Optimization of Crew Shielding Requirement in Reactor-Powered Lunar Surface Missions

A.F. BarghoutyNASA Marshall Space Flight CenterMa_l Code VP62, Huntsville, Al 35812Tel.: (258) 961-7508; Fax.: (256) 961-7522Email: abdulnaszer.f.barghouty_nasa.govAbstract - On the surface of the moon -and not only during heightened solar activities- theradiation environment As such that crew protection will be required for missions lasting in excessof six months. This study focuses on estimating the optimized crew shielding requirement for lunarsurface missions with a nuclear option. Simple, transpo_'t-simaiation based dose-depth relations of thethree (9alactic, solarl and fission) radiation sources am employed in a 1-dimensional optimizationscheme. The scheme is developed to estimate the total required mass of lunar-regolith separatingreactor from crew. The scheme was applied to both solar maximum and minimum conditions. Itis shown that savings of up to 30_ in regolith mass can be realized. It is argued, however_ thatinherent variation and unceT"tainty -mainly in lunar regolith attenuation properties in addition tothe 7ndiat'ion quality factor- can easily defeat this and sirrvilar optimization schemes.I. INTRODUCTIONIn addition to other flight risks and hazards, spaceflight beyond the confines of the Earth's magnetic fieldwill have to face the challenges of space radiation ex-posure. In ex_ended lunar surface missions protection ofcrew and systems require shielding strategies against var-ious sources of space radiation fields, both natural andmamintroduced. Due to various degrees of variability,unpredictability, as well as -in some critical axeas- lackof basic data, guaranteeing sate levels of exposure posesa special challenge.Exposure estimates for shielding solutions as well asfor safety assessment must be formnlated and optimizedbossed on incomplete data, constrained by both teclmicaland nml-technieal factors. One of the more consequen-tial constraints, albeit somewhat subjective, is that ofALARA, "as low as reasonably achievable."A main ta_k of mission designers is to minimize re-quirements on structure mid function while insuring max-imum protection ibr crew and systems, consistent withALARA. ALAIZA is currently NASA's accepted guide-line as welt as being a part of the legal requirements withregard to ionizing radiation exposure and crew healthmid protection.Slfielding solutions and dose and risk a.nsessments to beconsistent with ALAKA muai rely on robust and accurateexposure estimates. Objective comparisons among thesesolutions wiU clearly l_eed reliable estimates as well.To varions degrees, ,_uch estimates are hampered by in-herent uncnrtainties; in basic knowledge of tim radiationenvironment itself, its transport and interaction in var-tous media of complex geometry and composition, andmost critically, in the human biological response to suchexposure.In the absence of more empirical data, on the one hand,and the increasing complexity of the modality and appli-cations by which (and for which) one arrives at theseestimates, on the other, such estimates are best viewedas guidelines rather then predictions.Given the expected doses, this parametric study fo-cuses on estimating the optimal crew shielding require-ment in lunar surface missions with a nuclear option.Possible missions are assmned to take place during bothlow and high solar activity. Specifity due to the mission'slocation on the lunar surface is not taken into account.For this study's purposes, these missions are assmned toonly include a crew habitation module and powered bya small fission reactor placed at some distance from thismodule. No other details about the reactor or the habi-tation module, e.g., their geometric configuratlons andspecific structures or subsystem are either assumed orused.Independent of the exact type or chemical composi-tion of the shielding material, any shielding solution willrequire a certain amount of areal density to reduce theexpected crew exposures to acceptable levels. For thisstudy, lunar regolith, albeit in an idealized form, is as-sumed to be the shielding material of choice. 1The estimates and method presented here are meantto help mission designers put in perspective the ex-pected cumulative exposure -due to natural aud intro-duced sources- vis-5_-vis the amount of regolith mass re-quired for crew protection. For example, for logisticalconsiderations, one may want to minimize the separationdistance between habitat and reactor while maintainingmaximunl protection. Conversely, one may wm_t to min-imize the amount of regolith to be used by maximizingthe distance. Ideally, in both extremes as welt as fro' allestimates in between, required regolith mass needs to beoptimized for each separation distance.Since shielding will be required and can be used forboth reactor and crew, a self-consistent approach wouldbe to estimate, at a given distance, the optimal and also