Plutonium-242

Plutonium-242 (²⁴²Pu or Pu-242) is the second longest-lived isotope of plutonium, with a half-life of 375,000 years. The half-life of ²⁴²Pu is about 15 times that of ²³⁹Pu; so 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.^[4] As the direct parent of uranium-238 it is part of the uranium series decay chain.

Plutonium-242

General
Symbol	242Pu
Names	plutonium-242
Protons (Z)	94
Neutrons (N)	148
Nuclide data
Half-life (t1/2)	375000 years[1]
Isotope mass	242.059741[2] Da
Decay products	238U
Decay modes
Decay mode	Decay energy (MeV)
alpha decay	4.984[3]
Isotopes of plutonium Complete table of nuclides

It is not fissile (but it is fissionable by fast neutrons), and its neutron capture cross section is low. Like the other even isotopes of plutonium it has a significant rate of spontaneous fission.

In the nuclear fuel cycle

Transmutation flow in LWR

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 faster than linearly due to the intermediate isotopes' buildup.

²⁴²Pu has a particularly low cross section for thermal neutron capture; and it takes three neutron absorptions to become another fissile isotope (curium-245) and then one more neutron to undergo fission. Even then, there is a chance of the fourth neutron being absorbed instead of fissioning, leading to curium-246 (with again only a small neutron cross-section), 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 is transmuted during one cycle in a thermal reactor.