Plutonium-242
{{Short description|Isotope of plutonium}}
{{Infobox isotope
| alternate_names =
| symbol =Pu
| mass_number =242
| mass = 242.059
| num_neutrons =148
| num_protons =94
| abundance =
| halflife = {{val|375,000|u=years}}
| image =
| decay_product = Uranium-238
| decay_symbol = 238U
| decay_mass =
| decay_mode1 = alpha decay
| decay_energy1 =
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| parent =
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Plutonium-242 ({{sup|242}}Pu or Pu-242) is the second longest-lived isotope of plutonium, with a half-life of 375,000 years. The half-life of {{sup|242}}Pu is about 15 times that of {{sup|239}}Pu; so it is one-fifteenth as radioactive, and not one of the larger contributors to nuclear waste radioactivity. {{sup|242}}Pu's gamma ray emissions are also weaker than those of the other isotopes.{{cite web|url=http://www.wmsym.org/archives/2001/21B/21B-18.pdf|format=PDF|title=PLUTONIUM ISOTOPIC RESULTS OF KNOWN SAMPLES USING THE SNAP GAMMA SPECTROSCOPY ANALYSIS CODE AND THE ROBWIN SPECTRUM FITTING ROUTINE|access-date=2013-03-15|archive-date=2017-08-13|archive-url=https://web.archive.org/web/20170813191754/http://www.wmsym.org/archives/2001/21B/21B-18.pdf|url-status=dead}}
It is not fissile (but it is fissionable by fast neutrons), and its neutron capture cross section is low.
In the nuclear fuel cycle
Plutonium-242 is produced by successive neutron capture on Pu-239, Pu-240, and Pu-241. The odd-mass isotopes {{sup|239}}Pu and {{sup|241}}Pu 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 {{sup|242}}Pu is low at low burnup but increases nonlinearly.
{{sup|242}}Pu 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 {{sup|242}}Pu again, so the mean number of neutrons absorbed until fission is even higher than 4. Therefore, {{sup|242}}Pu 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, {{sup|242}}Pu's low cross section means that relatively little of it is transmuted during one cycle in a thermal reactor.