Back in 2011, a nuclear reactor in the Fukushima area of Japan melted down after a tsunami took out one of its cooling towers, causing widespread damage, nuclear leakage, and a wide disaster area. The disaster was the second largest ever, topped only by the 1986 nuclear disaster that took place in Chernobyl. It was the second nuclear event to ever be classified as a level 7 event on the International Nuclear Event Scale, and there have been no other such events since. Estimates put the time frame to completely fix things and make the land safe for use again at 35 to 40 years. You would think the nuclear radiation zone, especially where the actual disaster took place, would be somewhere that people wouldn't go except on relevant official business. There may, however, be a reason for the general public to walk there, albeit not a very good one.
According to the local government and Tepco, the local power company who owns the nuclear plants within the disaster zone, staff have been sent out to check the area for Pokemon in Pokemon GO, which, if present, could encourage people to walk into the disaster area. Naturally, they were found there. As such, they've written to Niantic to request removal of the Pokemon, which would ensure that only Tepco staff and government officials would have any good reasons to walk around in the disaster area. The Fukushima Daiichi disaster zone is strictly off limits to unauthorized persons, and has yet to report any security breaches attributed to Pokemon GO. After exactly that sort of thing happened to a nuclear power plant in Ohio, however, the Japanese government saw the writing on the wall and decided to take measures to protect Pokemon GO players from ending up in places they shouldn't be.
Wild Pokemon have also decided to settle down in the memorial set up to honor the victims of the Nagasaki nuclear attack that took place during World War 2, which has, predictably, resulted in people playing Pokemon GO in the memorial area. This has prompted the government to issue an official statement, calling the memorial area "a place for prayer", and asking for people to respect the memorial and not search for Pokemon in the area.