Атомная энергетика| Nuclear power plants
Final Certification |Итоговая Аттестация
Lesson 7
Read the text: Nuclear fuels
Materials whose ability to release energy derives from specific properties of the atom's nucleus. In
general, energy can be released by combining two light nuclei to form a heavier one, a process
called nuclear fusion; by splitting a heavy nucleus into two fragments of intermediate mass, a
process called nuclear fission; or by spontaneous nuclear decay processes, which are generically
referred to as radioactivity. Although the fusion process may significantly contribute to the world's
energy production in future centuries and although the production of limited amounts of energy by
radioactive decay is a well-established technology for specific applications, the only significant
industrial use of nuclear fuel so far utilizes fission. Therefore, the term nuclear fuels generally
designates nuclear fission fuels only.
Large releases of energy through a fission or a fusion reaction are possible because the stability of
the nucleus is a function of its size. The binding energy per nucleon provides a measure of the
nucleus stability. By selectively combining light nuclei together by a fusion reaction or by
fragmenting heavy nuclei by a fission reaction, nuclei with higher binding energies per nucleon can
be formed. The result of these two processes is a release of energy. The fissioning of one nucleus of
uranium releases as much energy as the oxidation of approximately 5 ? 107 atoms of carbon.
Many heavy elements can be made to fission by bombardment with high-energy particles. However,
only neutrons can provide a self-sustaining nuclear fission reaction. Upon capture of a neutron by a
heavy nucleus, the latter may become unstable and split into two fragments of intermediate mass.
This fragmentation is generally accompanied by the emission of one or several neutrons, which can
then induce new fissions. Only a few long-lived nuclides have been found to have a high probability
of fission: 233U, 235U, and 239Pu. Of these nuclides, only 235U occurs in nature as 1 part in 140 of
natural uranium, the remainder being mostly 238U. The other nuclides must be produced artificially:
233U from 232Th, and 239Pu from 238U. The nuclides 233U, 235U, and 239Pu are called fissile
materials since they undergo fission with either slow or fast neutrons, while 232Th and 238U are
called fertile materials. The latter, however, can also undergo the fission process at low yields with
energetic neutrons; therefore, they are also referred to as being fissionable.
The term nuclear fuel applies not only to the fissile materials, but often to the mixtures of fissile and
fertile materials as well. Using a mixture of fissile and fertile materials in a reactor allows capture of
excess neutrons by the fertile nuclides to form fissile nuclides. Depending on the efficiency of
production of fissile elements, the process is called conversion or breeding. Breeding is an extreme
case of conversion corresponding to a production of fissile material at least equal to its
consumption.
Nuclear fuel is any material that can be consumed to derive nuclear energy, by analogy to chemical
fuel that is burned to derive energy. By far the most common type of nuclear fuel is heavy fissile
elements that can be made to undergo nuclear fission chain reactions in a nuclear fission reactor;
nuclear fuel can refer to the material or to physical objects (for example fuel bundles composed of
fuel rods) composed of the fuel material, perhaps mixed with structural, neutron moderating, or
neutron reflecting materials. The most common fissile nuclear fuels are 235U and 239Pu, and the
actions of mining, refining, purifying, using, and ultimately disposing of these elements together
make up the nuclear fuel cycle, which is important for its relevance to nuclear power generation
and nuclear weapons.
Not all nuclear fuels are used in fission chain reactions. For example, 238Pu and some other
elements are used to produce small amounts of nuclear power by radioactive decay in radio-thermal
generators, and other atomic batteries. Light isotopes such as 3H (tritium) are used as fuel for
nuclear fusion. If one looks at binding energy of specific isotopes, there can be an energy gain from
fusing most elements with a lower atomic number than iron, and fissioning isotopes with a higher
atomic number than iron.
1. Match the left part with the right:
1. The result of these two process a) is generally accompanied by the emission of one
or several neutrons.
2. Not all nuclear fuels b) are used in fission chain reactions.
3. This fragmentation c) is heavy fissile elements.
4. By far the most common type of fuel d) is a release of energy.
2. Complete the sentences with the suggested words: fission, splitting, combining, intermediate,
fusion, spontaneous
Materials whose ability to release energy derives from specific properties of the atom's nucleus. In
general, energy can be released by _________ two light nuclei to form a heavier one, a process
called nuclear ________; by _________ a heavy nucleus into two fragments of __________ mass, a
process called nuclear ___________; or by ___________ nuclear decay processes, which are
generically referred to as radioactivity.