[en] The purpose of paper is the search of a technique for hardening the fast reactor neutron spectrum and by this way, increasing the burning of Np-237, Am-243, and other minor actinides (MA) fissionable preferentially under hard neutron spectrum. The main concern is for using lead isotope, Pb-208, as a reactor coolant instead of natural lead, Pb-nat, which is the mixture of isotopes. Pb-208 has the high inelastic scattering threshold of 2.61 MeV. So, it slowly moderates the fission spectrum. The contribution of liquid metal coolants: sodium, Pb-nat, and Pb-208, minor actinides: Np-237, Am- 241, and Am-243, elements of structural materials: Fe, Cr, Ni, etc, to the fast-reactor neutron spectrum for a blanket is analyzed. For this calculation using Monte-Carlo Code 'MMKFK' is conducted. As a result, it is shown that the incineration of MA in the blanket is increased from 18 to 26 percent when one uses Pb-208 as a coolant instead of the usual natural lead. (author)