Time evolution of criticality and burn-up grades of the Fixed Bed Nuclear Reactor (FBNR) are investigated for alternative fuels. These are: (1) low enriched uranium, (2) weapon grade plutonium, (3) reactor grade plutonium, and (4) minor actinides in the spent fuel of light water reactors (LWRs). The criticality calculations are conducted with SCALE 5.1 using S-8-P-3 approximation in 238 neutron energy groups with 90 groups in thermal energy region. The main results of the study can be summarized as follows: (1) Low enriched uranium (UO2): FBNR with an enrichment grade of 9% and 19% will start with k(eff) = 1.2744 and k(eff) = 1.36 and can operate similar to 8 and >15 years with the same fuel charge, where criticality drops to k(eff) = 1.06 and a burn-up grade of 54 000 and >110000 MW.D/t can be attained. (2) Weapon grade plutonium: Such a high quality nuclear fuel suggests to be mixed with thorium. Second series of criticality calculations are conducted with fuel compositions made of thoria (ThO2) and weapon grade PuO2, where PuO2 component has been varied from 1% to 100%. Criticality with k(eff) > 1.0 is achieved by similar to 2.5% PuO2. At 4% PuO2, the reactor criticality will become satisfactory (k(eff) = 1.1121), rapidly increasing with more PuO2. A reasonable mixture will by around 20% PuO2 and 80% ThO2 with a k(eff) = 1.2864. This mixed fuel would allow full power reactor operation for >20 years and burn-up grade can reach 136 000 MW.D/t. (3) Reactor grade plutonium: Third series of criticality calculations are conducted with fuel compositions made of thoria and reactor grade PuO2, where PuO2 is varied from 1% to 100%. Reactor becomes critical by 8% PuO2 content. One can achieve k(eff) = 1.2670 by 35% PuO2 and would allow full power reactor operation also for >20 years and burn-up grade can reach 123 000 MW.D/t. (4) Minor actinides in the spent fuel of LWRs: Fourth series of criticality calculations are conducted with fuel compositions made of thoria and MAO(2), where MAO(2) is varied from 1% to 100%. Reactor becomes critical by similar to 17% MAO(2) content. Reasonably high reactor criticality (k(eff) = 1.2673) is achieved by 50% MAO(2) for a reactor operation time of 15 years with a burn up of 86 000 MW.D/t without fuel change. On that way, the hazardous nuclear waste product can be transmuted as well as utilized as fuel. (C) 2010 Elsevier Ltd. All rights reserved.