China has already told explicitly several times that a nuclear war is not acceptable. China does not want to pay that price or a Russian victory, which will damage China economically too. The radioactivity may also end up in China, depending on weather conditions.
https://www.quora.com/If-Russia-uses-nukes-will-any-country-nuke-Russia
Adam Roach
Former Big Kahuna Author has 4.6K answers and 8.3M answer views Updated Nov 23
How a warrant for Putin puts new spin on Xi visit to Russia
https://apnews.com/article/us-russia-china-ukraine-icc-putin-57774b3a58d6ec1c75c921f71d9ebe90