Nuclear fusion is a process in which light-mass nuclei combine to form a heavier and more stable nucleus. Fusion produces even more energy than fission. In the sun and other stars, four hydrogen nuclei combine at extremely high temperatures and pressures to produce a helium nucleus. The concurrent loss of mass is converted into extraordinary amounts of energy see figure below. Fusion is even more appealing than fission as an energy source because no radioactive waste is produced and the only reactant needed is hydrogen.
No known materials can withstand such temperatures, so there is currently no feasible way to harness nuclear fusion for energy production, although research is ongoing. As we saw earlier, different types of radiation vary in their abilities to penetrate through matter. Alpha particles have very low penetrating ability and are stopped by skin and clothing.
Beta particles have a penetrating ability that is about times that of alpha particles. Gamma rays have very high penetrating ability, and great care must be taken to avoid overexposure to gamma rays. Radiation emitted by radioisotopes is called ionizing radiation. Ionizing radiation is radiation that has enough energy to knock electrons off the atoms of a bombarded substance and produce ions. The primary concern is that ionizing radiation can do damage to living tissues.
Radiation damage is measured in rems, which stands for roentgen equivalent man. A rem is the amount of ionizing radiation that does as much damage to human tissue as is done by 1 roentgen of high-voltage x-rays. Tissue damage from ionizing radiation can cause genetic mutations due to interactions between the radiation and DNA, which can lead to cancer.
You are constantly being bombarded with background radiation from space and from geologic sources that vary depending on where you live. Average exposure is estimated to be about 0.
The maximum permissible does of radiation exposure for people in the general population is 0. Some people are naturally at higher risk because of their occupations, so reliable instruments to detect radiation exposure have been developed. A Geiger counter is a device that uses a gas-filled metal tube to detect radiation see figure below. When the gas is exposed to ionizing radiation, it conducts a current, and the Geiger counter registers this as audible clicks.
The frequency of the clicks corresponds to the intensity of the radiation. A scintillation counter is a device that uses a phosphor-coated surface to detect radiation by the emission of bright bursts of light. Workers who are at risk of exposure to radiation wear small portable film badges. A film badge consists of several layers of photographic film that can measure the amount of radiation to which the wearer has been exposed.
Film badges are removed and analyzed at periodic intervals to ensure that the person does not become overexposed to radiation on a cumulative bases. Radioactive nuclides, such as cobalt, are frequently used in medicine to treat certain types of cancers. Atomic bombs are made up of a fissile element, such as uranium, that is enriched in the isotope that can sustain a fission nuclear chain reaction.
When a free neutron hits the nucleus of a fissile atom like uranium U , the uranium splits into two smaller atoms called fission fragments, plus more neutrons. Fission can be self-sustaining because it produces more neutrons with the speed required to cause new fissions. This creates the chain reaction. The very first uranium bomb, Little Boy, dropped on Hiroshima in , used 64 kilograms of 80 percent enriched uranium. In fission weapons, a mass of fissile material, either enriched uranium or plutonium, is assembled into a supercritical mass—the amount of material needed to start an exponentially growing nuclear chain reaction.
The implosion method is considered more sophisticated than the gun method and only can be used if the fissile material is plutonium. The inherent radioactivity of uranium will then release a neutron, which will bombard another atom of U to produce the unstable uranium, which undergoes fission, releases further neutrons, and continues the process.
The uranium atom can split any one of dozens of different ways, as long as the atomic weights add up to uranium plus the extra neutron. The following equation shows one possible split, namely into strontium 95 Sr , xenon Xe , and two neutrons n , plus energy:. Fission bomb assembly methods : Two methods have been applied to induce the nuclear chain reaction that produces the explosion of an atomic bomb. The gun-type assembly uses a conventional explosive to compress from one side, while the implosion assembly compresses from all sides simultaneously.
The immediate energy release per atom is about million electron volts Me. Of the energy produced, 93 percent is the kinetic energy of the charged fission fragments flying away from each other, mutually repelled by the positive charge of their protons.
This initial kinetic energy imparts an initial speed of about 12, kilometers per second. Here, their motion is converted into X-ray heat, a process which takes about a millionth of a second. By this time, the material in the core and tamper of the bomb is several meters in diameter and has been converted to plasma at a temperature of tens of millions of degrees.
This X-ray energy produces the blast and fire which are normally the purpose of a nuclear explosion. How to make an atomic bomb : An explanation of the fission process utilized in atomic bombs. A nuclear reactor is a piece of equipment in which nuclear chain reactions can be harnessed to produce energy in a controlled way.
The energy released from nuclear fission can be harnessed to make electricity with a nuclear reactor. A nuclear reactor is a piece of equipment where nuclear chain reactions can be controlled and sustained. The reactors use nuclear fuel, most commonly uranium and plutonium The amount of free energy in nuclear fuels is far greater than the energy in a similar amount of other fuels such as gasoline. In many countries, nuclear power is seen as an environmentally friendly alternative to fossil fuels, which are non-renewable and release large amounts of greenhouse gases.
However, nuclear reactors produce nuclear waste containing radioactive elements. When a large, fissile atomic nucleus such as uranium or plutonium absorbs a neutron, it may undergo nuclear fission. The nucleus splits into two or more lighter nuclei, releasing kinetic energy, gamma radiation, and free neutrons.
A portion of these neutrons may later be absorbed by other fissile atoms and trigger further fission events, which release more neutrons, and so on. This is known as a nuclear chain reaction. Nuclear chain reaction : A possible nuclear fission chain reaction. In the first step, a uranium atom absorbs a neutron, and splits into two new atoms fission fragments , releasing three new neutrons and a large amount of binding energy.
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. 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.
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