|Isotope mass||242.059 u|
|Decay mode||Decay energy (MeV)|
|Isotopes of plutonium |
Complete table of nuclides
Actinides and fission products by half-life
|Actinides by decay chain||Half-life
|Fission products of 235U by yield|
No fission products
|226RaNo||247Bk||1.3 k - 1.6 k|
|240Pu||229Th||246Cm?||243Am?||4.7 k - 7.4 k|
|245Cm?||250Cm||8.3 k - 8.5 k|
|230ThNo||231PaNo||32 k - 76 k|
|248Cm||242Pu||327 k - 375 k||79Se?|
|237Np?||2.1 M - 6.5 M||135Cs?||107Pd|
|236U||247Cm?||15 M - 24 M||129I?|
... nor beyond 15.7 M years
|232ThNo||238UNo||235U?No||0.7 G - 14.1 G|
Legend for superscript symbols
Plutonium-242 (242Pu) is one of the isotopes of plutonium, the second longest-lived, with a half-life of 373,300 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%.