50/50 Either Tallin or St Petersburg. One of them is not in Russia, but both are Russian-style (ie Chernobyl) water pressure reactors. One of them leaked in a massive scale, but thanksfully only twice.
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.