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Abstract. Usually the synthesis of two-dimensional and one-dimensional discrete ordinate calculations is used to evaluate neutron fluence on VVER-1000 reactor pressure vessel (RPV) for prognosis of radiation embrittlement. But there are some cases when this approach is not applicable. For example the latest projects of VVER-1000 have upgraded surveillance program. Containers with surveillance specimens are located on the inner surface of RPV with fast neutron flux maximum. Therefore, the synthesis approach is not suitable enough for calculation of local disturbance of neutron field in RPV inner surface behind the surveillance specimens because of their complicated and heterogeneous structure. In some cases the VVER-1000 core loading consists of fuel assemblies with different fuel height and the applicability of synthesis approach is also ambiguous for these fuel cycles. Also, the synthesis approach is not enough correct for the neutron fluence estimation at the RPV area above core top. Because of these reasons only the 3D neutron transport codes seem to be satisfactory for calculation of neutron fluence on the VVER-1000 RPV. The direct 3D calculations are also recommended by modern regulations. Description of 3D Code KATRINIn this paper the 3D discrete ordinate code KATRIN [1, 2] is tested for calculation of fast neutron fluence on VVER-1000 Reactor Pressure Vessel (RPV) by discrete ordinate method.The code KATRIN solves the multigroup transport equation for neutrons, photons and charged particles in 3D x,y,z and r, ,z geometries. The transport equation for charged particles can be solved in the continuous slowing-down approximation. The scattering anisotropy can be treated in the P L approximation. The adjoint solution of the problem can be also obtained (for neutral particles only). The principal application is the solving the deep-penetration transport problems, typical for radiation protection and shielding calculations. The fission problems (subcritical boundary value problem and k eff problem), problems with upscattering (the thermalization problem, etc.) and hadron cascade problems This is an Open Access article distributed under the terms of the Creative Commons Attribution License 2.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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