The next fusion experimental reactor such as ITER requires tritium breeding because of the high tritium cost and its limited availability from non-fusion sources, in addition to demonstrating breeding capability of commercial D-T reactors. A tritium fuel cycle model was developed to compute the required tritium breeding ratio(TBR) by taking into account reactor down-time. The results show that TBR should be unity to achieve 3 MW * Year/m2 of neutron fluence in 10 years for a steady-state reactor with 600 MW fusion power and 25% system availability provided 5 kg of initial tritium supply. If the external tritium supply is increased to 20 kg, the required TBR is 0.9. The estimated TBR is very sensitive to the variation of the tritium burn-up fraction in plasma and the tritium residence time in the tritium processing system. For example, decreasing the burn-up fraction from 5% to 1% leads to a 25% increase in the required TBR. Thus these parameters must be carefully examined in future work.