One of the U.S.S.R.'s nuclear accidents took place in the southern Urals on Sunday, September 29, 1957. The site was a highly guarded military location. According to Higginbotham's book, rumors of what had happened reached the west, and because the Soviet's refused to acknowledge anything had happened, their secretiveness eventually triggered the U-2 missions, and then on the second mission, Gary Francis Powers got shot down, and all hell broke loose.
Here's an article I found about the Chelybinsk-40 explosion:
The Most Contaminated Place on Earth