tim west said:
Uranium 238 is weapons grade but has a short life before reverting to the lesser effective 235.
Sorry Tim but that's all wrong. Uranium 238 has two properties that make it useful for weapons: it's very hard and very heavy. It may also be used as a container for nuclear bombs but is not fissile itself. It's half life is about 4,500,000,000 years and it's activity is a lot lower than that of the carbon 14 you eat every day.
It decays to thorium 234, and thus begins a line of disintegrations that ends with lead 206.
Uranium 235 is the fissile stuff that reactors typically run on. Its half life is 710,000,000 years which is long enough for some of it to be still around. It's the second of three naturally occurring radioactive nuclides that have been here since the solar system first formed and which are the source of most of the others. The third one is thorium 232.
Joe-90, I'll answer your question. There is no known way of altering the half life of plutonium. I've no doubt that some day we will learn how nuclear forces actually work and then we'll be able to do it - but not yet.
Meanwhile we do have one way of getting rid of plutonium and its really quite simple. Add a neutron.
The plutonium nucleus happily grabs the extra neutron then rearranges its internal structure and splits in two. That's how nuclear fission works, whether in a reactor or a bomb. It's where 80% of the energy comes from. (The remaining 20% comes from the decay of the fission products which have far too many neutrons in them.)
Plutonium can be used as fuel. It was used in the experimental fast breeder reactor which was intended to produce more plutonium than it consumed by adding the fission neutrons to the otherwise useless U 238 thus:
U 238 + n = U 239 -> Np 239 + beta -> Pu 239 + beta.
Like all the early magnox reactors it was built for the sole purpose of making plutonium for bombs. Anything that came out into the national grid was a bonus. If you don't put any U 238 in there you WON'T get more plutonium out than you put in.
We will eventually run out of U 235. We have a lot more U 238 which can, in theory at least, be converted to plutonium and then used as fuel. Right now I suspect that we won't want to do this because there's already too much post cold war plutonium in circulation and the sooner it's used up the better!