Conceptual design of a gas-cooled accelerator-driven reactor with very fast spectrum

A new reactor configuration is proposed under the requirements of providing a high value of breeding ratio and very high safety standards in all conditions, particularly in severe accidents. This double objective conveys the need of very fast neutron spectrum and the use of fuels with a relatively high content of transuranium nuclei, particularly plutonium isotopes. These facts define a narrow design window where a solution must be found to the safety problems created by the lack of Doppler effect and by a strong reduction in the percentage of delayed neutrons. The operational range in safe conditions almost vanishes, because of the lack of negative feedback mechanism and the very small reactivity margin before arriving to prompt-criticality. In order to improve operation conditions, the reactor is kept subcritical, and power level is kept constant with the help of an external source of neutrons, generated by spallations induced by accelerated protons. Main innovations are found in the fuel configuration, because the fuel is embodied into small disks with suitable cladding and different diameters, and the disks are fixed to a geometrical structure forming a virtual spherical surface or cage. The reactor core is therefore a pebble bed, but the pebbles are not actual pebbles, as already said, although they can be evacuated from the core by letting them fall down into a core catcher where subcriticality and decay heat cooling are guaranteed.