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Enhancing Nuclear Safety


Research

Thesis viva

Experimental study of a solid/liquid thermal interaction


Adrien Abbate will defend his thesis
on Monday 8th January 2018 at 9:45 am
at INSA Lyon, René Char amphitheater
20 avenue Albert Einstein
69100 Villeurbanne (France)


Abstract
During a reactivity insertion accident, the temperature and the pressure rapidly increase inside the rod and can lead to the rupture of the clad and the ejection of fuel toward the coolant. Since the fuel could be finely fragmented, the thermal interaction between fuel and coolant (FCI) could create a pressure wave as well as a large vapor volume. Safety-related consequences of the FCI may be related to both phenomena.

Past experimental studies concerning such a RIA related FCI are in-pile experiments in thermal hydraulics conditions that differ from PWR conditions. Therefore validation of a simulation tool from these data and extrapolation to reactors conditions is subject to uncertainties.

This experimental study is devoted to the violent thermal interaction between a hot material and a fluid. An experimental bench has been designed. It is mainly a cylindrical tube, where the interaction takes place, connected to a larger vessel as a compressibility tank. To reduce the required level of energy as well as temperature and pressure conditions, liquid carbon dioxide has been chosen to simulate water in PWR conditions. Respect of thermodynamics similarity criteria allows to lower pressure by a factor 3 and energy per unit mass fluid by a factor 5. To produce the energy pulse, a tungsten wire is heated by Joule effect from the discharge of a 27 mF ca pacity. Design of the tank allows for a relatively long mechanical relaxation of the coolant with regards to the heat transfer kinetics.

The pressure wave is recorded thanks to four dynamic pressure sensors along the tube. Two dual tip fiber optical probes allow characterizing the kinetics of vapor formation near the wire. The data acquisition system operates with a required frequency of the MHz range.

This test bench allows to record the local behavior of the fluid such as the pressurization of the liquid. A very clear pressure wave have been recorded just after weak energy pulse around 0.2 kJ. The influence of some major parameters on these quantities have been studied. For example, the liquid level in the tank is increased between two tests up to be totally full, so, the influence of the compressibility is highlighted. Also, three different wire diameters have been used to modify the heat transfer kinetics. Finally, several intensities of the energy pulse have been considered. All these studies hel p to im prove the understanding on the thermal interaction potentially involved in the nuclear reactor safety context.


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