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Name, symbol Plutonium-240,240Pu
Neutrons 146
Protons 94
Nuclide data
Natural abundance 0 (Artificial)
Half-life 6561(7) years[1]
Isotope mass 240.0538135 (20) [2] u
Decay mode Decay energy
Alpha decay 5.25575(14)[2] MeV

Plutonium-240 (240
/Pu-240) is an isotope of the actinide metal 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]

Pu-240 undergoes spontaneous fission as a secondary decay mode at a small but significant rate. The presence of Pu-240 limits the 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]

Nuclear properties[edit]

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

The isotope 240
has about the same thermal neutron capture cross section as 239
(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 240
captures a neutron, it is about 4500 times more likely to be become plutonium-241 than to fission. In general, isotopes of odd mass numbers are both 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[edit]

For producing weapon grade plutonium, the irradiated fuel needs to be as low in 240
as possible, usually less than 7% of the total plutonium.[8] This is because 240
undergoes spontaneous fission, and the resultant released neutrons from this process can cause the weapon to fizzle.[5] This naturally occurring phenomenon was extensively studied by the scientists of the Manhattan Project during World War II.[9] It threatened the design of gun-type nuclear weapons in which the assembly of fissile material into a supercritical mass is slow in comparison with the time-scale of the explosion.[10]

The minimization of the amount of 240
present in weapons grade plutonium 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% 239
and around 26%240
, the rest being made up of other plutonium isotopes, making it extremely difficult but not impossible to use it for the manufacturing of nuclear weapons.[11][4][8][12]

See also[edit]


  1. ^ Audi, G.; Bersillon, O.; Blachot, J.; Wapstra, A.H. (December 2003). "The Nubase evaluation of nuclear and decay properties". Nuclear Physics A. 729 (1): 3–128. doi:10.1016/j.nuclphysa.2003.11.001. 
  2. ^ a b Audi, G.; Wapstra, A.H.; Thibault, C. (December 2003). "The Ame2003 atomic mass evaluation". Nuclear Physics A. 729 (1): 337–676. 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 Commenorate the 50th Anniversary of the Discovery of Transuranium Elements. 
  4. ^ a b Şahin, Sümer (1981). "Remarks On The Plutonium-240 Induced Pre-Ignition Problem In A Nuclear Device". Nuclear Technology. 54 (1): 431–432. 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. ^ a b 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. ^ a b 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. doi:10.1080/08929880903368690. ISSN 0892-9882. 
  9. ^ Chamberlain, O.; Farwell, G. W.; Segrè, E. (1954). "Pu-240 and Its Spontaneous Fission". Physical Review. 94 (1): 156–156. 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. 
  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[edit]

Plutonium-240 is an
isotope of plutonium
Decay product of:
plutonium-239 (neutron capture)
curium-244 (α)
neptunium-240 (β-)
Decay chain
of plutonium-240
Decays to:
uranium-236 (α)