This spring, a Swedish scientist sparked international concern with a journal article saying that radioactive particles detected in 2010 showed North Korea had set off at least two small nuclear blasts — possibly in experiments designed to boost yields of much larger bombs. Separate claims surfaced that intelligence agencies suspected the detonations were done in cooperation with Iran. Now, a new paper says the tests likely never took place — or that if they did, they were too tiny to have any military significance.