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Decay properties

Plutonium-241 is a beta emitter with a half-life of 14.3 years, corresponding to a decay of about 5% of 241Pu nuclei over a one-year period. This decay has a Q-value of 20.78±0.17 keV and a mean of 5.227±0.043 keV, and does not emit gamma rays.¹ 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 half-life of 432 years; 241Am is a major contributor to the radioactivity of nuclear waste on a scale of hundreds or thousands of years. In its fully ionized state, the beta-decay half-life of 241Pu94+ decreases to 4.2 days, and only bound-state beta decay is possible.²

Plutonium-241 also has a rare alpha decay branch to uranium-237, occurring in about 0.002% of decays. With a Q-value of 5.055±0.005 MeV, it can emit Auger electrons and associated X-rays, unlike the beta-decay process.³

Role in nuclear fuel

Americium has lower valence and lower electronegativity than plutonium, neptunium or uranium, so in most nuclear reprocessing, americium tends to fractionate with the alkaline fission products – lanthanides, strontium, caesium, barium, yttrium – rather than with other actinides. Americium 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.

References

1. Basunia, M. S. (1 August 2006). "Nuclear Data Sheets for A = 237". Nuclear Data Sheets. 107 (8): 2323–2422. doi:10.1016/j.nds.2006.07.001. /wiki/Doi_(identifier) ↩

2. Takahashi, K.; Boyd, R. N.; Mathews, G. J.; Yokoi, K. (1 October 1987). "Bound-state beta decay of highly ionized atoms". Physical Review C. 36 (4): 1522–1528. doi:10.1103/PhysRevC.36.1522. https://www.researchgate.net/publication/13335547_Bound-state_beta_decay_of_highly_ionized_atoms ↩

3. Basunia, M. S. (1 August 2006). "Nuclear Data Sheets for A = 237". Nuclear Data Sheets. 107 (8): 2323–2422. doi:10.1016/j.nds.2006.07.001. /wiki/Doi_(identifier) ↩

4. 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. /wiki/Polonium ↩

5. Specifically from thermal neutron fission of uranium-235, e.g. in a typical nuclear reactor. /wiki/Thermal_neutron ↩

6. 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 [years]. No growth of Cf248 was detected, and a lower limit for the β− half-life can be set at about 104 [years]. No alpha activity attributable to the new isomer has been detected; the alpha half-life is probably greater than 300 [years]." /wiki/Bibcode_(identifier) ↩

7. This is the heaviest nuclide with a half-life of at least four years before the "sea of instability". /wiki/Sea_of_instability ↩

8. 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 eight quadrillion years. /wiki/Primordial_nuclide ↩