For sure: it does spawn without any monsters killed.
For sure: You can get more than one.
For sure: You can get it below lvl 14. (I didn't make past level 14 yet, but I had a game I had two nukes)
Guess: It has a spawning probability that increases by level, probably peaking around 16-18.
Guess: Only former humans spawn with inventory, and they are only spawned with their weapon, and some ammo. None of the monsters can spawn with any other item.
Guess: The game does not make sure that you get a nuke or not, it's just you are lucky or not.
Guess: Even if the nuke has been spawned, it might be destroyed is some blast.
Deduction: If the cheated game did not result in getting a nuke at level 19, after many trials, it may not be spawning on level 19.
Guess: It's spawning on level 19 (hence you were just unlucky to get it if you didn't)
Guess: Items probably have a min level of spawning, and after that, they get an increased probability of being created.
Guess: Items probably have a max level of spawning, and after that, they are not spawned anymore.
Guess: The items have a spawning chance of
|(their optimal level)- (Current Level)| / c
Where c is some constant.
Guess: Items are probably created with the following logic:
1- Each level has a number of items that it wants to create.
2- For each item, game decides it's type
3- For each item, game decides it's location.
The type decision is probably made by : an item being of type a is
(probability of a)/ (sum of all probabilities of all items)
Guess: probably at least 70% of this post is true. :)