Americium

Americium-241: Not fissionable to slow neutrons, used as the internal initiator for Americium fuel in RBMKs and its own RTG pellet.

Americium-242m: Extremely fissile, used for Americium fuel in RBMKs.

Reactor Grade Americium: Mixture between the two isotopes, used to make Low and Medium enriched Americium fuel.