What caused the chernobyl core to explode?

Reactor design flaws.

Reactor breach flaws.

Explanation:

The reactor #4 had a disastrous power increase which led to explosions in its core.

The extreme power increase made the reactor cause super heated water to turn into steam instantly. Which then the built up steam caused a destructive steam explosion, causing graphite fires which are still burning to this day after 33 years.

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