Any first attack with a nuclear weapon is an unprovoked nuclear attack. The best result, as Scott Ritter puts it, is that the nuclear genie will be out of the bottle and some big American city will be taken out. https://www.youtube.com/watch?v=8XQan1qo8T4
The worst possible outcome would be all-out nuclear war resulting in the destruction of human life. The movie "Fail Safe" considered the inadvertent launch of a nuke that took out Moscow. The US government nuked Manhattan to avoid an all-out nuclear war. Nuclear retaliation against a nuclear attack is the one and only justified reason for having nuclear weapons.
Since nuclear retaliation is a fair response to a nuclear attack, this punishment is very reasonable and should be established for an unprovoked nuclear attack: those who ordered the attack have to sit and watch (and hear) with eyes held open as their loved ones -- spouse, SOs, parents, children, siblings, grandchildren, nephews, nieces, and other close friends and relatives -- are tortured to death.
It is hoped that such a punishment would never have to occur. I would make an exception of an attack on a wide-open, sparsely populated area.