At the time of the Fukushima accident, 32 out of 548 fuels at Unit 3 were MOX (initial plutonium enrichment 3.9%).
Plutonium used in MOX fuel:
・Has a high boiling point, similarly to uranium used in regular fuel, and cannot easily be released into the environment even if the fuel melts.
・Generate, in nuclear fission, radioactive materials similar to those generated from uranium fuel.
For these reasons, the level of hazard in the event of core meltdown is similar between uranium fuel and MOX fuel.