PesWiki.com

Menu

PowerPedia:Nuclear fission

Lasted edited by Andrew Munsey, updated on June 14, 2016 at 9:03 pm.

  • 150 errors has been found on this page. Administrator will correct this soon.
  • This page has been imported from the old peswiki website. This message will be removed once updated.

:For the generation of electrical power by fission, see There was an error working with the wiki: Code[37]

Nuclear fission - also known as atomic fission - is a process in There was an error working with the wiki: Code[1] of an atom splits into two or more smaller nuclei as There was an error working with the wiki: Code[2]. The by-products include free There was an error working with the wiki: Code[3], and other nuclear fragments such as There was an error working with the wiki: Code[4] and There was an error working with the wiki: Code[5]. Fission of heavy elements is an There was an error working with the wiki: Code[38] and can release substantial amounts of useful Energy both as gamma rays and as Kinetic energy of the fragments (Heating the bulk material where fission takes place).

Nuclear fission is used to produce energy for Nuclear power and to drive explosion of There was an error working with the wiki: Code[39]s. Fission is useful as a power source because some materials, called There was an error working with the wiki: Code[40]s, both generate neutrons as part of the fission process and also undergo triggered fission when impacted by a free neutron. Nuclear fuels can be part of a self-sustaining There was an error working with the wiki: Code[41] that releases energy at a controlled rate in a There was an error working with the wiki: Code[42] or at a very rapid uncontrolled rate in a There was an error working with the wiki: Code[39].

The amount of There was an error working with the wiki: Code[6] of nuclear fission are highly There was an error working with the wiki: Code[7], giving rise to a There was an error working with the wiki: Code[8]. Concerns over nuclear waste accumulation and over the immense destructive potential of nuclear weapons counterbalance the desirable qualities of fission as an energy source, and give rise to intense ongoing There was an error working with the wiki: Code[9] debate over nuclear power.

Physical overview

Nuclear fission differs from other forms of There was an error working with the wiki: Code[10] There was an error working with the wiki: Code[11] (the isotope of There was an error working with the wiki: Code[12] (the isotope of There was an error working with the wiki: Code[13]/There was an error working with the wiki: Code[14] There was an error working with the wiki: Code[15] to There was an error working with the wiki: Code[16]s. In a There was an error working with the wiki: Code[44] or nuclear weapon, most fission events are induced by bombardment with another particle such as a neutron.

Typical fission events release several hundred There was an error working with the wiki: Code[17] of energy for each fuel atom that undergoes fission, which is why nuclear fission is used as an There was an error working with the wiki: Code[18] There was an error working with the wiki: Code[19]) release at most a few tens of There was an error working with the wiki: Code[20] per event, so nuclear fuel contains at least ten million times more usable energy than does chemical fuel. The energy of nuclear fission is released as Kinetic energy of the fission products and fragments, and as There was an error working with the wiki: Code[45] in the form of There was an error working with the wiki: Code[46]s in a nuclear reactor, the energy is converted to Heat as the particles and gamma rays collide with the atoms that make up the reactor and its There was an error working with the wiki: Code[47], usually Water or occasionally There was an error working with the wiki: Code[48].

Nuclear fission of heavy elements produces energy because the specific There was an error working with the wiki: Code[49] (binding energy per mass) of intermediate-mass nuclei with There was an error working with the wiki: Code[50]s and There was an error working with the wiki: Code[51]es close to 61Ni and 56Fe is greater than the specific binding energy of very heavy nuclei, so that energy is released when heavy nuclei are broken apart.

The total mass of the fission products (Mp) from a single reaction, after their kinetic energy has been dissipated, is less than the mass of the original fuel nucleus. The excess mass ?m is associated with the released energy which carries it away, according to Einstein's relation There was an error working with the wiki: Code[52], where the mass is ?m. In comparison, the specific binding energies of many lighter elements [elements 1 (hydrogen) through approximately 12 (magnesium)] are also significantly less than that of intermediate-mass nuclei, so if the lighter elements undergo There was an error working with the wiki: Code[53] (the counterpart to nuclear fission), this process also releases heat energy (is "exothermic").

