Radiation Shielding Options for the Affordable Fission Surface Power System

The Affordable Fission Surface Power System (AFSPS) is a proposed power source for an outpost capable of housing six humans for up to six weeks on the lunar surface and emphasizes the design principles of low risk and affordability over high performance. The radiation shield is the most massive component of the reactor system and its effect on launch mass greatly affects the affordability of the AFSPS. Potential shielding materials include lithium hydride, enriched boron-10 carbide, water, borated water, beryllium, boron-doped beryllium and zirconium hydride. Zirconium hydride is the most effective neutron attenuator and also significantly attenuates gamma radiation, but at a significant mass penalty. The other neutron attenuating materials all require the addition of a tungsten layer to provide significant gamma attenuation. Based on neutron radiation alone, lithium hydride is the lightest of the potential attenuators, followed by water and borated water. When gamma radiation is also considered, the lithium hydride/tungsten shield is shown to be the lightest composite shield with a combined mass of 3246 kg, followed by the borated water/tungsten shield (3479 kg). The boron carbide/tungsten shield has a total mass of 4129 kg, but represents significantly less development risk.