Nuclear Energy

Nuclear Energy is energy from the nucleus of an atom. In stars such as the sun, pairs of light atoms (mostly hydrogen) fuse together and release the radiation received on earth as solar energy. This nuclear fusion, the joining of the nuclei of atoms, is one form of nuclear energy. Another form is the splitting (fission) of heavy atoms such as uranium. Each atom of naturally occurring uranium has a very small probability of undergoing spontaneous fission at any given moment. When this happens, a pair of lighter atoms (known as fission products) are formed and 2 or 3 neutrons (subatomic particles from the original nucleus) are released. Nuclear reactions are fundamentally different from other common energy reactions. When a conventional fuel burns or when water flows through a hydroelectric generator, the atoms themselves are unaffected, although in the case of fuels they recombine chemically. Hence, the amount of matter remains the same. In nuclear reactions, the atoms themselves are altered and a tiny amount of matter is converted into energy.

To understand how the opposite processes, fusion and fission, can both release energy requires some knowledge of the "curve of binding energy" and Einstein's equation E=mc². The nuclei of all atoms consist of nucleons. A nucleon is either a proton, a subatomic particle with positive electric charge, or a neutron with a neutral charge. The mass of any nucleus is slightly less than the sum of the masses of its constituent nucleons. This difference, or "mass defect," represents the binding energy holding the nucleons together. According to Einstein's equation relating energy (E) to mass (m) through the square of the velocity of light (c), even tiny masses represent large energies: a mass of 100 kg completely converted to energy would supply all Canadian needs for one year. If the binding energy per nucleon is plotted against the number of nucleons in the nucleus, the humpbacked "curve of binding energy" is obtained. Starting at hydrogen (1 nucleon), the curve rises rapidly to oxygen (16 nucleons), then more slowly to arsenic (75 nucleons), before dropping slowly to uranium (238 nucleons). Thus, fusing 2 light nuclei into a heavier one releases some nuclear-binding energy; fissioning a very heavy nucleus into 2 intermediate ones releases a smaller amount of energy per nucleon but involves many more nucleons.

Radioactivity

Nuclear energy is also released as radioactivity, which is associated with naturally occurring radioactive minerals (eg, radium ores) and with man-made radioisotopes used in medicine and industry. Most fission-product nuclei are radioactive. All radioactive nuclei are unstable and, sooner or later, will decay through the emission of subatomic particles accompanied by gamma radiation (similar to X radiation). The particle released may be an alpha particle, a particularly stable combination of 2 neutrons and 2 protons, or a beta particle (also known as an electron), which is a negatively charged subatomic particle formed when a neutron transforms into a proton. After emission the nucleus may remain radioactive, or may be stable. Just when any given radioactive nucleus will decay is unpredictable. However, in a large number of nuclei of the same kind, half will decay in a period characteristic of that kind of nucleus, its "half-life." Half the remainder will decay in a second half-life period, and so on. Consequently, only about 1/1000 of the original amount of any radioactive material will remain after 10 half-lives. Geothermal energy, the heat flowing to Earth's surface from its core, results from the radioactive decay of heavy nuclei such as uranium and is therefore another form of nuclear energy.

Radioisotopes are extremely reliable sources of heat for certain applications. An isotope of plutonium (plutonium-238) formed as a by-product in nuclear fission reactors is used to power heart pacemakers and space satellites. Cobalt-60 can be used to power navigation buoys. If the radioisotope emits gamma radiation, equipment using it must incorporate shielding to protect anyone nearby. A fundamental limitation is inherent in radioactive sources: the more intense the radioactivity (hence, the greater the heat produced), the shorter the source's half-life. For instance, cobalt-60 has a half-life of 5.27 years; therefore after 5.27 years it will produce half as much heat and radiation as it did initially.

Nuclear Fission

A vital contribution to the understanding of radioactivity was made by Ernest Rutherford and Frederick Soddy working at McGill University in Montréal early in this century. It was they who first suggested the manner in which a nucleus of one element became a nucleus of another element, ie, by radioactive emissions. In 1904 Rutherford conjectured "that an enormous store of latent energy is resident in the atoms of the radio-elements" and that this energy could be tapped if the rate of radioactive disintegration could be controlled. While it has not been possible to control that rate, human control of nuclear fission has proved practicable. This fact, first demonstrated by Enrico Fermi in a squash court at the University of Chicago in 1942, has made this particular nuclear reaction so important.

