neutron neutron neutron
Figure 8.16 Nuclear fission chain reaction
Other fissile nuclei (Uranium 235 or Plutonium 239)
Other fissile nuclei (Uranium 235 or Plutonium 239)
Figure 8.16 Nuclear fission chain reaction
energy. If controlled, this chain reaction can be used to produce a large and sustained amounts of energy. This possibility was demonstrated in 1942, following Leo Szilard's original suggestion, by an outstanding team of scientists led by Enrico Fermi, who constructed the first nuclear reactor in a squash court under the stands of the University of Chicago's football field. At that time, the project funded by the U.S. government was directed to investigate the possibility of making an atomic bomb based on nuclear fission. This first nuclear reactor, called "atomic pile" consisted of highly purified graphite, uranium, uranium oxide and cadmium control rods and was about the size of a two-car garage. To control the chain reaction, it was necessary to control the neutron flow. When a 235U nucleus is broken by fission, the neutrons emitted have high kinetic energy, meaning that they have high speeds in the order of 20000 km s-1. These fast-moving neutrons will most probably be captured by 238U atoms which still constitute the major part of the uranium, but do not undergo fission. Slower-moving neutrons have a much lower energy content. They are still able to split 235U nuclei and sustain the chain reaction, but are less likely to undergo reactions with other kinds of atoms. To reduce the neutron's speed, so-called moderators are used. The best moderators are low-mass atoms such as deuterium, helium or carbon which de-
Table 8.2 Energy content of various fuels.
Average energy content in 1 g [kcal]
Wood Coal Oil LNG
Uranium (LWR, once through)
crease efficiently, by successive collisions, the neutron's energy, keeping neutron losses at a minimum and do not undergo fission themselves. Many early atomic reactors used graphite as a moderator because it is inexpensive and easy to handle. Water and heavy water (water in which the hydrogen atoms have been replaced by deuterium, D2O), however, are more efficient moderators and are the most widely used for nuclear reactors today. To control the rate of the chain reaction, control rods able to absorb neutrons without re-emitting them, are also necessary. These generally contain cadmium or boron, and can be moved in and out the reactor to regulate the flux of neutrons and thus the amount of energy produced. Fully inserted, the control rods will stop the chain reaction.
Using a combination of uranium, uranium oxide, graphite moderator and cadmium control rods, Fermi's first nuclear reactor when tested successfully in December 1942, and produced only about a few watts of energy, but proved that a controlled use of atomic energy was possible.
Most importantly at the time, it also showed that the construction of an atomic bomb was achievable, triggering the Manhattan project, which led to the explosion of the first atomic bomb in 1945 and put an end to World War II. In the early years, military applications dominated the use of nuclear energy. In 1954 for example, the first nuclear submarine, the U.S.S. Nautilus was launched; this was able to stay underwater without refueling for long periods of time -an unimaginable situation before the advent of this new energy source.
Interestingly, Fermi's nuclear reactor was not really the first one on Earth. As we now recognize, about 2 billion years ago, natural nuclear reactors operated in a rich deposit of uranium near what is now Oklo, Gabon. At the time, the concentration of 235U in all natural uranium was above 3% instead of today's value of 0.7%, due to radioactive decay. A natural chain reaction started spontaneously in the presence of water acting as moderator and continued for about 2 million years before stopping. During that time, fission products as well as plutonium and other transuranic element were naturally formed .
The world's first commercial-scale nuclear power plant opened in 1956 in the United Kingdom. It was equipped with a Magnox reactor using graphite as moderator and CO2 gas as a coolant. It used, like Fermi's reactor, non-enriched natural uranium containing only 0.7% 235U. In the United States, the first commercial nuclear power began operation in 1957 in Shippingport, Pennsylvania. It was a so-called pressurized water reactor (PWR), which is still the technology used in 60% of the plants currently in operation worldwide. The heat generated by the fission reaction is used to heat water in which the fuel rods are immersed. Reaching temperatures of around 300 °C, the water, however, does not boil because it is kept under high pressure. The pressurized water serves both as a moderator and coolant. Via a heat exchanger it is used to boil water in a secondary loop, producing steam to propel a turbine which in turn spins a generator to produce electric power.
