Investigation of Thermal Mixing in the Control Rod Top Tube Using Large Eddy Simulations

  • Reijo Pegonen KTH Royal Institute of Technology, Roslagstullsbacken 21, SE-10691 Stockholm, Sweden
  • Nicolas Edh Forsmarks Kraftgrupp AB, SE-74203 Östhammar, Sweden
  • Kristian Angele Vattenfall Research & Development AB, SE-81426 Älvkarleby, Sweden
  • Henryk Anglart KTH Royal Institute of Technology, Roslagstullsbacken 21, SE-10691 Stockholm, Sweden


Thermal mixing and thermal fatigue has led to component failures in the nuclear industry. The thermal fatigue phenomenon is intimately linked with the mixing of streams of different temperatures in proximity to a solid wall. Due to conjugate heat transfer, temperature fluctuations are induced in the wall. One of the key issues is to predict the amplitude and the frequency of the fluctuations. This paper presents pre-calculations of the thermal mixing experiments that are under preparation at the KTH Royal Institute of Technology as part of the THEMFE project (Thermal Mixing and Fatigue Experiment).The proposed geometry is a simplification of a reactor control rod and consists of a top-tube and control rod stem, which are modeled as concentric cylinders. In addition there are only two hot inlet jets and two cold inlet jets, whereas in reality there are 8 upper inlets and 4 lower inlets for hot bypass water and the cold flow is annular.Thermal mixing was studied by using a transient Computational Fluid Dynamics (CFD) solver for the incompressible filtered Navier-Stokes equations and employing a Large Eddy Simulation model of turbulence implemented in OpenFOAM. The aim was to verify that the proposed simplified geometry and the flow conditions of the experiment will lead to low frequent temperature fluctuations of the order of 0.1-1 Hz, as seen in previous experiments with the real geometry. Such low frequencies are typical for the thermal fatigue phenomenon. The study was focused on the region near the control rod stem and therefore a refined grid was used in that region. The final mesh consisted of over one million cells.The results did indeed reveal low frequent temperature fluctuations in the lower part of the mixing region near the control rod stem. The results of this paper indicate that the length of the mixing region is 23 cm, which is large enough to be resolved in the experiment. It was also found that the most dangerous region, where the dominant high amplitude temperature fluctuations have a frequency of the order of 0.1 Hz, is 4 cm long. As expected, the instant flow field is asymmetric with large secondary flows. The present results verify that the proposed geometry and flow conditions can be applied in the experiment.


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How to Cite
PEGONEN, Reijo et al. Investigation of Thermal Mixing in the Control Rod Top Tube Using Large Eddy Simulations. Journal of Power Technologies, [S.l.], v. 94, n. 1, p. 67--78, oct. 2013. ISSN 2083-4195. Available at: <>. Date accessed: 29 july 2021.


Thermal mixing, Mixing region, Low frequency temperature fluctuations, Computational Fluid Dynamics, Large Eddy Simulation, Incompressible flow.

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