1 Shielding1.1 Mission Statement1.1.1 Dose Limit1.1.2 Criteria1.2 Radiation Interactions1.3 Natural Shielding1.4 Neutron Shielding Material1.4.1 Choice Summary1.4.2 Dose Rates without Shielding1.4.3 Material Selection1.4.4 Boron Carbide Performance and Burn-Up Modeling1.5 Gamma Shielding Materials1.5.1 Choice Summary1.5.2 Gamma Dose Rates without Shielding1.5.3 Material Selection1.5.4 Tungsten Performance Modeling1.6 Shielding Design1.6.1 Summary1.6.2 Geometry1.6.3 Design Discussion1.7 Alternate Design – The Three Layer Shield1.7.1 Shielding with Lunar Surface Material1.7.2 Shielding Options on Mars1.8 Limitations and Future Work1.9 Summary1.10 ReferencesMSR – Shielding1 ShieldingThere are several types of shielding for space systems, however in this context, shieldingis primarily to protect against biologically damaging ionizing radiation resulting fromfission and fission product decay. Ionizing radiation includes charged particles (protons,alpha and beta particles), neutrons and gamma rays. Each type of radiation interactsuniquely with different materials; therefore, we must consider the attenuation of eachtype of radiation separately. The secondary function of shielding is to protect the reactor against transients fromincoming radiation, from natural forces (i.e. temperature swings and dust storms), andfrom corrosion due to energetic particle bombardment. 1.1 Mission StatementThe objective of the shielding group was to design a shielding system that will reduce thenominal radiation dose received, from the reactor core, by crew and radiation-sensitiveinstrumentation to as reasonably low a level as possible. While the secondary functions ofcore protection are important, detailed analyses in these areas are out of the immediatescope of this shielding design and will be an area for further work. It is important to note here that constraints of a particular mission or campaign dictatesthe shielding design. An optimal shield design is dependent upon the local topography,soil composition, distance from habitat, and exploration region of the crew. Here thedesign team will chose the most robust and flexible shielding design for the MSR insteadof an optimized design for a particular type of mission.1.1.1 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 5000 mrem in a year (or anaverage dose rate of 0.57 mrem/hr) [1]. This value very nearly approaches the estimated0.6 mrem/hr for naturally occurring radiation on the lunar surface due to galactic cosmicradiation [2]. For astronauts, however, the acceptable dose increases as NASA stipulates amaximum occupational dose of 50 rem/yr (5.9 mrem/hr). Thus, if the core radiationoutput is reduced to a compromise magnitude of 2.0 mrem/hr, the same system thatprotects crew from natural ionizing radiation can easily be adapted to protect them fromthe remaining core radiation.It is true that if the crew is receiving 0.6 mrem/hr from GCR and 2.0 mrem/hr from thereactor core, they will in fact be receiving a total does of 2.6 mrem/hr – which is still wellwithin the allowed dosage for astronauts. Mainly, though, it is important for the reactorshielding system to bring reduce the dose to the right order of magnitude, because toattenuate radiation by a multiplicative requires adding a particular thickness whereas- 1 -MSR – Shieldingattenuating radiation by multiple orders of magnitude requires multiplying the availableshielding thickness. For both the above reasons, 2.0 mrem/hr will be the chosen target formaximum radiation output of the combined core/shielding system.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 this shieldingdesign is to create a robust shield that can protect the crew regardless of mission type.However, it is unfeasible to shield to a dose rate of 2.0 mrem/hr at a distance of one meterfrom the core because the shielding system will become prohibitively large. Thus, theshield design will necessarily include exclusion zones where the mass/volume constraintsoverride the need for the crew to get close to the core.1.1.2 CriteriaTo achieve the declared limit, the shielding group will have to make several key designdecisions. These include whether to construct the shield on Earth and launch it into space,whether to shield gamma rays with the same unit used to shield neutrons, what materialsto use, and what geometry to implement. To reach these decisions, the shielding group will have to take into account several designconstraints, which include lander weight tolerance, radiation emitted from fissioningnuclei in the core (both neutrons and gammas), radiation output from daughter nuclei,and effects of neutron reflection on core reactivity. By paying due attention to theseelements, the shielding group has developed a design that accomplishes the stated goalwhile still permitting plausible integration with the other surface reactor systems.The shielding design process differs from the design process of other major reactorcomponents in that the only real complexity in decision-making is determining what thedose limit should be. Other decisions come down to finding a solution that will reach thisdose limit while minimizing the mass of the system. Therefore, in this section, the formaldecision methodology presented in Chapter X is not applicable; only mass andattenuation characteristics drive the shielding design choice.1.2 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 LET