Abstract: In order to check the conceptual design of the subcritical blanket in fusion-fission hybrid reactor, an integral experiment was carried out in alternate depleted uranium/polyethylene shells with 14 MeV neutron using activation technique. The ²³⁸U(n,f) and ²³⁵U(n,f) reaction rates at 90° direction to the incident D beam were determined by measuring the 293.3 keV γ ray emitted from ¹⁴³Ce which is generated by ²³⁸U(n,f) and ²³⁵U(n,f) reactions. The experiment was simulated using MCNP5 code with ENDF/B-Ⅵ library, and the calculated ²³⁸U(n,f) and ²³⁵U(n,f) reaction rates are generally 5% higher than experimental results.