Immediate dismantling begins soon after final shutdown, once fuel is removed. It uses the operating organisation's knowledge while it still exists, but requires the disposal route to be available early.
Deferred dismantling places the plant in a safe enclosure for decades so that short-lived activation products decay. It reduces dose and waste volume but transfers cost and knowledge to a later generation.
Entombment encases radioactive material in place. IAEA treats it as an option only in exceptional circumstances, since it creates a disposal facility that was never sited or designed as one.