Radioisotope and Nuclear Technologies for Space Exploration

Radioisotope heat sources and power systems, traditionally fuelled by 238Pu, have been developed and used for spacecraft thermal management and to provide electrical power during many deep space and planetary science missions. The use of fission reactors in space, however, has been limited to high power applications in Earth orbit. Previous ground based research programs conducted by the U.S. Atomic Energy Commission demonstrated the principal of nuclear thermal rocket propulsion but to date, flight heritage of nuclear propulsion has been limited to nuclear-electric propulsion. The development of space nuclear systems and tributary components that are capable of meeting the rigors of space flight is of paramount importance. Performance, lifetime and operational safety under all foreseeable conditions are essential considerations that must be made. The selection of appropriate materials and environmental compatibility is vital to the success of any given design. The ability for radioisotope heat sources to survive the extreme temperatures and mechanical loads associated with launch related accidents, is both legally mandated and necessary for the protection of life and the Earth’s environment. Nuclear fuels for fission systems must provide equal protection during accidents while the integral design ensures that a reactor remains in a safe configuration. A historical overview of nuclear systems for space is presented. Traditional and modern system designs and fabrication techniques are discussed. Applicable solid state and mechanical power conversion methods are described and their performances are evaluated. Consideration is made for the effect of radioisotope selection and heat source encapsulation architecture upon radiation safety. The identification of 241Am as an alternative isotope fuel is made. Other candidate isotopes such as 210Po, 242Cm and 244Cm are assessed. The development of encapsulation methods that are resistant to the extraction and dispersion of the radioactive materials enclosed is increasingly attractive for security reasons. Spark Plasma Sintering (SPS) processes are presented as novel, simple and rapid techniques for the encapsulation of radioisotopic materials within tungsten ceramic-metallic or cermet matrices. Computational modelling via Monte-Carlo simulation has shown that the encapsulation of radioisotopes within heterogeneous tungsten cermet matrices may reduce the neutron, X-ray and Gamma-ray radiation dose delivered to the localised environment. The prevention of fabrication related volatilisation of radioisotopic compounds is fundamental to the success of the encapsulation process. SPS is empirically demonstrated via the use of CeO2 as an inert simulant for radioisotopic compounds such as PuO2, AmO2 and UO2. The chemical compatibility of americium oxides within a tungsten matrix is also demonstrated through pressureless sintering within a Differential Scanning Calorimetric furnace. The techniques developed for radioisotope encapsulation are also demonstrated in context of cermet fuel fabrication for high temperature space power and propulsion reactor systems. The use of tungsten cermet fuels may eliminate material incompatibilities and failures experienced by historical nuclear thermal propulsion programs. Finally, three novel concept applications of nuclear energy as an enabling technology for planetary exploration are presented. Melt penetration of icy surfaces and long range mobility on planetary surfaces is proposed via the use of pulsed high power heat capacitive radioisotope sources. In-situ resource utilization is considered for propellant production. The use of CO2 is proposed as a propellant for a radioisotope thermal rocket in the context of a ‘Mars Hopper’. A CO2 propellant is also considered in the context of a high temperature (3000°C) nuclear thermal propulsion system for a single stage surface ascent vehicle under a Mars sample return mission.