Spaceflights have been a significant development in technology in our modern age. Development of space crafts and voyages to space are, however, faced with several challenges. One of the biggest challenges is dealing with space radiation. Astronauts in spacecraft traveling beyond the low earth orbit are bound to face radiation problems since they are moving outside the earth's protective shield; the atmosphere and magnetic field of the earth. This entails exposure to a broad spectrum of space radiation. Space radiation differs from that of the earth in several ways. On earth humans are exposed to non-ionizing radiation; radiation that is of little energy though it can cause minor effects. In space, on the other hand, due to lack of the atmosphere and magnetosphere, astronauts become exposed to ionizing radiations such as galactic cosmic radiation from exploding stars, radiation trapped in the earth's magnetic field and solar energetic particles. The effects of exposure can be acute or chronic depending on the period of exposure, intensity of the radiation and the energy of the radiation. Acute effects include acute radiation syndrome that is characterized by nausea, vomiting, and fatigue. Chronic effects include cancer (Okunade, 2002).
The consequences of radiation exposure necessitate the development of countermeasures. One of the best countermeasures is the development of a radiation shield for the space crafts to ensure that the radiation limits that the individuals in the craft are exposed to are within the standard levels; levels that are not harmful to human beings. To ensure the efficiency of the radiation shield, it is essential to come up with a shield design that will ensure that meets the required standards (Kimura et al., 2014). In this paper, we look at a shielding design project. The aim of the project is to develop a radiation shield for a spacecraft running a reactor powered by uranium fuel. As a result, the design of the shield should not only cater for space radiation but also encompass protection from nuclear radiation from the uranium. Considering that the space travel is bound to take ten years, the design should also ensure that levels of exposure to radiation should be within the standards set by the Canadian Nuclear Safety Commission. The project also aims at designing a shield that will not melt in extreme temperatures to avoid loss of structural integrity, and the shield should not collapse under its weight. Despite all the other factors to consider in the shield design, the ultimate goal is to design a shield that is efficient but still costs effective.
There are several designs that can be adopted for the development of magnetic shields. For this project the available shielding methods or design included; electrostatic fields, plasma shielding, confined magnetic field and unconfined magnetic field. Each of these designs had to be analyzed in depth regarding requirements, advantage, disadvantages and cost effectiveness; this would enable selecting a suitable shielding method (Atwell, Saganti, Cucinotta & Zeitlin, 2004).
In this shielding design, an electrostatic field is confined within concentric spheres. Within these concentric spheres, the electrostatic field points outwards towards away from the positively charged inner sphere and to the negatively charged negative sphere. The equations for the design can be represented as follows:
Vr=Qa4poa+Qb4pob raVr=Qa4por+Qb4pob a<r<bVr=Qa+Qb4por rbIn the above equation Q is the sphere's charge, a is the inner and positively charged sphere's radius, b is the radius of the outer sphere that is negatively charged, and the final variable is e0 is free vacuum space permittivity. The outer sphere repels electrons of energies close to 1MeV. This is done at a low potential to ensure that the effect on incoming positively-charged ions is minimal. The magnitude of the electric field intensity at the surface of both shells is limited to 3* 107 V/m to ensure that the vacuum does not breakdown. The radii of the shells should be kept at a minimum of the order of several hundred to prevent the breakdown of the vacuum. Vacuum breakdown would result in charge conduction between the shell hence equalizing their potentials and eliminating the protective deflecting field.
The electrostatic field shield design is quite effective though it becomes unreasonable for interplanetary space-crafts due to its unsuitability for GCR shielding, its requirement for the electrostatic potential that exceeds the state of the art by a large margin, and the potential of the vacuum to breakdown rendering the shield useless. To overcome the challenges resulting from concentric shells the configuration of the shells can be altered into an asymmetric electrostatic radiation shield configuration. The basic design of this configuration involves the construction of a shield using a linear quadruple arrangement with a negative pole at each end and positive pole at the middle where the protected zone is positioned (Washburn, Blattnig, Singleterry & Westover, 2015).
The concept of this shield focuses on utilization of a large electrostatic field to repel positively charged particles. Secondly, the design encompasses a lower intensity magnetic field used to prevent the positive potential from attracting and accelerating the space electrons to very high energies. This magnetic field controls a cloud of free electrons which repels incoming electrons. As a result, the design shields SPEE protons. This design is advantageous due to a substantial reduction in weight of shielding material. Despite the substantial weight reduction, the shield faces technological hitches such as achieving electrostatic potential exceeding 200 MV is difficult in a spacecraft, plasma instabilities, and large energy quantity in the plasma. The mechanism utilized for radiation shielding, therefore, make plasma shield not feasible for deep space exploration.
