From: Andy Soos, ENN
Published July 30, 2010 11:54 AM

Nuclear Fusion

Fusion power is the power generated by nuclear fusion reactions. In this kind of reaction, two light atomic nuclei fuse together to form a heavier nucleus and in doing so, release a large amount of energy. The major difference between fission and fusion reactors is that there is no possibility of a catastrophic accident in a fusion reactor resulting in major release of radioactivity to the environment. The primary reason is that nuclear fusion requires precisely controlled temperature, pressure, and magnetic field parameters to generate net energy. If the reactor were damaged, these parameters would be disrupted and the heat generation in the reactor would rapidly cease. In contrast, the fission products in a fission reactor continue to generate heat through beta-decay for several hours or even days after reactor shut-down, meaning that melting of fuel rods is possible even after the reactor has been stopped due to continued accumulation of heat.

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Purdue researchers have discovered mechanisms critical to interactions between hot plasma and surfaces facing the plasma inside a thermonuclear fusion reactor, part of work aimed at developing coatings capable of withstanding the grueling conditions inside the reactors.

A fusion power plant would produce 10 times more energy than a conventional nuclear fission reactor, and because the deuterium fuel is contained in seawater, a fusion reactor's fuel supply would be virtually inexhaustible.

Research at Purdue University focuses on the "plasma-material interface," a crucial region where the inner lining of a fusion reactor comes into contact with the extreme heat of the plasma. Nuclear and materials engineers are harnessing nanotechnology to define tiny features in the coating in work aimed at creating new "plasma-facing" materials tolerant to radiation damage, said Jean Paul Allain, an assistant professor of nuclear engineering.

One lining being considered uses lithium, which is applied to the inner graphite wall of the reactor and diffuses into the graphite, creating an entirely new material called lithiated graphite. The lithiated graphite binds to deuterium atoms in fuel inside fusion reactors known as tokamaks. The machines house a magnetic field to confine a donut-shaped plasma of deuterium, an isotope of hydrogen.

During a fusion reaction, some of the deuterium atoms strike the inner walls of the reactor and are either "pumped," causing them to bind with the lithiated graphite, or returned to the core and recycled back to the plasma. This process can be "tuned" by these liners to control how much deuterium fuel is retained.

Findings have been detailed in two research papers presented during the 19th International Conference on Plasma-Surface Interactions in May, and another paper will be presented during the Fusion Nuclear Science and Technology/Plasma Facing Components meeting on August 2-6 at the University of California at Los Angeles.

Purdue is working with researches at Princeton University in the Princeton Plasma Physics Laboratory, which operates the nation's only spherical tokamak reactor, known as the National Spherical Torus Experiment. The machines are ideal for materials testing.

A major challenge in finding the right coatings to line fusion reactors is that the material changes due to extreme conditions inside the reactors, where temperatures reach millions of degrees. The energy causes tiny micro- and nano-scale features to "self-organize" on the surface of the lithiated graphite under normal plasma-surface interaction conditions. But the surface only continues this pumping action for a few seconds before being compromised by damage induced by the extreme internal conditions, so researchers are trying to improve the material durability, Allain said.

"The key is to understand how to exploit these self-organizing structures and patterns to provide the recycling and also to self-heal, or replenish the pumping conditions we started with," he said.

Allain's group is working at Purdue's Birck Nanotechnology Center to analyze tiles used in the Princeton Plasma Physics Laboratory tokamak.

The Purdue team also will study materials inserted into the tokamak using a special "plasma-materials interface probe." The materials will then be studied at the Princeton laboratory using a specialized "in situ surface analysis facility laboratory" that will be assembled at Purdue and transported to Princeton later this summer.

Future work will include research to study the role played by specific textures, the nanometer-scale structures formed in the tokamak linings.

A tokamak is a type of machine that uses a magnetic field to confine a plasma in the shape of a torus (donut shaped). Achieving a stable plasma equilibrium requires magnetic field lines that move around the torus in a helical shape.

The concept of controlled fusion power is not here yet but one day it will be. The current work takes the process a few steps further.

For further information: http://www.purdue.edu/newsroom/research/2010/100727AllainFusion.html

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