As a rule materials of small atomic weight (light and heavy water, graphite, and so on) are used as neutron moderators and reflectors. A new very heavy atomic weight moderator is proposed—radiogenic lead consisting mainly of isotope208Pb. It is characterized by extremely low neutron radiative capture cross-section (0.23 mbarn for thermal neutrons, i.e., less than that for graphite and deuterium) and highest albedo of thermal neutrons. It is evaluated that the use of radiogenic lead makes it possible to slow down the chain fission reaction on prompt neutrons in a fast reactor. This can increase safety of the fast reactors and reduce as well requirements pertaining to the fuel fabrication technology. Radiogenic lead with high208Pb content as a liquid-metal coolant of fast reactors helps to achieve a favorable (negative) reactivity coefficient on coolant temperature. It is noteworthy that radiogenic lead with high208Pb content may be extracted from thorium (as well as thorium-uranium) ores without isotope separation. This has been confirmed experimentally by the investigations performed at San Paulo University, Brazil.
Analysis of the radiological burden of the spallation products (SP) was performed for various spallation targets (lead, tungsten, tin). The radiological burden was discussed in terms of toxicity based upon the concept of Annual Limit on Intake (ALI) shows that alpha-emitting rare earths (146 Sm, 148 Gd, 150 Gd, 154 Dy) are dominant. Their toxicity was estimated at equilibrium state with and without their transmutation. It is concluded that, in terms of toxicity, accumulation of SP in the target is quite comparable with transmutation of fission products (FP) in the blanket of Accelerator-Driven System (ADS).
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