Chadwick D. Barklay

Considerations in the fabrication, assembly, and testing of radioisotopic thermo‐photovoltaic (RTPV) generators for future space missions

To increase energy output with a smaller size and mass than the radioisotopic thermoelectric generators (RTGs) that were previously used on deep space missions, a radioisotopic thermophotovoltaic (RTPV) system is being developed for the ‘‘Pluto Express’’ flyby mission. To minimize cost and development time, some facilities and components currently used for RTG production can be used to produce RTPVs. Production options also include out‐sourcing and use of off‐the‐shelf hardware. Necessary modifications to tooling, production equipment, testing and shipping methods can be achieved in a timely manner so that the RTPV will be ready well before the planned launch of ‘‘Pluto Express.’’

Investigation of Effects of Neutron Irradiation on Tantalum Alloys for Radioisotope Power System Applications

Tantalum alloys have been used by the U.S. Department of Energy as structural alloys for space nuclear power systems such as Radioisotopic Thermoelectric Generators (RTG) since the 1960s. Tantalum alloys are attractive for high temperature structural applications due to their high melting point, excellent formability, good thermal conductivity, good ductility (even at low temperatures), corrosion resistance, and weldability. A number of tantalum alloys have been developed over the years to increase high‐temperature strength (Ta‐10%W) and to reduce creep strain (T‐111). These tantalum alloys have demonstrated sufficient high‐temperature toughness to survive the increasing high pressures of the RTG’s operating environment resulting from the alpha decay of the 238‐plutonium dioxide fuel. However, 238‐plutonium is also a powerful neutron source. Therefore, the RTG operating environment produces large amounts of 3‐helium and neutron displacement damage over the 30 year life of the RTG. The literature to date s...

Performance testing of the EU & QU MMRTG

The Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) Lifecycle Testing Laboratory is operated by the University of Dayton Research Institute (UDRI), which is dedicated to conducting life-cycle testing of electrically heated versions of the MMRTG. Since there are only two MMRTG electrically-heated thermoelectric generators (ETG) available for testing, a Test Plan Development Working Group was established to determine and prioritize the performance testing that is being conducted with the Engineering Unit (EU) and Qualification Unit (QU) ETGs. This working group is comprised of subject matter experts from the U.S. Department of Energy (DOE), the National Aeronautics and Space Administration (NASA) Glenn Research Center (GRC), NASA Jet Propulsion Laboratory (JPL), Idaho National Laboratory (INL), Oak Ridge National Laboratory (ORNL), UDRI, Aerojet Rocketdyne, and Teledyne Energy Systems. The highest priority testing was concluded by the working group to be: 1) To determine the impact of thermal cycling on thermoelectric components by characterizing the evolution of the thermoelectric/electrical properties of the EU as a result of thermal cycling the ETG through a Martian Sol repeatedly; 2) to characterize the effect of a simulated cruise-phase environment on the QU by evaluating the change in performance of the ETG before and after a cruise-phase simulation; and 3) characterize and clear any potential MMRTG internal shorts to chassis by integrating the JPL derived active short technique between the internal electrical power circuit and chassis frame of the MMRTG. The data and risk mitigation techniques derived from this testing can potentially be incorporated into future missions that would employ the MMRTG or successor thermoelectric radioisotope power systems.

Investigation of Stress Rupture Tested Neutron Irradiated Tantalum Alloys

Irradiation of metals with high‐energy particles produces nano‐scale defects that act as obstacles to dislocation glide. This paper presents the effects of low‐level neutron radiation on the stress rupture and microstructural properties of two tantalum alloys, Ta‐10%W and Ta‐8%W‐2%Hf (T‐111), which have been used to encapsulate radioactive fuel for space Radioisotope Power Systems (RPS). Ta‐10%W and T‐111 test specimens were exposed to a neutron fluence level (1.2×1015 nvt) at temperatures less than <0.2 Tm, which is equivalent to the cumulative fluence associated with the 30‐year mission life of a RPS. This fluence level results in an atomic displacement damage of approximately 3.0×10−7 dpa in both alloys. The atomic displacement damage produces an approximate two‐order of magnitude increase in the stress rupture time, and a two‐order of magnitude reduction in steady state creep rate. These observations are statistically significant at the 0.05 significance level. Transmission electron microscopy of rupt...

Materials Technology Support for Radioisotope Power Systems Final Report

Over the period of this sponsored research, UDRI performed a number of materials related tasks that helped to facilitate increased understanding of the properties and applications of a number of candidate program related materials including; effects of neutron irradiation on tantalum alloys using a 500kW reactor, thermodynamic based modeling of the chemical species in weld pools, and the application of candidate coatings for increased oxidation resistance of FWPF (Fine Weave Pierced Fabric) modules.

