Feasibility study on the production of ²²⁸ Th as a ²¹² Pb generator using the experimental fast reactor Joyo

The growing interest in targeted alpha therapy has highlighted the need for stable and efficient production methods for multiple alpha-emitting nuclides. This study investigates an efficient method for producing 228Th, a parent nuclide for 212Pb generator, via multiple neutron capture reactions using 226Ra as the target nuclide under spectrum-tailored irradiation in a fast reactor. By installing moderators into the radial reflector region, the effects of moderator materials, moderator volume fraction, and stepwise irradiation schemes on the transmutation pathway from 226Ra to 228Th were systematically evaluated. The results indicate that irradiation of a 1-g 226Ra target for four cycles using a beryllium moderator at a 100% volume fraction, which is optimal for the 226Ra (n,γ) reaction, followed by one cycle using a ZrH1.65 moderator at a 50% volume fraction, which is optimal for the 227Ac (n,γ) reaction, enables annual production of approximately 37 Ci of 228Th per gram of 226Ra. This activity corresponds to the 228Th inventory after five irradiation cycles and includes radioactive decay during irradiation and cooling periods.