The second most common type of nuclear reactor is the so-called boiling water reactor (BWR), with more than 90 units operating worldwide. The design of the BWR has many similarities with the PWR, except that the water cooling the core is allowed to boil and the steam generated is used directly to drive turbines. After condensation, the water is returned to the reactor to close the cycle. This system has a simpler design than the PWR, but as the water around the core is contaminated with traces of radioactive material, although generally with a short half-life, the turbine must be shielded in order to avoid the escape of radiation. For safety reasons, PWRs are thus the preferred reactors in the Western World. In France, for example, all 58 nuclear reactors in operation are of the PWR type.
Both the PWR and BWR, representing together more than 80% of the commercial nuclear reactors worldwide, are light-water (H2O) moderated and use uranium enriched at 3-5% in fissile 235U isotope. Because light-water not only slows neutrons but can also absorb them, it is not as selective as a moderator than heavy-water (D2O) or graphite. Therefore, the CANDU (Canada deuterium uranium) pressurized water reactors developed in Canada, using natural uranium (0.7% 235U) are moderated with heavy-water (D2O). The cost of uranium enrichment is avoided, but extensive amounts of expensive D2O have to be employed. About 40 CANDU reactors are presently operating in seven different countries, including Canada, India, South Korea, and China. In the United Kingdom, advanced gas-cooled reactors (AGR) derived from the earlier Magnox reactors using graphite as moderator, CO2 as coolant, and uranium enriched at 2.53.5% in 235U are in operation.
The commercial reactors used today (PWR, BWR, CANDU, AGR, etc.) constructed between the years 1970s and 2000 are considered as second-generation reactors (Fig. 8.17). Currently, the transition to a third generation of reactors is under way. Two units have already been completed in Japan, and several others are under construction or planned in countries such as Taiwan, France, or Korea. They are an evolution from the second-generation reactors, but feature enhanced safety systems and are less expensive to build, maintain and operate. At the same time, revolutionary designs known as generation IV systems, which have new and innovative reactor or fuel cycle systems are well under development .
Most of the commercial reactors currently in operation use enriched uranium in a once-through cycle (Fig. 8.18); this means that the uranium is used only once and must then be disposed of. This cycle is the most uranium reserve-intensive, as only 235U contributes by fission to the production of energy. 238U, which constitutes up to 97% of the fuel, and 99.7% of natural uranium is left almost untouched. The solution to limit this waste of resources is to use a different fuel cycle. In the typical reactor, fast neutrons are slowed down by moderators to increase the probability of collision between these slow neutrons and the fissile 235U nucleus, and thus increase the amount of energy generated by fission. Fast neutrons, however, have the ability to convert the 238U isotope, which does not directly undergo fission, to plutonium 2 39 (239Pu), a fissile material which can thus produce energy. Therefore, 238U is referred to as a "fertile" isotope. 239Pu was formed during the universe creation, but due to his its half-life of 24110 years it has disappeared a long time ago in Nature. In the existing fuel cycle, the enriched uranium, once used, still contains 1% 235U but also 4% fission products, 0.1% of minor actinides, and 1% 239Pu. The spent fuel can be repro cessed and the useful 235U sent back to the enrichment plant. 239Pu also can be separated and mixed with uranium in the form of oxides to form a mixed oxides fuel commonly known as MOx. MOx is currently in regular use to generate electricity in a number of countries, including Germany, Belgium, and France. In the United States, the reprocessing of nuclear fuel was banned in 1977 by president
Jimmy Carter, and despite a lift of the ban in 1981, until now no reprocessing of spent nuclear fuel has been carried out. The use of 239Pu in nuclear reactors to produce energy allows the destruction of highly radioactive 239Pu which otherwise would have to be stockpiled or disposed of, and also induces significant savings in 235U.
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