Light materials with small atomic mass (light or heavy water, graphite, and so on) are usually used as a neutron reflector and moderator. The present paper proposes using a new, heavy element as neutron moderator and reflector, namely, “radiogenic lead” with dominant content of isotope 208Pb. Radiogenic lead is a stable natural lead. This isotope is characterized by extremely low micro cross-section of radiative neutron capture (~0.23？mb) for thermal neutrons, which is smaller than graphite and deuterium cross-sections. The reflector-converter for a fast reactor core is the structure capable of transforming some part of prompt neutrons leaked from the core into the reflected neutrons with properties similar to those of delayed neutrons, that is, sufficiently large contribution to reactivity at the level of effective fraction of delayed neutrons and relatively long lifetime, comparable with lifetimes of radionuclides-emitters of delayed neutrons. It is evaluated that the use of radiogenic lead makes it possible to slow down the chain fission reaction on prompt neutrons in the fast reactor. This can improve the fast reactor safety and reduce some requirements to the technologies used to fabricate fuel for the fast reactor. 1. Introduction Importance of such physical characteristic as prompt neutron lifetime for nuclear reactor safety is well known for a long time and was reflected in numerous publications, for example, in one of such fundamental works as [1]. The longer prompt neutron lifetime could produce the most favorable effects on nuclear reactor safety under conditions of the reactivity-induced accidents. The positive role to be played by the radiogenic lead, that is, lead with a dominant content of isotope 208Pb, as a coolant for fast reactor safety was first noted in works [2, 3], where a possibility for significant improvement of the coolant temperature reactivity coefficient was shown. Later, some possibilities for improving other neutron-physical and thermal-hydraulic parameters of power fast reactors were considered at usage of the radiogenic lead as a coolant [4]. This direction of researches has been developed in work [5], where, in addition to the aforementioned possibilities, the prospects of the radiogenic lead applications for developing high-flux accelerator-driven systems capable to transmute radioactive wastes and for upgrading proliferation resistance of advanced Pu-based fuel compositions were investigated too. Extension of prompt neutron lifetime in fast reactors with the radiogenic lead as a neutron reflector was first proposed
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