|Isotope mass||242.059 u|
|Decay mode||Decay energy (MeV)|
|Isotopes of plutonium |
Complete table of nuclides
|Actinides by decay chain||Half-life
|Fission products of 235U by yield|
No fission products
... nor beyond 15.7 Ma
Legend for superscript symbols
Plutonium-242 (242Pu) is one of the isotopes of plutonium, the second longest-lived, with a half-life of 375,000 years. The half-life of 242Pu is about 15 times longer than that of 239Pu; 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.
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.
Plutonium-242 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 will be transmuted during one cycle in a thermal reactor.
Plutonium-242 primarily decays into uranium-238 via alpha decay, before continuing along the uranium series. Plutonium-242 will occasionally decay via spontaneous fission with a rate of 5.5 × 10−4%.