 | |
| General | |
|---|---|
| Symbol | 240Pu |
| Names | plutonium-240 |
| Protons(Z) | 94 |
| Neutrons(N) | 146 |
| Nuclide data | |
| Natural abundance | Trace |
| Half-life(t1/2) | 6561(7) years[1] |
| Isotope mass | 240.053812[2]Da |
| Decay products | 236U |
| Decay modes | |
| Decay mode | Decay energy (MeV) |
| Alpha decay | 5.256[3] |
| Isotopes of plutonium Complete table of nuclides | |
Plutonium-240 (240
Pu orPu-240) is anisotope of plutonium formed whenplutonium-239 captures aneutron without undergoing fission. The detection of itsspontaneous fission led to its discovery in 1944 atLos Alamos and had important consequences for theManhattan Project.[4]
As with the other major plutonium isotopes, the normal decay leads to a more-stable isotope of uranium (236U) and in effect no further decay chain on human timescales. Over geologic time it would follow thethorium series.
240Pu undergoes spontaneous fission as a secondary decay mode at a small but significant rate. The presence of240Pu limits plutonium's use in anuclear bomb, because the neutron flux from spontaneous fission initiates thechain reaction prematurely, causing an early release of energy that physically disperses the core before fullimplosion is reached (a "fizzle").[5][6]
About 62% to 73% of the time when239Pucaptures a neutron, it undergoesfission; the remainder of the time, it forms240Pu. The longer anuclear fuel element remains in anuclear reactor, the greater the relative percentage of240Pu in the fuel becomes.
The isotope240Pu has about the same thermal neutron capturecross section as239Pu (289.5±1.4 vs.269.3±2.9barns),[7][8] but only a tiny thermal neutron fission cross section (0.064 barns). When the isotope240Pu captures a neutron, it is about 4500 times more likely to becomeplutonium-241 than to fission. In general, isotopes of oddmass 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 athermal reactor.
The inevitable presence of some240Pu in a plutonium-based nuclear warhead core complicates its design, and pure239Pu is considered optimal.[9] This is for a few reasons:
The spontaneous fission problem was extensively studied by the scientists of theManhattan Project duringWorld War II.[10] It blocked the use of plutonium ingun-type nuclear weapons in which the assembly offissile material into its optimalsupercritical mass configuration can take up to a millisecond to complete, and made it necessary to developimplosion-style weapons where the assembly occurs in a few microseconds.[11] Even with this design, it was estimated in advance of theTrinity test that240Pu impurity would cause a 12% chance of the explosion failing to reach its maximum yield.[9]
The minimization of the amount of240
Pu, as inweapons-grade plutonium (less than 7%240Pu) is achieved byreprocessing 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 (the short-lived 238 and 241 are problematic with respect to handling, storage, and decay heat), making it more difficult to use it for the manufacturing of nuclear weapons.[5][9][12][13] For nuclear weapon designs introduced after the 1940s, however, there has been considerable debate over the degree to which240
Pu poses a barrier for weapons construction; see the articleReactor-grade plutonium.
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
| Lighter: plutonium-239 | Plutonium-240 is an isotope ofplutonium | Heavier: plutonium-241 |
| Decay product of: curium-244 (α) neptunium-240(β−) | Decay chain of plutonium-240 | Decays to: uranium-236 (α) |