Plutonium-240

Last updated
Plutonium-240, 240Pu
General
Symbol 240Pu
Names plutonium-240, 240Pu, Pu-240
Protons (Z)94
Neutrons (N)146
Nuclide data
Natural abundance Trace
Half-life (t1/2)6561(7) years [1]
Isotope mass 240.0538135(20) [2] Da
Decay modes
Decay mode Decay energy (MeV)
Alpha decay 5.25575(14) [2]
Isotopes of plutonium
Complete table of nuclides

Plutonium-240 (240
Pu
 or Pu-240) is an isotope of plutonium formed when plutonium-239 captures a neutron. The detection of its spontaneous fission led to its discovery in 1944 at Los Alamos and had important consequences for the Manhattan Project. [3]

Contents

240Pu undergoes spontaneous fission as a secondary decay mode at a small but significant rate. The presence of 240Pu limits plutonium's use in a nuclear bomb, because the neutron flux from spontaneous fission initiates the chain reaction prematurely, causing an early release of energy that physically disperses the core before full implosion is reached. [4] [5] It decays by alpha emission to uranium-236.

Nuclear properties

About 62% to 73% of the time when 239Pu captures a neutron, it undergoes fission; the remainder of the time, it forms 240Pu. The longer a nuclear fuel element remains in a nuclear reactor, the greater the relative percentage of 240Pu in the fuel becomes.

The isotope 240Pu has about the same thermal neutron capture cross section as 239Pu (289.5±1.4 vs. 269.3±2.9 barns), [6] [7] but only a tiny thermal neutron fission cross section (0.064 barns). When the isotope 240Pu captures a neutron, it is about 4500 times more likely to become plutonium-241 than to fission. In general, isotopes of odd mass numbers are more likely to absorb a neutron, and can undergo fission upon neutron absorption more easily than isotopes of even mass number. Thus, even mass isotopes tend to accumulate, especially in a thermal reactor.

Nuclear weapons

The inevitable presence of some 240Pu in a plutonium-based nuclear warhead core complicates its design, and pure 239Pu is considered optimal. [8] This is for a few reasons:

The spontaneous fission problem was extensively studied by the scientists of the Manhattan Project during World War II. [9] It blocked the use of plutonium in gun-type nuclear weapons in which the assembly of fissile material into its optimal supercritical mass configuration can take up to a millisecond to complete, and made it necessary to develop implosion-style weapons where the assembly occurs in a few microseconds. [10] Even with this design, it was estimated in advance of the Trinity test that 240Pu impurity would cause a 12% chance of the explosion failing to reach its maximum yield. [8]

The minimization of the amount of 240
Pu
, as in weapons-grade plutonium (less than 7% 240Pu) is achieved by reprocessing the fuel after just 90 days of use. Such rapid fuel cycles are highly impractical for civilian power reactors and are normally only carried out with dedicated weapons plutonium production reactors. Plutonium from spent civilian power reactor fuel typically has under 70% 239Pu and around 26% 240
Pu
, the rest being made up of other plutonium isotopes, making it more difficult to use it for the manufacturing of nuclear weapons. [4] [8] [11] [12] For nuclear weapon designs introduced after the 1940s, however, there has been considerable debate over the degree to which 240
Pu
 poses a barrier for weapons construction; see the article Reactor-grade plutonium.

See also

Related Research Articles

<span class="mw-page-title-main">Nuclear fission</span> Nuclear reaction splitting an atom into multiple parts

Nuclear fission is a reaction in which the nucleus of an atom splits into two or more smaller nuclei. The fission process often produces gamma photons, and releases a very large amount of energy even by the energetic standards of radioactive decay.

<span class="mw-page-title-main">Nuclear chain reaction</span> When one nuclear reaction causes more

In nuclear physics, a nuclear chain reaction occurs when one single nuclear reaction causes an average of one or more subsequent nuclear reactions, thus leading to the possibility of a self-propagating series or "positive feedback loop" of these reactions. The specific nuclear reaction may be the fission of heavy isotopes. A nuclear chain reaction releases several million times more energy per reaction than any chemical reaction.

In nuclear engineering, fissile material is material that can undergo nuclear fission when struck by a neutron of low energy. A self-sustaining thermal chain reaction can only be achieved with fissile material. The predominant neutron energy in a system may be typified by either slow neutrons or fast neutrons. Fissile material can be used to fuel thermal-neutron reactors, fast-neutron reactors and nuclear explosives.

<span class="mw-page-title-main">Uranium-238</span> Isotope of uranium

Uranium-238 is the most common isotope of uranium found in nature, with a relative abundance of 99%. Unlike uranium-235, it is non-fissile, which means it cannot sustain a chain reaction in a thermal-neutron reactor. However, it is fissionable by fast neutrons, and is fertile, meaning it can be transmuted to fissile plutonium-239. 238U cannot support a chain reaction because inelastic scattering reduces neutron energy below the range where fast fission of one or more next-generation nuclei is probable. Doppler broadening of 238U's neutron absorption resonances, increasing absorption as fuel temperature increases, is also an essential negative feedback mechanism for reactor control.

