Bad design - I'm not entirely sure of the particulars, but the power plant overheated and triggered a steam explosion in what cooling system they had. This explosion blew open the interior of the reactor where the potentially dangerous radioactive materials are located, spewing them into the surrounding environment.
Better-designed nuclear power plants simply become unusable when they melt down, instead of steam explosions. In no case, however, can a nuclear power plant explode the way that a nuclear bomb can.
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