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MIT 22 33 - Shielding

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1 Shielding1.1 Introduction1.2 Dose Limit1.3 Radiation Interactions1.4 Natural Shielding1.5 Neutron Shielding Material1.5.1 Choice Summary1.5.2 Dose Rates without Shielding1.5.3 Material Selection1.5.4 Boron Carbide Performance and Burn-Up Modeling1.6 Gamma Shielding Materials1.6.1 Choice Summary1.6.2 Gamma Dose Rates without Shielding1.6.3 Material Selection1.6.4 Tungsten Performance Modeling1.7 Shielding Design1.7.1 Summary1.7.2 Geometry1.7.3 Design Discussion1.8 Alternate Design – The Three Layer Shield1.8.1 Shielding with Lunar Surface Material1.8.2 Shielding Options on Mars1.9 Future Work1.10 Summary1.11 ReferencesMSR – Shielding1 Shielding1.1 IntroductionThere are several types of shielding for space systems, however in this context, shieldingis needed primarily to protect against biologically damaging ionizing radiation resultingfrom fission and fission product decay emitted by the core. The objective of this groupwas to design a system that will reduce the nominal radiation dose received by crew andradiation-sensitive instrumentation to as low as reasonably achievable (ALARA). Thischapter establishes dose limits to the crew and describes various methods of shielding theMSR to reduce the nominal radiation dose to these limits. It is important to note here that constraints of a particular mission or campaign dictate theshielding design. An optimal shield is dependent upon the local topography, soilcomposition, distance from habitat, and exploration region of the crew. Instead oftailoring a design for a particular mission type, the team will chose the most robust andflexible shielding system.1.2 Dose LimitWhile the question of how much radiation is too much is contentious, the occupationalguidelines of the United States Department of Energy offer a suitable limit. These rulesstipulate that a radiation worker cannot receive greater than 5 rem in a year (or an averagedose rate of 0.57 mrem/hr) [1]. This value very nearly approaches the estimated 0.6mrem/hr for naturally occurring radiation on the Lunar and Martian surfaces due togalactic cosmic radiation (GCR) [2]. For astronauts, however, the acceptable doseincreases as NASA stipulates a maximum occupational dose of 50 rem/yr (5.9 mrem/hr).Thus, if the core radiation output is reduced to a compromise magnitude of 2.0 mrem/hr,the same system that protects crew from natural ionizing radiation can easily be adaptedto protect them from the remaining core radiation as radiation attenuation is exponentialwith shielding thickness.Dose rates due to radiation from the reactor are a function of distance from the reactor;therefore, it is meaningless to declare a limit on dosage without specifying a distance atwhich the astronauts will receive this dose. As stated above, the goal of the shield is toprotect the crew regardless of mission type. However, it is unfeasible to shield to a doserate of 2.0 mrem/hr at distances very close to the core because the shielding system willbecome prohibitively large. To appropriately integrate with the rest of the MSR, we haveimposed a mass constraint of 2 MT on the shielding system. Thus, exclusion zones arerequired.- 1 -MSR – Shielding1.3 Radiation InteractionsIn order to lay the groundwork for choosing appropriate shielding materials we will firstexamine the interaction of various types of radiation with matter. Charged particles areeasily attenuated, or absorbed, and are thus inconsequential in this shielding analysis.Gamma rays, on the other hand, are the most challenging to attenuate, as photonspenetrate matter more effectively than particulate radiation at a given energy. Neutrons,while slightly easier to shield than gammas, make up the most potentially damagingradiation component due to a high and varying linear energy transfer and possible neutronactivation of nuclei.Materials comprised of high Z elements provide the high electron density and nuclearcharge crucial to effective gamma attenuation. Gamma rays interact primarily viainteraction with orbital electrons in the form of photoelectric absorption, Comptonscattering and electron-positron pair production. By offering more loci for photon-electron interactions, high Z materials generally attenuate gammas most effectively. The most effective neutron shields are those which have a low atomic mass. Materialscomposed of low Z elements slow neutrons primarily via elastic scattering. Collisions ofneutrons with nuclei similar in mass transfer more energy per scatter than collisions withheavy nuclei, and so require fewer scattering events for the same average energy loss.Thus, hydrogenous materials, such as concrete and water, are often the neutron shieldingof choice for terrestrial reactors. Since neutrons and gamma rays make up the primary sources of biologically hazardousradiation from a reactor, a shielding system must consist of both low and high Zmaterials. In addition, because neutron attenuation produces secondary photons throughinelastic scattering events, it is suggested that the gamma-shielding layer be theoutermost layer of any two-component design in order to stop secondary gammaradiation produced from neutron attenuation. 1.4 Natural ShieldingThe first decision the design team had to make, after determining the dose limit, waswhether it is best to construct a shield on Earth and launch it with the system, or take theapproach of in situ resource utilization – piling Lunar/Martian soil on the reactor as ashield. In this section, we will examine the possibility of natural shielding. This methodcould substantial reduce the weight of the MSR, however it will limit available shieldingmaterials to the surface soil composition and require bringing machines capable ofdigging and transporting metric tons of Lunar or Martian rock.The limiting factor for deploying any shielding technology is launch mass, becausetherein lies the problem of cost: estimated launch costs are several thousand dollars perkilogram [7]. To launch a shield massive enough to sufficiently attenuate ionizingradiation from the reactor core, the price tag will be large. In light of the mass-prohibitivenature of heavy and bulky shielding systems, the use of “natural”

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