Book Description

Nuclear thermal propulsion (NTP) is a viable technology for near-term interplanetary exploration. Of the candidate mission architectures that are currently available for a crewed Mars mission, NTP permits the greatest flexibility in launch window selection and provides the shortest possible trip time, thereby reducing risks associated with cosmic ray exposure, microgravity, isolation, and life-support system reliability. A set of methods and tools were developed for designing and evaluating the radiation shielding required to enable the use of nuclear propulsion technologies. The tools produced in this work are coupled with MCNP6 to perform Monte Carlo radiation transport simulations. A method was developed for calculating the time-dependent dose rate that varies dramatically as the propellant (which acts as an inherent shield) is consumed, and as the delayed fission product sources of photons are produced during engine operation and decayed after shutdown. The radiation environment surrounding the nuclear engines was evaluated and mapped into useful iso-line contours, and was recorded and reimplemented as a source term for evaluation of shielding strategies. A multiobjective optimization algorithm (MOEA) was developed to explore candidate shield designs. The MOEA allows simultaneous changes in both discrete parameters (e.g. material selection) and continuous parameters (e.g. thickness and diameter). The design space is thus efficiently explored such that a set of optimal design solutions are produced that perform best at both minimizing the effects of penetrating radiation and doing so at minimal mass expense. Rather than produce and optimize single point-designs, this method produces curves of optimal solutions that are most useful in early-stage design trades where performance requirements are still an uncertainty. A tool for visualization of the design space following an optimization case was produced, and two example scenarios were evaluated to demonstrate the use of those methods produced in this work. Radiation shield design is a complex and iterative process that must be considered early in the design of both a nuclear engine and a full propulsion stage. There are many significant interfaces introduced by penetrating nuclear radiation that are not typical of most flight systems, and the methods employed in this work may be used to characterize their effects.