E=M_{U^{235}}~c^2- M_P~c^2

The variation in specific binding energy with atomic number is due to the interplay of the two fundamental Forces acting on the component There was an error working with the wiki: Code[54]s (There was an error working with the wiki: Code[55]s and There was an error working with the wiki: Code[56]s) that make up the nucleus. Nuclei are bound by an attractive There was an error working with the wiki: Code[57] between nucleons, which overcomes the There was an error working with the wiki: Code[58] between protons. However, the strong nuclear force acts only over extremely short ranges, since it follows a There was an error working with the wiki: Code[59]. For this reason large nuclei are less tightly bound per unit mass than small nuclei, and breaking a very large nucleus into two or more intermediate-sized nuclei releases energy. In practice, as noted, most of this energy appears as kinetic energy as the two smaller nuclei mutually repel and fly away from each other at high speed.

In nuclear fission events the nuclei may break into any combination of lighter nuclei, but the most common event is not fission to equal mass nuclei of about mass 120 the most common event (depending on isotope and process) is a slightly unequal fission in which one daughter nucleus has a mass of about 90 to 100 u and the other the remaining 130 to 140 u Unequal fissions are energetically more favorable because this allows one product to be closer to the energetic minimum near mass 60 u (only a quarter of the average fissionable mass), while the other nucleus with mass 135 u is still not far out of the range of the most tightly bound nuclei (another statement of this, is that the atomic [[binding energy] curve is slightly steeper to the left of mass 120 u than to the right of it).

Because of the short range of the strong binding force, large nuclei must contain proportionally more neutrons than do light elements, which are most stable with a 1-1 ratio of protons and neutrons. Extra neutrons stabilize heavy elements because they add to strong-force binding without adding to proton-proton repulsion however this process works better for heavier elements which have room in outer nuclear orbitals for the necessary extra neutrons. Fission products have, on average, about the same ratio of neutrons and protons as their parent nucleus, and are therefore usually unstable because they have proportionally too many neutrons compared to stable isotopes of similar mass. This is the fundamental cause of the problem of There was an error working with the wiki: Code[21], There was an error working with the wiki: Code[22] fast-moving Electrons to conserve Electric charge as excess neutrons convert to protons inside the nucleus of the fission product atoms.

The most common nuclear fuels, 235U and 239Pu, are not major radiologic hazards by themselves: 235U has a There was an error working with the wiki: Code[60] of approximately 700 million years, and although 239Pu has a half-life of only about 24,000 years, it is a pure There was an error working with the wiki: Code[61] emitter and hence not particularly dangerous unless ingested. Once a There was an error working with the wiki: Code[62] has been used, the remaining fuel material is intimately mixed with highly radioactive fission products that emit energetic There was an error working with the wiki: Code[63] and There was an error working with the wiki: Code[64]. Some fission products have half-lives as short as seconds others have half-lives of tens of thousands of years, requiring long-term storage in facilities such as There was an error working with the wiki: Code[65] until the fission products decay into non-radioactive stable isotopes.

Spontaneous and induced fission chain reactions

Many heavy elements, such as There was an error working with the wiki: Code[23], slow moving neutron are also called There was an error working with the wiki: Code[66]. A few particularly fissile and readily obtainable isotopes (notably 235U and 239Pu) are called There was an error working with the wiki: Code[67]s because they can sustain a chain reaction and can be obtained in large enough quantities to be useful.

All fissionable and fissile isotopes undergo a small amount of spontaneous fission which releases a few free neutrons into any sample of nuclear fuel. The neutrons typically escape rapidly from the fuel and become a There was an error working with the wiki: Code[68], with a There was an error working with the wiki: Code[69] of about 15 minutes before they decay to protons and beta rays. The neutrons usually impact and are absorbed by other nuclei in the vicinity before this happens. However, some neutrons will impact fuel nuclei and induce further fissions, releasing yet more neutrons. If enough nuclear fuel is assembled into one place, or if the escaping neutrons are sufficiently contained, then these freshly generated neutrons outnumber the neutrons that escape from the assembly, and a sustained nuclear chain reaction will take place.

