My understanding is the Fukushima reactor was releasing steam from the low pressure coolant water in order to reduce pressure. This would also release heat which helps.
However coolant water surrounds and is completely separated from the inside sealed reactor system itself. That system is closed and has its own cooling water (under high pressure) which transfers heat by conduction to the surrounding cooling water.
They do not mix.
So why is some steam released in an emergency, radioactive?