Nuclear safety of the fast reactor with 208Pb-reflector at neutron flash above effective fraction of delayed neutrons

This paper aims at improving fast reactors safety through slowing-down their power runaways. The method is surrounding the core by the neutron reflector made of materials of heavy atomic weight and extremely low neutron absorption. The power runaways can be slowed down because of a long way for leakage neutrons to come back from distant layers of neutron reflector to the core. Time interval needed for leakage neutrons to come back to the core could become comparable with lifetime of delayed neutrons. So, this kind of reflector is able to transform some fraction of prompt leakage neutrons into additional group of delayed neutrons. It has been demonstrated that mean prompt neutron lifetime can be elongated roughly by three orders of magnitude with appropriate slowing-down the reactor power runaway. The most significant slowing-down the power runaways can be reached at the most dangerous reactivity insertion, i.e. at the reactivity larger than effective fraction of delayed neutrons. Thanks to the elongated mean prompt neutron lifetime and to the shortened time constant of fuel rods the negative consequences of the accidental reactivity insertion above effective fraction of delayed neutrons could be substantially weakened because, in addition to the Doppler-effect, the negative feedbacks on coolant and cladding temperatures receive enough time to actuate.