I have a story where world finds out that North Koreans have built a fusion power plant that produces 4 gigawatts of electricity (*) and they don't reveal it's existence, for unknown reasons they don't want to boast about superiority of Juche. It's unimportant to me how they built it, what I need to know how the world detects it without North Koreans spilling the beans. The story is told from the point of the world which is kept in dark. No defectors with knowledge about the plant is accessible to the journalists, spy agencies etc.
I placed this in North Korea because they're a closest thing to information black hole in this world and still have functional government. So please no hacking, defectors or careless public sources. The knowledge about plant is kept secret as the death of the previous dear leader.
Would there be some kind of radiation or some other clue that would tip us off?
(*) The number of 4 gigawatts is arbitrary, any usable amount would do if that changes something