Nuclear Waste Transmutation
Particle accelerators, and the neutrons that they can produce, allow today to transform nuclear waste in less radioactive material, in other words to transform a chemical element in another, though the old alchemist dream (of transforming metal into gold) is still not at hand! Nuclear spallation (the frantumation of heavy nuclei hit by protons or neutrons) is the physical process enabling the transmutation of elements. As it often happens, this is one of the possible applications of researches that are motivated from the thirst of knowledge by scientists addressing the properties of materials and chemical compounds.
The spallation process allows to understand the structure of matter in neutron sources like ESS, but can be also used for nuclear energy applications, which is a primary sector for the world economy.
In the modern version of the alchemist dream of transmuting elements this time the goal is not to transform them into the noble metal, but to try to solve the issue of the geological stocking of nuclear waste, which remains radioactive for millions of years, without chances of contaminating the biosphere. Bombarding an element with a neutron beam it is possible to induce a change to its atomic nucleus and to “transform” it in another element (transmutation). A high enough neutron flux is able to transmute the long-lived nuclear waste produced by conventional reactors into material which is still radioactive, but with much smaller half-lives. The waste can be recycled and used as nuclear fuel in special nuclear fission reactors, called Accelerator Driven Systems (ADS).
ADS have an important difference with respect to conventional nuclear reactors: the energy producing chain reaction is not self-sustained, but needs an external neutron source, provided by a powerful superconducting proton accelerator by spallation. The intrinsic safety derives from the fact that when the external source is interrupted, the nuclear reaction cannot proceed autonomously, and the reactor switches off immediately.
The European nuclear industry and several Countries are jointly carrying out feasibility studies for ADS projects (like MYRRHA in Belgium), working together in research programs financed by Euratom (e.g. MAX), in which INFN is involved. These projects allow the construction of prototypes of the main components, to develop a new mode of generating nuclear power, in a new safe and environmentally friendly way. This would avoid to delegate to future generations the issue of living with nuclear waste, potentially harmful for millions of years.
LASA has been involved in the realization of superconducting proton cavities prototypes for ADS since the mid 90s and participated to the commissioning of significative technological demonstration tests during several Euratom Projects.
The fuel cycle closure with ADS
Nuclear waste from conventional reactors is chemically separated: Uranium is retrieved from the spent fuel bars (used for new fuel) and separated from the fission products and transuranic elements. The former, possibly after a “cooling” period, are directed to the geological repository and the latter are reprocessed to form the fuel of an ADS system, where they are transmuted by fission. Reprocessing of the spent ADS fuel allows an efficient transmutation process. The very small part that is left from the process is sent to underground repositories. The superconducting linear proton accelerator that produces through spallation the neutrons needed for the fission process is fed by a portion of the electricity produced by the ADS system, while the rest is sent to the electrical grid.
