Feasibility of $^{225}$Ac & $^{227}$Th production and fast neutron utilization in a versatile thorium target system

Abstract

A versatile thorium target system is proposed, providing rare α-emitters and a fast neutron source simultaneously. The system adopts affordable electron source and natural thorium target to generate (γ,xn) photonuclear reaction and fast neutrons (E > 0.8 MeV). From the MCNP-assisted depletion calculation, we found that this system can extract $^{225}$Ac and $^{227}$Th semi-permanently from their mother nuclei with periodic chemical separation procedure. Parametric studies regarding electron’s energy, beam diameter and dispersion were done to get optimal isotopic yields. A sufficient amount of mother nuclei can be retained from one-year beam irradiation and, for a 500 kW electron beam, the yearly yield of $^{225}$Ac and $^{227}$Th can be ~8.5 GBq and 49 GBq, respectively. We also evaluated the fast neutron flux from thorium target for electron beam bombardment. In order to utilize the intense and fast neutron source, we adopted split Th model and neutron-target is placed in-between separated Th targets. The reduction of $^{225}$Ac and $^{227}$Th yields is inevitable in the split Th model, we found that the yield loss can be minimized when the upper target’s thickness is higher than 1 cm. For a 2 cm thickness of upper Th target, about 7.3 % and 7.9 % reduction are expected in 225Ac and 227Th yields, respectively. However, the fast neutron flux in the neutron-target is about 2.21E+13/cm$^2$-sec, which is comparable to HANARO, the only research reactor in South Korea. Lastly, parametric studies were done to evaluate a heat removal capability of water-cooled thorium target system based on preliminary computational fluid dynamics (CFD) analyses and we found the possibility of managing the localized heat of a 500 kW electron beam.