There was an interesting (if short) conversation on another list about how the US was able to spot small scale nuclear events (even things like a criticality event in a poorly shielded reactor) The speculation revolved around some unidentified sensors that were on the GPS satellite at the San Diego Air & Space museum, since removed (the sensors not the model)) and the way in which GPS satellites could tell you where things were relative to them, just as easily as your car can figure out where it is relative to the satellite.

There are a number of monitoring systems spelled out in the various disarmament treaties, and the infrasound sensors were part of the test ban treaty, so I don't doubt that if there had been a nuclear component, someone would know about it :-)

Yeah but this isn't a random location. Chelyabinsk being home to Russia's nuclear research, and relatively close to Baikonur.