Russia has about 2,000 of them and using them is part of their military agenda if they consider their homeland under threat . They are between 1 and 50 kilotons . Hiroshima was about 15 kt and the biggest conventional bomb dropped about one hundredth of a kt (0.01). They could best be used on a battlefield where there is a large congregation of men ....my impression is Ukraine largely avoid these .
The difference between tactical nukes and big conventional bombs isn't just scale but also the residual radioactivity left behind at the site . Two were used to end WW2 in the Pacific when the US was the only country having nuclear weapons - that didn't last long of course - and have never been used since . (Mainly for fear of retaliation / escalation .)
Their use in Ukraine by Putin would mean destroying a lot of people he consider rightfully part of the russian empire and also land he considers part so would be hard to reconcile with stated objectives in Ukraine . (In his eyes at least if not in relaity ,) annexing those parts of Ukraine and having Ukraine trying to retake them could be used as an excuse to use them to defend the homeland ...even if Ukrainians never cross the border into russia .
Putin has threatened several times - widely seen as a way of keeping US involvement at a minimum . Putin knows that if used there would be a huge backlash from almost all countries -I'd imagine China included and so wouldn't use lightly . But he has shown a propensity to take big gambles and if desperate ...... who knows .
