While I was a graduate student at UT Austin, I attended a talk By Dr. C.P.J. Barty, Chief Technology Officer for the National Ignition Facility and Photon Science Directorate at the Lawrence Livermore National Laboratory, titled “Extreme Gamma-ray Sources and the Dawn of Nuclear Photonics”. It was quite fascinating. The basic idea is to use a linear accelerator to accelerate packets of electrons to relativistic speeds, and then to interact those packets with an infrared laser pulse (inside the laser cavity), to generate tunable extreme Gamma-ray laser pulses. They were able to demonstrate burning of nuclear waste products, as well as distinguishing between different elemental isotopes based on their absorption.

I haven't heard anything about the project since, but it sounded like a promising approach for burning nuclear waste, particularly when using a superconducting accelerator to do energy recycling.