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. Fast neutron reactors may be designed as breeders to yield more fissile material than they consume, or to be plutonium burners to dispose of excess plutonium. A plutonium burner would be designed without a breeding blanket, simply with a core optimised for plutonium fuel, and this is the likely shape of future fast neutron reactors, even if they have some breeding function.
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Recommended Videos Problem 2. Problem 3. Problem 4. Problem 5. Problem 6. Problem 7. Problem 8. Problem 9. Problem Plutonium can also be used in fast neutron reactors, where a much higher proportion of Pu fissions and in fact all the plutonium isotopes fission, and so function as a fuel. As with uranium, the energy potential of plutonium is more fully realized in a fast reactor. Four of the six 'Generation IV' reactor designs currently under development are fast neutron reactors and will thus utilize plutonium in some way see page on Generation IV Nuclear Reactors.
In these, plutonium production will take place in the core, where burn-up is high and the proportion of plutonium isotopes other than Pu will remain high. In pure form plutonium exists in six allotropic forms or crystal structure — more than any other element. As temperature changes, it switches forms — each has significantly different mechanical and electrical properties.
One is nearly twice the density of lead The alpha phase is hard and brittle, like cast iron, and if finely divided it spontaneously ignites in air to form PuO 2. Beta, gamma and delta phases are all less dense. Alloyed with gallium, plutonium becomes more workable.
Russia has maintained a positive policy of civil plutonium utilization. Apart from its formation in today's nuclear reactors, plutonium was formed by the operation of naturally-occurring nuclear reactors in uranium deposits at Oklo in what is now west Africa, some two billion years ago. Civil plutonium stored over several years becomes contaminated with the Pu decay product americium see page on The Many Uses of Nuclear Technology , which interferes with normal fuel fabrication procedures.
After long storage, Am must be removed before the plutonium can be used in a MOX fuel fabrication plant because it emits intense gamma radiation in the course of its alpha decay to Np The European Space Agency is paying NNL to produce Am for watt e radioisotope thermoelectric generators RTGs using very pure Am recovered from old civil plutonium, as the isotope is much less expensive than Pu now scarce.
Of some 2, types of radioisotopes known to humankind, only 22 are capable of powering a deep-space probe, according to a study by the US National Academy of Sciences. Of these, all but Pu are problematical due to being too expensive, emitting too much radiation to work with, or lacking enough heat output however, note European use of Am in above section on Plutonium and americium.
The decay heat of Pu 0. These spacecraft have operated for over 35 years and are expected to send back signals powered by their RTGs through to The Cassini spacecraft carried three generators with 33 kg of plutonium oxide providing watts power as it orbited around Saturn, having taken seven years to get there.
See also information page on Nuclear Reactors and Radioisotopes for Space. Plutonium is made by irradiating neptunium, recovered from research reactor fuel or special targets, in research reactors. Np is formed and quickly decays to Pu Pu was then recovered by further reprocessing at the H Canyon plant there. This was essentially Cold War-origin material. Currently, supplies of high-purity Pu are scarce. Since the early s after production ceased at Savannah River in , the USA was buying all its supply for spacecraft from Russia — some INL supplies the neptunium and does some of the irradiation.
It uses the High Flux Isotope Reactor, irradiating neptunium targets for 72 days. The plutonium is then chemically separated and purified to produce an oxide powder.
ORNL expects production to ramp up to 1. OPG would use a similar process to that at its Pickering units to produce cobalt These would be irradiated at Darlington then returned to Chalk River for processing. Production target is reportedly 5 kg Pu per year by about , but the project is yet to receive regulatory approval.
Early heart pacemakers used Pu as the power source, and after 30 years some were still running well. It takes about 10 kilograms of nearly pure Pu to make a bomb though the Nagasaki bomb in used less. Producing this requires 30 megawatt-years of reactor operation, with frequent fuel changes and reprocessing of the 'hot' fuel.
Allowing the fuel to stay longer in the reactor increases the concentration of the higher isotopes of plutonium, in particular the Pu isotope, as can be seen in the Table above. For weapons use, Pu is considered a serious contaminant, due to higher neutron emission and higher heat production.
It is not feasible to separate Pu from Pu The operational requirements of power reactors and plutonium production reactors are quite different, and so therefore is their design. An explosive device could be made from plutonium extracted from low burn-up reactor fuel i. Typical 'reactor-grade' plutonium recovered from reprocessing used power reactor fuel has about one-third non-fissile isotopes mainly Pu d. In the UK, the Magnox reactors were designed for the dual use of generating commercial electricity as well as being able to produce plutonium for the country's defence programme.
A report released by the UK's Ministry of Defence MoD says that both the Calder Hall and the Chapelcross power stations, which started up in and respectively, were operated on this basis 3.
The government confirmed in April that production of plutonium for defence purposes had ceased in the s at these two stations, which are both now permanently shutdown.
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