Space Nuclear Reactor Shields for Manned and Unmanned Applications BARBARA I. McKISSOCK & HARVEY S. BLOOMFIELD Summary Missions which use nuclear reactor power systems require radiation shielding of pay load and/or crew areas to predetermined dose rates. Since shielding can become a significant fraction of the total mass of the system, it is of interest to show the effect of various parameters on shield thickness and mass for manned and unmanned applications. Algorithms were developed to give the thicknesses needed if reactor thermal power, separation distances and dose rates are given as input. The thickness algorithms were combined with models for four different shield geometries to allow tradeoff studies of shield volume and mass for a variety of manned and unmanned missions. The shield design tradeoffs presented in this study include the effects of: higher allowable dose rates; radiation hardened electronics; shorter crew exposure times; shield geometry; distance of the payload and/or crew from the reactor; and changes in the size of the shielded area. Specific NASA missions that were considered in this study include unmanned outer planetary exploration, manned advanced/evolutionary space station and advanced manned lunar base. Introduction Potential mission options as identified and studied by NASA [1, 2] to expand human presence in space and increase scientific knowledge of our solar system require power sources with capabilities beyond those launched to date. Nuclear reactor power systems enable or enhance many missions that require high capacity and long operational lifetimes, as well as providing independence from variations in solar energy availability. Nuclear reactor power systems include shielding to reduce radiation doses at the payload and/or crew areas to allowable levels. This shielding can be a significant portion of the total power system mass, especially for manned missions. Due to this potentially large impact on system mass, and therefore impact on launch capability and cost, shield design and analysis is carried out with an emphasis on finding a minimum mass configuration that meets radiation dose requirements. The design of minimum mass nuclear reactor shielding for manned and unmanned space mission applications is a complex undertaking that requires detailed design considerations of reactor source spectrum, power plant and payload geometry, as well as a knowledge of the particular mission application constraints. Application and comparative studies of space nuclear power plant applications will, however, require Barbara I. McKissock & Harvey S. Bloomfield, NASA, Lewis Research Center, Cleveland, OH 44135, USA.
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