Welcome to the MedLibrary.org Wikipedia Supplement on Isotopes of plutonium -- Please Click to Return to Front Page

Plutonium (Pu) has no stable isotopes. A standard atomic mass cannot be given.

Decay modes

Twenty plutonium radioisotopes have been characterized. The most stable are Pu-244, with a half-life of 80.8 million years, Pu-242, with a half-life of 373,300 years, and Pu-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, though none are very stable (all have half-lives less than one second).

The isotopes of plutonium range in atomic weight from 228.0387 u (Pu-228) to 247.074 u (Pu-247). The primary decay modes before the most stable isotope, Pu-244, are spontaneous fission and alpha emission; the primary mode after is beta emission. The primary decay products before Pu-244 are uranium and neptunium isotopes (neglecting the wide range of daughter nuclei created by fission processes), and the primary products after are americium isotopes.

Production and uses

A pellet of plutonium-238, glowing under its own light, used for radioisotope thermoelectric generators.

Transmutation flow between ²³⁸Pu and ²⁴⁴Cm in LWR.¹
Transmutation speed not shown and varies greatly by nuclide.
²⁴⁵Cm–²⁴⁸Cm are long-lived with negligible decay.

Pu-239, a fissile isotope which is the second most used nuclear fuel in nuclear reactors after U-235, and the most used fuel in the fission portion of nuclear weapons, is produced from U-238 by neutron capture followed by two beta decays.

Pu-240, Pu-241, Pu-242 are produced by further neutron capture. The odd-mass isotopes Pu-239 and Pu-241 have about a 3/4 chance of undergoing fission on capture of a thermal neutron and about a 1/4 chance of retaining the neutron and becoming the following isotope. The even-mass isotopes are fertile material but not fissile and also have a lower overall probability (cross section) of neutron capture; therefore, they tend to accumulate in nuclear fuel used in a thermal reactor, the design of all nuclear power plants today. In plutonium that has been used a second time in thermal reactors in MOX fuel, Pu-240 may even be the most common isotope. All plutonium isotopes and other actinides, however, are fissionable with fast neutrons. Pu-240 does have a moderate thermal neutron absorption cross section, so that Pu-241 production in a thermal reactor becomes a significant fraction as large as Pu-239 production.

Pu-241 has a halflife of 14 years, and has slightly higher thermal neutron cross sections than Pu-239 for both fission and absorption. While nuclear fuel is being used in a reactor, a Pu-241 nucleus is much more likely to fission or to capture a neutron than to decay. Pu-241 accounts for a significant proportion of fissions in thermal reactor fuel that has been used for some time. However, in spent nuclear fuel that does not quickly undergo nuclear reprocessing but instead is cooled for years after use, much or most of the Pu-241 will beta decay to americium-241, one of the minor actinides, a strong alpha emitter, and difficult to use in thermal reactors.

Pu-242 has a particularly low cross section for thermal neutron capture; and it takes four neutron absorptions to become another fissile isotope (either curium-245 or Pu-241) and fission. Even then, there is a chance either of those two fissile isotopes will fail to fission but instead absorb the fourth neutron, becoming curium-246 (on the way to even heavier actinides like californium, which is a neutron emitter by spontaneous fission and difficult to handle) or becoming Pu-242 again; so the mean number of neutrons absorbed before fission is even higher than 4. Therefore Pu-242 is particularly unsuited to recycling in a thermal reactor and would be better used in a fast reactor where it can be fissioned directly. However, Pu-242's low cross section means that relatively little of it will be transmuted during one cycle in a thermal reactor. Pu-242's halflife is about 15 times as long as Pu-239's halflife; therefore it is 1/15 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.²

Pu-243 has a halflife of only 5 hours, beta decaying to americium-243. Because Pu-243 has little opportunity to capture an additional neutron before decay, the nuclear fuel cycle does not produce the extremely long-lived Pu-244 in significant quantity.

Pu-238 is not normally produced in as large quantity by the nuclear fuel cycle, but some is produced from neptunium-237 by neutron capture (this reaction can also be used with purified neptunium to produce Pu-238 relatively free of other plutonium isotopes for use in radioisotope thermoelectric generators), by the (n,2n) reaction of fast neutrons on Pu-239, or by alpha decay of curium-242 which is produced by neutron capture from Am-241. It has significant thermal neutron cross section for fission, but is more likely to capture a neutron and become Pu-239.

Manufacture

Pu-240, Pu-241 and Pu-242

The activation cross section for ²³⁹Pu is 270 barns, while the fission cross section is 747 barns for thermal neutrons. The higher plutonium isotopes are created when the uranium fuel is used for a long time. It is the case that for high burnup used fuel that the concentrations of the higher plutonium isotopes will be higher than the low burnup fuel which is reprocessed to obtain bomb grade plutonium.

