Ukraine's reactors are pretty old and I wouldn't put it at 100% that they will all survive such ordeal without melting.
There are many consequences of a Russian attack at Ukrainian nuclear infrastructure that will bear negative cost for Russia but I don't have the time to explain it here at length. I'll mention just the most obvious and pertinent:
per adopted NATO policy any disruption to nuclear safety in Ukraine caused by Russian military activity triggers article 5. All that is required is either for Poland and Romania to file the motion which I assure you will be automatic from our side.
The basic response that has been agreed is to
"ensure that Russian attacks against Ukrainian nuclear sites do not constitute a radiation threat to NATO states".
Because apart from Enerhodar all other facilities are only vulnerable to air and missile strikes the measures undertaken will establish a
de jure NATO-enforced no-fly zone over nuclear sites in Ukraine, which will effectively translate into a
de-facto no-fly zone over all of Ukraine. It will be achieved through the following measures:
- destruction of Russian air defenses within the territory of Republic of Ukraine, including Crimea
- destruction of Russian air bases within the territory of Republic of Ukraine, including Crimea
- destruction of Russian Black Sea fleet at sea and within the territory of Republic of Ukraine, including Crimea
- destruction of any air defense assets defending against above actions regardless of their location
This would still be fairly limited response considering that Pentagon has communicated through reliable channels (you know which ones if you are a person in the know, and not an attention-starved idiot making noise online) that the reaction to Russia's use of tactical nuclear weapon in Ukraine would be the destruction of Black Sea Fleet. Black Sea Fleet includes vessels currently in the Mediterranean. This is why the choice of phrasing was particular. This is the response to threats of nuclear weapon being used against a state that is not a member of the alliance. Tactical nuclear weapons have low risk of fallout that would affect NATO states unless they are used against targets near NATO territory.
What's more important: points 1-4 will effectively ensure persistent NATO presence in the air over Russian forces for a period of at least 1-2 weeks which will effectively have a similar effect to NATO providing air support to Ukrainian forces on the ground during the time, even though the targets will be limited.
As such it is most likely that any offensive operation undertaken by Ukrainian forces at such time will not meet any meaningful resistance which will end with a total rout of Russian forces from Ukraine since two weeks is all that is necessary to clear the southern region of the front between Zaporozhia and Donetsk which will then result in the collapse of defenses on the right side of Dnepr, a disorganized rout into and out of Crimea.
Furthermore point 4 effectively makes it possible to attack Russian facilities in Syria simply by using the excuse that they constitute threat to USAF bombers from Diego Garcia and USAF will ensure it by flying bombers as close to Latakia as possible. It
won't happen to demonstrate that it is "all about nuclear safety in Ukraine".
Russian air and naval capabilities were never a match for NATO even before the war. The problem was always ground forces and nuclear weapons. After eight months of war attrition and with the intelligence that NATO has gathered in that time it will be a repetition of Desert Storm that will cost Russia almost all of its naval and air assets in Southern Military District.
There is a thin line established through numerous crises in the past that both sides stay clear of which ensures that the war in Ukraine remains a proxy conflict convenient to both sides. If Russia crosses that line it will bear the consequences. All you need to remember is the Iranian missile strike on US forces in Iraq following the assassination of Soleimani. Those are the rules of the game that need to be followed if the game is to continue. New game, new rules and a lot of unpredictability and chaos.
Putin is many things but he is not an idiot. There is no miscalculation possible because NATO communicated its position clearly numerous times. He also knows the consequences if he gave the order to use nuclear weapons as a deterrent which will be the last order given by him as the President of Russia. Other than that Russia has nothing to counter points 1-4 and the immediate fallout. He also knows that there are many people in the Pentagon that secretly dream of the opportunity. And the fact that he will be seen as responsible for it, while the natural response (nuclear) will not be available will result in collapse of his regime, to be replaced by a de facto military dictatorship. Again this is down to GRU vs FSB dynamic within the Russian state security apparatus.
So unless someone is actively sabotaging Putin from within there will be no strikes that affect nuclear sites.
One more thing - the incidents at Enerhodar were likely caused by both sides as means of testing responses. It had nothing to do with whatever it was the media or "independent analysts" online were claiming. It was always about exploring and reinforcing the rules of engagement.