Heterogeneous Target to Reduce the Toxicity of Spallation Products in an Accelerator-Driven System

This paper addresses radiological issues that are unique for accelerator-driven neutron generation with much attention given to the limited area in a spallation target that encloses the propagation of high-energy cascade reactions. At certain beam options, a cascade of neutron-producing processes leaves the alpha-emitting spallation products belonging to the class of rare earths, like ₆₂¹⁴⁶Sm, ₆₄¹⁴⁸Gd, ₆₄¹⁵⁰Gd, and ₆₆¹⁵⁴Dy, whose overall toxicity in a lead target might overrun the alpha-emitting activation product ₈₄²¹⁰Po. To suppress their accumulation, the concept of a heterogeneous liquid-metal spallation module is proposed. This concept envisages the separation of a spallation target into two zones with specifically designated roles of neutron production and neutron multiplication. The main idea is to localize the proton-induced neutron production in a material with Z number <60 so as to exclude accumulation of problematic rare earths. Radioactive ₅₀¹²⁶Sn from fission products is considered as a material for this zone. Such a configuration not only lifts the great deal of spallation product burden from the lead target but also helps in eliminating the most troublesome long-lived fission ash, and what is important is that, compared to the bulk lead target, there is no appreciable detrimental effect on the overall neutron production.