Plutonium-242

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Actinides and fission products by half-life
Actinides[1] by decay chain Half-life
range (a)		 Fission products by yield[2]
4n 4n+1 4n+2 4n+3
4.5–7% 0.04–1.25% <0.001%
228Ra№ 4–6 † 155Euþ
244Cm 241Puƒ 250Cf 227Ac№ 10–29 90Sr 85Kr 113mCdþ
232Uƒ 238Pu 243Cmƒ 29–97 137Cs 151Smþ 121mSn
248Bk[3] 249Cfƒ 242mAmƒ 141–351

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

241Am 251Cfƒ[4] 430–900
226Ra№ 247Bk 1.3k–1.6k
240Pu 229Th 246Cm 243Am 4.7k–7.4k
245Cmƒ 250Cm 8.3k–8.5k
239Puƒ 24.1k
230Th№ 231Pa№ 32k–76k
236Npƒ 233Uƒ 234U№ 150k–250k ‡ 99Tc₡ 126Sn
248Cm 242Pu 327k–375k 79Se₡
1.53M 93Zr
237Np 2.1M–6.5M 135Cs₡ 107Pd
236U 247Cmƒ 15M–24M 129I₡
244Pu№ 80M

...nor beyond 15.7M[5]

232Th№ 238U№ 235Uƒ№ 0.7G–14G

Legend for superscript symbols
₡  has thermal neutron capture cross section in the range of 8–50 barns
ƒ  fissile
m  metastable isomer
№  naturally occurring radioactive material (NORM)
þ  neutron poison (thermal neutron capture cross section greater than 3k barns)
†  range 4a–97a: Medium-lived fission product
‡  over 200ka: Long-lived fission product

Pu-242 is one of the isotopes of plutonium, the second longest-lived, with a half-life of 373,300 years. 242Pu'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.[6]

It is not fissile (though it is fissionable by fast neutrons) and its neutron capture cross section is also low.

In the nuclear fuel cycle

Transmutation flow in LWR

Pu-242 is produced by successive neutron capture on Pu-239, Pu-240, and Pu-241. 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.

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 undergo 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 242Pu 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, 242Pu's low cross section means that relatively little of it will be transmuted during one cycle in a thermal reactor.

References

  1. ↑ 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 isotopes have half-lives of at least four years (the longest-lived isotope in the gap is radon-222 with a half life of less than four days). Radium's longest lived isotope, at a notable 1600 years, thus merits the element's inclusion here.
  2. ↑ Specifically from thermal neutron fission of U-235, e.g. in a typical nuclear reactor.
  3. ↑ 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. 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 β− 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."
  4. ↑ This is the heaviest isotope with a half-life of at least four years before the "Sea of Instability".
  5. ↑ Excluding those "classically stable" isotopes 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.
  6. ↑ "PLUTONIUM ISOTOPIC RESULTS OF KNOWN SAMPLES USING THE SNAP GAMMA SPECTROSCOPY ANALYSIS CODE AND THE ROBWIN SPECTRUM FITTING ROUTINE" (PDF). 
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