Russian nuclear use would therefore be a massive gamble for limited gains that would not achieve Putin’s stated war aims. At best, Russian nuclear use would freeze the front lines in their current positions and enable the Kremlin to preserve its currently occupied territory in Ukraine. Russian nuclear use would not enable Russian offensives to capture the entirety of Ukraine (the Kremlin’s original objective for their February 2022 invasion). Russian military doctrine calls for the Russian Armed Forces to be able to effectively fight on a nuclear battlefield, and the “correct” doctrinal use of tactical nuclear weapons would involve tactical nuclear strikes to punch holes in Ukrainian lines, enabling Russian mechanized units to conduct an immediate attack through the targeted area and drive deep into Ukrainian rear areas.[9] The degraded, hodgepodge Russian forces currently operating in Ukraine cannot currently conduct effective offensive operations even in a non-nuclear environment. They will be flatly unable to operate on a nuclear battlefield. DNR/LNR proxy units, Wagner Group fighters, BARS reservist units, and the depleted remnants of the Russian conventional units that actually exercised fighting on a nuclear battlefield in annual exercises—not to mention newly mobilized replacements shipped to the front lines with less than a week of training—will not have the equipment, training, and morale necessary to conduct offensive operations following nuclear use. NATO is additionally likely to respond to Russian nuclear weapon use in Ukraine with conventional strikes on Russian positions there. Russian use of multiple weapons (which would be required to achieve decisive operational effects) would only increase the likelihood and scale of a Western conventional response.
ISW has an extensive analysis https://www.understandingwar.org/backgrounder/special-report-assessing-putin%E2%80%99s-implicit-nuclear-threats-after-annexation Here is the last paragraph: