD-DEM and Euler-granular techniques are yet
to be further explored in the LMFR framework. A proper definition of the forces involved
between particles and with the liquid metal will require extensive experimental and numer-
ical programs.
l Solidification
Challenge
Liquid metals considered in reactor applications have melting temperatures above the ambi-
ent temperature. In certain accidental scenarios with overcooling in primary heat exchangers
or through the vessel walls, the liquid-metal coolants can solidify. This might result in partial
or complete blockage altering the coolant flow paths in the reactor. Solidification might also
lead to mechanical stresses in components as a result of thermal contraction and expansion.
Additionally, in lead-bismuth eutectic, volume change due to recrystallization can occur
leading to mechanical stresses (Glasbrenner et al., 2005). Hence, it is important to know
if and where solidification can occur during accidental transients and how the solidification
front moves.
State of the art
Solidification and remelting experiments to assess mechanical effects have been performed
in the framework of the Russian submarine program (Pylchenkov, 1999). Tests on basic
structures, reactor components, and integral structures indicate that most of the damage
in solidification and remelting cycles are caused by local overheating during the remelting
stage and due care should be taken. The precise behavior of components could however
depend on the peculiarities of the components. In the current LMFR framework, activities
are undertaken to expand the knowledge on solidification and its propagation in typical
liquid-metal reactor coolants (Roelofs et al., 2016; Tarantino, 2017). Hereto, fundamental
experiments are performed, also providing validation data for numerical models of
solidification.
Development needs
Fundamental experiments will have to be complemented with applied experiment represen-
tative of reactor components and configurations to assess the effect of solidification from a
mechanical and a thermal-hydraulic system behavior point of view. Numerical models need
to be further developed and/or validated in parallel.

2.4.2 Core thermal hydraulics
The category of core thermal hydraulics includes the following topics:
Normal operation:
l Fuel assembly (normal and deformed) (see also Section 6.2.3)
Challenge
Thermal-hydraulic characterization of a fuel assembly is needed under all circumstances
with a main focus on the maximum fuel cladding temperature. This needs to be achieved