So, there are a couple of choices to go about acquiring the material for a nuclear weapon. Option A is a uranium bomb. It’s an easier, more efficient, process but an obvious one and very difficult to miniaturize a weapon with. The later is not desirable in modern warfare because it amounts to a giant clunky bomb you need a specialized aircraft to deliver. But, for that, all you have to do is continuously enrich uranium until you get weapons-grade material.
Would we detect the radiation of that material in this attack if it was produced or kept at Fordow? No. As mentioned, Fordow is a subterranean facility deep below a mountain and shielded by its solid granite core and layers of igneous rock. There’s no chance a conventional kinetic bomb like a MOP could penetrate that deep into the mountain or cause sufficient damage to the facility to eject radioactive material. But I don’t think Iran is doing that anyway.
Option B is a plutonium bomb. It’s more difficult and time consuming to produce, but it is easier to hide and miniaturize for all of the ballistic missiles Iran has designed and produced for the purpose of carrying a nuclear warhead. It also makes sense as Iran’s nuclear weapons program is modeled after and aided by North Korea and their first test detonation was a plutonium bomb. To make one of these, you enrich uranium to reactor grade (which we know Fordow and the other facilities are doing and looks innocent enough on the surface) and harvest the plutonium byproduct from the reactor.
But here’s the rub: plutonium bombs iz hard because not just any plutonium will do. One of the things you get when you put enriched uranium in a reactor is Plutonium-239. That is good. That’s what you want. But after awhile, it starts creating Plutonium-240 and 241. Those are bad. They are highly unstable and if you build a bomb with those it could spontaneously detonate at any moment. Problem is, there’s no rule that says when that happens. It depends on the fuel and the reactor.
The relevance of Israel’s assassination of Iran’s nuclear scientists is not that they’re nuclear scientists. It’s that those individuals know things. Any fool with a physics degree can make a uranium bomb, but it requires time sensitive processes and a great deal of specialized expertise and precision to make a plutonium bomb. You need standardization, you have to know the enrichment process end to end, and you have to know every beep, boop, and quirk of the reactor you’re putting it in. Or else you’ll blow yourself to the moon.