Nuclear fission and fusion
Nuclear Power Plants
The generation of electricity with nuclear power plants has made a rapid development since the commissioning of the first fission reactor in 1942. The rapid growth was hampered in April 1986 because of the Chernobyl (USSR) accident. Public opinion reacted against the use of nuclear energy, in particular in the industrialized countries. Large-scale application of renewable energy sources was expected to solve the energy problem and bring the world an environmentally friendly source of energy, while the generation of electricity through nuclear technology was widely out of favor internationally.
The nuclear power plants that are in operation have fission reactors that split uranium nuclei, and during that splitting process, mass is lost and converted into energy in accordance with Einstein’s relation E = mc2. Even when the losses in mass are very small, the released energy is enormous. The energy of the fission process is used to heat water and to produce steam as in conventional gas or coal-fired thermal power plants. Since the Second World War, another nuclear technology has also come into the picture: nuclear fusion. It is called fusion because it is based on fusing light nuclei, such as hydrogen isotopes, to release energy. The process is similar to that which powers the sun and other stars. Nuclear fusion is the holy grail that could solve the world’s energy problem. Fusion power not only offers the potential of an almost limitless source of energy for future generations but also presents some formidable scientific and engineering challenges.
Nuclear Fission
In 1942, Enrico Fermi demonstrated the first nuclear chain reaction at the University of Chicago. After Fermi’s experiment, the secret Manhattan project started that developed and built the atomic bomb. It was the atomic bomb that destroyed Hiroshima and Nagasaki in August 1945. Since its development, mankind has also focused on the peaceful uses of nuclear fission. In July 1957 an experimental reactor in California produced electricity for the first time using a nuclear reactor, and in 1958, the first large-scale nuclear power plant was commissioned in Pennsylvania, United States.
The uranium used as fuel in nuclear plants is formed into ceramic pellets, about the size of a little finger. These pellets are inserted into long vertical tubes within the reactor core. When the uranium atoms in these pellets are struck by atomic particles, they can split – or fission – to release particles of their own. Uranium is not very stable by nature, and by inserting an extra particle, a neutron, into its nucleus, the latter becomes so unstable that it splits spontaneously into two pieces, releasing an enormous amount of heat and emitting three neutrons. These newly produced neutrons strike other uranium atoms, splitting them. This sequence of one fission triggering others, and those triggering still more, is called a chain reaction.
If each emitted neutron would trigger a new fission, the released energy would increase too rapidly. The nuclear reaction inside the reactor is controlled by rods that are inserted among the tubes that hold the uranium fuel. These control rods are made up of material that absorbs neutrons and prevents them from hitting atoms that can fission. The control rods are made from hafnium, a very good absorber of neutrons. In this way, the nuclear reaction can be speeded up or slowed down by varying the number of control rods that are withdrawn/inserted and to what degree they are withdrawn/inserted.
There are two main types of commercial power plants: boiling water reactors and pressurized water reactors. In boiling water reactors, schematically illustrated in Figure 1, the water is heated by the nuclear fuel and boils to steam in the reactor vessel. It is then piped directly to the steam turbine.
In pressurized water reactors, the water is heated by the nuclear fuel but kept under pressure to prevent it from boiling. Outside the reactor the heat of the water is transferred to a separate supply of water that boils and makes steam. The pressurized water reactor is schematically illustrated in Figure 2.
The solid uranium fuel contains two kinds, or isotopes, of uranium atoms: 235U and 238U. 235U makes up less than 1% of natural uranium and fissions easily, but 238U, which makes up most of natural uranium, is practically non-fissionable. Through a process known as “enrichment,” the concentration of 235U in the uranium is increased to about 3–5% before it is used as reactor fuel.
Most of the fragments of fission, the particles left over after the atom has split, are radioactive. During the life of the fuel, these radioactive fragments collect within the fuel pellets. The fuel remains in the reactor for 3–4 years; most of the 235U is fissioned then and trapped fission fragments reduce the efficiency of the chain reaction. The fuel removed from the reactors, which we call “nuclear waste,” is stored underwater in large concrete and stainless-steel containers or above ground in steel and lead containers.
In 2016, 450 nuclear reactors in 30 countries around the world were used for electricity generation. These nuclear power plants provided in 2012 some 11% of the world’s electricity production. France, Ukraine, Slovakia and Hungary produce more than 50% of its electricity with nuclear power plants.
similar to that which powers the sun and other stars. Nuclear fusion is the holy grail that could solve the world’s energy problem. Fusion power not only offers the potential of an almost limitless source of energy for future generations but also presents some formidable scientific and engineering challenges.
