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The existence of Tl-208 in thorium fuel cycle is a double-edged sword. Tl-208 is a strong 2.6 MeV gamma emitter, which acts as an effective proliferation barrier while in the same time complicating the spent fuel handling. To ensure the safety of the latter, the buildup of Tl-208 and its parent isotope, U-232, are necessary to be understood. This paper attempts to analyze U-232 and Tl-208 buildup in the Experimental Power Reactor (Reaktor Daya Eksperimental, RDE) fuel based on thorium cycle, using various U-233 isotopic vectors simulated with ORIGEN2.1 code. The simulation result shows that U-232-contaminated fresh fuels ended up with larger U-232 and Tl-208 activities at the end of cycle (EOC) compared with uncontaminated fresh fuel. However, their U-232 build-up rate are lower and even negative at one case. Then, lower U-233 purity resulted in larger U-232 and Tl-208 activities at EOC. This result implies a considerable difference of isotope buildup between the various U-233 vectors. Consequently, the thorium cycle-based RDE spent fuel handling should consider the isotopic vector of U-233 used in fresh fuel.