Confined magnetic field
In this arrangement, concentric spheres are sued to limit the spatial extent of the magnetic field within some localized finite region of space around the space craft's volume that is inhabited. It makes use of Lorentz force to repel incoming particles from radiation environment in space. In the magnetic field, the motion of the charged particles is such that the magnetic field alters only the direction of travel of the particles, particles that miss the inhabited volume are said to be shielded. As a result of deflection and motion in the field, the particles emit bremsstrahlung radiation. The effects of this radiation are not significant if the particles involve are protons or heavier ions. However, in the case of incident electrons the bremsstrahlung radiation yields are quite significant due to the small mass of electrons.
For a charged particle moving under the influence of a magnetic field, the momentum per unit charge can be calculated as follows:
R=pcZeWhere p represents the momentum of the particle, Ze represents the charge and c is the speed of light in vacuum. It Is essential to not that the particle rigidity is also related to Larmor radius r (radius of curvature) of the trajectory of the particle in the magnetic field and magnetic field intensity as R=0.003Br, where B is he magnetic field's component that is perpendicular to the momentum of the particle. The units of r are meters, and R is gigavolts (GV).
Considering that the particle motion for heavy charged particles is often described regarding the kinetic energy of the particle per nucleon instead of its total momentum, the equation for R can be rewritten as follows:
R=A T2+2mc2TZeGiven that T is the kinetic energy of the particle per nucleon (GeV/nucleon), A is its mass number, and m is the rest of the mass of the nucleon. (mc2 = 0.939 GeV).
With this kind of design, the confined magnetic field becomes suitable for crew quarter in a craft to be used for deep space radiation environment and interplanetary exploration. This is due to the use of superconducting coils arranged in a Faraday cup arrangement. At the moment, this design is not able to shield against the GCR and SPE but shortly, it will be able to shield effectively against these radiations (Atwell, Saganti, Cucinotta & Zeitlin, 2004).
Unconfined Magnetic Field
Mimicking of the dipole-like magnetic field of the earth can be used to provide radiation shielding in space; this forms the concept behind the design. In the design, the shape of craft is assumed to be cylindrical, and it is also assumed that craft is surrounded by a dipole-like magnetic field. The m origin of the magnetic field is assumed to be the current passing through the coils on the spacecraft or the skin of spacecraft. It is a good design for shielding large space colonies. However, the exposure levels are still quite high than those applicable to Earth's general population. Recent adjustments have been proposed for the design to provide directional protection against SPE protons. However, the design is still flawed by the lack of shielding against GCR particles (Sahin & Mehmet Sahin, 2001).
Shielding effectiveness criteria
When selecting a shielding design, it is important to determine which design is most effective for the intended space exploration. For instance, to determine if a proposed electromagnetic shield configuration is more feasible compared to a passive, bulk material shield, it is essential to consider the appropriate radiation environment. Most of the proposed design for active radiation shielding consider only the space radiation environment and omit other essential components that are part and parcel of the complete spectrum that needs to be addressed by a spacecraft's shield configuration. Furthermore, the merit figure used to efficiency evaluation must be relevant to radiation protection purposes. It is also essential to compare the masses and costs of various shielding alternatives to arrive at an engineering solution to the shielding problem. However, the actual engineering criteria are based on the best estimates of the actual risk of crew members developing health hazards such as fatal cancer as a result of exposure to the space radiation environment (Washburn, Blattnig, Singleterry & Westover, 2015).
Considering the human risk of contracting health hazards, three criteria were used in selecting the most suitable shield design for the spacecraft. These measures needed to meet the National Council on Radiation Protection and Measurements (NCRP) standards and those of the Canadian Nuclear Safety Commission. The criteria for comparing shielding methods, therefore, involved:
Making sure that the radiation environment used included radiation from solar particles events, galactic cosmic rays and the uranium fuel used for the spacecraft engine.
Secondly, the appropriate target radiation levels for establishing estimated shielding requirements for comparison of shielding methods needed to be a realistic one and based on the threats that humans face in space travel.
Mass estimation included not only the shield components but also the inherent shielding provided by the structure of the spacecraft and internal components.
Choice of Shield Materials
From computation done in previous researches, it is evident that lead is an excellent choice of material for construction of radiation protective shield barrier. Other shielding materials are usable. However, use of other materials results in greater weight and volume of the structure than the one that would have been used to achieve radiation protection using the lead material. There are various forms of lead that can be used...
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