Unless they're literally housed inside the containment building, this is useless against precision guided missiles and bombs. Are you talking about deterrence to nuclear bombing of Chernobyl? Then it is a moot point because the fallout from the weapon itself would dwarf the dormant fissile material in the containment facility.
Sorry, but this makes no sense. If Russia wants to deter EU with fallout, they already have such a mechanism - their massive stockpile of nuclear weapons to deter aggression by EU.
I would imagine that even a small conventional bomb landing anywhere within a few miles of Chernobyl would unearth and spread contamination.
I remember reading somewhere that all the ground was full of small radioactive particles in the area surrounding Chernobyl. So basically what they had to do was dig up the first few feet of dirt and flip it over, burying the contaminated dirt. Any small bomb would undo that. Then the wind would carry it and we'd have a whole mess.
No this doesn't have anything to do with Nuclear Weapon deterrence. Much of the liquidation effort was literally burying contaminated topsoil in the area. The explosions from conventional bombs would kick radioactive particles back up into the atmosphere. Europe probably does not want to deal with that, so I imagine they will refrain from attacking directly.
If I understand correctly, your central thesis is about deterring EU from attacking. It misses the point that Russia already has a huge stockpile of nuclear weapons to deter EU from attacking them. So, why would they rely on an 'offchance' 2nd-hand threat of fallout when they can just threaten to use their guaranteed-to-deter stockpile of weapons?
I think I'm using "Allied Forces" and "Europeans" too loosely. I was lumping Ukrainian forces into that label. They're already under attack so any conventional retaliation on invading forces is already a given. I'm basically saying that NATO/EU would put pressure on Ukraine NOT to attack the staging area because any fallout could waft over into Western Europe. This is in addition to the already present downside of Ukraine re-contaminating their own backyard.
Sorry, but this makes no sense. If Russia wants to deter EU with fallout, they already have such a mechanism - their massive stockpile of nuclear weapons to deter aggression by EU.