Helium bubbling in a Molten Salt Fast Reactor
A flotation process
Master of Science Thesis
Delft University of Technology Faculty of Applied Sciences
Dept. of Radiation Science & Technology Nuclear Energy and Radiation Applications
The Molten Salt Reactor (MSR) is one of the Generation IV nuclear reactor concepts. The molten salt reactor uses a liquid fuel, which gives the possibility to reprocess the fuel while operating the reactor. Reprocessing the fuel during operation strongly decreases the radiotoxicity of the waste and optimizes the use of natural resources.
The first part of this thesis is to examine the reprocessing steps in a Molten Salt Reactor. There are five reprocessing steps, which can be divided in on-line and off-line reprocessing. Helium bubbling is used as the on-line reprocessing technique. Fluorination, protactinium removal, actinide extraction and lanthanide extraction are used as the off-line reprocessing techniques.
It was found that the reprocessing steps show a promising molten salt reactor concept, but the individual techniques are still to be optimized. And the different steps in the reprocessing of the salt must still be connected together to form a continuous process. It was also found that the helium bubbling technique, which was initially used to extract the gaseous fission products, also extracted a part of the noble and semi-noble metals from the molten salt. These metals will not dissolve in the salt and plate out on metal surfaces in the reactor. If the plate-out is excessive, the decay heat from the noble and semi-noble metals may damage the reactor. The extraction of these metals with helium is proposed to be achieved by a flotation process. The factors that play a role in a flotation process are described by Schulze [Schulze, 1984] and Nguyen [Nguyen, 2004].
The second part and final goal of this thesis was to develop an experimental method to determine the influence of the bubble size and gas flow in a flotation process. A flotation process is described as an extraction of solids from a liquid with gas bubbles. The bubble size and gas flow are the variables that are adjustable in the helium bubbling technique in a Molten Salt Reactor. The development of this experimental method included the making of a new set-up with a newly fabricated piece of equipment and developing a method to work with this set-up.
The result of the set-up is a modified version of a Hallimond tube. The set-up is built to use the methods to examine a flotation process, which are described by Nguyen [Nguyen, 2004]. The methods are a direct and an indirect. The indirect method is based on the extraction of a known amount of particles that are added to the set-up. The direct method is filming the direct interaction of solids with gas bubbles and is used to get a better understanding of how the process works. It was found that the developed set-up is able to get good results for the direct method and promising results for the indirect method. This new equipment and the new experimental method offer the possibility for a number of follow-up studies on the topic of a flotation process.