Background Information

Fusion is a process in which two smaller nuclei are combined to form a larger nucleus, causing an elemental change at the subatomic level. Looking at the stability of atoms graph ( Figure 1 ), we can see that smaller atoms are more unstable than mid sized atoms. By fusing together these lighter elements, some of the mass of the reactants is converted into energy. Fusion is potential alternative source of energy to fossil fuel combustion as the deuterium fuel fusion requires is cheap and abundantly present in the oceans of the earth, the energy output by a fusion reaction is extremely large (10 million times more energy per kilogram fuel), and the reaction is relatively safe. If we compare the theoretical yield of the coal power plant, the fission nuclear power plant, and potential future fusion reactors ( Figure 2 ), we can see that there is a huge difference in energy output.

Figure 1

Figure 2

The isotope of hydrogen, deuterium, is found in 1 out of every 6500 hydrogen atoms in water. With the huge amounts of water contained in the earth's four oceans, a virtual limitless supply of fusion fuel exists on the earth. Another advantage of fusion reactions is the minimal environmental impact. The reactor cannot ‘blow up', because loss of containment would just lead to cessation of reaction. As well, the by-products of the reactor is tritium, which has a half life of only twelve years.

The equation E=mc 2 states the relationship between mass and energy. In nuclear reactions, part of the mass converts to energy during the reaction. For a fusion reaction or fission reaction to release energy, the binding energy of the products must be greater than the binding energy of the reactants (and hence the overall reaction will be exothermic). This is true for lighter elements when fusion takes place, and with heavier elements when fission takes place. Fusion occurs when the nuclei of two atoms are so energized that they possess enough kinetic energy and speed to overcome the electromagnetic repulsion between the two atoms' respective nuclear protons and come within 2x10 -15 m to each other, where the strong nuclear force begins to act and fuses the two nuclei into a single, bigger nucleus.

Do to the extremely high activation energy of the reaction, fusion can only occur at temperatures upward of 100 000 000 °C: 100 million degrees, or the equivalent of that energy in accelerating force. The only places in the universe where this high of a temperature exists is in the centre of stars, where temperatures of 15 000 000°C and crushing gravitational forces causes the spontaneous fusion of hydrogen atoms into helium atoms (also known as the proton-proton chain in most stars) ( Figure 3 ). These reactions release many neutrinos and gamma rays, as well as light waves and infrared heat, and are what power life on the earth.

Figure 3

Many scientists have attempted to harness the energy of fusion by using the light elements, namely isotopal hydrogen due to its lower binding energy ( Figure 4 ), to create an artificial fusion reaction. Serious research began at the end of the Second World War, with Britain successfully developing the thermonuclear, or hydrogen, bomb in 1953, which used a nuclear fission explosion to generate enough energy to set off the fusion reaction. However, creating a sustained fusion reactor that would produce more power than it used was more of a challenge.

Figure 4

One of the first reactor ideas was presented by G.P. Thomson, the son of famous Physicists J.J. Thomson, which suggested a toroidal (doughnut-shaped) reactor that would contain the deuterium plasma using the magnetic “pinch” principle: an electric current passing through ionized gases would produce its own magnetic field. Thomson's theoretical reactor was expensive and difficult to build from an engineering standpoint, and was ultimately abandoned.

The next scientist to take up the challenge was an Australian named Peter Thonemann. After having worked on the American and British hydrogen bomb project, he proposed the development of the ‘Perhapsatron', an inductively driven toroidal system. It too ultimately failed.

The first fusion reactor to gain fame in the public domain was the 120 tonne British ZETA (Zero-Energy Thermonuclear Assembly) machine, built in 1958. It claimed to have created a successful sustained fusion reaction. However, the claim was false: originating from overzealous politicians intent on showing up the Soviets.

However, the development of the Soviet Tokamak reactor in 1968 revived hopes that sustaining a fusion reaction was possible. The tokamak reactor had a very similar design to Thomson's original idea, but used additional magnetic coils around the doughnut belt to further stabilize the plasma. This machine was able to suspend the heated plasma for several tenths of a second before they hit the metal surface of the ring and cooled. In that time, measureable amounts of energy could actually be produced.

Around the same time, another type of fusion reactor, the inertial reactor, was developed. This reactor used the principle of the hydrogen bomb, but at a much smaller scale, to generate energy. By using lasers to compress small sections of plasma, it was able to produce thousands of small thermonuclear explosions.

The two reactors enjoyed relative success over the next few decades, and in 1991, a magnetic containment reactor generated 1 MW of power in a short experiment. Fusion reactors are no longer a pipe dream, but fast becoming reality…

Types of Plasma