Why are nuclear accidents in Russia international problems?

You are probably thinking of Chernobyl. This is actually in the Ukraine, but at the time of the disaster there it was part of the Soviet Union. I'm not aware of any severe accidents in Russia as it is now. However accidents like Chernobyl have international repercussions because the fallout can be carried across national boundaries by the weather. In that case, with the Soviet government's secrecy policy, the fact it had happened only became known by picking up traces in countries like Sweden. The fallout covered Scandinavia and as far as the UK, we had a long period when sheep grazing in Scotland and the north of England were contaminated.