When uranium atoms capture neutrons, then either nuclear fission or the formation of heavier basic elements occurs. The heavier elements are, generally speaking, highly radioactive and emit dangerous radiation during a relatively long period. If these heavier elements are exposed to radiation, they are able to decay faster and because of that the storage time in a deep repository can be reduced. This process is called transmutation.
The technique used for transmutation is not completely uncomplicated and there is still no really good solution. The principle is that there is a particle accelerator which accelerates protons that strike lead. The lead in turn releases neutrons that subsequently expose the heavy radioactive elements to radiation. This process leads to the material being transformed to an element with a shorter half-life – a transmutation has occurred.