Well…I assume that any worker at a uranium/plutonium refining plant would be under more supervision and have more clearly defined procedures than to simply allow workers to melt stuff togeather as they want.
In any case, in order for a nuclear explosion, you would need a super critical amound of either plutonium-239 or uranium-235 (highly enriched uranium). Regular (natural) uranium and reactor-grade uranium cannot be used to build a weapon. There are other isotopes that can be used for fission, but U-235 and Pu-239 are the commonly used ones.
Just bringing together enough material to be above critical mass does not gaurentee an explosion though. They have to be brought togeather very rapidly and under the right conditions. If a nuclear weapon is destroyed in an explosion or fire, for example, it’s highly unlikely that it would detonate, because in order for it to detonate it woud have to have all ignition points in the explposive initiator detonate in the correct order to create a symetircal implosion effect.
In some cases, if nuclear material is brought togeather in imperfect conditions, such as a flawed weapons design, it will “fissle.” This means that fission begins but the material is blown apart or depleated before a full-sized explosion occures. However, it may cause a small (by nuclear weapon standards) explosion.
Even if you didn’t have a full-scale explosion though, it could easily kill you. I heard a story about a worker during the manhattan project who accidentally brought togeather enough material for it to become supercrtical. Eventhough it fizzled, it instantly killed him by producing a super-intense pulse of neutrons and gamma radiation.
AS far as using uranium and plutonium togeather. Theoretically, you probably could use them both as fission fuel in a mixture for a weapon, though it might yeild difficult to predict results. There are weapons which contain a uranium and a plutonium component, but not in a mixture. (at least not that I know of). For example, the Teller design for the H-bomb contains a plutonium first stage and a fusion stage with a uranium “spark plug” in the middle.
Also, some power reactors have been run on Mixed Oxide Fuel or MOX, which is a combination of plutonium and uranium, but nowhere near weapons grade. Also, any reactor that runs on uranium for long enough will build up some plutonium in the fuel through breeding of U-238. In practice, this plutonium does contribute to the power of the reactor,
-Steve