Most (all?) power reactor designs produce plutonium as a by-product, as with many small-scale reactors. For example, Canada built a small research reactor [1]

Most (all?) power reactor designs produce plutonium as a by-product, as with many small-scale reactors. For example, Canada built a small research reactor [1] in India in 1954, similar to many Canadian heavy water reactor designs. It was supplied with natural uranium. It was repurposed to manufacture several kilograms of plutonium per year. As I understand it, any country with power reactors and a decent industrial-engineering base - such as Finland, Canada, Japan, Argentina, etc. -- could, in principle, build a fission weapon on rather short notice. Separation of plutonium from reactor waste is a horribly nasty business, but not challenging in the same way as uranium enrichment. [1] https://en.wikipedia.org/wiki/CIRUS_reactor https://en.wikipedia.org/wiki/CIRUS_reactor