Lead and tin targets for reducing polonium waste

The purpose of paper is in studying perspectives of new target materials and coolants for accelerator-driven systems (ADS). The main concern is for their improvement using an isotopic tailoring technique to reduce post irradiation activity. Two types of materials are explored. The first one is molten lead used in fast reactors as a coolant. Calculation using the FISPACT-3 code shows that 30 years irradiation of natural lead in a hard neutron spectrum, leads to accumulation of long-lived toxic radionuclides, Bi-207, Bi-208, and Pb-210, that extends the cooling time of activity to the clearance level up to 50 years. For using lead isotope, Pb-206, instead of Pb-nat it is possible to shorten this time. In this case, the concentration of the most toxic radionuclide polonium isotope, Po-210, essentially decreases. Another material which can be isotopically tailored is molten tin. A complete absence of alpha-active nuclides, high boiling point, wide operation temperature range gives some advantages for tin over lead. Tin target can compete with lead target as an effective proton-to-neutron converter. In order to enhance the neutron yield, heavy tin isotope of Sn-124 is to be used. The calculation performed with DISCA code shows that the neutron yield of Sn-124 is 15 % less than the one of Pb-natural for protons with the energy of 400 to 800 MeV. From this data lead enriched by isotope, Pb-206, and tin enriched by isotope, Sn-124, can be considered as an effective target-converter materials for reducing toxic alpha-active wastes such Po-210. (author)