RADIOGENIC LEAD WITH DOMINANT CONTENT OF ~(208)Pb: NEW COOLANT AND NEUTRON MODERATOR FOR INNOVATIVE NUCLEAR REACTORS

摘要

The advantages of radiogenic lead with dominant content of ~(208)Pb as a reactor coolant with respect to natural lead are caused by unique nuclear properties of ~(208)Pb which is a double-magic nucleus with closed proton and neutron shells. This results in significantly lower micro cross section and resonance integral of radiative neutron capture by ~(208)Pb than those for numerous light neutron moderators. The extremely weak ability of ~(208)Pb to absorb neutrons results in the following effects. Firstly, neutron moderating factor (ratio of scattering to capture cross sections) is larger than that for graphite and light water. Secondly, age and diffusion length of thermal neutrons are larger than those for graphite, light and heavy water. Thirdly, neutron lifetime in ~(208)Pb is comparable with that for graphite, beryllium and heavy water what could be important for safe reactor operation. The paper presents some results obtained in neutronics and thermal-hydraulics evaluations of the benefits from the use of radiogenic lead with dominant content of ~(208)Pb instead of natural lead as a coolant of fast breeder reactors. The paper demonstrates that substitution of radiogenic lead for natural lead can offer the following benefits for operation of fast breeder reactors. Firstly, improvement of the reactor safety thanks to the better values of coolant temperature reactivity coefficient and, secondly, improvement of some thermal-hydraulic reactor parameters. Radiogenic lead can be extracted from thorium sludge without isotope separation as ~(208)Pb is a final isotope in the decay chain of ~(232)Th.