Uranium-233 is a fissile isotope of uranium that is bred from thorium-232 as part of the thorium fuel cycle. Uranium-233 was investigated for use in nuclear weapons and as a reactor fuel. It has been used successfully in experimental nuclear reactors and has been proposed for much wider use as a nuclear fuel. It has a half-life of 160,000 years.

<span class="mw-page-title-main">Plutonium-239</span> Isotope of plutonium

Plutonium-239 is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 is also used for that purpose. Plutonium-239 is also one of the three main isotopes demonstrated usable as fuel in thermal spectrum nuclear reactors, along with uranium-235 and uranium-233. Plutonium-239 has a half-life of 24,110 years.

Promethium (61Pm) is an artificial element, except in trace quantities as a product of spontaneous fission of 238U and 235U and alpha decay of 151Eu, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no stable isotopes. It was first synthesized in 1945.

Caesium (55Cs) has 41 known isotopes, the atomic masses of these isotopes range from 112 to 152. Only one isotope, 133Cs, is stable. The longest-lived radioisotopes are 135Cs with a half-life of 1.33 million years, 137
Cs
 with a half-life of 30.1671 years and 134Cs with a half-life of 2.0652 years. All other isotopes have half-lives less than 2 weeks, most under an hour.

Tin (50Sn) is the element with the greatest number of stable isotopes. Moreover, tin is not only the element with the greatest number of observationally stable isotopes, but also the element with the greatest number of theoretically stable isotopes. This is probably related to the fact that 50 is a "magic number" of protons. In addition, twenty-nine unstable tin isotopes are known, including tin-100 (100Sn) and tin-132 (132Sn), which are both "doubly magic". The longest-lived tin radioisotope is tin-126 (126Sn), with a half-life of 230,000 years. The other 28 radioisotopes have half-lives of less than a year.

Naturally occurring palladium (46Pd) is composed of six stable isotopes, 102Pd, 104Pd, 105Pd, 106Pd, 108Pd, and 110Pd, although 102Pd and 110Pd are theoretically unstable. The most stable radioisotopes are 107Pd with a half-life of 6.5 million years, 103Pd with a half-life of 17 days, and 100Pd with a half-life of 3.63 days. Twenty-three other radioisotopes have been characterized with atomic weights ranging from 90.949 u (91Pd) to 128.96 u (129Pd). Most of these have half-lives that are less than a half an hour except 101Pd, 109Pd, and 112Pd.

Neptunium (93Np) is usually considered an artificial element, although trace quantities are found in nature, so a standard atomic weight cannot be given. Like all trace or artificial elements, it has no stable isotopes. The first isotope to be synthesized and identified was 239Np in 1940, produced by bombarding 238
U
 with neutrons to produce 239
U
, which then underwent beta decay to 239
Np
.

Plutonium (94Pu) is an artificial element, except for trace quantities resulting from neutron capture by uranium, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no stable isotopes. It was synthesized long before being found in nature, the first isotope synthesized being 238Pu in 1940. Twenty plutonium radioisotopes have been characterized. The most stable are plutonium-244 with a half-life of 80.8 million years, plutonium-242 with a half-life of 373,300 years, and plutonium-239 with a half-life of 24,110 years. All of the remaining radioactive isotopes have half-lives that are less than 7,000 years. This element also has eight meta states; all have half-lives of less than one second.

<span class="mw-page-title-main">Minor actinide</span> Category of elements in spent nuclear fuel

A minor actinide is an actinide, other than uranium or plutonium, found in spent nuclear fuel. The minor actinides include neptunium, americium, curium, berkelium, californium, einsteinium, and fermium. The most important isotopes of these elements in spent nuclear fuel are neptunium-237, americium-241, americium-243, curium-242 through -248, and californium-249 through -252.

<span class="mw-page-title-main">Spent nuclear fuel</span> Nuclear fuel thats been irradiated in a nuclear reactor

Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor. It is no longer useful in sustaining a nuclear reaction in an ordinary thermal reactor and, depending on its point along the nuclear fuel cycle, it will have different isotopic constituents than when it started.

<span class="mw-page-title-main">Weapons-grade nuclear material</span> Nuclear material pure enough to be used for nuclear weapons

Weapons-grade nuclear material is any fissionable nuclear material that is pure enough to make a nuclear weapon and has properties that make it particularly suitable for nuclear weapons use. Plutonium and uranium in grades normally used in nuclear weapons are the most common examples.

Plutonium-241 is an isotope of plutonium formed when plutonium-240 captures a neutron. Like some other plutonium isotopes, 241Pu is fissile, with a neutron absorption cross section about one-third greater than that of 239Pu, and a similar probability of fissioning on neutron absorption, around 73%. In the non-fission case, neutron capture produces plutonium-242. In general, isotopes with an odd number of neutrons are both more likely to absorb a neutron, and more likely to undergo fission on neutron absorption, than isotopes with an even number of neutrons.

