Author ORCID Identifier
Doctor of Philosophy
Mechanical and Nuclear Engineering
As future crewed, deep space missions are being planned, it is important to assess how spacecraft design can be used to minimize radiation exposure. Collectively with shielding material, vehicle shape and astronaut position must be used to protect astronauts from the two primary sources of space radiation: Galactic Cosmic Rays (GCR) and Solar Particle Events (SPE). GCRs, which are composed of low intensity, highly energetic, and fully ionized stable and meta-stable isotopes, are considered a chronic source of radiation risk to the astronauts. SPEs, which originate from solar coronal mass ejections, are composed mostly of high intensity protons that can vary in magnitude and duration and are considered an acute source of radiation risk to the astronauts.
A manned space vehicle is exposed to both high energy radiation sources in deep space and therefore both boundary conditions are used in this research. As a proxy to human risk from these sources, the whole body effective dose equivalent (ED) response function is used. The primary objectives of this research is to assess how shielding material, vehicle shape and astronaut position can be used to lower astronaut exposure. Additionally, secondary objectives were added: to evaluate the various configurations within OLTARIS (On-Line Tool for the Assessment of Radiation in Space) and to quantify the impact of the simplifications used in its development.
In addition to NASA's OLTARIS code that contains HZETRN (High Z and Energy TRaNsport) as its particle transport engine, this research uses Los Alamos National Laboratory's MCNP transport code for its simplification analysis. MCNP and OLTARIS are both versatile Boltzmann transport equation solutions with the ability to account for numerous atomic and nuclear reactions in shielding materials and biological systems to accommodate the GCR and SPE sources. Each accomplishes the transport of particles through materials differently. MCNP is a 3D code with one energy dimension, two angular dimensions, and one time dimension transport solution while OLTARIS is a 1D with one energy dimension transport solution with a diffusion-like solution for coupled neutrons and protons. Approximations are used with and in OLTARIS to analyze spacecraft in three spatial dimensions. MCNP utilizes the Monte-Carlo solution method and OLTARIS uses a power series solution to the one dimensional method of characteristics transport equations.
For cancerous effects, an ED career limit of 150 mSv is used as a proxy to the current risk limit of 3% REID (Risk of Exposure Induced Death) with a 95% confidence interval to express results in the number of safe days in deep space. Since ED depends on vehicle geometry, shielding material and shielding thickness, each variable is used in a comprehensive evaluation. Two vehicle shapes (spherical and right circular cylindrical), 68 common and future aerospace materials, material thicknesses ranging from 0.01 to 1000 g/cm2 and 5 astronaut positions are evaluated. Additionally the thickness, mass and volume profiles are calculated to further assess the validity of OLTARIS' ray trace method. Potentially, these results could be used to recommend the placement of rooms for an astronaut's everyday activity, such as living areas, work areas, and sleeping quarters. Furthermore, this research gives a basis for and outlines a methodology to further investigate the simplifications and configurations within OLTARIS, and the heavy-ion charged particle interactions within MCNP.
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Available for download on Friday, October 29, 2021