Just nitpicking, but AFAIK nuclear power plants already "go critical" whenever they're operating (according to https://en.wikipedia.org/wiki/Criticality_(status) "Criticality is the normal operating condition of a nuclear reactor, in which nuclear fuel sustains a fission chain reaction.")
Because a reactor explosion is a small chemical reaction, with the yield of a single medium-sized conventional bomb, that also happens to scatter some amount of radioactive crap into the surrounding environment.
Whereas a nuclear explosion is a self-sustaining nuclear reaction, which produces a fireball that can engulf an entire city.
Given the option, I'd far rather stand a mile downwind of an exploding reactor[1], than a mile from ground zero of any modern nuclear weapon[2]. The solution to the first problem is walking away, and trying not to breathe too much of the crap in. There is no solution to the second problem, because you, and everything in miles around you would be instantly incinerated.
[2] https://youtu.be/r3l0G3XOUv4 - the fireball is a mile across. Anyone for miles outside the fireball zone will be turned into charcoal. Each modern ICBM carries 6-12 of these. That is what nuclear war will look like. Both sides in a nuclear conflict will fire hundreds of them, against opposing cities.
Because the effects of a nuclear meltdown are closer to that of a chemical attack and far away from that of a thermonuclear detonation. Blurry red lines cease to be lines.
When we have the stomach to go through with a plan so overwhelming that it would cause Putin to submit in fear.
That is a very high threshold.
No doubt there are people working on black flag political assassination attempts, but really the only other options are convincing non NATO members to intervene or launching bombers towards Moscow and making an ultimatum that won’t be perceived as a bluff.
Ultimately though i think Ukraine has to mostly handle this on its own and if it doesn’t win the only thing to be done is let it happen and completely isolate Russia from the global economy until they are broken. It is not a pretty picture when your only options are backing a nuclear power into a corner.
