Stalking the Fusion Doughnut
Source: The Green Economy, 3/20/2011
The search for utility scale energy that is cheap, carbon neutral, plentiful and safe has been this millennia’s search for the alchemist’s dream to turn lead into gold.
Nuclear fission, using heavy atoms like uranium, is one approach that has failed to meet the dream. The other is fusion, in which light atoms, like hydrogen, are fused at high temperature with an accompanying release of energy. This is the dream that has energized scientists at Princeton Plasma Physics Lab, (PPPL) working with the US DOE Fusion Project, for decades. It is a dream that has inspired 23 nations to come together to build a fusion plant in France, called ITER, latin for “The Way”.
The Donut
To visualize the process, imagine a very hot donut (100 million degrees Celsius) in a cylindrical container. The donut (plasma) is suspended by magnets so that it never touches the sides of the container. The reason is that the donut would cease to be a donut if it touched the sides, immediately cooling below the temperature needed to remain a plasma.
(PPPL) has fired up their donut countless times—although for a duration of a second or less—which has enabled them to study the plasma, creating both photographic and computer simulated images.
The Win
According to PPPL, the abundance of raw materials, their wide distribution, and the environmental acceptability make fusion a win for all. They believe that the costs of development are balanced by the expectation that fusion will be a safe, economical source of electricity.
Abundant Fuel Supply
The major fuel, deuterium, can be readily extracted from ordinary water. The tritium could be produced from lithium, which is available in land deposits or from sea water which contains thousands of years’ supply.
No Risk of a Nuclear Accident
The amounts of deuterium and tritium in the fusion reaction zone are considered to be too small to result in a large uncontrolled release of energy. In the event of a malfunction, the plasma would strike the walls of its containment vessel and cool.
No Air Pollution
Since no fossil fuels are used, there would be no release of chemical combustion products.
No High-level Nuclear Waste
Similarly, there would be no fission products to present a handling and disposal problem. It is intended that careful materials selection would minimize the handling and ultimate disposal of radioactivity produced by neutrons interacting with the reactor structure.
No Generation of Weapons Material
The by-products of fusion are not suitable for use in the production of nuclear weapons.
ITER
Fusion is still a dream, despite plans going forward in France. Decades away, energy deployment strategies may make large single source energy production obsolete. However, the dream continues, producing research that can lead to unknown technological innovations.
The Science
To produce net power, fusion reactions must take place at high temperatures. The power production process which can occur at the lowest temperature and, hence, the most readily attainable fusion process on earth is the combination of a deuterium nucleus with one of tritium. The products are energetic helium-4 (He4) the common isotope of helium (which is also called an alpha particle), and a more highly energetic free neutron (n). The helium nucleus carries one-fifth of the total energy released and the neutron carries the remaining four fifths.
Since nuclei carry positive charges, they normally repel one another. The higher the temperature, the faster the atoms or nuclei move. When they collide at high speeds, they overcome the force of repulsion of the positive charges, and the nuclei fuse. In such collisions energy is released. The difficulty in producing fusion energy has been to develop a device which can heat the deuterium-tritium fuel to a sufficiently high temperature and then confine it for a long enough time so that more energy is released through fusion reactions than is used for heating.
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