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.
Plutonium-242 has a remarkably low propensity for thermal neutron absorption; and it undergoes a series of transformations requiring three neutron absorptions to become another fissile isotope (either curium-245 or plutonium-241), followed by one more neutron for fission.