Control of the fission process depends on the existence of a chain reaction. Naturally occurring uranium consists of 99.28% of the uranium-238 isotope, 0.71% of uranium-235 and very small amounts of other isotopes. When hit by a neutron, the nucleus of a uranium-235 atom has a high probability of fissioning; if the atom is uranium-238 the probability is very low. This induced fission process was first discovered in 1938 by the Germans Otto Hahn and Fritz Strassmann. When a uranium-235 atom fissions, it emits 2 or 3 neutrons. If one of these neutrons hits and thus causes fission in another uranium-235 atom, more neutrons are emitted, one of which could possibly cause a further fission, and so on in a chain reaction. Thus, once started, the fission process can be self-sustaining. If, on the average, exactly one neutron from each fission results in one other fission, the process is in equilibrium and a steady level of heat is produced. The other neutrons escape from the mass of uranium or are absorbed by materials, other than uranium-235, within the mass. This is the situation in a nuclear reactor operating at steady power.

To increase the power, some of the competitive absorbers of neutrons are removed, allowing the chain reaction to diverge until the desired power level is attained. The equilibrium production of neutrons is then restored to stabilize the power. To decrease the power or shut down the reactor, more absorbers are introduced.

However much natural uranium is heaped up, no significant fission will result, because there are not enough fissile uranium-235 atoms present to sustain a chain reaction. The few neutrons produced are absorbed by the much more abundant uranium-238 atoms and so are unavailable to cause further fission. One solution is to increase the proportion of uranium-235 atoms artificially; this is done in uranium-enrichment plants which exploit the small differences in physical properties between the 2 uranium isotopes. A more subtle solution is to divide up the uranium into small packets, each surrounded by a "moderator" which slows down the neutrons emitted from one packet of uranium before they hit the next packet. Slow neutrons cause fission in uranium-235 much more readily than faster ones. Generally, elements with light atoms are good moderators: ordinary water is not good enough to sustain a chain reaction with natural uranium; very pure graphite (carbon) is better; heavy water, a compound of deuterium and oxygen is best. Deuterium, the heavy isotope of hydrogen, is present in all naturally occurring hydrogen (about one part in 7000). Heavy water is produced by enriching the deuterium content of natural water in heavy-water plants.

A neutron absorbed by a uranium-238 atom is not lost, merely stored. The resulting compound nucleus subsequently transforms spontaneously by radioactive decay into an isotope of another element, plutonium-239. Although uranium-238 is not fissile, plutonium-239 is. Thus, uranium-238 is said to be fertile. Plutonium-239 can be used to sustain a nuclear fission reaction in the same way as uranium-235. Thorium, another naturally occurring nuclear fuel that is somewhat like uranium, consists almost entirely of the fertile isotope thorium-232, which can yield the fissile uranium-233 by absorbing a neutron. These alternative nuclear fuels and moderators can be combined to produce heat and electricity in nuclear power plants.

History

Natural nuclear reactors predated the man-made variety by about 2 billion years. At that time, nuclear chain reactions generating considerable heat occurred in several rich uranium deposits at Oklo, Gabon, West Africa. This prehistoric event, which has been deduced recently from chemical analysis of the remaining uranium, illustrates basic principles of radioactivity and fission. Since uranium-235 is radioactive, with a half-life of 0.7 billion years, natural uranium then would have contained over 5% uranium-235, a sufficiently high fissile concentration to sustain a chain reaction with ordinary water as a moderator.

The nuclear reaction presumably started when groundwater seeped into the deposits. When the chain reaction and the associated fission heat built up to a sufficient level to boil the water and expel it, the resulting lack of a moderator would have caused the reaction to shut down automatically. This cycle must have repeated itself many times, like a gigantic coffee pot percolating away over hundreds of thousands of years. Analysis shows that, despite the absence of any deliberate means of retention, the plutonium produced in these natural reactors remained trapped in the uranium deposits until it had decayed away by its own radioactivity.

Since the early work of Rutherford and Soddy, Canada has contributed significantly to the science and application of nuclear energy. In 1933 Gilbert Labine brought into production Canada's first radium mine at Port Radium, NWT, on Great Bear Lake. Uranium, always found in association with radium, was then considered a waste product. In 1940 George Laurence started experiments in the National Research Council's Ottawa laboratories with uranium and a graphite moderator. Had his materials been purer, he might have achieved a chain reaction before Fermi. The Port Radium mine of Eldorado Gold Mines Ltd was reopened in 1942 to produce uranium.

