is it possible to safely dispose of nuclear waste by bombarding it with neutrons, and if so, what energy/temperature of neutron is required? Are thermal neutrons sufficient or must they be fast? Thanks!
Yes, but it depends on what type of waste you are talking about. Nuclear fuel produces two kinds of "waste".
The first is fission products, which are the product of fission. Fission products have relatively short half-lives and decay fairly quickly. Since they have short half-lives, they also decay the fastest and will be gone in something on the order of 100 years. This is the "easy" waste to get rid of.
The second type of waste is the "actinides" (plutonium, neptunium, curium, etc.) that are produced when uranium and other actinides absorb a neutron. The actinides have very long half-lives, on the order of billions of years, and are therefore the longest-lived isotopes.
If you reprocess nuclear fuel and put the actinides back into the reactor, you will generally destroy them. This is especially true in a fast reactor where the fission to absorption ratio of actinides is higher than in thermal reactors.
In general, fuel reprocessing will remove the fission products for disposal and put the actinides back into the fuel stream.
Bombarding substances with neutrons would be counter-productive. That bombardment would create more radioactive nuclides, in general. If there is some specific nuclide, it might be possible to transform some of it to (if it's neutron deficient) to a stable nuclide. Unfortunately, you wouldn't be able to prevent the stable nuclide from absorbing more neutrons and becoming radioactive again.
There isn't general energy which would be effective for absorption by all radionuclides.
Bottom line: it won't work.
Erm -- no.
Anything coming out of a nuclear reactor has already been pretty thoroughly bombarded by neutrons. Bombarding non-radioactive nuclei with neutrons has the tendency to transmute them into radioactive nuclei, at least until they decay again. Google "s-process", or perhaps "nucleosythesis via the s-process" for how the stars do it.
In fact, while much of the really long-lived isotopes from fission reactors is fission byproducts from the fuel, there's a lot of shorter-lived isotopes that come from everything else in the reactor being bombarded by neutrons.