Transmutation instead of storage is the promising approach to turn highly radioactive waste into non-radioactive material, or at least shorten the half-life period to manageable periods. Vacuum is one of the requirements for this process.
Half-life of 15 million years
About one percent of spent fuel rods is problematic material, including radioactive plutonium and other highly radioactive isotopes, which can have half-lives of up to 15 million years. Today, reprocessing plants already recycle the plutonium and remaining fissile uranium to make new fuel rods. The remaining highly hazardous materials were previously candidates for a near-infinite final storage process. But it is also possible to chemically separate them and then subject them to physical changes (transmutation).
Transmutation takes place in what is known as an accelerator-driven system (ADS). The core element of the ADS is a hundred-metre-long particle accelerator in which protons are accelerated to speeds near the speed of light. The particles may not collide with other particles in this process. For this reason, special vacuum pumps in the system generate an ultra-high vacuum of 10-6 to 10-10 hPa.
The protons hit a mixture of heavy metals with an enormous amount of kinetic energy and its nuclei burst. This releases neutrons that now also hit the particles of atomic waste with high energy. Their bombardment starts many decay processes in the atomic nuclei of the radioactive isotope. To a large extent, they transmute into stable – non-radioactive – isotopes or into radioactive particles with significantly shorter half-lives. The number of critical isotopes can be successively reduced in several rounds.
In contrast to nuclear fission, the transmutation cannot escalate out of control. If the proton beam is turned off, the chain reaction stops. In principle, the process can still generate more energy than it requires. This has worked in laboratories for a while now. Researchers have been studying ADS at an industrial scale since the 1990s. The first pilot system is expected to begin work in Japan in 2020. A second is expected to commence operations in 2023 in Mol, Belgium. An atomic waste recycling power plant could process the highly radioactive waste from ten nuclear power plants each year. The problem of final storage for atomic waste would finally have manageable dimensions.
Spent fuel rods contain 95 percent uranium and one percent plutonium. They are first mechanically crushed, then dissolved in nitric acid. Chemical reactions separate the uranium, plutonium and the other remaining materials from each other. Approximately 10 percent of the uranium can be enriched again for use in new fuel rods. The plutonium is also processed for nuclear fuel.
However, about 90 percent of the material remaining after this recycling process is waste that consists of radioactive isotopes of many elements, from arsenic to terbium. A tiny amount of material that can be used as radioactive sources for medical or scientific purposes can still be extracted from these radioisotopes. The remaining waste is subsequently separated into low-, intermediate- and high-level radioactive material. About seven percent is highly radioactive waste and around one percent is "problem waste" that would need to be stored for millions of years in some cases, if not for transmutation. The volume of atomic waste that has to be stored for millennia is greatly reduced through reprocessing.