Interesting post, Matt. How big do you think that Pei Kauyung's vest pocket nuke was? Now obviously you are not going to slip something the mass of a truck in strapped under some priest's cassock.
So how big does it have to be? We are not talking about something the size of the one that was dropped on Hiroshima. All we want to do is thoroughly trash the inside of the temple including the basement and we are working on the presumption that the inside of the temple is not nearly so strong as the shell.
Or would something other than a nuke work better? I'm no genius at this stuff. But it does seem to me that there are two constraints here. The first has to do with electricity. Electricity transmits magnetic waves along the wire carrying the juice, but that can in large measure be minimized by good shielding around the center wire carrying the juice. So a small current carried in coaxical cable would only transmit very weakly, perhaps weakly enough to get lost in the hash from the stuff the temple already has going. That at much seems straight forward, although I don't know how weak the current would need to be. But the better the shielding, the less gets transmitted for a sensor to pick up. Or, perhaps one avoids juice altogether and uses a spring wind up timer that releases a pin onto primer that explodes on impact which is how I was visualizing in my previous post.
The second issue you mentioned in your post. That of radiation from the nuclear material. But that brings us back to my original question...how big? Something the size of a pineapple enclosed in a lead encased box would seem doable, sealed up so that only the timer would be visible outside the box. Taking your figure of one half inch casing would make it a bit heavy, but probably not unmanageable. But then on the other hand, I'm not sure just how heavy our so called pineapple would be!
To conclude my musings, it is obvious that the only persons in the Safehold universe who could construct such a thing would be Nimue and Merlin. They would do it in the cave using materials that you and I have never heard of so all of this speculating is probably pretty much off the mark! Oh well, it was fun anyway!
Don