Energy from Nuclear Fusion

The protons and neutrons in the core, or nucleus of atoms are held together by the so-called strong nuclear force. This is one of the fundamental forces of nature and is called strong because it can overcome the great electromagnetic repulsive force between the positively charged protons. By Einstein’s mass-energy equivalency formula, E=MC2, this binding energy in the nucleus appears as extra mass in the nuclei of some atoms as compared to others. The key to releasing nuclear energy is to release this strong force by either splitting apart heavy nuclei, which have an abundance of binding energy, or by fusing light nuclei into slightly heavier ones. Iron, element 26 on the periodic table, has the least binding energy of any nucleus, thus it is the end product in any energy-releasing fission or fusion nuclear chain reaction. The cores of stars are largely composed of iron because of the process of nuclear fusion which gives them their energy.

In a fusion reaction, light isotopes of say hydrogen are combined by the application of external energy. They are forced together until they are within range of the strong nuclear force, at which point they combine. Any difference in mass between the end products and the reactants is released as energy either in the form of gamma rays, which are high energy photons of light,or as kinetic energy in any neutrons left over from the reaction. The main problem in extracting energy from a fusion reaction is to convert these hard radiations into usable work. The usual means of accomplishing this is to have a blanket of some heavy element to absorb the radiation and heat up, thus enabling steam power to turn some electrical generating equipment.

In a fusion bomb energy is released in a burst form for destructive purposes. The fusion reaction is derived from the energy supplied from a fission bomb which produces fast neutrons and x-rays. The fast neutrons split nuclei of an isotope of lithium, lithium 6, into tritium. The lithium is chemically in the form of lithium deuteride, which is a compound made from the lithium and deuterium from heavy water. The combination of deuterium and tritium is heated and compressed inside of a casing by the x-rays from the fission bomb, resulting in the release of more energy from the fusion reaction than was supplied by the trigger devise.

In magnetic confinement fusion generators, such as the Tokomak, there is a large donut-shaped vacuum chamber which is wound with heavy coils of superconducting wire. Ports in the chamber lead from magnetrons, like the ones in a microwave oven, which supply radio frequency energy to heat the deuterium-tritium plasma to high temperatures. The magnetic field in the superconducting windings of the tokomak are ramped up, compressing the hydrogen plasma into a thin ring in the center of the toroidal chamber. When pressure, density, and temperature reach a critical level, the hydrogen fuses into helium and energy is released. Tokomak type reactors have reached the so called “break even” point recently, where the energy released equals the energy supplied to the reactor, but they have not yet been constructed as generators of usable power.

In laser confinement fusion, a small pellet of glass, hollow, filled with deuterium and tritium gas, is dropped into a spherical chamber. At the center of the chamber are focussed many high powered, short pulse laser beams. When the pellet reaches the center of the chamber, the lasers fire. Energy from the lasers heats and vaporizes the glass pellet and the jets of vaporized glass act like rocket engines to drive the hydrogen fuel together at great pressure and temperature. The result is a small fusion bomb which releases energy. Like the Tokomak, laser fusion is still in the research stages, though the break-even point has been reached. Some facilities also use electron beams to impinge the fuel pellets, relpacing the lasers with particle accelerators.

Cold fusion is an excellent example of bad science. A palladium electrode is immersed in heavy water and electricity is run through the solution much as in any common electrochemical cell. The water is decomposed to deuterium and oxygen, with the deuterium evolving at the surface of the palladium electrode. The idea that fusion was occuring was triggered by the observation that heat was evolved at the palladuim electrode, in some geometries small explosions even occured. However, palladium is well known to form hydrides with hydrogen gas, the hydrogen occupies interstitial locations in the crystalline structure of the electrode. Palladium is also a catalyst, and acts to cause the hydrogen in it’s structure to recombine with oxygen in solution in the water, thus generating chemical energy. This exact reaction was observed by Michael Farady around 1850.

Colliding particle beams could be used to cause fusion reactions, but the ability to focus beams of a high enough current to provide useful power is still in the future. Much of the research done into fusion reactions has been accomplished with particle accelerators impinging on stationary targets, as these provide means to actually measure required activation energies and reaction cross sections.

There is much energy that could be released in useful form by fusion processes, however, more research needs to be done to deal with the high reaction energies needed and produced by these processes.