China might pose a nuclear threat if they weren't actually a "nuclear pygmy", and if they tipped over into the kind of irrationality you might find in a few other places. Russia? No. Large arsenal, but not suicidal.
Basically, you'll never see a nuclear exhange between the significant nuclear powers, for the simple reason that it would be too destructive. And strange as it may seem, Israel has the wherewithal to inflict unacceptable damage on any attacker if the conflict goes nuclear.
Put another way, the US is supposed to end up with something like 1300 warheads when all is said and done regarding armament levels, Russia about the same. Both of these arsenals are deemed to be large enough to effectively kill the rest of the world. China, in comparison, is a dwarf - far less capable than Israel which has at least 400 warheads, or something around 30% of the clout packed by the two "big ones".
Bottom line: you'd not nuke any of the major nuclear powers if you wanted your country (and your people) to go on existing.
Of course, none of the big league would want to nuke Israel anyhow, so the point is really moot. But running this kind of scenario with a "host" of countries lobbing nukes is actually at least as outlandish as the space alien cr*p :)).
Well, you learn something new everyday!
Thanks, Cachelot.