Uranium-236 (236U) is an isotope of uranium that is neither fissile with thermal neutrons, nor very good fertile material, but is generally considered a nuisance and long-lived radioactive waste. It is found in spent nuclear fuel and in the reprocessed uranium made from spent nuclear fuel.

Plutonium-242 is one of the isotopes of plutonium, the second longest-lived, with a half-life of 375,000 years. The half-life of 242Pu is about 15 times that of 239Pu; so it is one-fifteenth as radioactive, and not one of the larger contributors to nuclear waste radioactivity. 242Pu's gamma ray emissions are also weaker than those of the other isotopes.

Reactor-grade plutonium (RGPu) is the isotopic grade of plutonium that is found in spent nuclear fuel after the uranium-235 primary fuel that a nuclear power reactor uses has burnt up. The uranium-238 from which most of the plutonium isotopes derive by neutron capture is found along with the U-235 in the low enriched uranium fuel of civilian reactors.

Long-lived fission products (LLFPs) are radioactive materials with a long half-life produced by nuclear fission of uranium and plutonium. Because of their persistent radiotoxicity, it is necessary to isolate them from humans and the biosphere and to confine them in nuclear waste repositories for geological periods of time.

References

  1. ↑ Audi, Georges; Bersillon, Olivier; Blachot, Jean; Wapstra, Aaldert Hendrik (December 2003). "The Nubase evaluation of nuclear and decay properties". Nuclear Physics A. 729 (1): 3–128. Bibcode:2003NuPhA.729....3A. CiteSeerX   10.1.1.692.8504 . doi:10.1016/j.nuclphysa.2003.11.001.
  2. 1 2 Audi, Georges; Wapstra, Aaldert Hendrik; Thibault, Catherine (December 2003). "The Ame2003 atomic mass evaluation". Nuclear Physics A. 729 (1): 337–676. Bibcode:2003NuPhA.729..337A. doi:10.1016/j.nuclphysa.2003.11.003.
  3. ↑ Farwell, G. W. (1990). "Emilio Segre, Enrico Fermi, Pu-240, and the atomic bomb". Symposium to Commemorate the 50th Anniversary of the Discovery of Transuranium Elements.
  4. 1 2 Şahin, Sümer (1981). "Remarks On The Plutonium-240 Induced Pre-Ignition Problem In A Nuclear Device". Nuclear Technology. 54 (1): 431–432. doi:10.13182/NT81-A32795. The energy yield of a nuclear explosive decreases by one and two orders of magnitude if the 240 Pu content increases from 5 (nearly weapons-grade plutonium) to 15 and 25%, respectively
  5. 1 2 Bodansky, David (2007). "Nuclear Bombs, Nuclear Energy, and Terrorism". Nuclear Energy: Principles, Practices, and Prospects. Springer Science & Business Media. ISBN   978-0-387-26931-3.
  6. ↑ Mughabghab, S. F. (2006). Atlas of neutron resonances : resonance parameters and thermal cross sections Z=1-100. Amsterdam: Elsevier. ISBN   978-0-08-046106-9.
  7. ↑ "Actinide data: Thermal neutron cross sections, resonance integrals, and Westcott factors". Nuclear Data for Safeguards. International Atomic Energy Agency . Retrieved 2016-09-11.
  8. 1 2 3 4 5 6 Mark, J. Carson; Hippel, Frank von; Lyman, Edward (2009-10-30). "Explosive Properties of Reactor-Grade Plutonium" (PDF). Science & Global Security. 17 (2–3): 170–185. Bibcode:2009S&GS...17..170M. doi:10.1080/08929880903368690. ISSN   0892-9882. S2CID   219716695.
  9. ↑ Chamberlain, O.; Farwell, G. W.; Segrè, E. (1954). "Pu-240 and Its Spontaneous Fission". Physical Review. 94 (1): 156. Bibcode:1954PhRv...94..156C. doi:10.1103/PhysRev.94.156.
  10. ↑ Hoddeson, Lillian (1993). "The Discovery of Spontaneous Fission in Plutonium during World War II". Historical Studies in the Physical and Biological Sciences. 23 (2): 279–300. doi:10.2307/27757700. JSTOR   27757700.
  11. ↑ Şahin, Sümer; Ligou, Jacques (1980). "The Effect of the Spontaneous Fission of Plutonium-240 on the Energy Release in a Nuclear Explosive". Nuclear Technology. 50 (1): 88. doi:10.13182/NT80-A17072.
  12. ↑ Şahi̇n, Sümer (1978). "The effect of Pu-240 on neutron lifetime in nuclear explosives". Annals of Nuclear Energy. 5 (2): 55–58. doi:10.1016/0306-4549(78)90104-4.
Lighter:
plutonium-239 		Plutonium-240 is an
isotope of plutonium 		Heavier:
plutonium-241
Decay product of:
curium-244 (α)
neptunium-240 (β −)		 Decay chain
of plutonium-240		 Decays to:
uranium-236 (α)
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