An assembly that supports a sustained nuclear chain reaction is called a There was an error working with the wiki: Code[24] or, if the assembly is almost entirely made of a nuclear fuel, a There was an error working with the wiki: Code[25]. The word "critical" refers to a There was an error working with the wiki: Code[70] in the behavior of the There was an error working with the wiki: Code[71] that governs the number of free neutrons present in the fuel: if less than a critical mass is present, then the amount of neutrons is determined by There was an error working with the wiki: Code[72], but if a critical mass or more is present, then the amount of neutrons is controlled instead by the physics of the chain reaction. The actual Mass of a critical mass of nuclear fuel depends strongly on the geometry and surrounding materials.

Not all fissionable isotopes can sustain a chain reaction. For example, 238U, the most abundant form of uranium, is fissionable but not fissile: it undergoes induced fission when impacted by an energetic neutron with over 1 MeV of kinetic energy. But too few of the neutrons produced by 238U fission are energetic enough to induce further fissions in 238U, so no chain reaction is possible with this isotope. Instead, bombarding 238U with slow neutrons causes it to absorb them (becoming 239U) and decay by There was an error working with the wiki: Code[26] to 239Pu that process is used to manufacture 239Pu in There was an error working with the wiki: Code[73]s, but does not contribute to a neutron chain reaction.

Fissionable, non-fissile isotopes can be used as fission energy source even without a chain reaction. Bombarding 238U with fast neutrons induces fissions, releasing energy as long as the external neutron source is present. That effect is used to augment the energy released by modern There was an error working with the wiki: Code[74]s, by jacketing the weapon with 238U to react with neutrons released by There was an error working with the wiki: Code[75] at the center of the device.

Fission reactors

Critical fission reactors are the most common type of There was an error working with the wiki: Code[76]. In a critical fission reactor, neutrons produced by fission of fuel atoms are used to induce yet more fissions, to sustain a controllable amount of energy release. Devices that produce engineered but non-self-sustaining fission reactions are subcritical fission reactors. Such devices use There was an error working with the wiki: Code[77] or There was an error working with the wiki: Code[78]s to trigger fissions.

Critical fission reactors are built for three primary purposes, which typically involve different engineering trade-offs to take advantage of either the heat or the neutrons produced by the fission chain reaction:

There was an error working with the wiki: Code[27]s are intended to produce heat for nuclear power, either as part of a Electricity generation or a local power system such as a There was an error working with the wiki: Code[79].

There was an error working with the wiki: Code[80]s are intended to produce neutrons and/or activate radioactive sources for scientific, medical, engineering, or other research purposes.

There was an error working with the wiki: Code[28]s are intended to produce nuclear fuels in bulk from more abundant There was an error working with the wiki: Code[81]. The most common type makes 239Pu (a nuclear fuel) from the naturally very abundant 238U (not a nuclear fuel).

While, in principle, all fission reactors can act in all three capacities, in practice the tasks lead to conflicting engineering goals and most reactors have been built with only one of the above tasks in mind. (There are several early counter-examples, such as the There was an error working with the wiki: Code[29] There was an error working with the wiki: Code[30], now decommissioned). Power reactors generally convert the kinetic energy of fission products into heat, which is used to heat a There was an error working with the wiki: Code[82] and drive a Heat engine that generates mechanical or electrical power. The working fluid is usually water with a steam turbine, but some designs use other materials such as gaseous There was an error working with the wiki: Code[83]. Research reactors produce neutrons that are used in various ways, with the heat of fission being treated as an unavoidable waste product. Breeder reactors are a specialized form of research reactor, with the caveat that the sample being irradiated is usually the fuel itself, a mixture of 238U and 235U.

For a more detailed description of the physics and operating principles of critical fission reactors, see There was an error working with the wiki: Code[84]. For a description of their social, political, and environmental aspects, see There was an error working with the wiki: Code[85].

Fission bombs

One class of There was an error working with the wiki: Code[86], a fission bomb, otherwise known as an atomic bomb or atom bomb, is a fission reactor designed to liberate as much energy as possible as rapidly as possible, before the released energy causes the reactor to explode (and the chain reaction to stop). Development of nuclear weapons was the motivation behind early research into nuclear fission: the There was an error working with the wiki: Code[87] of the There was an error working with the wiki: Code[88] during There was an error working with the wiki: Code[89] carried out most of the early scientific work on fission chain reactions, culminating in the There was an error working with the wiki: Code[90] and There was an error working with the wiki: Code[91] bombs that were exploded over There was an error working with the wiki: Code[92] and There was an error working with the wiki: Code[93], There was an error working with the wiki: Code[94] in August of There was an error working with the wiki: Code[95].

