The possibility of spent nuclear fuel rejuvenation in fusion reactors is investigated for both (D,T) and catalysed (D,D) modes. The analysis is conducted for light water reactor (LWR) spent nuclear fuel which was used up to a total enrichment grade (fissionable plutonium plus U-235) Of 2.174% The behaviour of the spent fuel is observed during 48 months for discrete time intervals of Delta t = 6 months. The cooling of the fissile fuel zone is considered with three different coolants, namely pressurized helium ('gas'), Flibe (Li2BeF4) ('molten salt') and natural lithium ('liquid metal'). The best rejuvenation performance is obtained with Flibe, followed by helium and natural lithium. The catalysed (D,D) mode has a better rejuvenation capability than the conventional (D,T) mode. A rejuvenation time of 12 months is evaluated for a final fissile fuel enrichment grade of around 3.5% with Flibe coolant in the fissile zone under a first-wall fusion neutron current load of 10(14) (2.45 MeV n cm(-2) s) and 10(14) (14, 1 MeV n cm(-2) s), corresponding to 2.64 MW m(-2) by a plant factor of 75% for the catalysed (D,D) fusion reactor mode. The rejuvenation time increases to 24 months for the same fissile fuel enrichment grade using the (D,T) fusion reactor mode under a first-wall fusion neutron current load of 10(14) (2.45 MeV n cm-2 s), corresponding to 2.25 MWm(-2) by a plant factor of 75%. This enrichment would be sufficient for reutilization in a LWR. Longer rejuvenation periods (up to 48 months) increase the fissile fuel enrichment level of the LWR spent fuel to much higher degrees (> 5% for gas coolant and > 7% for Flibe), opening possibilities for reutilization of the spent fuel in compact LWRs and in multiple cycles. Although the nuclear quality of the plutonium increases steadily during rejuvenation, the isotopic percentages of Pu-239 and Pu-240 remain < 70% and > 20%, respectively, during spent fuel rejuvenation, so that the plutonium component cannot be considered of nuclear weapon grade quality. This is a positive factor with regard to safeguarding. (C) 1998 Elsevier Science Ltd. All rights reserved.