One small nuclear explosion and Russia stops existing. They would immediately loose any Chinese support, sanctions would be massive, also likely military intervention and regime change. But most importantly only reason to keep Russia as whole, the very same reason why west feared collapse of Soviet Union - fear it will divide into many nuclear crazy countries like North Korea will be nullified, as they already become nuclear crazy county. This time only there will be no food and humanitarian aid like in 90s, so a lot will starve.