Abstract <p>When solving the problem of validation of shielding codes, the data of the JASPER experiment (IAEA SINBAD-2010 database) can be effectively used. The JASPER experiment simulates the shielding of a fast neutron reactor (steel, boron carbide). However, the group source of neutrons with the angular distribution of the flux is determined in a nonstandard way: not volumetric, not point, but emanating from the emitting surface (surface) and at the same time emanating into the air (into a weakly scattering medium). When calculating the experiment in a three-dimensional formulation using the FRIGATE program, a numerical effect consisting in the fact that the use of numerical DDL schemes of the DS_N method of discrete ordinates for calculating the radiation transfer from such a surface source leads to an underestimation of the calculated results in the surface layers of the shielding composition by up to 50% for a spherical source has been revealed. In this paper, to reduce the methodical error of the numerical scheme, it has been proposed to apply a well-known method for representing a surface source as a volumetric one in a small region. The proposed method has been tested on a model problem and has proved to be very effective.</p>