Plutonium-240
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Plutonium-240 ( or Pu-240) is an isotope of plutonium formed when plutonium-239 captures a
neutron The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...
. The detection of its spontaneous fission led to its discovery in 1944 at Los Alamos and had important consequences for the
Manhattan Project The Manhattan Project was a research and development program undertaken during World War II to produce the first nuclear weapons. It was led by the United States in collaboration with the United Kingdom and Canada. From 1942 to 1946, the ...
. 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 A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission or atomic bomb) or a combination of fission and fusion reactions (thermonuclear weapon), producing a nuclear exp ...
, 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 239Pu captures a neutron, it undergoes fission; the remainder of the time, it forms 240Pu. The longer a
nuclear fuel Nuclear fuel refers to any substance, typically fissile material, which is used by nuclear power stations or other atomic nucleus, nuclear devices to generate energy. Oxide fuel For fission reactors, the fuel (typically based on uranium) is ...
element remains in a
nuclear reactor A nuclear reactor is a device used to initiate and control a Nuclear fission, fission nuclear chain reaction. They are used for Nuclear power, commercial electricity, nuclear marine propulsion, marine propulsion, Weapons-grade plutonium, weapons ...
, 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 ( vs. barns), 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 number The mass number (symbol ''A'', from the German word: ''Atomgewicht'', "atomic weight"), also called atomic mass number or nucleon number, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus. It is appro ...
s 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. This is for a few reasons: * 240Pu 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 240Pu would thus randomly cause fizzles, with an explosive yield well below the potential yield. * Isotopes besides 239Pu release significantly more radiation, which complicates its handling by workers. * Isotopes besides 239Pu 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 The Manhattan Project was a research and development program undertaken during World War II to produce the first nuclear weapons. It was led by the United States in collaboration with the United Kingdom and Canada. From 1942 to 1946, the ...
during
World War II World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...
. 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 240Pu impurity would cause a 12% chance of the explosion failing to reach its maximum yield. The minimization of the amount of , 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% , 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 poses a barrier for weapons construction; see the article Reactor-grade plutonium.


See also

* Burnup * Isotopes of plutonium


References


External links


NLM Hazardous Substances Databank – Plutonium, Radioactive
{{Isotopes of plutonium Actinides Isotopes of plutonium Fertile materials