Use of raw Martian and Lunar soils for surface-based reactor shielding

For several decades, the idea of flying and landing a less-than-man-rated nuclear reactor for planetary surface applications has been considered. This approach promises significant mass savings and therefore reduction in launch cost. To compensate for the lack of shielding, it has been suggested the use of in-situ materials for providing radiation protection. This would take the form of either raw dirt walls or processed soil materials into blocks or tile elements. As a first step in determining the suitability of this approach, it is necessary to understand the neutron activation characteristics of these soils. A simple assessment of these activation characteristics was conducted for both Martian and Lunar soils using ORIGEN2.2. An average composition for these soils was assumed. As a baseline material, commonly used NBS-03 concrete was compared against the soils. Preliminary results indicate that over 2.5 times more gamma-radiation production of these soils vs. concrete took place during the irradiation phase (a baseline of 2.4 x 1011 neutrons/sec-cm2 was assumed). This was due primarily to radiative capture on Na23 and Mn55 and subsequent decay of their activation products. This is does not necessarily disqualify these materials as potential shielding material since the -radiation output was only in the order of 4.2 x 108 photons/cm3-sec. Furthermore, these soils did not show any significant activity after shutdown of the neutron source (the reactor), since all activation products had very short half lives. Their performance in this area was comparable to that of NBS-03 concrete.