How much nuke-metal can you get out of Reactor X? As it turned out, nuclear proliferation scholar David Albright came up with an oversimplified rough formula, which he wrote in a briefing.

You need:

1. The thermal energy created by the reactor. Note that thermal megawatts are different and greater than electrical megawatts.
2. The capacity factor. As I’m not any kind of scientist, I would say just go with Albright’s recommendations or make it even lower than 0.5 if you’re using a press-ganged power reactor to simulate the extra work needed.
3. The conversion factor. This is obvious in the slide for purpose-built production reactors, but for adapted ones, you need to look a little deeper. Fortunately, the same presentation has a comparative slide.

(Again, do not quote me on this. The very presentation says “this is for production not adapted reactors”, but oh well.)

So:

200mw production reactor: 200×0.6x365x.85=37kg of weapons grade plutonium in a year.

Adapted LWR power reactor: 1,400 mwth, wastage: 1400×0.35x365x.51= 91.2 kg of weapons grade plutonium in a year.

A Fat Man-level warhead is estimated to need anything from 6.5 kg (very low technology, used in said bomb itself) to 3kg. See here and here.