Even the first fission bombs were thousands of times more There was an error working with the wiki: Code[31], destroying a large part of the city of There was an error working with the wiki: Code[96]. Modern nuclear weapons (which include a thermonuclear fusion as well as one or more fission stages) are literally hundreds of times more energetic for their weight than the first pure fission atomic bombs, so that a modern single missile warhead bomb weighing less than 1/8th as much as Little Boy (see for example There was an error working with the wiki: Code[97]) has a yield of 475,000 tons of TNT, and could bring destruction to 10 times the city area.

While the fundamental physics of the fission There was an error working with the wiki: Code[32], was intended to work by exploding fission bombs behind a massively padded vehicle!).

The There was an error working with the wiki: Code[98] importance of nuclear weapons is a major reason why the There was an error working with the wiki: Code[99] of nuclear fission is politically sensitive. Viable fission bomb designs are within the capabilities of bright undergraduates (see There was an error working with the wiki: Code[100]), but nuclear fuel to realize the designs is thought to be difficult to obtain (see There was an error working with the wiki: Code[101] and There was an error working with the wiki: Code[102]).'

History

The results of the bombardment of uranium by neutrons had proved interesting and puzzling. First studied by There was an error working with the wiki: Code[103] and his colleagues in There was an error working with the wiki: Code[104], they were not properly interpreted until several years later.

On There was an error working with the wiki: Code[33]-occupied Denmark in a small boat, along with thousands of other Danish Jews, in large scale operation.) Just before Bohr left Denmark, two of his colleagues, There was an error working with the wiki: Code[34], as previously used up to this point, was a term which was borrowed from biology, where it was and is used to describe the splitting of one living cell into two).

The occasion for this There was an error working with the wiki: Code[105] was the important discovery of There was an error working with the wiki: Code[106] and There was an error working with the wiki: Code[107] in Germany (published in Naturwissenschaften in early January There was an error working with the wiki: Code[108]) which proved that an isotope of barium was produced by neutron bombardment of uranium. Bohr had promised to keep the Meitner/Frisch interpretation secret until their paper was published to preserve priority, but on the boat he discussed it with There was an error working with the wiki: Code[109], but forgot to tell him to keep it secret. Rosenfeld immediately upon arrival told everyone at There was an error working with the wiki: Code[110], and from them the news spread by word of mouth to neighboring physicists including There was an error working with the wiki: Code[111] at There was an error working with the wiki: Code[112]. As a result of conversations among Fermi, There was an error working with the wiki: Code[113], and There was an error working with the wiki: Code[114], a search was undertaken at Columbia for the heavy pulses of ionization that would be expected from the flying fragments of the There was an error working with the wiki: Code[115] nucleus. On There was an error working with the wiki: Code[116], There was an error working with the wiki: Code[108], there was a conference on theoretical physics at There was an error working with the wiki: Code[118], sponsored jointly by the There was an error working with the wiki: Code[119] and the There was an error working with the wiki: Code[120] of Washington.

