[en] The SERA treatment planning system is being adapted to the NG-12I fast neutron facility used at Snezhinsk Neutron Therapy Center, Russia. This requires a computer model of the neutron facility that would match the SERA input format and provide adequately accurate results within a reasonable time. The detailed description of the facility cannot be used in SERA calculations because of its awkwardness, long time for dose calculation, and the requirement that the source must be defined as an energy-angle distribution of neutrons on a plane. First we made the detailed description of the neutron facility in the format of input data for the universal code PRIZMA which is a main tool for Monte-Carlo neutron transport simulations at RFNC-VNIITF. The detailed model of the generator was verified through comparison between calculated and experimental data. Then we considered a number of simplified models in which the neutron source was defined as a source on a plane. The source plane was defined in three positions: at the collimator's outlet, at the collimator's inlet, and near the real source. Only the third model provided a good agreement of results. Further analysis into the importance of neutrons in different regions of the third model showed the regions where they were of low importance that allowed us to limit the neutron tracking region by a truncated cone and significantly reduce the dose calculation time. Doses calculated along the neutron beam axis by PRIZMA, SERA and MNCP5 agree well. (authors)