Close calls can certainly be hidden, but serious nuclear accidents that result in appreciable release of radioactive materials are another story. Those are ver

Close calls can certainly be hidden, but serious nuclear accidents that result in appreciable release of radioactive materials are another story. Those are very hard to hide for long (there have certainly been attempts though.) I think by now we know about about most of the accidents. For instance, the British government initially tried to cover up the Windscale fire, but they couldn't hide the release of Polonium.