Science

Opinion please – Thorium, safest nuclear fuel?

Thorium is a better nuclear fuel than uranium. It is the safest and relatively inexhaustible energy source for the near future, according to an open letter published last week in the NRC newspaper.


Interest in thorium is greater than ever. Only the word itself has been googled three times more than average since the recent disaster at Japan’s Fukushima nuclear reactor. The reason behind this increased interest is the sudden loss of confidence in uranium. But if thorium had been used instead of uranium as the fuel in Japan’s nuclear power plants, would the risk of a meltdown have been minimized?


Associate professor and section leader of TU Delft’s faculty of Applied Sciences’ physics of nuclear reactors group, Jan Leen Kloosterman, responds negatively to this question. “Thorium is not a nuclear fuel itself, but rather a fertile material,” he says. “This means you can use thorium to produce an isotope of uranium and to use that as a nuclear fuel. Besides, thorium is not directly related to the safety of nuclear energy, as safety is determined by the type of reactor.”


So what type of reactor would be safer than the current uranium-based reactors? According to Kloosterman, the answer is Liquid Fluoride Thorium Reactor (LFTR), a specific type of Molten Salt Reactor, which is the name used for all types of reactors in which the fuel is dissolved in salt, chlorine or fluoride salts. In the LFTR, a graphite block with cooling channels contains a mixture of liquid fluoride thorium (Th-232) dissolved in fluoride salts, which also serve as a coolant. To start the reactor, a bit of uranium (U-235) is initially incorporated in order to make nuclear fuel, as thorium is not fissile. In principle, the reaction that follows –  in which U-233 is produced and consumed – continues indefinitely.


If an LFTR had been built in Fukushima, there would not have been any danger of overheating. “If the electricity fails in this thorium reactor, a passive cooling system gains momentum,” Kloosterman explains. “The primary circuit is linked with a pipeline to underneath tanks, which are passively chilled by air flowing around it. The pipeline is closed by a plug of frozen salt that is maintained by a refrigeration system. If the power fails, the cooling stops, the prop melts and the salt flows into the storage tanks, where it can cool. The problem in Fukushima and other nuclear power plants is that the heat cannot be removed”. 


Another advantage of Liquid Fluoride Thorium Reactors is the high energy potential. “The plant in Borssele needs 50 tons of natural uranium each year, while an LFTR with the same size would require only 500 kilos. This means 100 times more energy,” Kloosterman further explains. “Thorium is fully used, while in current nuclear power plants only 1 percent of uranium is utilized. This goes hand in hand with the emergent form of nuclear waste, which has to be stored for only 500 years instead of 10,000 years.”

The Dutch government is planning to build a second nuclear power plant in Borssele. Should they consider an LFTR? Kloosterman: “No, that would premature. In about two decades we hope to have finished all research and development of this new type of reactor. After that we can then start to replace all 442 nuclear power plants in the world that are based on uranium.”

Gently a finger curls around my hand. It smoothly follows every movement I make. Only the cold metal touch reveals that this is a robotic finger.

PhD student, Gert Kragten, of the biomechanical engineering department (Mechanical, Maritime and Materials Engineering), makes the finger curl by pulling a rod at the other end, just like a puppeteer. Specially designed pulleys at the joints of the finger ensures that the power is distributed over the three phalanxes, although not per se evenly. The way the power is distributed depends on the shape of the object the finger encounters, yet it always results in an accurate grip.

This technique is called under-actuated grasping. Within robotics this is a rather new discipline. The fingers or gripers (if you combine more fingers) can move in many more directions than there are motors. “In robotics, we are accustomed to developing machines that generate a desired movement and we put a motor at every hinge”, says Kragten’s colleague, dr. Just Herder. “In contrast, here we only use one motor. Therefore, in a sense you could say that you can’t really steer an under-actuated finger.”

Herder has been working on under-actuated fingers at the TU for about ten years. He is one of the pioneers in the field, which is still very small. Last weekend he, Kragten and a couple of other colleagues were in Montreal to attend a conference dedicated to this subject. There were about 40 attendees. “Almost everybody who works in this field was present,” says Kragten, who organized the conference. “We are a small group of rebels within robotics”, Herder adds.

Earlier this year, the research on gentle robot hands resulted in a spin-off company, called Lacquey, which was set up by dr. Richard van der Linde and dr. Martijn Wisse. This company is specialized in gripers that can handle soft vegetables and fruit for industrial purposes.

Herder focuses his research on prostheses: “With an under-actuated prosthesis, people only need to learn how to make one motor move, which is quite simple. And if you don’t use the prostheses – if you are resting – the robotic hand is soft, just like a normal hand. So it looks and feels much more natural than a regular robotic hand.”

Together with Peter Steutel, a student, Herder has developed a finger consisting of one solid piece of titanium. The thin structure is very flexible and does not require hinges. He now wants to develop the other fingers as well, combining them to form a hand. But that is still very challenging. Among other things, the construction needs to become stronger. Herder: “With the type of finger we have now, a person could only hold something as heavy as a glass of milk.”
Ultimately Herder’s goal is to make soft, inexpensive plastic hands.  

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