Search both here and on Google for the Sino-Soviet Border Wars or Clashes.
It’s mostly a lot of speculation as neither party is talking. They don’t seem to have had a nuclear exchange, and there’s no hard evidence other than some characteristic seismograph recordings here in the West (nuclear detonations make distinctive patterns on seismographs, the larger the more distinctive) and some few Soviet documents that survived the multiple purges.
See this thread for a little more info: http://www.freerepublic.com/focus/f-news/1690314/posts?
One detonation seems to have been a nuclear land mine or some such around 1969. The other likely event was in 1972 and wasn’t as distinctive - likely someone salvage-fusing a device to keep it from getting captured when the Chinese overran them.
Thanks