Nuclear Engineering (student) here.
>The decay was an atom splitting into two smaller atoms and energy, which is fission.
Fission in the context of engineering refers to the use of neutrons to force atoms to split, not to naturally decaying isotopes.
>The question was of why would fukishima need active cooling when passive cooling is so "easy" to do.
Because it wasn't designed to use passive cooling. Passive cooling requires your reactor to be designed to facilitate it (all gen 3+ are designed like thisâ"I believe the NRC refuses to certify anything that is nonpassive). Passive cooling refers to not requiring power to run the coolant pumps or anything. The AP1000 is designed to using convection of steam inside the containment building to cool the reactor.