That's inaccurate. The plutonium Japan has is from MOX fuel, which means it's contaminated with Pu-240. If U-238 is excessively irradiated, the Pu-239 (the weapons grade stuff) absorbs an extra neutron and becomes Pu-240 (which spoils nuclear weapons). Weapons reactors are designed to pass the uranium through the core relatively quickly to avoid this problem, a regular civilian reactor keeps the fuel in the core much longer. The plutonium Japan has is too contaminated with Pu-240 to be usable in nuclear weapons.Look at Japan, they don't have nuclear weapons but people say Japan can make a thousand nukes in a few months if they decided to.
It is possible in theory to separate the isotopes and isolate the Pu-239, but centriguation is not feasible because the mass difference is too small (1 vs. 3 in the case of U-238/U-235 separation). It's possible to use laser enrichment, but it's one of those technologies that governments keep behind 7 proxies, so no one knows whether or not it works.