Fission chambers loaded with high enriched uranium are used for spent fuel measurements in the socalled Fork detector. The Fork detector is one of the work-horses used by safeguards inspectors for spent fuel measurements during verification activities in the framework of the Non-Proliferation and Euratom Treaties. Having an accurate and validated model of the measurement equipment is beneficial for the investigation of this type of applications. SCK_CEN is carrying out a significant effort to model the Fork detector with the MCNPX code. However, scarce information is known about the fission chambers. This work describes the impact of the design information of the fission chamber on the calculated detector sensitivity and, consequently, on the overall Fork detector response for neutrons, using Monte Carlo simulations. The heavy ions transport in the active layer of the fission chamber was also studied and the resulting fission product energy spectra were compared with the available experimental data.