Thermodynamic Prediction of Compositional Phases Confirmed by Transmission Electron Microscopy on Tantalum‐Based Alloy Weldments

Tantalum alloys have been used by the U.S. Department of Energy as structural alloys for radioisotope based thermal to electrical power systems since the 1960s. Tantalum alloys are attractive for high temperature structural applications due to their high melting point, excellent formability, good thermal conductivity, good ductility (even at low temperatures), corrosion resistance, and weldability. Tantalum alloys have demonstrated sufficient high‐temperature toughness to survive prolonged exposure to the radioisotope power‐system working environment. Typically, the fabrication of power systems requires the welding of various components including the structural members made of tantalum alloys. Issues such as thermodynamics, lattice structure, weld pool dynamics, material purity and contamination, and welding atmosphere purity all potentially confound the understanding of the differences between the weldment properties of the different tantalum‐based alloys. The objective of this paper is to outline the th...

Compatibility issues of potential payloads for the USA/9904/B(U)F-85 RTG transportation system (RTGTS) for the “Pluto Express” mission

The specific electric power system for the “Pluto Express” mission has yet to be specified. However, electric power will be provided by either radioisotopic thermoelectric generators (RTG), radioisotope thermophotovoltaic systems (RTPV), alkali metal thermal to electrical conversion (AMTEC) systems, radioisotope Stirling systems, or a combination of these. The selected radioisotopic power system will also be transported using the USA/9904/B(U)F-85, Radioisotope Thermoelectric Generator (RTG) Transportation System (RTGTS). As a result, all of the potential payloads present uniquely different environmental and physical configuration requirements. This paper presents the major compatibility issues of the potential payloads for the USA/9904/B(U)F-85 RTG Transportation System for the “Pluto Express” mission.

Logistical concepts associated with international shipments using the USA/9904/B(U)F RTG Transportation System (RTGTS)

Over the last 30 years, radioisotopes have provided heat from which electrical power is generated. For space missions, the isotope of choice has generally been 238PuO2, its long half-life making it ideal for supplying power to remote satellites and spacecraft like the Voyager, Pioneer, and Viking missions, as well as the recently launched Galileo and Ulysses missions, and the presently planned Cassini mission. Electric power for future space missions will be provided by either radioisotopic thermoelectric generators (RTG), radioisotope thermophotovoltaic systems (RTPV), alkali metal thermal to electrical conversion (AMTEC) systems, radioisotope Stirling systems, or a combination of these. The type of electrical power system has yet to be specified for the “Pluto Express” mission. However, the current plan does incorporate the use of Russian launch platforms for the spacecraft. The implied tasks associated with this plan require obtaining international certification for the transport of the radioisotopic p...

Pelletization and encapsulation of general purpose heat source (GPHS) fueled clads for future space missions

Mankind must continue to explore the universe in order to gain a better understanding of how we relate to it and how we can best use its resources to our benefit. Because of the significant costs of this type of exploration, it can more effectively be accomplished through an international team effort. This unified effort must include the design, planning, and execution phases of future space missions, extending down to such activities as the processing, pelletization, and encapsulation of the fuel that will be used to support the spacecraft electrical power generation systems. Over the last 30 years, radioisotopes have provided heat from which electrical power is generated. For space missions, the isotope of choice has generally been 238PuO2, its long half‐life making it ideal for supplying power to remote satellites and spacecraft like the Voyager, Pioneer, and Viking missions, as well as the recently launched Galileo and Ulysses missions, and the presently planned Cassini mission. Electric power for future space missions will be provided by either radioisotopic thermoelectric generators (RTG), radioisotope thermophotovoltaic systems (RTPV), radioisotope Stirling systems or a combination of these. However, all of the aforementioned systems will be thermally driven by General‐Purpose Heat Source (GPHS) fueled clads in some configuration. Each GPHS fueled clad contains a 150‐gram pellet of 238PuO2, and each pellet is encapsulated within an iridium‐alloy shell. Historically, the fabrication of the iridium‐alloy shells has been performed at EG&G Mound, and Oak Ridge National Laboratory, and the girth welding of the GPHS capsules has been performed at Westinghouse Savannah River Corporation, and Los Alamos National Laboratory. This paper describes a cost effective alternative method for the production of GPHS capsules. Fundamental considerations such as the potential production options, the associated support activities, and the methodology to transport the welded fueled clads are discussed.