(converting nuclear material from bombs into civilian nuclear reactor fuel - a good way to get rid of them bombs)
I thought the high radioactive material for nukes was a by-product from nuclear powerplants not the other way around?
https://www.nytimes.com/2009/11/10/business/energy-environment/10nukes.html
From a technical perspective, of all the uranium atoms in uranium ore dug out of the ground, about 0.7% is U235 which is "fissile" and the other 99.2% is U238 which is not fissile, but is "fertile" - that is, it can undergo "transmutation" by neutron capture yielding Plutonium isotope 239 (Pu239).
Some heavy water moderated (eg CANDU) or graphite moderated (eg Hanford B) reactors can run on unenriched uranium fuel, but generally speaking, light water moderated reactors (ie most of the world's civilian reactors) require the uranium to be enriched to about 3% to 5% U235 content. If you want to make a "Little Boy" gun-type atomic bomb (what was dropped on Hiroshima), then you require something like +90% U235 content in the fuel. Uranium is enriched by separating the U235 atoms from the U238 atoms, typically with centrifuges and the uranium in the form of Uranium Hexafluoride gas (a U235 atom is three neutrons lighter than a U238 atom). Uranium enrichment to +90% for bomb fuel is really hard, expensive work, and U235 has a rather high critical mass so you need quite a lot of it to make a bomb. On the other hand, Pu239 has a critical mass about an order of magnitude smaller than that of U235 and is cheaper "per bomb" to produce, but the NRE costs are higher (plutonium implosion bombs are much more complex).
When you fission U235/Pu239 in a reactor, it releases neutrons, and some of the U238 in the fuel gets transmuted into Pu239 (remember that you start with fuel that is something like 5% fissile material and 95% U238). Pu239 is fissionable, and can be used either as reactor fuel, or as the fuel for an implosion type atomic bomb (what was dropped on Nagasaki, and what is used as the initiator for thermonuclear fusion aka "Hydrogen" bombs).
However, bomb-grade Pu239 is harder to manufacture than reactor grade Pu239. Basically, in a civilian power reactor, you fission the fuel for a year or two until the build up of fission products tends to absorb so much of the neutron flux that the fuel losses "reactivity" (kinda like a battery going flat). Then you pull the spent fuel from the reactor, replace it and repeat. To a certain extent the spent fuel can be recycled (separate the unfissioned Uranium/Plutonium from the spent fuel for reuse and send the waste fission products to high level waste disposal).
On the other hand if you are seeking bomb-grade plutonium, then you don't want much "burnup" in the reactor (fuel only in the reactor for weeks or months before plutonium extraction processing) to ensure that the Pu239 doesn't get excessively contaminated with heavier plutonium isotopes like Pu240 which tend to reduce the yield of a nuclear bomb by prematurely initiating the reaction before the fuel is fully compressed. It is really really hard to separate Pu239 from Pu240 (much harder than separating U235 from U238), and spent high-burnup fuel is really hard to handle because it is very strongly radioactive because of all the fission products in it. Also, Pu239 that is contaminated with short half-life fission products is kinda hard on all the troops that have to handle the weapon containing it (the idea is to threaten to irradiate the enemy, not your own forces!).
My understanding is that most weapons grade Pu239 has come from low-burnup "weapons production" reactors like Windscale and Hanford B, oh and also from frequent online refuelling of RBMK reactors (which have a notorious reputation because of what happened in the 1980's in what is now Ukraine...)
https://en.wikipedia.org/wiki/RBMK