Why can't isotopes with long half-lives be radiated with free protons, neutron radiation and gamma rays (photodisintegration) in order to transmutate those isotopes into something either stable (or stable-ish, with extremely long half-lives) or highly unstable (short half-life?)
Other posters have asked similar questions about using free neutrons, but not the other two options....
Transmutation is a very difficult art...
You have absolutely no control over the final product, in 99% of cases, from a radioactive isotope, you will obtain another radioactive isotope. Free neutrons and protons do not exist. the reactions have very low probabilities, very large flows of particles are needed which are only found in an accelerator or a nuclear reactor.
The costs of using these machines are high.
Even in the reactor, the irradiation volumes available are very low (of the order of a few liters).
One can wonder about the interest of operating a reactor that produces its own waste...
The irradiation times will be extremely long, they will be counted in months, even years.
You must transmute atom by atom: the target will never be fully transmuted.
The transmuted part in turn becomes a transmuting target!
The problem has now existed for almost 100 years, there are no credible industrial alternatives to natural radioactive decay: I think that in 100 years, it will still be the same.
Research into Accelerator Transmutation of Nuclear Waste has been going on for many decades, at least back to J.R. Harris's report on "The Transmutation of Radioactive Reactor Waste". Harris estimated that for electrons and gammas, "the energy required for the transmutation is greater than the electrical energy obtained from the reactor." Proton induced transmutation was better, but Harris estimated it would still require roughly 4/7 of the energy produced in the reactor, which is too large to be economically feasible. Using protons to produce spallation neutrons, however, did have some promise, since a 1 GeV proton incident on a uranium target will on average produce 39 neutrons that could be used to transmute waste.
For the specific transmutation of $^{137}$Cs, another estimate was that electrons would require $\sim 4000$ MeV/nucleus, much more than the $\sim 1000$ MeV of fission energy released with the production of each atom of $^{137}$Cs. The estimate for protons as more optimistic, with only $\sim 60$ MeV required, but this ignored all the efficiency factors for converting fission thermal power to proton beam energy.
One active area of research is using proton accelerator produced neutrons to drive a subcritical reactor. MYRRHA: An accelerator driven system to manage radioactive waste is currently under construction in Belgium, and can burn minor actinides as up to 30% of their fuel. These minor actinides are the major long-lived components of nuclear waste.
It is not so easy to transmute short-lived fission products such as $^{137}\textrm{Cs}$ or $^{90}\textrm{Sr}$ because of the small neutron cross-sections for such processes, so there is a large literature on using proton beams. Despite the pessimistic assessments above for electron beams, research is ongoing in areas such as transmutation of long-lived nuclear waste by a laser plasma electron accelerator or hybrid photo-neutron systems. There have even been proposals for using negative muons for nuclear waste transmutation via muon capture decays!
