In this process, the uranium was chemically combined with fluorine to form a hexafluoride gas prior to separation by diffusion. This is not a practical method for extracting radioisotopes for scientific and medical use. It was extremely expensive and could only supply naturally occurring isotopes. A more efficient approach is to artificially manufacture radioisotopes.
This can be done by firing high-speed particles into the nucleus of an atom. When struck, the nucleus may absorb the particle or become unstable and emit a particle. In either case, the number of particles in the nucleus would be altered, creating an isotope. One source of high-speed particles could be a cyclotron. A cyclotron accelerates particles around a circular race track with periodic pushes of an electric field.
The particles gather speed with each push, just as a child swings higher with each push on a swing. When traveling fast enough, the particles are directed off the race track and into the target. A cyclotron works only with charged particles, however. Another source of bullets are the neutrons already shooting about inside a nuclear reactor. The neutrons normally strike the nuclei of the fuel, making them unstable and causing the nuclei to split fission into two large fragments and two to three "free" neutrons.
These free neutrons in turn make additional nuclei unstable, causing further fission. The result is a chain reaction. Too many neutrons can lead to an uncontrolled chain reaction, releasing too much heat and perhaps causing a "meltdown.
In this way the surplus neutrons are used to create radioactive isotopes of the materials placed in the targets. With practice, scientists using both cyclotrons and reactors have learned the proper mix of target atoms and shooting particles to "cook up" a wide variety of useful radioisotopes. 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.
For instance, the Fast Breeder Reactor was originally conceived to extend the world's uranium resources, and could do this by a factor of about Although several countries ran extensive fast breeder reactor development programs, major technical and materials problems were encountered.
To the extent that these programs permitted, it was not established that any of the designs would have been commercially competitive with existing light water reactors. An important aspect of fast reactor economics lies in the value of the plutonium fuel which is bred; unless this shows an advantage relative to contemporary costs for uranium, there would be little benefit from the use of this type of reactor.
This point was driven home in the s and s by recognition of the abundance of uranium in geological resources and its relatively low price then. Fast reactors have a strong negative temperature coefficient the reaction slows as the temperature rises unduly , an inherent safety feature, and the basis of automatic load-following in some new designs, by controlling the coolant flow.
Today there is renewed interest in fast neutron reactors for three reasons. First is their potential roles in burning long-lived actinides recovered from light water reactor used fuel, secondly a short-term role in the disposal of ex-military plutonium, and thirdly enabling much fuller use of the world's uranium resources even though these re abundant.
In all respects the technology is important to long-term considerations of world energy sustainability. For more information, see page on Fast Neutron Reactors. Fission of U nuclei typically releases 2 or 3 neutrons, with an average of almost 2.
One of these neutrons is needed to sustain the chain reaction at a steady level of controlled criticality; on average, the others leak from the core region or are absorbed in non-fission reactions. Neutron-absorbing control rods are used to adjust the power output of a reactor.
When they are slightly withdrawn from their position at criticality, the number of neutrons available for ongoing fission exceeds unity i. When the power reaches the desired level, the control rods are returned to the critical position and the power stabilises. The ability to control the chain reaction is entirely due to the presence of the small proportion of delayed neutrons arising from fission 0.
Without these, any change in the critical balance of the chain reaction would lead to a virtually instantaneous and uncontrollable rise or fall in the neutron population. It is also relevant to note that safe design and operation of a reactor sets very strict limits on the extent to which departures from criticality are permitted. These limits are built in to the overall design. While fuel is being burned in the reactor, it is gradually accumulating fission products and transuranic elements which cause additional neutron absorption.
The control system has to be adjusted to compensate for the increased absorption. When the fuel has been in the reactor for three years or so, this build-up in absorption, along with the metallurgical changes induced by the constant neutron bombardment of the fuel materials, dictates that the fuel should be replaced.
This effectively limits the burn-up to about half of the fissile material, and the fuel assemblies must then be removed and replaced with fresh fuel. Fuel life can be extended by use of burnable poisons such as gadolinium, the effect of which compensates for the build-up of neutron absorbers. A moderator material comprising light atoms thus surrounds the fuel rods in a reactor.
Without absorbing too many, it must slow down the neutrons in elastic collisions compare it with collisions between billiard balls on an atomic scale. In a reactor using natural unenriched uranium the only suitable moderators are graphite and heavy water these have low levels of unwanted neutron absorption.
With enriched uranium i. Water is also commonly used as a coolant, to remove the heat and generate steam. Other features may be used in different reactor types to control the chain reaction. For instance, a small amount of boron may be added to the cooling water and its concentration reduced progressively as other neutron absorbers build up in the fuel elements.
For emergency situations, provision may be made for rapidly adding an excessive quantity of boron to the water. Commercial power reactors are usually designed to have negative temperature and void coefficients. The significance of this is that if the temperature should rise beyond its normal operating level, or if boiling should occur beyond an acceptable level, the balance of the chain reaction is affected so as to reduce the rate of fission and hence reduce the temperature.
One mechanism involved is the Doppler effect, whereby U absorbs more neutrons as the temperature rises, thereby pushing the neutron balance towards subcritical. Another important mechanism, in light water reactors, is that the formation of steam within the water moderator will reduce its density and hence its moderating effect, and this again will tilt the neutron balance towards subcritical.
In naval reactors used for propulsion, where fuel changes are inconvenient, the fuel is enriched to higher levels initially and burnable poisons — neutron absorbers — are incorporated and the initial fuel load may last the life of the vessel. Hence as the fission products and transuranic elements accumulate, the 'poison' is depleted and the two effects tend to cancel one another out. Gadolinium is incorporated in the ceramic fuel pellets.
An alternative is zirconium bromide integral fuel burnable absorber IFBA as a thin coating on normal pellets. Uranium naturally contains all three isotopes U, U and U , and it rarely varies more than 0. To produce fuel-grade uranium, the uranium has to be processed to produce uranium dioxide and to enrich or concentrate the U in the fuel pellets.
During this processing, depleted uranium DU , enriched in U and depleted in U, is produced. DU and enriched uranium have numerous civilian and military uses. Since U is the most radioactive isotope of uranium, the removal of it to makes DU the least radioactive phase of uranium, but it still has heavy metal toxicity issues. Despite any processing, enriched, depleted or natural uranium all behave the same chemically.
U, when bombarded by neutrons, fissions or splits into two smaller nuclei and releases energy and starts a nuclear chain reaction. Because of the energy released, U is efficient for power generation and the only isotope of uranium that can sustain these reactions.
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