MSR Shielding 1 Shielding 1 1 Introduction There are several types of shielding for space systems however in this context shielding is needed primarily to protect against biologically damaging ionizing radiation resulting from fission and fission product decay emitted by the core The objective of this group was to design a system that will reduce the nominal radiation dose received by crew and radiation sensitive instrumentation to as low as reasonably achievable ALARA This chapter establishes dose limits to the crew and describes various methods of shielding the MSR to reduce the nominal radiation dose to these limits It is important to note here that constraints of a particular mission or campaign dictate the shielding design An optimal shield is dependent upon the local topography soil composition distance from habitat and exploration region of the crew Instead of tailoring a design for a particular mission type the team will chose the most robust and flexible shielding system 1 2 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 5 rem 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 and Martian surfaces due to galactic cosmic radiation GCR 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 as radiation attenuation is exponential with 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 at which the astronauts will receive this dose As stated above the goal of the shield is to protect the crew regardless of mission type However it is unfeasible to shield to a dose rate of 2 0 mrem hr at distances very close to the core because the shielding system will become prohibitively large To appropriately integrate with the rest of the MSR we have imposed a mass constraint of 2 MT on the shielding system Thus exclusion zones are required 1 MSR Shielding 1 3 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 linear energy transfer and possible neutron activation of nuclei Materials comprised of high Z elements provide the high electron density and nuclear charge crucial to effective gamma attenuation Gamma rays interact primarily via interaction with orbital electrons in the form of photoelectric absorption Compton scattering and electron positron pair production By offering more loci for photonelectron interactions high Z materials generally attenuate gammas most effectively The most effective neutron shields are those which have a low atomic 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 heavy 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 shielding of choice for terrestrial reactors Since neutrons and gamma rays make up the primary sources of biologically hazardous radiation from a reactor a shielding system must consist of both low and high Z materials In addition because neutron attenuation produces secondary photons through inelastic scattering events it is suggested that the gamma shielding layer be the outermost layer of any two component design in order to stop secondary gamma radiation produced from neutron attenuation 1 4 Natural Shielding The first decision the design team had to make after determining the dose limit was whether it is best to construct a shield on Earth and launch it with the system or take the approach of in situ resource utilization piling Lunar Martian soil on the reactor as a shield In this section we will examine the possibility of natural shielding This method could substantial reduce the weight of the MSR however it will limit available shielding materials to the surface soil composition and require bringing machines capable of digging and transporting metric tons of Lunar or Martian rock The limiting factor for deploying any shielding technology is launch mass because therein lies the problem of cost estimated launch costs are several thousand dollars per kilogram 7 To launch a shield massive enough to sufficiently attenuate ionizing radiation from the reactor core the price tag will be large In light of the mass prohibitive nature of heavy and bulky shielding systems the use of natural shielding becomes 2 MSR Shielding attractive A remaining possibility is the use of a mixed system of artificial shielding to stop most of the radiation and surrounding it with a natural barrier for bringing the dose down to our specified limit of 2 0 mrem hr Natural Shielding on the Moon By utilizing material already existing on the moon s surface the weight requirement will fall substantially Given its barren landscape interspersed with mountains and valleys and a surface comprising a powdery soil the moon offers little for a makeshift shield other than the bare ground itself With basalt rock of an average density of 3 4 g cc 8 a shield of arbitrary thickness can in principle be constructed without the need for launching any extra weight other than the tools used for digging or blasting into the surface Lunar rock composition includes many oxides mostly silicon based but also oxides of refractory elements including calcium aluminum and titanium all of which are