The danger comes when this naturally occurring radioactive gas gets trapped and concentrated, as can happen in mines and in the lower floors of some homes. (health.com)
But the fuel still inside the reactors must be cooled with water to prevent it overheating and releasing radioactive gases. (newscientist.com)
In the unlikely event that an overheating core causes a reactor breach, the pressure could potentially cause an explosive venting of radioactive gases into the environment. (discovermagazine.com)