AbstractA fusion breeder is presented for the rejuvenation of spent nuclear fuel. A (D, T) fusion reactor acts as an external high energetic (14.1 MeV) neutron source. The fissile fuel zone, containing ten rows in radial direction, covers the cylindrical fusion plasma chamber. The first three fuel rod rows contain Canadian deuterium uranium (CANDU) reactor spent nuclear fuel which was used down to a total enrichment grade of 0.418%. The following seven fuel rod rows contain light water reactor (LWR) spent nuclear fuel, which was used down to a total enrichment grade of 2.17%. This allows a certain degree of fission power flattening. Fissile zone is cooled with pressurised helium gas with volume ration of Vcoolant/Vfuel=2 in the fissile zone. Spent fuel rejuvenation occurs through the neutron capture reaction in 238U. The new fissile material increases the nuclear quality of the spent fuel which can be described as the cumulative fissile fuel enrichment (CFFE) grade of the nuclear fuel which is the sum of the isotopic ratios of all fissile material (235U+239Pu+241Pu) in the mixed oxide (MOX) fuel. Under a first-wall fusion neutron current load of 1014 (14.1-MeV n/cm2 s), corresponding to 2.25 MW/m2 and by a plant factor of 100%, the CANDU spent fuel can achieve an enrichment degree of 1% after ∼7 months, suitable for reutilization in a CANDU reactor. LWR spent fuel requires >15 months to reach an enrichment grade ∼3.5%, suitable for reutilization in a LWR. A longer rejuvenation period (up to 48 months) increases the fissile fuel enrichment levels of the spent fuel reactor to much higher degrees (>3% for CANDU spent fuel and over 5% for LWR spent fuel), opening possibilities an increased burn-up in critical reactors and a re-utilization in multiple cycles.