In 1943, as part of the Allied war effort, a joint Canadian-UK team, with important French participation, was established at Montréal to pursue the concept of nuclear reactors with heavy water. In the same year heavy water was first produced in Canada at the synthetic ammonia fertilizer plant of the Consolidated Mining and Smelting Corp at Trail, BC, using a Norwegian process. In 1944, C.J. Mackenzie, who was then in charge of the Canadian program, wrote with great foresight to C.D. Howe, who was the minister responsible for it: "In my opinion Canada has a unique opportunity to become involved in a project which is not only of the greatest immediate military importance but which may revolutionize the future world in the same degree as did the invention of the steam engine and the discovery of electricity."

The Chalk River Nuclear Laboratories were established in 1944, and in 1945 the Zero Energy Experimental Pile (ZEEP), the first reactor outside the US, also started up there. In 1946, W. Bennett Lewis, who subsequently was primarily responsible for the technical development of the Canadian CANDU reactor system, became technical director at Chalk River, replacing John Cockcroft who went on to lead the United Kingdom's program. The first radioisotopes produced in the NRX reactor at Chalk River were marketed in 1949. In 1951 the world's first cobalt radiotherapy units for the treatment of cancer, using radioactive cobalt produced in the NRX reactor, were developed by Harold Johns and others. These units were installed in the Victoria Hospital in London, Ontario, and in the University Hospital in Saskatoon, Saskatchewan.

Since then Canada has exported more than 1300 units, and the associated cobalt, to more than 80 countries. These units are credited with saving 13 million person-years of life for the patients involved. In 1962 Canada's first nuclear power plant, the Nuclear Power Demonstration Plant, was opened at Rolphton, Ontario. This plant demonstrated all-important principles for the CANDU design of reactor.

Future

Although it has not yet been possible to control fusion, the required conditions are reasonably well established. It is known that fusing atoms of ordinary hydrogen (the reaction that occurs in the sun) would be extremely difficult. Instead the hydrogen isotopes deuterium and tritium are used in fusion experiments. The deuterium-tritium combination is believed to offer fusion more easily than deuterium-deuterium. First, the atoms of deuterium and tritium must be at very high temperatures, about 100 million °C; then, the atoms must be together long enough for fusion to occur. The time needed is least for densely packed atoms and increases as the density decreases. The minimum requirement commonly quoted for the product of density and time for fusion of deuterium and tritium is 10 atom s/cm³.

Since no structural materials can operate at the high temperatures required for the fusion reaction, other means of confining the reacting atoms had to be found. At these temperatures the atoms are ionized, ie, electrically charged, and subject to forces when they move in a magnetic field. Hence, magnetic fields can be designed to keep the hot atoms "bottled up" through magnetic confinement. In inertial confinement a small pellet of solid deuterium-tritium would be bombarded from all sides by high-energy beams of laser light or charged particles. The intense beams would heat the pellet and, by causing shock waves, compress it to about 1/1000 of its original volume. The increased density means that a shorter period of confinement is necessary.

Until controlled fusion has been demonstrated and a practical fusion reactor designed, it would be premature to estimate costs. Like solar energy and nuclear fission, the fuel is abundant and cheap, but the cost of the equipment needed to provide the energy in usable form will be appreciable. Long before fusion becomes an alternative to fission as an energy source, the 2 may complement each other in a hybrid system. In addition to releasing energy, the fusion reaction provides high-energy neutrons which could, through other nuclear reactions, be multiplied into many neutrons of lower energy. These, in turn, could be absorbed by fertile materials, such as uranium-238 or thorium, to produce fissile materials to fuel conventional fission reactors.For this purpose another nuclear reaction, spallation, must be regarded as an alternative to fusion. In spallation, heavy atoms (eg, lead) bombarded by light particles (eg, hydrogen nuclei) emit high-energy neutrons which can be used in the same way to provide fuel. Unlike fusion, spallation has already been demonstrated in the laboratory.

Nuclear energy offers a new energy source just when the limits of the chemical fuels, oil, natural gas and coal, are being realized. If recycled, the world's nuclear fuels are virtually inexhaustible. However, the radiation from nuclear energy, like the fire of chemical energy, has its hazards as well as its benefits.

Like fire, radiation should be respected but not feared. All life has evolved in a sea of radiation that existed from the start of time. To ensure the safety of the public and of workers, and to protect the environment, the federal government regulates the electrical utilities and the hospitals, universities and other institutions which use nuclear energy and radioisotopes. The regulations are based on internationally agreed standards; in Canada the regulatory body is the Atomic Energy Control Board. In addition, more than 10 public inquiries have been held in Canada, dealing with various aspects of the nuclear industry, from uranium prospecting and mining, through reactor safety to the disposal of nuclear hazardous waste. The overwhelming conclusion of these examinations has been that it is in the public interest to continue with the exploitation of nuclear energy, subject to proper regulation.