Japan itself had a Level 4 accident in 1999, when three workers preparing fuel for a reactor at a Japan Nuclear Fuel Conversion plant triggered a nuclear chain reaction, called a criticality.

I deliberately chosen not to include those because I wanted to show you that nuclear plants have containments and reactor or power conversion systems which typically steam turbines.

It governs the behaviour of subatomic particles zipping around powerful accelerators at close to the speed of light and its equations foresaw the conversion of mass into energy in nuclear bombs.