Fusion Power - The Cons

While fusion power may be the answer to our energy prayers, it is not currently a viable option. While there are no obvious drawbacks to the use of fusion power, we are years if not decades away from harnessing this power. Scientists must achieve the necessary temperatures, which has been done; demonstrate sustained reactions where substantial amounts of energy are produced, and complete operation of a demonstration fusion power plant. To better understand what needs to be done, one needs to understand the fusion process.

To produce net power the fusion reactions of a gaseous deuterium-tritium fuel must be heated to about 100 million degrees Celsius (more than six times hotter than the interior of the sun). Since nuclei normally repel one another because they carry positive charges, their movement will coincide with their temperature - i.e. the higher the temperature, the faster they move. At a faster speed, they overcome the force of repulsion of the positive charges and fuse. These collisions release energy. To date, scientists have attained temperatures as high as 510 million degrees to create these collisions. | | The problem in producing fusion energy is in developing a device that can heat the above fuel to a high enough temperature and then confine it for a length of time that will result in more energy being released through the fusion reactions than it took to actually heat the nuclei. Nature lends a hand in solving this problem through ionization and plasma. At the high temperatures required, all the electrons of light atoms become separated from the nuclei, which are positively charged (ions). The negatively charged free electrons combine with the ions and create a hot gas known as plasma. Now that we have this plasma, we need to confine it and harness its energy. A magnetic field will cause plasma to follow spiral paths about the field lines which will in turn confine them and prevent them striking the container's walls. If there is no magnetic field, the charged particles in the plasma move randomly, striking the walls, which cools the plasma and inhibits the fusion reactions.

Current magnetic fusion experiments do not create sufficient fusion energy to maintain the plasma temperature. As a result, the devices operate in short pulses and the plasma must be heated afresh in every pulse. Current research offers three types of plasma heating to aid in this process: (1) Ohmic Heating - this is the same kind of heating that occurs in an electric heater or light bulb; however the highest attainable temp to date is 20-30 million degrees Celsius; (2) Neutral-beam injection - which involves the introduction of high-energy atoms into the ohmically heated plasma whereby the atoms are immediately ionized and are trapped by the magnetic field, whereby the ions then transfer part of their energy to the plasma particles in repeated collisions, thus increasing the plasma temperature; and (3) radio-frequency heating - whereby oscillators generate high frequency waves whose energy is transferred to the charged particles in the plasma, which in turn collides with other plasma particles and increases the temperature of the bulk plasma. | | | | The most likely scenario for a fusion power plan is one in which the fuel or deuterium-tritium mixture is sent an evacuated reactor chamber, ionized and heated to thermonuclear temperatures. The fuel will be held away from chamber wall by magnetic forces long enough for a useful number of reactions to take place. The charged helium nuclei will then collide with the newly injected cold fuel atoms which are then ionized and heated, thus sustaining the fusion reaction. The neutrons, having no charge, will move in straight lines through the thin walls of a vacuum chamber, with little energy loss. The neutrons and their energy will then be absorbed in a "blanket" containing lithium which surrounds the fusion chamber. The neutrons collide with the lithium nuclei thus releasing their energy an creating heat. This heat is conveyed to a conventional steam electric plant. The neutrons themselves ultimately enter into nuclear reactions with lithium to generate tritium which is separated and fed back into the reactor as a fuel. While scientists have been able to attain the initial temperatures, they still have to refine the above process. Further research will be required to ascertain the which materials will be resistant to energetic neutron bombardment, thermal stress, and magnetic forces. Additionally, someone will have to design a system to remove the spent gas. The Department of Energy has been working with numerous laboratories and research facilities to solve these problems and thus turn the fusion dream into a reality. Once that is done, we have a wonderful source of energy.

References:
Energy.gov.2011. US Department of Energy. Natural Gas. Retrieved on March 24, 2011, from http://www.energy.gov/energysources/fusion.htm
Princeton Plasma Physics Lab. Retrieved on 26 March, 2011, from http://www.pppl.gov/fusionreactions.cfm