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Plutonium-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.

Pu undergoes spontaneous fission as a secondary decay mode at a small but significant rate. The presence of Pu 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. It decays by alpha emission to uranium-236.

Nuclear properties

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

The isotope Pu has about the same thermal neutron capture cross section as Pu (289.5±1.4 vs. 269.3±2.9 barns), but only a tiny thermal neutron fission cross section (0.064 barns). When the isotope Pu 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 Pu in a plutonium-based nuclear warhead core complicates its design, and pure Pu is considered optimal. This is for a few reasons:

• Pu has a high rate of spontaneous fission. A single stray neutron that is introduced while the core is supercritical will cause it to detonate almost immediately, even before it has been crushed to an optimal configuration. The presence of Pu would thus randomly cause fizzles, with an explosive yield well below the potential yield.
• Isotopes besides Pu release significantly more radiation, which complicates its handling by workers.
• Isotopes besides Pu produce more decay heat, which can cause phase change distortions of the precision core if allowed to build up.

The spontaneous fission problem was extensively studied by the scientists of the Manhattan Project during World War II. 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. Even with this design, it was estimated in advance of the Trinity test that Pu impurity would cause a 12% chance of the explosion failing to reach its maximum yield.

The minimization of the amount of
Pu
, as in weapons-grade plutonium (less than 7% Pu) 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% Pu and around 26%
Pu
, the rest being made up of other plutonium isotopes, making it more difficult to use it for the manufacturing of nuclear weapons. For nuclear weapon designs introduced after the 1940s, however, there has been considerable debate over the degree to which
Pu
poses a barrier for weapons construction; see the article Reactor-grade plutonium.

See also

• Burnup
• Isotopes of plutonium

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. ^ 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. ^ 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. ^ 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. ^ 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.

External links

• NLM Hazardous Substances Databank – Plutonium, Radioactive

• Actinides
• Isotopes of plutonium
• Fertile materials