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In the nuclear fuel cycle

Plutonium-242 is produced by successive neutron capture on 239Pu, 240Pu, and 241Pu. The odd-mass isotopes 239Pu and 241Pu 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 proportion of 242Pu is low at low burnup but increases nonlinearly.

242Pu has a particularly low cross section for thermal neutron capture; and it takes three neutron absorptions to become another fissile isotope (either curium-245 or plutonium-241) and then one more neutron to undergo fission. Even then, there is a chance either of those two fissile isotopes will absorb the fourth neutron instead of fissioning, 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 242Pu again, so the mean number of neutrons absorbed until fission is even higher than 4. Therefore, 242Pu 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, 242Pu's low cross section means that relatively little of it is transmuted during one cycle in a thermal reactor.

Decay

242Pu alpha decays into uranium-238, before continuing along the uranium series. 242Pu decays by spontaneous fission in about 5.5 × 10−4% of cases.⁷

References

1. "PLUTONIUM ISOTOPIC RESULTS OF KNOWN SAMPLES USING THE SNAP GAMMA SPECTROSCOPY ANALYSIS CODE AND THE ROBWIN SPECTRUM FITTING ROUTINE" (PDF). Archived from the original (PDF) on 2017-08-13. Retrieved 2013-03-15. https://web.archive.org/web/20170813191754/http://www.wmsym.org/archives/2001/21B/21B-18.pdf ↩

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. /wiki/Polonium ↩

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

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 [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) ↩

5. This is the heaviest nuclide with a half-life of at least four years before the "sea of instability". /wiki/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 eight quadrillion years. /wiki/Primordial_nuclide ↩

7. Chart of all nuclei which includes half life and mode of decay https://web.archive.org/web/20000817205239/http://nucleardata.nuclear.lu.se/nucleardata/toi/pdf/chart.pdf ↩