Some of these neutrons are absorbed by other atoms of uranium In turn, these atoms split apart, releasing more energy and more neutrons. A typical reaction is: The brackets around U indicate that it has a highly unstable nucleus. Under proper conditions, the fission of a few nuclei of uranium sets in motion a chain reaction Figure 4. In fact, this reaction is the source of energy in the atomic bomb. Each fission results in two or more neutrons that can react with other uranium atoms so that the number of nuclear fissions occurring soon reaches an enormous number.
In nuclear power plants, the energy released by the controlled fission of uranium is collected in the reactor and used to produce steam in a heat exchanger. The steam then drives a turbine to produce electricity.
Energy generation can be regulated by inserting control rods between the fuel rods in the reactor to absorb excess neutrons, thereby controlling the rate of the chain reaction. A typical nuclear power plant in operation today uses about 2 kg uranium to generate megawatts of electricity. About tons 5. Uranium natural abundance 0. The much more abundant uranium does not undergo fission and therefore cannot be used as a fuel for nuclear reactors. Whereas the U nucleus is 'fissile', that of U is said to be 'fertile'.
This means that it can capture one of the neutrons which are flying about in the core of the reactor and become indirectly plutonium, which is fissile. Pu is very much like U, in that it fissions when hit by a neutron and this yields a similar amount of energy. Because there is so much U in a reactor core most of the fuel , these reactions occur frequently, and in fact about one-third of the fuel's energy yield comes from 'burning' Pu But sometimes a Pu atom simply captures a neutron without splitting, and it becomes Pu Because the Pu is either progressively 'burned' or becomes Pu, the longer the fuel stays in the reactor the more Pu is in it.
The significance of this is that when the spent fuel is removed after about three years, the plutonium in it is not suitable for making weapons but can be recycled as fuel. Uranium ore can be mined by underground or open-cut methods, depending on its depth.
After mining, the ore is crushed and ground up. Then it is treated with acid to dissolve the uranium, which is recovered from solution. Uranium may also be mined by in situ leaching ISL , where it is dissolved from a porous underground ore body in situ and pumped to the surface.
This is the form in which uranium is sold. Before it can be used in a reactor for electricity generation, however, it must undergo a series of processes to produce a useable fuel. For most of the world's reactors, the next step in making the fuel is to convert the uranium oxide into a gas, uranium hexafluoride UF 6 , which enables it to be enriched.
Enrichment increases the proportion of the uranium isotope from its natural level of 0. This enables greater technical efficiency in reactor design and operation, particularly in larger reactors, and allows the use of ordinary water as a moderator.
After enrichment, the UF 6 gas is converted to uranium dioxide UO 2 which is formed into fuel pellets. These fuel pellets are placed inside thin metal tubes, known as fuel rods, which are assembled in bundles to become the fuel elements or assemblies for the core of the reactor. In a typical large power reactor there might be 51, fuel rods with over 18 million pellets. For reactors which use natural uranium as their fuel and hence which require graphite or heavy water as a moderator the U 3 O 8 concentrate simply needs to be refined and converted directly to uranium dioxide.
When the uranium fuel has been in the reactor for about three years, the used fuel is removed, stored, and then either reprocessed or disposed of underground see Nuclear Fuel Cycle or Radioactive Waste Management.
This amounts to over TWh each year, as much as from all sources of electricity worldwide in It comes from about nuclear reactors with a total output capacity of about , megawatts MWe operating in 32 countries.
About 50 more reactors are under construction and about are planned. Over the 60 years that the world has enjoyed the benefits of cleanly-generated electricity from nuclear power, there have been over 18, reactor-years of operational experience. See also Nuclear Generation by Country. Uranium is widespread in many rocks, and even in seawater. However, like other metals, it is seldom sufficiently concentrated to be economically recoverable.
Where it is, we speak of an orebody. In defining what is ore, assumptions are made about the cost of mining and the market price of the metal.
Uranium reserves are therefore calculated as tonnes recoverable up to a certain cost. NB: the figures in this table are liable to change as new data becomes available. Mining methods have been changing. From the new Canadian mines increased it again. In situ leach ISL, also called in situ recovery, ISR mining has been steadily increasing its share of the total, mainly due to Kazakhstan, and in accounted for over half of production:.
Uranium is sold only to countries which are signatories of the Nuclear Non-Proliferation Treaty NPT , and which allow international inspection to verify that it is used only for peaceful purposes. Many people, when talking about nuclear energy, have only nuclear reactors or perhaps nuclear weapons in mind.
Few people realise the extent to which the use of radioisotopes has changed our lives over the last few decades. Using relatively small special-purpose nuclear reactors, it is possible to make a wide range of radioactive materials radioisotopes at low cost. For this reason the use of artificially-produced radioisotopes has become widespread since the early s, and there are now about 'research' reactors in 56 countries producing them.
