Plutonium-242

Transmutation flow in LWR

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.^[1]

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

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