MSR Shielding 1 Shielding There are several types of shielding for space systems however in this context shielding is primarily to protect against biologically damaging ionizing radiation resulting from fission and fission product decay Ionizing radiation includes charged particles protons alpha and beta particles neutrons and gamma rays Each type of radiation interacts uniquely with different materials therefore we must consider the attenuation of each type of radiation separately The secondary function of shielding is to protect the reactor against transients from incoming radiation from natural forces i e temperature swings and dust storms and from corrosion due to energetic particle bombardment 1 1 Mission Statement The objective of the shielding group was to design a shielding system that will reduce the nominal radiation dose received from the reactor core by crew and radiation sensitive instrumentation to as reasonably low a level as possible While the secondary functions of core protection are important detailed analyses in these areas are out of the immediate scope 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 dictates the shielding design An optimal shield design is dependent upon the local topography soil composition distance from habitat and exploration region of the crew Here the design team will chose the most robust and flexible shielding design for the MSR instead of an optimized design for a particular type of mission 1 1 1 Dose Limit While the question of how much radiation is too much is contentious the occupational guidelines of the United States Department of Energy offer a suitable limit These rules stipulate that a radiation worker cannot receive greater than 5000 mrem in a year or an average dose rate of 0 57 mrem hr 1 This value very nearly approaches the estimated 0 6 mrem hr for naturally occurring radiation on the lunar surface due to galactic cosmic radiation 2 For astronauts however the acceptable dose increases 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 adapted to protect them from the remaining core radiation It is true that if the crew is receiving 0 6 mrem hr from GCR and 2 0 mrem hr from the reactor core they will in fact be receiving a total does of 2 6 mrem hr which is still well within the allowed dosage for astronauts Mainly though it is important for the reactor shielding system to bring reduce the dose to the right order of magnitude because to attenuate radiation by a multiplicative requires adding a particular thickness whereas 1 MSR Shielding attenuating radiation by multiple orders of magnitude requires multiplying the available shielding thickness For both the above reasons 2 0 mrem hr will be the chosen target for maximum 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 at which the astronauts will receive this dose As stated above the goal of this shielding design 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 meter from the core because the shielding system will become prohibitively large Thus the shield design will necessarily include exclusion zones where the mass volume constraints override the need for the crew to get close to the core 1 1 2 Criteria To achieve the declared limit the shielding group will have to make several key design decisions 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 materials to use and what geometry to implement To reach these decisions the shielding group will have to take into account several design constraints which include lander weight tolerance radiation emitted from fissioning nuclei 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 these elements the shielding group has developed a design that accomplishes the stated goal while still permitting plausible integration with the other surface reactor systems The shielding design process differs from the design process of other major reactor components in that the only real complexity in decision making is determining what the dose limit should be Other decisions come down to finding a solution that will reach this dose limit while minimizing the mass of the system Therefore in this section the formal decision methodology presented in Chapter X is not applicable only mass and attenuation characteristics drive the shielding design choice 1 2 Radiation Interactions In order to lay the groundwork for choosing appropriate shielding materials we will first examine the interaction of various types of radiation with matter Charged particles are easily attenuated or absorbed and are thus inconsequential in this shielding analysis Gamma rays on the other hand are the most challenging to attenuate as photons penetrate matter more effectively than particulate radiation at a given energy Neutrons while slightly easier to shield than gammas make up the most potentially damaging radiation component due to a high and varying LET and possible neutron activation of nuclei Materials comprised of high Z elements posses the high electron density crucial to effective gamma attenuation Gamma rays interact primarily via interaction with orbital 2 MSR Shielding electrons in the form of photoelectric absorption Compton scattering and electronpositron pair production In this type of reactor with high Z fuel elements and a fast neutron spectrum pair production is the dominant mode of photon attenuation 3 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 atomics mass Materials composed of low Z elements slow neutrons primarily via elastic scattering Collisions of neutrons with nuclei similar in mass transfer more energy per scatter than collisions with