Estonia would have immediately shared the information about the accident with the allies. Russia always (without exceptions) tries to hide it from the world
Yes, I was wrong. The 2nd nearby is Murmansk, and that's to far away for that measurements. So it's clear that it was from the stone aged St Petersburg plant, a design before Chernobyl 3+4 (which was iter 2). Their broken Unit 1 is shut down, but the old 2-4 are still online, plus 4 modern ones.
https://en.wikipedia.org/wiki/Leningrad_Nuclear_Power_Plant
Or that military ship Akademik Aleksandrov which was nearby at that time. https://news.ycombinator.com/item?id=23664056