Page 55 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

30 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

State of the art
Preliminary simulation work using system thermal hydraulics and CFD codes was carried
out for different reactor types.
Development needs
For a thorough analysis of final designs, further numerical simulations are needed, especially
to guarantee sufficient cooling in all possible operational situations. These should be carried
out with codes that are validated for the purpose.
Off-normal operation:
l Sloshing
Challenge
Pool-type liquid-metal-cooled reactors present large volumes of (high density) liquid coolant
in large-size vessels. Sloshing of the liquid metal in the reactor pool, for example, due to
seismic events, gives rise to dynamic loads on the reactor vessel and components that need
to be taken into account in the reactor design phase. It is thus important to predict these loads
accurately.
State of the art
Limited studies, both experimentally and numerically have been reported on this topic, using
scaled facilities and simulant coolants. Recently, Myrillas et al. (2016) reported about the
development, construction, and exploitation of a small-scale experimental setup that sup-
ports CFD model development and validation while carefully taking care of scaling effects
caused by difference in size and fluid. The growing capabilities of CFD allow to move
from analytic studies based on simple mechanical models that have their limitations for
the analysis of complex 3-D geometries to numerical simulation of sloshing problems. Dif-
ferent approaches have been proposed: finite element, finite volume, and smooth particle
hydrodynamics.
Development needs
Validation on the basis of relevant experimental data is the key development need for the
numerical approaches mentioned above. Once validated in such a way, CFD methods can
be applied to other similar designs with reasonable confidence as well.
l Jet-stratification interaction (see also Section 6.2.4)
Challenge
A jet, for example, exiting from the core in the upper pool or from a pump and/or heat
exchanger in the lower pool, may affect and change the flow patterns in a large plenum sig-
nificantly, interacting with any stratification that might be present and thus changing the con-
tact zone of liquid and vessel wall or components leading to thermal striping or fatigue
issues. Note that this is also important in normal operation conditions as striping and fatigue
are long-term effects.
State of the art
At a small scale, LES and DNS simulations of jet behavior have been successfully validated
in recent years against both water- and liquid-metal experiments. However, the numerical
cost of these methods currently prevents their application to larger scales, such as the com-
plete core outlet region or the hot/cold pool themselves. For these cases, RANS simulations
remain the only option. However, these models often require careful calibration in order to
correctly reproduce carefully the average behavior of the jet and are typically unable to pro-
vide information on its fluctuating behavior.