EVOLUTION OF FAST REACTOR CORE SPECTRA IN CHANGING A HEAVY LIQUID METAL COOLANT BY MOLTEN PB-208

摘要

In the paper neutron spectra of fast reactor cooled with lead-bismuth or lead-208 are given. It is shown that in changing the coolant from lead-bismuth to lead-208 the core neutron spectra of the fast reactor FR RBEC-M are hardening in whole by several percents when a little share of low energy neutrons (5eV - 50 keV) is slightly increasing. The shift of spectra to higher energies permits to enhance the fuel fission while the increased share of low energy neutrons provides more effective conversion of uranium-238 into plutonium due to peculiarity of ~(238)U neutron capture cross section. Good neutron and physical features of molten ~(208)Pb permit to assume it as perspective coolant for fast reactors and accelerator driven systems. The one-group cross sections of neutron radiation capture, <σ(n,g)>, by ~(208)Pb, ~(238)U, ~(99)Tc, mix of lead and bismuth, ~(nat)Pb-Bi, averaged over neutron spectra of the fast reactor RBEC-M are given. It is shown that one-group cross sections of neutron capture by material of the liquid metal coolant consisted from lead enriched with the stable lead isotope, ~(208)Pb, are by 4-7 times smaller than <σ(n,g)> for the coolant ~(nat)Pb-Bi. The economy of neutrons in the core cooled with ~(208)Pb can be used for reducing reactor's initial fuel load, increasing fuel breeding and transmutation of long lived fission products, for example ~(99)Tc. Good neutron and physical features of lead enriched with ~(208)Pb permit to consider it as a perspective low neutron absorbing coolant for fast reactors and accelerator driven systems.