Most of the reactors built to produce plutonium in the US did not generate electricity. Their sole purpose was to produce plutonium, and everything else (IE many gigawatts of heat that could theoretically have been used for power-generation) was a waste product. IMO it's extremely misleading to refer to such a source as "spent fuel", because it implies that a typical nuclear power station's spent fuel (IE the waste byproduct of electrical power generation) could be used as a source of weapons-grade plutonium.
IMO it's sort of like describing orange juice as coming from "spent oranges". Yes, you have to "spend" oranges to make orange juice, but you're not going to get any substantial amount of usable orange juice from oranges that have already been "spent" in some other way.
Apparently the Russians built a number of dual-purpose reactors, so maybe the claim makes more sense in the context of that part of the world. I don't know how efficient such a system is, but AFAIK there was only ever one reactor in the US (the N Reactor at Hanford) that could produce both weapons-grade plutonium *and* electricity, and it was a political disaster (WPPSS).