The uranium used as fuel in nuclear plants is formed into ceramic pellets, about the size of a little finger. These pellets are inserted into long vertical tubes within the reactor core. When the uranium atoms in these pellets are struck by atomic particles, they can split – or fission – to release particles of their own. Uranium is not very stable by nature, and by inserting an extra particle, a neu- tron, into its nucleus, the latter becomes so unstable that it splits spontaneously into two pieces, releasing an enormous amount of heat and emitting three neu- trons. These newly produced neutrons strike other uranium atoms, splitting them. This sequence of one fission triggering others, and those triggering still more, is called a chain reaction.
If each emitted neutron would trigger a new fission, the released energy would increase too rapidly. The nuclear reaction inside the reactor is controlled by rods that are inserted among the tubes that hold the uranium fuel. These control rods are made up of material that absorbs neutrons and prevents them from hitting atoms that can fission. The control rods are made from hafnium, a very good absorber of neutrons. In this way, the nuclear reaction can be speeded up or slowed down by varying the number of control rods that are withdrawn/inserted and to what degree they are withdrawn/inserted.
There are two main types of commercial power plants: boiling water reactors and pressurized water reactors. In boiling water reactors, schematically illus- trated in Figure 2.8, the water is heated by the nuclear fuel and boils to steam in the reactor vessel. It is then piped directly to the steam turbine. In pressurized water reactors, the water is heated by the nuclear fuel but kept under pressure to prevent it from boiling. Outside the reactor the heat of the water is transferred to a separate supply of water that boils and makes steam. The pressurized water reactor is schematically illustrated in Figure 2.9.
The solid uranium fuel contains two kinds, or isotopes, of uranium atoms: 235U and 238U. 235U makes up less than 1% of natural uranium and fissions easily, but 238U, which makes up most of natural uranium, is practically non- fissionable. Through a process known as “enrichment,” the concentration of 235U in the uranium is increased to about 3–5% before it is used as reactor fuel.
Most of the fragments of fission, the particles left over after the atom has split, are radioactive. During the life of the fuel, these radioactive fragments collect within the fuel pellets. The fuel remains in the reactor for 3–4 years; most of the 235U is fissioned then and trapped fission fragments reduce the efficiency of the chain reaction. The fuel removed from the reactors, which we call “nuclear waste,” is stored underwater in large concrete and stainless-steel containers or above ground in steel and lead containers.
In 2016, 450 nuclear reactors in 30 countries around the world were used forelectricity generation. These nuclear power plants provided in 2012 some 11%of the world’s electricity production. France, Ukraine, Slovakia and Hungaryproduce more than 50% of its electricity with nuclear power plants (see also Table 2.1).
Nuclear Fusion
The enormous potential of nuclear fusion is hidden in Einstein’s relation E=mc2: a small amount of matter can result in an enormous amount of energy. In 1920 the astronomer Arthur Eddington reasoned that the nuclear fusion from hydrogen to helium was the source of energy that powers the sun. Two atoms of hydrogen combine together, or fuse, to form an atom of helium. In the process of fusion of the hydrogen, matter is converted into energy. The easiest fusion reaction is combining deuterium (or heavy hydrogen) with tritium to make helium and a neutron, as visualized in Figure 3. Deuterium is plentifully available in ordinary water, and tritium can be produced by combining the fusion neutron with the light metal lithium. So, the fuel for the fusion process is not a problem, and that makes it so very attractive. The bad news is that to make fusion happen, the atoms of hydrogen must be heated to some 100 million degrees so they are ionized and form a plasma. At this high temperature, the ionized atoms have sufficient energy to fuse, but the plasma has to be held together for the fusion to occur. The sun and stars do this by gravity. To achieve this on Earth, a strong magnetic field is used to hold the ionized atoms together while they are heated by, for instance, microwaves. The technical concept to perform this was developed by the Russians in 1959.They called their design “tokamak,” which means toroidal magnetic chamber in the Russian language. The tokamak forms the basic element in the majority of the fusion reactors. In 1997, 16 MW of power was released during one second in the Joint European Torus (JET) in Culham (near Oxford) in the United Kingdom. In 2006, the decision was taken to build a 500 MW experimental reactor in Cadarache in the south of France. In this International Thermonuclear Experimental Reactor (ITER) project, the countries of the European Union, Japan, China, the former Soviet Union, and the United States are working together toward the next step on the long road to realizing the dream of nuclear fusion as the solution to the world’s energy problems.