Abstract
The National Aeronautics and Space Administration (NASA) is currently studying a variety of mission scenarios designed to establish a permanent human presence on the surface of Mars. Nuclear Electric Propulsion (NEP) is one of the possible designs for this program. During the initial stages of vehicle design work, careful consideration must be given to, not only the shielding requirements of natural space radiations, but to the shielding and configuration requirements of the on-board reactors. In this work, the radiation transport code MCNP has been used to make initial estimates of crew exposures to reactor radiation fields for a specific manned NIFP vehicle design. In this design, three 25 MWt scaled SP-1 00-class reactor radiations are shielded by three identical shields. Each shield layers beryllium, tungsten, and lithium hydride between the reactor and the crew compartment. In a related project, separate calculations are made of both the exiting neutron and gamma fluxes from the reactors during beginning-of-life, full-power operation. This data is then used as the source terms for particle transport in MCNP. The total gamma and neutron fluxes exiting the reactor shields are recorded and separate transport calculations are then performed for a 10 g cm-2 crew compartment aluminum thickness. Estimates of crew exposures have been assessed for various thicknesses of the shield tungsten and lithium hydride layers. A minimal tungsten thickness of 20 cm is required to shield the reactor photons below the 0.05 Sv y-I limit. In addition to a 20-.cm thick tungsten layer, a 40-cm thick lithium hydride layer is required to shield the reactor neutrons below the annual lin-At. If the tungsten layer is 30-cm thick, the lithium hydride layer should be at least 30-cm thick. These estimates do not take the photons generated by neutron interactions inside the shield into account because the neutron MCNP cross sections did not allow reliable estimates of photon production.
Delisle, Christine Louise (1994). An assessment of crew exposures during nuclear-powered manned Mars missions. Master's thesis, Texas A&M University. Available electronically from
https : / /hdl .handle .net /1969 .1 /ETD -TAMU -1994 -THESIS -D354.