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Actinides and fission products by half-life
Actinides[2] by decay chain Half-life
range (y)
Fission products of 235U by yield[3]
4n 4n+1 4n+2 4n+3
4.5-7% 0.04-1.25% <0.001%
228RaNo 4-6 + 155Euþ
244Cm? 241Pu? 250Cf 227AcNo 10-29 90Sr 85Kr 113mCdþ
232U? 238Pu? 243Cm? 29-97 137Cs 151Smþ 121mSn
248Bk[4] 249Cf? 242mAm? 141-351

No fission products
have a half-life
in the range of
100-210 k years ...

241Am? 251Cf?[5] 430-900
226RaNo 247Bk 1.3 k - 1.6 k
240Pu 229Th 246Cm? 243Am? 4.7 k - 7.4 k
245Cm? 250Cm 8.3 k - 8.5 k
239Pu? 24.1 k
230ThNo 231PaNo 32 k - 76 k
236Np? 233U? 234UNo ? 99Tc? 126Sn
248Cm 242Pu 327 k - 375 k 79Se?
1.53 M 93Zr
237Np? 2.1 M - 6.5 M 135Cs? 107Pd
236U 247Cm? 15 M - 24 M 129I?
244Pu 80 M

... nor beyond 15.7 M years[6]

232ThNo 238UNo 235U?No 0.7 G - 14.1 G

Legend for superscript symbols
?  has thermal neutron capture cross section in the range of 8-50 barns
metastable isomer
No  primarily a naturally occurring radioactive material (NORM)
þ  neutron poison (thermal neutron capture cross section greater than 3k barns)
+  range 4-97 y: Medium-lived fission product
?  over 200,000 y: Long-lived fission product

Plutonium-241 (241Pu, Pu-241) is an isotope of plutonium formed when plutonium-240 captures a neutron. Like 239Pu but unlike 240Pu, 241Pu is fissile, with a neutron absorption cross section about 1/3 greater than 239Pu, and a similar probability of fissioning on neutron absorption, around 73%. In the non-fission case, neutron capture produces plutonium-242. In general, isotopes with an odd number of neutrons are both more likely to absorb a neutron, and more likely to undergo fission on neutron absorption, than isotopes with an even number of neutrons.

Decay to americium

241Pu has a half-life of 14 years, corresponding to a decay of about 5% of 241Pu nuclei over a one-year period. The longer spent nuclear fuel waits before reprocessing, the more 241Pu decays to americium-241, which is nonfissile (although fissionable by fast neutrons) and an alpha emitter with a halflife of 432 years; 241Am is a major contributor to the radioactivity of nuclear waste on a scale of hundreds or thousands of years.

Americium has lower valence and lower electronegativity than plutonium, neptunium or uranium, so in most nuclear reprocessing, Am tends to fractionate not with U, Np, or Pu, but with the alkaline fission products: lanthanides, strontium, caesium, barium, yttrium, and is therefore not recycled into nuclear fuel unless special efforts are made.

In a thermal reactor, 241Am captures a neutron to become americium-242, which quickly becomes curium-242 (or, 17.3% of the time, 242Pu) via beta decay. Both 242Cm and 242Pu are much less likely to absorb a neutron, and even less likely to fission; however, 242Cm is short-lived (half-life 160 days) and almost always undergoes alpha decay to 238Pu rather than capturing another neutron. In short, 241Am needs to absorb two neutrons before again becoming a fissile isotope.


  1. ^ Table de Radionucleides Pu-241, Laboratoire National Henri Becquerel
  2. ^ Plus radium (element 88). While actually a sub-actinide, it immediately precedes actinium (89) and follows a three-element gap of instability after polonium (84) where no nuclides have half-lives of at least four years (the longest-lived nuclide in the gap is radon-222 with a half life of less than four days). Radium's longest lived isotope, at 1,600 years, thus merits the element's inclusion here.
  3. ^ Specifically from thermal neutron fission of U-235, e.g. in a typical nuclear reactor.
  4. ^ Milsted, J.; Friedman, A. M.; Stevens, C. M. (1965). "The alpha half-life of berkelium-247; a new long-lived isomer of berkelium-248". Nuclear Physics. 71 (2): 299. Bibcode:1965NucPh..71..299M. doi:10.1016/0029-5582(65)90719-4.
    "The isotopic analyses disclosed a species of mass 248 in constant abundance in three samples analysed over a period of about 10 months. This was ascribed to an isomer of Bk248 with a half-life greater than 9 y. No growth of Cf248 was detected, and a lower limit for the ?- half-life can be set at about 104 y. No alpha activity attributable to the new isomer has been detected; the alpha half-life is probably greater than 300 y."
  5. ^ This is the heaviest nuclide with a half-life of at least four years before the "Sea of Instability".
  6. ^ Excluding those "classically stable" nuclides with half-lives significantly in excess of 232Th; e.g., while 113mCd has a half-life of only fourteen years, that of 113Cd is nearly eight quadrillion years.

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