Unmanned Missions One of the unmanned spacecraft/mission applications considered for small reactors is the Mariner Mark II Cassini spacecraft and mission [6]. Since the radiation hardness for the electronics and instrumentation was undefined, a range of integrated gamma and neutron exposures from 7.5 krad and 1.5 X 10" nvt to 300 krad and 6X 1012 nvt were studied. Unmanned conical shield masses for shielded spot size diameters of 8-18 m and a range of separation distances from 10-40 m were compared using a 20 kWt fast spectrum reactor. The shield masses needed at the two dose extremes for the range of shielded spot size diameters are shown in Fig. 6, for a payload separation distance of 20 m. Figure 7 shows the effect of different payload separation distances on shield mass at the two dose extremes for a 10 m shielded spot size diameter. Manned Missions Two manned missions that have been considered for possible applications of nuclear power are a manned lunar base [7] and a growth space station [8], Different shielding options were considered for each of these applications which will be discussed separately. The shielding for the lunar base is a truncated conical shadow shield, with a halfangle determined by the separation distance and the size of the shielded area to which manned activities would be limited. Analytically, it is treated as a shadow shield, neglecting surface backscattering and secondary radiation effects. Results from a preliminary study of attenuation and scattering by the lunar surface material showed negligible backscattering effects, and sufficient attenuation to produce shield equivalent doses within 7 m of soil. Dose rates of 5 and 25 rem/30 days were considered for
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