Fission or Fusion I think that right now, fission is the only way that we can get more energy out of a nuclear reaction than we put in. First, the energy per fission is very large. In practical units, the fission of 1 kg (2. 2 lb) of uranium-235 releases 18. 7 million kilowatt-hours as heat.

Second, the fission process initiated by the absorption of one neutron in uranium-235 releases about 2. 5 neutrons, on the average, from the split nuclei. The neutrons released in this manner quickly cause the fission of two more atoms, thereby releasing four or more additional neutrons and initiating a self-sustaining series of nuclear fissions, or a chain reaction, which results in continuous release of nuclear energy. Naturally occurring uranium contains only 0. 71 percent uranium-235; the remainder is the non-fissile isotope uranium-238. A mass of natural uranium by itself, no matter how large, cannot sustain a chain reaction because only the uranium-235 is easily fissionable.

The probability that a fission neutron with an initial energy of about 1 MeV will induce fission is rather low, but can be increased by a factor of hundreds when the neutron is slowed down through a series of elastic collisions with light nuclei such as hydrogen, deuterium, or carbon. This fact is the basis for the design of practical energy-producing fission reactors. In December 1942 at the University of Chicago, the Italian physicist Enrico Fermi succeeded in producing the first nuclear chain reaction. This was done with an arrangement of natural uranium lumps distributed within a large stack of pure graphite, a form of carbon. In Fermi's 'pile,' or nuclear reactor, the graphite moderator served to slow the neutrons.

Nuclear fusion was first achieved on earth in the early 1930 s by bombarding a target containing deuterium, the mass-2 isotope of hydrogen, with high-energy deuterons in a cyclotron. To accelerate the deuteron beam a great deal of energy is required, most of which appeared as heat in the target. As a result, no net useful energy was produced. In the 1950 s the first large-scale but uncontrolled release of fusion energy was demonstrated in the tests of thermonuclear weapons by the United States, the USSR, Great Britain, and France. This was such a brief and uncontrolled release that it could not be used for the production of electric power. In the fission reactions I discussed earlier, the neutron, which has no electric charge, can easily approach and react with a fissionable nucleus, for example, uranium-235.

In the typical fusion reaction, however, the reacting nuclei both have a positive electric charge, and the natural repulsion between them, called Coulomb repulsion, must be overcome before they can join. This occurs when the temperature of the reacting gas is sufficiently high, 50 to 100 million ^0 C (90 to 180 million ^0 F). In a gas of the heavy hydrogen isotopes deuterium and tritium at such temperature, the fusion reaction occurs, releasing about 17. 6 MeV per fusion event.

The energy appears first as kinetic energy of the helium-4 nucleus and the neutron, but is soon transformed into heat in the gas and surrounding materials. If the density of the gas is sufficient-and at these temperatures the density need be only 10-5 atm, or almost a vacuum-the energetic helium-4 nucleus can transfer its energy to the surrounding hydrogen gas, thereby maintaining the high temperature and allowing subsequent fusion reactions, or a fusion chain reaction, to take place. Under these conditions, 'nuclear ignition' is said to have occurred. The basic problems in attaining useful nuclear fusion conditions are to heat the gas to these very high temperatures, and to confine a sufficient quantity of the reacting nuclei for a long enough time to permit the release of more energy than is needed to heat and confine the gas. A subsequent major problem is the capture of this energy and its conversion to electricity.

At temperatures of even 100, 000^0 C (180, 000^0 F), all the hydrogen atoms are fully ionized. The gas consists of an electrically neutral assemblage of positively charged nuclei and negatively charged free electrons. This state of matter is called a plasma. A plasma hot enough for fusion cannot be contained by ordinary materials.

The plasma would cool very rapidly, and the vessel walls would be destroyed by the temperatures present. However, since the plasma consists of charged nuclei and electrons, which move in tight spirals around strong magnetic field lines, the plasma can be contained in a properly shaped magnetic field region without reacting with material walls. In any useful fusion device, the energy output must exceed the energy required to confine and heat the plasma. This condition can be met when the product of confinement time t and plasma density n exceeds about 1014. The relationship t n ^3 1014 is called the Lawson criterion. Numerous schemes for the magnetic confinement of plasma have been tried since 1950 in the United States, the former USSR, Great Britain, Japan, and elsewhere.

Thermonuclear reactions have been observed, but the Lawson number rarely exceeded 1012. One device, however, the toka mak, originally suggested in the USSR by Igor T amm and Andrey Sakharov, began to give encouraging results in the early 1960 s. The confinement chamber of a toka mak has the shape of a 'torus', with a minor diameter of about 1 m (about 3. 3 ft) and a major diameter of about 3 m (about 9. 8 ft). A toroidal magnetic field of about 50, 000 gauss is established inside this chamber by large electromagnets.

A longitudinal current of several million amperes is induced in the plasma by the transformer coils that link the torus. The resulting magnetic field lines, spirals in the torus, stably confine the plasma. Based on the successful operation of small at several laboratories, two large devices were built in the early 1980 s, one at Princeton University in the United States and one in the USSR. In the toka mak, high plasma temperature naturally results from resistive heating by the very large toroidal current, and additional heating by neutral beam injection in the new large machines should result in ignition conditions. Another possible route to fusion energy is that of inertial confinement.

In this concept, the fuel, tritium or deuterium, is contained within a tiny pellet that is then bombarded on several sides by a pulsed laser beam. This causes an implosion of the pellet, setting off a thermonuclear reaction that ignites the fuel. Several laboratories in the United States and elsewhere a recurrently pursuing this possibility. Progress in fusion research has been promising, but the development of practical systems for creating a stable fusion reaction that produces more power than it consumes will probably take decades to realize.

The research is expensive, as well. However, some progress has been made in the early 1990 s. In 1991, for the first time ever, a significant amount of energy, about 1. 7 million watts, was produced from controlled nuclear fusion at the Joint European Torus (JET) Laboratory in England. In December 1993, researchers at Princeton University used the Tokamak Fusion Test Reactor to produce a controlled fusion reaction that output 5.

6 million watts of power. However, both the JET and the Tokamak Fusion Test Reactor consumed more energy than they produced during their operation. If fusion energy does become practical, it offers the many a limitless source of fuel, deuterium from the ocean, no possibility of a reactor accident, as the amount of fuel in the system is very small, and waste products much less radioactive and simpler to handle than those from fission systems. I conclude, that even though fusion is much better, cleaner, and safer, than fission, we do not have the knowledge of how to create and contain the energy realised in a fusion reaction. So, until we do, fission is the only way we can use the atom to create power.