Fermi left New York to attend this meeting before the Columbia fission experiments had been tried. At the meeting Bohr and Fermi discussed the problem of fission, and in particular Fermi mentioned the possibility that neutrons might be emitted during the process. Although this was only a guess, its implication of the possibility of a nuclear chain reaction was obvious. "There was an error working with the wiki: Code[121]s" at that time were a known phenomenon in chemistry, but the analogous process in nuclear physics using neutrons had been foreseen as early as 1933 by There was an error working with the wiki: Code[122], although Szilard at that time had no idea with what materials the process might be initiated. Now, with the discovery of neutron-induced fission of heavy elements, a number of sensational articles were published in the press on the subject of nuclear chain reactions. Before the meeting in Washington was over, several other experiments to confirm fission had been initiated, and positive experimental confirmation was reported from four laboratories (There was an error working with the wiki: Code[123], Carnegie Institution of Washington, There was an error working with the wiki: Code[124], There was an error working with the wiki: Code[125]) in the There was an error working with the wiki: Code[126] There was an error working with the wiki: Code[127], issue of the Physical Review. By this time Bohr had heard that similar experiments had been made in his laboratory in Copenhagen about There was an error working with the wiki: Code[128]. (Letter by Frisch to Nature dated There was an error working with the wiki: Code[129] There was an error working with the wiki: Code[127], and appearing in the There was an error working with the wiki: Code[131] issue.) There was an error working with the wiki: Code[132] in There was an error working with the wiki: Code[133] had also published his first results in the Comptes Rendus of There was an error working with the wiki: Code[134] There was an error working with the wiki: Code[127]. From this time on there was a steady flow of papers on the subject of fission, so that by the time (There was an error working with the wiki: Code[136] There was an error working with the wiki: Code[127]) L. A. Turner of Princeton wrote a review article on the subject in the Reviews of Modern Physics nearly one hundred papers had appeared. Complete analysis and discussion of these papers have appeared in Turner's article and elsewhere.

A major focus of early fission research was on producing a controllable nuclear chain reaction, which would mark the first harnessing of nuclear power. This led to the development of There was an error working with the wiki: Code[138], the world's first man-made critical nuclear reactor (which used There was an error working with the wiki: Code[139], the only natural nuclear fuel available in macroscopic quantities), and then to the There was an error working with the wiki: Code[140] to develop a nuclear weapon.

Producing a fission chain reaction in uranium fuel is far from trivial. Early nuclear reactors did not use isotopically enriched uranium, and in consequence they were required to use large quantities of highly purified graphite as neutron moderation materials. Use of ordinary water (as opposed to There was an error working with the wiki: Code[35] the chain reaction).

Production of such materials at industrial scale had to be solved for nuclear power generation and weapons production to be accomplished. Up to There was an error working with the wiki: Code[141], the total amount of uranium metal produced in the USA was not more than a few grams and even this was of doubtful purity of metallic beryllium not more than a few kilograms concentrated deuterium oxide (There was an error working with the wiki: Code[142]) not more than a few kilograms and finally carbon had never been produced in quantity with anything like the purity required of a moderator.

The problem of producing large amounts of high purity uranium was solved by There was an error working with the wiki: Code[143] using the There was an error working with the wiki: Code[144] process. There was an error working with the wiki: Code[145] was established in 1942 to produce the large amounts of natural (unenriched) uranium that would be necessary for the research to come. The success of the There was an error working with the wiki: Code[146] which used unenriched (natural) uranium, like all of the atomic "piles" which produced the plutonium for the atomic bomb, was also due specifically to Szilard's realization that very pure graphite could be used for the moderator of even natural uranium "piles". In wartime Germany, failure to appreciate the qualities of very pure graphite led to reactor designs dependent on heavy water, which in turn was denied the Germans by allied attacks in Norway, where There was an error working with the wiki: Code[147] was produced. These difficulties prevented the Nazis from building a nuclear reactor capable of criticality during the war.

Unknown until 1972, when French physicist There was an error working with the wiki: Code[36], nature had beaten humans to the punch by engaging in large-scale uranium fission chain reactions, some 2,000 million years in the past. This ancient process was able to use normal water as a moderator, only because 2,000 million years in the past, natural uranium was "enriched" with the shorter-lived fissile isotope 235U, as compared with the natural uranium available today. For more detail on the early development of There was an error working with the wiki: Code[148]s and There was an error working with the wiki: Code[149]s, see There was an error working with the wiki: Code[150].

External srticles and references

The Effects of Nuclear Weapons

Annotated bibliography for nuclear fission from the Alsos Digital Library, alsos.wlu.edu search

The Discovery of Nuclear Fission Historical account complete with audio and teacher's guides from the American Institute of Physics History Center

atomicarchive.com Nuclear Fission Explained

There was an error working with the wiki: Code[1], Wikipedia: The Free Encyclopedia. Wikimedia Foundation.

Nuclear Files.org What is Nuclear Fission?

Nuclear Fission Animation

The nuclear fission process A simple explanation of the process of nuclear fission

Comments