Page 45 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

20 Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Thermal fluctuations/striping (see also Section 6.2.4)
l
Challenge
Incomplete mixing of nonisothermal flow streams can lead to random fast temperature fluc-
tuations in adjoining structural wall surfaces and in this way induce high-cycle thermal
fatigue. This can also be the case for fluctuating free surface levels or thermal stratification
layers. The attenuation of the fluctuations on the walls depends on their frequency and on the
heat transfer coefficient. The latter is particularly high in liquid-metal-cooled reactors.
State of the art
Several experimental works in air, water, and sodium have been performed in the past
to understand the thermal striping phenomenon (Choi et al., 2015). Today, only large eddy
simulations can sufficiently resolve and predict the mixing behavior of thermal striping,
including temperature fluctuation and fluctuation frequencies. However, their very high
computational costs exclude the use of CFD for large engineering applications. In view
of this, other approaches are currently explored. Thermal striping limits on maximum allow-
able temperature fluctuations in liquid metals are being developed from structural mechanics
considerations, mostly for sodium (Chellapandi et al., 2009). These limits strongly depend
on the attenuation and hence on the heat transfer coefficient.
Development needs
Turbulence models that allow application to industrial cases with an accuracy that is
sufficient for thermal striping need to be further developed and validated. Heat transfer
coefficients considered in setting thermal striping limits need to be confirmed through
thermal-hydraulic analysis for all liquid metals considered.
Mechanical fluctuations (see also Section 6.2.2)
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Challenge
Failures of components resulting from excessive vibrations are affecting the performance of
nuclear power plants. Pettigrew et al. (1998) describe that generally, problems caused by
excessive vibration are fatigue cracks and fretting-wear damage. Tube failures due to fretting
wear in heat exchangers and vibration-related damage in nuclear fuel pins are of particular
concern. According to Weaver et al. (2000), reports of flow-excited failures of heat
exchanger tubes began appearing in the 1950s.
State of the art
Today, flow-induced mechanical fluctuations are still not fully understood, and simulation
methodologies coupling CFD and computational structural mechanics (CSM) are still under
development (see, e.g., Degroote and Vierendeels, 2012; Blom et al., 2015). Internationally,
firstly, developments are focusing on developing the methodologies and validating them
(Roelofs et al., 2015b). Secondly, pragmatic methods are under development that allows
simulation of large systems to a reasonable accuracy (Longatte et al., 2013).
Development needs
Especially for large systems, such as a complete fuel assembly or a heat exchanger, one
should take into account that computational resources for the strongly coupled simulations
are putting a restriction on the application and will require a longer-term effort. Both, an
experimental and a simulation program need to be set up to reach fully developed and
validated models both in simulant fluids and in the actual liquid-metal coolant.
l Bubble transport
Challenge
Correctly, predicting the transport of gas bubbles in pool-type liquid-metal-cooled reactors
plays an important role in assessing the risk of gas accumulation in the reactor. Gas bubbles