Producing weapons grade nuclear materials isn't that difficult. Iran is believed to only need a few weeks if they want to. (IRAN has known stock piles of just below weapons, but well above power plant grade Uranium). Many of us could probably do it in our basement if we could get our hands on enough Uranium. (there are a number of ways to die a painful death doing this as most people work in their basement, but we could probably get enough to produce a bomb before we died). With the knowledge anyone has and Ukraine's current nuclear power plants and thus nuclear knowledge it is reasonable to assume they could get the weapons grade nuclear material if they really wanted.