Investigating the Effect of Freeze Plug Inclination Angle on Melting Behaviour with the Linearised Enthalpy Method

Master thesis (2023)

Authors

C.Z. van Dijk Applied Sciences

Contributors

D. Lathouwers RST/Reactor Physics and Nuclear Materials - AS (mentor)
B.J. Kaaks RST/Reactor Physics and Nuclear Materials - AS (graduation committee member)
Faculty
Applied Sciences, Applied Sciences
Copyright: © 2023 Chris van Dijk
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Published Date

25-01-2023

Language

English

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Faculty

Applied Sciences

Abstract

The Molten Salt Fast Reactor (MSFR) is a next generation nuclear reactor. The MSFR may play an important role in combatting global warming whilst ensuring energy safety considering fossil fuels’ finiteness. A key safety mechanism for the MSFR is the freeze plug, a valve underneath the reactor core designed to melt during reactor malfunction, letting the fuel salt drain into an emergency draining system.

In this thesis, the effect of the inclination angle of the freeze plug on its melting behaviour was investigated. This was done via the Linearised Enthalpy Method for phase change in OpenFOAM 8, a Finite Volume Method software package. First, two benchmark cases - one of inclined gallium melting and one of a heater copper fin in air - were performed. These showed reasonable agreement with established results.

A cylindrical freeze plug consisting of LiF-ThF4 salt was defined, with copper and hastelloy annuli around it to simulate the cooling and the draining pipe, respectively. Steady-state simulations were performed for inclination angles of 𝜃 = 0°, 15°, 30°, 45°, 60°, and 90°. Via simulations on meshes with 40,500 and 1.1 million cells, it was shown that the freeze plug did not form at 𝜃 = 60° and 90°. For the other four inclination angles, the resulting freeze plug was found to be mesh convergent via a third mesh of 3.6 million cells.

Then, a transient simulation mimicking reactor malfunction was performed for the inclination angles 0° ≤ 𝜃 ≤ 45°. It was found that the start and end opening times - the times where the melt front first and last reached the freeze plug’s bottom - were reduced by 74% and 52%, respectively for 𝜃 = 45° with respect to 𝜃 = 0°. The other two 𝜃’s also showed lower opening times compared to 𝜃 = 0°. The inclination angle thus was found to have a significant effect on the freeze plug’s melting behaviour, shortening the opening times.

Physical improvements for the model include the incorporation of molten salt leakage after start opening time, and settling. Computational improvements are also possible, such as the refinement of the mesh near the salt boundary or adaptive mesh refinement near the solid-liquid boundary. Symmetry can be exploited to speed up results.