Pu-239

Main article: Plutonium-239

Plutonium-239 is one of the three fissile materials used for the production of nuclear weapons and in some nuclear reactors as a source of energy. The other fissile materials are uranium-235 and uranium-233. Plutonium-239 is virtually nonexistent in nature. It is made by bombarding uranium-238 with neutrons in a nuclear reactor. Uranium-238 is present in quantity in most reactor fuel; hence plutonium-239 is continuously made in these reactors. Since plutonium-239 can itself be split by neutrons to release energy, plutonium-239 provides a portion of the energy generation in a nuclear reactor.

A ring of weapons-grade electrorefined plutonium, with 99.96% purity. This 5.3 kg ring is enough plutonium for use in an efficient nuclear weapon.

Pu-238

Main article: Plutonium-238

There are small amounts of Pu-238 in the plutonium of usual plutonium-producing reactors. However, isotopic separation would be quite expensive compared to another method: when a U-235 atom captures a neutron, it is converted to an excited state of U-236. Some of the excited U-236 nuclei undergo fission, but some decay to the ground state of U-236 by emitting gamma radiation. Further neutron capture creates U-237 which has a half-life of 7 days and thus quickly decays to Np-237. Since nearly all neptunium is produced in this way or consists of isotopes which decay quickly, one gets nearly pure Np-237 by chemical separation of neptunium. After this chemical separation, Np-237 is again irradiated by reactor neutrons to be converted to Np-238 which decays to Pu-238 with a half-life of 2 days.

Pu-240 as obstacle to nuclear weapons

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 nuclear bomb potential because the neutron flux from spontaneous fission, initiates the chain reaction prematurely and reduces the bomb's power by exploding the core before full implosion is reached. Plutonium consisting of more than about 90% Pu-239 is called weapons-grade plutonium; plutonium from spent nuclear fuel from commercial power reactors generally contains at least 20% Pu-240 and is called reactor-grade plutonium. However, modern nuclear weapons use fusion boosting which mitigates the predetonation problem; if the pit can generate a nuclear weapon yield of even a fraction of a kiloton, which is enough to start deuterium-tritium fusion, the resulting burst of neutrons will fission enough plutonium to ensure a yield of tens of kilotons.

Pu-240 contamination is the reason plutonium weapons must use the implosion method. Theoretically, pure Pu-239 could be used in a gun-type nuclear weapon, but achieving this level of purity is prohibitively difficult. Pu-240 contamination has proven a mixed blessing to nuclear weapons design. While it created delays and headaches during the Manhattan Project because of the need to develop implosion technology, those very same difficulties are currently a barrier to nuclear proliferation. Implosion devices are also inherently more efficient and less prone toward accidental detonation than are gun-type weapons.

Table

Notes

• Values marked # are not purely derived from experimental data, but at least partly from systematic trends. Spins with weak assignment arguments are enclosed in parentheses.
• Uncertainties are given in concise form in parentheses after the corresponding last digits. Uncertainty values denote one standard deviation, except isotopic composition and standard atomic mass from IUPAC which use expanded uncertainties.

References

• Isotope masses from Ame2003 Atomic Mass Evaluation by G. Audi, A.H. Wapstra, C. Thibault, J. Blachot and O. Bersillon in Nuclear Physics A729 (2003).
• Isotopic compositions and standard atomic masses from Atomic weights of the elements. Review 2000 (IUPAC Technical Report). Pure Appl. Chem. Vol. 75, No. 6, pp. 683-800, (2003) and Atomic Weights Revised (2005).
• Half-life, spin, and isomer data selected from these sources. Editing notes on this article's talk page.
  • Audi, Bersillon, Blachot, Wapstra. The Nubase2003 evaluation of nuclear and decay properties, Nuc. Phys. A 729, pp. 3-128 (2003).
  • National Nuclear Data Center, Brookhaven National Laboratory. Information extracted from the NuDat 2.1 database (retrieved Sept. 2005).
  • David R. Lide (ed.), Norman E. Holden in CRC Handbook of Chemistry and Physics, 85th Edition, online version. CRC Press. Boca Raton, Florida (2005). Section 11, Table of the Isotopes.

Wikipedia content modification information:

• This page was last modified on 26 October 2008, at 23:54.

Wikipedia Authorship and Review

Wikipedia content provided here is not reviewed directly by MedLibrary.org. Wikipedia content is authored by an open community of volunteers and is not produced by or in any way affiliated with MedLibrary.org.

Wikipedia Usage Guidelines

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article on "Isotopes of plutonium".

The URL for this specific entry is:

• http://medlibrary.org/medwiki/Isotopes_of_plutonium

All Wikipedia text is available under the terms of the GNU Free Documentation License. (See Copyrights for details). Wikipedia® is a registered trademark of the Wikimedia Foundation, Inc.