[en] The solution of the simplest model problems of neutron transport in P₁ and 2P₀ approximations is considered. On the basis of the analysis of obtained expressions possible errors arising when using the spherical harmonics method for calculation of nuclear reactor parameters are studied. It is concluded that in the case of detailed group analysis (narrow groups) diffusion approximation underestimates a reflector efficiency in upper groups. If the cross sections of neutron leakage in these groups are high enough the reflector thickness increase results in the flux decrease. In the 2P₀ approximation in this case negative fluxes directed from a boundary occur. When calculating the fast reactors this effect effect can result in general underestimating of the reflector role and as a sequence in underestimating of neutron efficient multiplication factor ksub(eff). As the total number of neutrons in a group is provided by the diffusion approximation in general correctly and the number of reflected neutrons is underestimated in the case discussed the overestimating of the number of neutrons slowing-down in a reflector and penetrating through it is to be expected. For thermal reactors the overestimating of ksub(eff) is to be expected if neutron slowing-down in a reflector is essential in the general bahance and the reflector is thick