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Fusion Reactor Experiment Succeeds on Chinese Tokamak


Susan Mumm

Fusion Reactor Experiment Succeeds on Chinese Tokamak

Scientists meeting in an international fusion energy conference Friday proclaimed the success of an NPRE-led innovation for fusion reactors.

NPRE Prof. David Ruzic presented results on “Lithium-Metal Infused Trenches: A New Way to Remove Diverter Heat Flux,” at the 24thInternational Atomic Energy Agency (IAEA) Fusion Energy Conference held in San Diego, California. Ruzic’s paper reflected success from an experiment conducted earlier in the week, on Wednesday, October 11, 2012, on an HT-7 Tokamak reactor in Hefei, China.

Later, selected to summarize the conference’s fusion technology and power plant designs contributions, Dr. Stanley Milora of Oak Ridge National Laboratory noted that the Illinois group’s experiment “is installed on HT-7 in China and is working very, very satisfactorily.”

The success in China is the culmination of a decade of work aimed at making fusion reactors more economical.

Ruzic and his students in the Center for Plasma Material Interactions (CPMI) have designed a method for molten lithium to self-circulate along the surface of a fusion reactor’s diverter, where excess heat is collected and removed. Such an innovation, Ruzic believes, could reduce the size of a fusion reactor’s radius by a third, while allowing it to produce the same volume of energy.

 “We’re looking at a factor of 10 in cost reductions, at least,” Ruzic said.

This demonstration of a flowing molten lithium limiterin an actual major fusion device is a first and a major breakthrough.

The concept, known as LiMIT (Lithium/Metal Infused Trenches), previously was successfully conducted in CPMI laboratories. Ruzic believes the success in China shows that LiMit works on an actual fusion device as well as in the laboratory. “It did flow – at the rate we predicted. It did not cause any type of failure, and it even improved their energy confinement by 10 percent. Given that this was a small module and was not the main “limiter,” it is very likely that a larger LiMITS closer to the plasma would have improved confinement and other parameters by an even greater degree.”

In addition to the Chinese, the U.S. Department of Energy has been interested in the work, and had awarded a $600,000 grant to fund experiments in the CPMI laboratories.

For decades, scientists have been dreaming of harnessing nuclear fusion, the power of the stars and sun, to provide safe, clean and virtually unlimited energy. But among deterrents, the size that a fusion reactor facility needs to be to sustain plasma so it is hot enough to fuse particles is so great (about 100 million degrees Celsius) that it is not cost-effective. According to Ruzic, an entire fission nuclear reactor and steam generator could fit inside the central core of ITER, the International Thermonuclear Experimental fusion Reactor being built in France.

In comparison, a fission reactor now costs 10 times less than a fusion reactor to build, and lasts for 50 years as opposed to a fusion reactor’s few months of life.

The key to CPMI’s proposed solution to reduce the facility’s size lies in using molten lithium as the material in contact with the plasma in the diverter portion of a fusion reactor. Most of the heat striking the walls of a fusion reactor is collected and removed at the diverter.

While a thin shell of evaporated lithium on a major portion of the reactor’s donut-shaped walls helps fusion work better, the surface temperature normally gets too high in the area of the diverter. Lithium vaporizes easily around 450 degrees Celsius, and the temperature of static lithium would rise to that degree too quickly where the diverter is located.

To counteract this, Ruzic and his students have used flowing molten lithium in the radial trenches of metal tiles lining the diverter. In this way, the lithium flows out of the high-heat zone before it gets too hot. One of the main challenges the research group encountered was getting the lithium to flow, realizing it could not be pumped through, nor would it fall by gravity because the magnetic field of the fusion device would stop it from flowing.

The key innovation was Ruzic’s group’s use of thermo-electric magnetohydrodynamics in which they employed the magnetic field itself and the heat from the tokamak to make the lithium flow in the direction they want it to go. “The hotter the lithium gets, the faster it flows,” Ruzic said.

The work combines LiMIT with an experiment called SLiDE (Solid/Liquid Lithium Diverter Experiment). SLiDE uses an electron beam to test liquid metal plasma facing components (PFCs) under a constant heat flux similar to those found in fusion devices. It has been the primary means for testing the LiMIT concept in the laboratory.

The work also involves Ruzic’s group’s DEVeX (Diverter Erosion and Vapor shielding eXperiment). In this work, the researchers simulate the types of plasmas that occur in a fusion reactor when there is instability. It also looks at lithium’s ability to shield a surface from the power flux occurring on the surface. “DEVeX is a plasma canon,” Ruzic said.

Ruzic said of the collective work: “This general concept could be significant in getting fusion reactors to be practical. It’s certainly the most significant problem that I’ve worked on” in this area.

For over 10 years Ruzic has been thinking about ideas for using lithium in fusion reactors, and he recalls sketching the basics for LiMIT on a napkin in May 2010. Among students who have played a role in this work have been J.P. Allain (MS 00, PhD 01), now an associate professor of nuclear engineering at Purdue University, and Michael Jaworski (BS 02 Mechanical Engineering, MS 06, PhD 09) a researcher at the Princeton Plasma Physics Laboratory. Current graduate students working with Ruzic on the experiments are Soon Wook Jung, Peter Fiflis and WenYu Xu.

The lure of making fusion power a reality was one of the reasons Ruzic was drawn to nuclear physics as a student in the 1970s.

“When I started grad school they said fusion energy was 25 years away. Now (scientists are) saying it’s 35 years away from now. If they had told me when I started that it would take 70 years I probably wouldn’t have gone into the field!” Ruzic said. “I’m glad I did though. This type of concept could finally lead us to having fusion as an economically viable power source.”