It's even harder than that. Unless you remove the fuel rods quickly, in 1-2 months, you get too much thermally hot plutonium-238, and too much plutonium-240, w

It's even harder than that. Unless you remove the fuel rods quickly, in 1-2 months, you get too much thermally hot plutonium-238, and too much plutonium-240, which is the isotope that prevented using reactor bred plutonium in a gun assembly bomb and required developing the implosion design. Another isotope quickly decays into a fierce gamma ray emitter. At best, from what I've read, you'd have a bomb that prior to detonation would dissipate 100 kW, requiring serious refrigeration and therefore size, and you'd be lucky to get a yield greater than 1 kT. That's of course quite capable of ruining your whole day, but it's perhaps more accurately viewed as a super dirty bomb, that's likely how you could achieve the most damage with it. Not a city killer, but a contaminator.