Several reasons why I think it was a fake:

- North Korea does not have the technology to contain the radiation tight enought that nobody can detect it.

- North Korea doesn't give a shit about the environment or it's people. Even if they knew how to, they wouldn't spend the resources to contain the radiation beyond the immediate short term safety concerns.

- Kim Jong Il is known for grand-standing. If it was real he would have made sure everyone could confirm it.

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