Radiation Shielding and Orbital Safety Strategies for a Space Nuclear Propulsion System

Nuclear propulsion systems represent a key option for future deep space missions, since they are characterised by high thrust and specific impulse alongside with the capability for extended power generation when operated in bimodal Nuclear Thermal Propulsion (NTP)/Nuclear Electric Propulsion (NEP) mode. The following work investigates the design and optimisation of the radiation shielding for a spacecraft equipped with a bimodal reactor concept developed at the University of Pisa, with the parallel objective of determining the departure and final disposal Nuclear Safe Orbit (NSO) necessary to guarantee environmental and health safety upon atmospheric re-entry. Following a brief historical overview and a discussion on the fundamental principles of orbital mechanics relevant for propulsion mission analysis, a state of the art review of NTP systems is presented, focusing on shielding strategies, safety considerations, NSO determination, and the assessment of two postulated accident scenarios: inadvertent re-entry and criticality. Building on this review, this study focuses on a specific case study: a lunar cargo mission featuring two hours of NTP operation and 190 days of NEP cruise. Using the OpenMC transport code, the shielding geometry for the University of Pisa bimodal reactor was defined and iteratively optimised to reduce neutron flux and photon dose rates to acceptable levels for spacecraft electronics while limiting mass penalties. The reactor burnup and the resulting activity inventory were then calculated to evaluate the NSO altitudes required for both departure and final disposal, ensuring compliance with safety standards. The results presented in this work should be regarded as a first-order analysis tool, providing reference values for shielding design and mission planning while maintaining computational efficiency through the use of simplified models in the preliminary phases of the analysis. The optimised shielding configuration will subsequently be tested on the complete reactor geometry to confirm its effectiveness under realistic operational conditions, providing a solid basis for further and more detailed studies.