A thought occurred to me, and it's so logical, I concluded that it must be true.
Russia will not detonate a nuclear weapon in Ukraine, fearing repercussion.
Russia will detonate a nuclear weapon in Russia. In other words, Russia will do a nuclear test. Like North Korea did.
As far as I know, there is no international law against Russia doing a nuclear test. Russia is a recognized nuclear weapon state of NPT, unlike North Korea. The test will give weight to Putin's words that he is not bluffing.
Not sure if it considered a "international law" but there are treaties about nuclear testing :
https://en.wikipedia.org/wiki/Nuclear_weapons_testing#Treaties_against_testing
Like this one :
https://en.wikipedia.org/wiki/Comprehensive_Nuclear-Test-Ban_Treaty
Now, I'm curious about what the community will think if you put it on metaculus, could be interresting.
As the article explains, CTBT is not in force.