Criticality and burn up behavior of the Fixed Bed Nuclear Reactor (FBNR) are investigated for the mixed fuel (UO2)-U-233/ThO2 as an alternative to low enriched (UO2)-U-235 fuel. CERMET fuel with a zirconium matrix and cladding has been used throughout the study. The main results of the study can be summarized as follows: Reactor criticality is already achieved by similar to 2% (UO2)-U-233 with the mixed (UO2)-U-233/ThO2 fuel. At higher U-233 fractions, reactor criticality rises rapidly and exceeds k(eff)>1.5 already by 9% (UO2)-U-233. With 100% (UO2)-U-233, start up criticality can reach k(eff)=2.0975. Time dependent reactor criticality k(eff) and fuel burn up have been investigated for two different mixed fuel (UO2)-U-233/ThO2 compositions, namely: 4% (UO2)-U-233 + 96% ThO2 for a reactor power of 40 MWel (120 MWth) and 9% (UO2)-U-233 + 91% ThO2 for a reactor power of 70 MWel (210 MWth). Sufficient reactor criticality (k(eff) > 1.06) for continuous operation without fuel change can he sustained during similar to 5 and 12 years with 4% and 9% (UO2)-U-233 fractions in the mixed fuel, leading to burn ups of similar to 36000 and >105000 MWD/t, respectively. Thorium based fuel produces no prolific uranium. Plutonium production remains negligible.