Abstract
The purpose of the present study is the justification of the possibility of improving fast reactor safety by surrounding reactor cores with reflectors made of material with special neutron physics properties. Such properties of 208Pb lead isotope as heavy atomic weight, small neutron absorption cross section, and high inelastic scattering threshold result in certain peculiarities in neutron kinetics of the fast reactor equipped with 208Pb reflector, which can significantly enhance reactor safety. The reflector will also make possible generation of additional delayed neutrons characterized by the “dead” time. This will improve the resistibility of the fission chain reaction to stepwise reactivity excursions and exclude prompt supercriticality. Let us note that generation of additional delayed neutrons can be shaped by reactor designers. The relevance of the study amounts to the fact that generation of additional delayed neutrons in the reflector will make it possible mitigating the consequences of a reactivity accident even if the introduced reactivity exceeds the effective fraction of delayed neutrons. At the same time, the role of the fraction of delayed neutrons as the maximum permissible reactivity for reactor safety is depreciated. Scientific originality of the study pertains to the fact that the problem of yield of additional neutrons with properties close to normal delayed neutrons, has not been posed before. The authors suggest a new method for enhancing safety of fast reactors by increasing the fraction of delayed neutrons due to the time delay of prompt neutrons during their transfer in the reflector. In order to benefit from the expected advantages, the following combination is acceptable: lead enriched by 208Pb is used as a neutron reflector while natural lead or other material (sodium, etc.) is used as a coolant in the reactor core.
Highlights
Kulikov GG et al.: Safety features of fast reactor. It is known (Bell and Glasstone 1970) that kinetics of nuclear reactors is comparatively slow with introduction of positive reactivity smaller than the effective fraction of delayed neutrons since the kinetics is determined by the average lifetime of neutrons (0.3–80 s)
In the case of introduction of reactivity exceeding the fraction of delayed neutrons the kinetics becomes fast, since it is determined by significantly shorter average lifetime of prompt neutrons: ~ 0.1–1 μs for fast reactors and ~ 0.1–1 ms for thermal reactors
Customary lead with constant isotopic composition (1.4% 204Pb, 24.1% 206Pb, 22.1% 207Pb and 52.4% 208Pb) as well as radiogenic lead characterized with variable isotopic composition since 208Pb, 206Pb and 207Pb isotopes are the final products of decay chains started with 232Th, 238U and 235U, respectively, are found in geological formations
Summary
It is known (Bell and Glasstone 1970) that kinetics of nuclear reactors is comparatively slow with introduction of positive reactivity smaller than the effective fraction of delayed neutrons since the kinetics is determined by the average lifetime of neutrons (0.3–80 s). In the case of introduction of reactivity exceeding the fraction of delayed neutrons the kinetics becomes fast, since it is determined by significantly shorter average lifetime of prompt neutrons: ~ 0.1–1 μs for fast reactors and ~ 0.1–1 ms for thermal reactors It is the use of light moderators in terms of atomic weight such as, for instance, water and graphite, which allows increasing the average neutron lifetime in thermal reactor by three–four orders of magnitude as compared with fast reactors and correspondingly slowing down the development of fission chain reaction on prompt neutrons. The first condition will allow retaining hard neutron spectrum while the second condition allows increasing the average neutron lifetime Such possibility is opened by application of heavy moderators in terms of their atomic weight with low neutron absorption
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