The problem is that the Soviets had complete Nuclear and Conventional superiority over the Chinese. Thus the most common scenarios of NATO/WP facing military collpase so makes use of tactical and then strategic nuclear weapons isn't going to happen. Neither is a massive first strike by one side because they're paranoid that the other is preparing to do it. The Soviet Union has the capability to defeat the PRC using Conventional Weapons and thus they will.
The best scenario is if Mao for some reason goes insane with rage after his defeat in the border war and launches an all out invasion into Siberia, causing one jumpy Soviet commander to launch tactical nuclear weapons when he is being overrun (both NATO and WP guidelines advised firing tactical nukes if they are surrounded and unable to escape), resulting in hundreds of thousands of Chinese deaths. Despite Soviet apologies and offers for a cease fire, Mao has now gone totally beserk and orders nuclear strikes against Vladivostock and other Soviet targets, killing hundreds of thousands more. This leads to a limited Soviet retaliation of around 50 nukes on Chinese cities. Millions die, China collapses into anarchy.
If the Soviet Union decides to try and conquer. China for some insane reason, Mao might also use nukes.