ADVANTAGES OF THE FAST REACTOR CORE SURROUNDED BY THE PHYSICALLY THICK NEUTRON REFLECTOR MADE OF LEAD-208
The paper analyzes the effects produced by the use of lead isotope 208Pb instead of natural lead as a coolant and neutron reflector on neutron-physical parameters of fast reactor. A possibility is demonstrated to reach substantial (up to three orders of magnitude) elongation of mean prompt neutron lifetime in the fast reactor with physically thick <sup>208</sup>Pb-reflector. So large elongation of prompt neutron lifetime opens an opportunity for remarkable enhancement of the reactor safety under conditions of reactivity-induced accidents. Numerical analyses have shown that it became possible to form high flux of slowed down and epithermal neutrons in <sup>208</sup>Pb-reflector even at large distances from the reactor core. Spacious area of high neutron flux out of the reactor core may be used to transmute long-lived fission products and to produce significant quantities of necessary isotopes. Weak neutron absorption in <sup>208</sup>Pb makes it possible to reach remarkable (up to 20 %) reduction of plutonium fraction in mixed uranium-plutonium fuel composition. It is proposed to place a layer with neutron absorber in <sup>208</sup>Pb-reflector above the reactor core for the reactivity control with no variations in neutron-physical properties of the reactor core. If reactivity of the control rods in the reactor core is lower than effective fraction of delayed neutrons, then the reactor is obviously subcritical on prompt neutrons of the reactor core. In this case the danger of the reactivity-induced accident on prompt neutrons of the reactor core disappears. If the reactivity-induced accident is caused by joint action of the neutrons produced in the reactor core and in <sup>208</sup>Pb-reflector, then the accident propagation will be substantially slowed down because of the longer lifetime of neutrons in <sup>208</sup>Pb-reflector.