when we start building "breeder" reactors again. Plutonium is a byproduct of the breeder reactors when they create the type of uranium that can be used for nuclear bombs (this is one of the reasons that breeder reactors aren't popular). I was talking with one of the scientists at Ames last night, and apparently this is pretty much the last of NASA supply, since the US has significantly cut down on their production of weaponizeable uranium.
That's not really accurate, but rather than explaining why, I'll just give the abbreviated rundown on plutonium:
Plutonium 238 is a powerful alpha emitter with a half-life of 87.7 years, making it a great element for powering mars rovers. It's produced by exposing Neptunium 237 to neutron flux. You can get Np-237 out of nuclear waste from ordinary reactors. The US has mostly been buying Pu-238 from Russia, but we're running out, and starting up our own production again is kind of expensive. We can do it, though.
Plutonium 239 is the kind that gets used in bombs. It's fissile. It's produced by exposing Uranium 238 ("depleted uranium") to neutron flux in a nuclear reactor. It's tricky to make weapons-grade Pu-239, because it tends to be contaminated with Pu-240.
The type of Uranium used in nuclear weapons (fissile U235) isn't created in reactors; it's purified from natural Uranium, which is almost entirely non-fissile U238 but contains a small proportion of U235.
Most commerical enrichment of Uranium uses gas centrifuges to separate the heavier isotopes from the lighter.
