only a first-cut or system level estimate of shield mass to provide mission planners with preliminary estimates. Thickness Algorithms The NASA Lewis Research Center has been conducting feasibility studies and tradeoff analyses of space nuclear reactor power system applications since the early 1960s. These studies were primarily focused on manned interplanetary missions where shield mass is a significant fraction of the total power plant mass. In an attempt to estimate the shield mass more accurately, an extensive development of Monte Carlo shielding codes was undertaken to replace the point kernel methods previously used [3]. Further attempts to refine estimates of shield thickness and mass in optically thick manned shields were directed toward optimization of shield geometry and increased confidence in radiation attenuation calculations, with the incorporation of a variety of variance reduction techniques [4]. These efforts resulted in the development of a simple algorithm for shield thickness as a function of reactor power level. The algorithm is based on a thermal reactor spectrum and a combined neutron and gamma biological dose constraint. A six- layer shield configuration, composed of three layers each of tungsten and lithium hydride slabs, was required to meet the dose constraint, which included secondary gamma ray production within the shield. A similar algorithm for ‘instrument rated’ or unmanned shield thickness was developed from a series of MCNP and TWODANT computer code calculations carried out by the Los Alamos National Laboratory [5] for a two-layer shield composed of tungsten and lithium hydride slabs. Reactor leakage source terms were based on a fast spectrum SP-100 reactor core, and payload radiation constraints are in terms of neutron fluence and absorbed gamma dose. Application of these algorithms to estimate shield thickness for other dose constraints or separation distances is accomplished by use of the exponential attenuation law and the inverse distance squared law, respectively. Geometry Considerations Shield thickness estimates must be combined with geometrical considerations to provide the shield mass estimates required for mission planning studies. Geometrical considerations and constraints for instrument-rated shields differ from those of manrated shields. Typically, instrument-rated shield configurations must satisfy a constant dose constraint for radiation-sensitive instruments and electronics that are located on the power plant side of the reactor source. Therefore, shielding configurations that protect the power plant side of the reactor source would be required. In the case of man-rated shielding, typical configurations must satisfy a dose constraint that varies with the location of specific manned activities. These activities are not generally limited to a single hemisphere or side of the reactor source. In order to accommodate the wide range of possible manned and unmanned shield configurations that may be encountered, a variety of shield geometry options are employed. The ones discussed in this paper are the cone, the truncated cone, the 4^ and the in with a conical insert. The cone geometry, shown in Fig. 1, consists of a frustum of a right circular cone. This geometry can be used with either the manned or unmanned thickness algorithms for cases with only one dose constraint. The truncated cone geometry, shown in Fig. 2, is a frustum of a right circular cone that has been cut
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