[en] The technique for the two-group diffusion problem solution in the region consisting of rectilinear hexahedral prisms with different group cross sections is given. The technique is realized in the BIPR-6 programm, using the coarse pitch of the difference grid (one point on the prism-cassette in the view) and intended for WWER-type reactor physical calculations. Using characteristic methodical examples for the WWER-reactors it is shown that the BIPR-6 programm permits to obtain the problem solution with an error less than 1% for the values of the average cassete energy release