Plutonium-242
Isotope
Plutonium-242 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 that of ²³⁹Pu; so it is one-fifteenth as radioactive, and not one of the larger contributors... Wikipedia