6. Radiation environment for manned systems: reactor systems developed to date are for unmanned power sources and the only radiation shielding has been for protection of electronic equipment. If operational missions involving manned vehicles advance, shielding weights will also likely increase to provide additional protection for humans. Advanced Multi-megawatt Space Nuclear Power Concepts A number of reactor concepts have been proposed as part of the Multi-megawatt Program (MMW) jointly sponsored by the US Department of Energy and the Strategic Defense Initiative Office (SDIO). The requirements driving the design of these concepts are primarily those of the SDI, but there are several aspects of the concepts which are consistent with the requirements of NASA. Table I is a listing of these concepts with a description of the type of system, power range (exact power levels are classified), and the developing vendor. The concepts are described in detail in the following sections. Boeing System description. Boeing has developed a hydrogen cooled open Brayton cycle system in the 10s of MWe range. This is a new reactor design and uses a fuel pin type core.
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