[en] A method is described to calculate Ksub(ef) and the spatial-energy distributions of neutrons in the multizone HTGR type reactors. The diffusion algorithm is drawn in terms of the finite difference method to compute the three-dimensional elliptical equations. The numerical solution is realized by mixed iterations using the Yang-Frankel method. The algorithm is translated into action through the VAK code written in FORTRAN for a BESM-6 computer. The code does not limit the number of planes of node points along the reactor axis. When calculating in the two-group diffusion approximation the maximum number of nodes per plane is 4000. The physical model of neutron diffusion put into the VAK code can be used to calculate physical parameters of different nuclear reactors unless the diffusion approximation is valid