I cannot see him nuking Ukraine for the simple reason he will destroy not only the
Ukrainian wheat production by irradiating the soil and crops, but he would wipe out a lot of Russias too wind dependant.
His sunflower production
And his Corn production.
In fact a lot of Russias food production capability, see
https://ipad.fas.usda.gov/rssiws/al/rs_cropprod.aspx