In nuclei with an odd number of neutrons, such as U, the fission cross-section becomes very large at the thermal energies of slow neutrons. We therefore say that the fission cross-section of those nuclei is much reduced at high neutron energies relative to its value at thermal energies for slow neutrons. It is nonetheless possible to use this so-called fast fission in a fast neutron reactor whose design minimises the moderation of the high-energy neutrons produced in the fission process.
See below. This nucleus is relatively unstable, and it is likely to break into two fragments of around half the mass. These fragments are nuclei found around the middle of the Periodic Table and the probabilistic nature of the break-up leads to several hundred possible combinations. Creation of the fission fragments is followed almost instantaneously by emission of a number of neutrons typically 2 or 3, average 2. Alpha particles from the decay cause a release of neutrons from the beryllium as it turns to carbon However, in solid fuel they can only travel a microscopic distance, so their energy becomes converted into heat.
The balance of the energy comes from gamma rays emitted during or immediately following the fission process and from the kinetic energy of the neutrons. Some of the latter are immediate so-called prompt neutrons , but a small proportion 0. The longest delayed neutron group has a half-life of about 56 seconds. The delayed neutron release is the crucial factor enabling a chain reacting system or reactor to be controllable and to be able to be held precisely critical.
At criticality the chain reacting system is exactly in balance, such that the number of neutrons produced in fissions remains constant. This number of neutrons may be completely accounted for by the sum of those causing further fissions, those otherwise absorbed, and those leaking out of the system. Under these circumstances the power generated by the system remains constant. To raise or lower the power, the balance must be changed using the control system so that the number of neutrons present and hence the rate of power generation is either reduced or increased.
The control system is used to restore the balance when the desired new power level is attained. The number of neutrons and the specific fission products from any fission event are governed by statistical probability, in that the precise break up of a single nucleus cannot be predicted.
However, conservation laws require the total number of nucleons and the total energy to be conserved. The fission reaction in U produces fission products such as Ba, Kr, Sr, Cs, I and Xe with atomic masses distributed around 95 and Examples may be given of typical reaction products, such as:. Both the barium and krypton isotopes subsequently decay and form more stable isotopes of neodymium and yttrium, with the emission of several electrons from the nucleus beta decays.
It is the beta decays, with some associated gamma rays, which make the fission products highly radioactive. This radioactivity by definition! This contrasts with 4 eV or 6. This must be allowed for when the reactor is shut down, since heat generation continues after fission stops. It is this decay which makes used fuel initially generate heat and hence need cooling, as very publicly demonstrated in the Fukushima accident when cooling was lost an hour after shutdown and the fuel was still producing about 1.
Neutrons may be captured by non-fissile nuclei, and some energy is produced by this mechanism in the form of gamma rays as the compound nucleus de-excites. The resultant new nucleus may become more stable by emitting alpha or beta particles. Neutron capture by one of the uranium isotopes will form what are called transuranic elements, actinides beyond uranium in the periodic table.
Since U is the major proportion of the fuel element material in a thermal reactor, capture of neutrons by U and the creation of U is an important process. As already noted, Pu is fissile in the same way as U, i. It is the other main source of energy in any nuclear reactor.
If fuel is left in the reactor for a typical three years, about two-thirds of the Pu is fissioned with the U, and it typically contributes about one-third of the energy output. The masses of its fission products are distributed around and atomic mass units. One difference is that Pu fission in a thermal reactor results in 2. In a fast reactor, Pu produces more neutrons per fission e.
The main transuranic constituents of used fuel are isotopes of plutonium, curium, neptunium and americium, the last three being 'minor actinides'. These are alpha-emitters and have long half-lives, decaying on a similar time scale to the uranium isotopes. They are the reason that used fuel needs secure disposal beyond the few thousand years or so which might be necessary for the decay of fission products alone.
Apart from transuranic elements in the reactor fuel, activation products are formed wherever neutrons impact on any other material surrounding the fuel. Activation products in a reactor and particularly its steel components exposed to neutrons range from tritium H-3 and carbon, to cobalt, iron and nickel The latter four radioisotopes create difficulties during eventual demolition of the reactor, and affect the extent to which materials can be recycled.
In a fast neutron reactor the fuel in the core is Pu and the abundant neutrons which leak from the core breed more Pu in a fertile blanket of U around the core. A minor fraction of U might be subject to fission, but most of the neutrons reaching the U blanket will have lost some of their original energy and are therefore subject only to capture and thus breeding of Pu Cooling of the fast reactor core requires a heat transfer medium which has minimal moderation of the neutrons, and hence liquid metals are used, typically sodium.
Such reactors can be up to times more efficient at converting fertile material than ordinary thermal reactors because of the arrangement of fissile and fertile materials, and there is some advantage from the fact that Pu yields more neutrons per fission than U Although both yield more neutrons per fission when split by fast rather than slow neutrons, this is incidental since the fission cross sections are much smaller at high neutron energies.
While the conversion ratio the ratio of new fissile nuclei to fissioned nuclei in a normal reactor is around 0.
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