Plutonium-242
No fission products
have a half-life
in the range of
100–210 k years ...
... nor beyond 15.7 M years
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 4–97 y: Medium-lived fission product
‡ over 200,000 y: Long-lived fission product
Plutonium-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 one-fifteenth 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.
It is not fissile (though it is fissionable by fast neutrons) and its neutron capture cross section is also low.
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.
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Wikipedia