This work is aimed at running the first IRIS reactor core with mixed thorium dioxide fuel (ThO2-UO2 and ThO2-PuO2).Calculations are performed by using Dragon 4.0.4 and Citation codes.The results show the multiplication factor (Keff) for central and peripheral assemblies as a function of buup.To ensure the proliferation resistance,the value of 235U enrichment is ＜ 20％.The Keff is calculated using Dragon 4.0.4 for a single fuel rod and the model developed to fuel assembly,while the whole core was calculated using Citation code.For a fuel buup,the use of increased enrichment fuel in the IRIS core leads to high reserve of reactivity,which is compensated with an integral fuel buable absorber.The self-shielding of boron is in an IRIS reactor fuel.The effect of increased enrichment to the bu-up rates,and buable poison distribution on the reactor performance,are evaluated.The equipment used in traditional light water reactors is evaluated for designing a small unit IRIS reactor.