[en] A fiber-optic Cerenkov radiation sensor (FOCRS), which utilizes transparent core of the optical fiber as a sensing material of gamma ray, has been evaluated a feasibility in application of determining radiation release characteristic of spent nuclear fuel by Monte Carlo simulation with GEANT4 simulation toolkit. A simulation model with Westinghouse type 17 x 17 fuel assembly and the FOCRS were constructed with GEANT4 and the nuclide composition of spent nuclear fuel was evaluated with ORIGEN-ARP. The simulation results exhibit that the FOCRS could describe the gamma ray distribution emitted from spent nuclear fuel assembly. The FORCS, therefore, can be used to evaluate gamma ray distribution at extremely radiation environment around the spent nuclear fuel assembly. (author)