Advanced AMTEC radioisotope power systems for deep space applications

Design optimization and integration of nickel/Haynes-25 AMTEC cells into radioisotope power systems

The Alkali Metal Thermal to Electric Converter (AMTEC) is a static energy conversion device that operates at high thermal to electric conversion efficiencies that are essentially independent of size, have reached 19% and are expected to reach 25% to 30% in 1997. AMTEC systems have been chosen by NASA and DOE for spacecraft applications and have considerable promise for a wide variety of terrestrial applications. Reduction of parasitic heat losses in AMTEC systems related to radiative heat transfer from the hot side to the condenser can make a substantial contribution to system efficiency. Through design, analysis and the fabrication and testing of cells and systems, the proposed program to develop a Directed Reflectivity Condenser (DRC) has investigated the feasibility of an improved AMTEC condenser component. Phase 1 work showed the potential for adding from 4% to 7% to overall system efficiency for identical operating conditions using the concept. A detailed thermal analysis of several DRC capped cell designs was carried out and some of the conditions under which a DRC, used as the condenser at an end cap of a cylindrical converter, can reduce thermal radiation related losses were determined. A model experimental converter was built and tested to compare DRC and planar condenser surfaces. The results of both analysis and experiment indicate that for moderate aspect ratios of a cylindrical, end condensed converter, the DRC can reduce overall thermal losses by up to 4%. The initial effort in Phase 2 extended the analysis to a novel 150 watt radial AMTEC cell design. This analysis indicated that for the effective aspect ratio of this new converter design, the system performance at the 100+ watt level was not significantly improved by use of a DRC type condenser surface. Further analyses however showed that for cylindrical, end-condensed converters, optimized for use with internal radiation shields, the use of DRC surfaces on the side walls of the converter could be more effective than on the condenser end surface itself. The experimental work in Phase 2 was intended to incorporate a DRC into this cell design and use its measured performance to refine the state-of-the-art AMTEC analytical models. Because the analysis had indicated that the new radial converter design, which may be useful for systems at the {approx} 100 watt level was not much assisted by the DRC properties, this program was redirected toward the simpler cylindrical converter design with the corner cube surfaces on the side walls. The Phase II program was proposed and planned with a funding level substantially below the maximum potentially available for Phase II programs at that time. At the time, there were two other funded government sponsored programs at AMPS for which positive results of the analyses described in this report were expected to lead to incorporation of the DRC concept into converters scheduled to be built for these programs. The programs of interest were the Air Force program titled ''Radiation Tolerant, Eclipse Compatible, Solar AMTEC System'' (F29601-99-C-0132) and the DOE/NASA Advanced Radioisotope Power System (ARPS) program. Shortly after its start, the Air Force program was canceled due to elimination of AF SBIR funds at AFRL and the ARPS program was reduced to a level that could not support introduction of novel concept testing. As a result of these two circumstances, the direct testing of the DRC concept in a full up converter was not completed in the Phase II period.

A series of previous OSC papers described: 1) a novel methodology for the coupled thermal, fluid flow, and electrical analysis of multitube AMTEC (Alkali Metal Thermal-to-Electric Conversion) cells; 2) the application of that methodology to determine the effect of numerous design variations on the cell’s performance, leading to selection and performance characterization of an OSC-recommended cell design; and 3) the design, analysis, and characterization of an OSC-generated power system design combining sixteen of the above AMTEC cells with two or three GPHS (General Purpose Heat Source) radioisotope heat source modules, and the applicability of those power systems to future space missions (e.g. Pluto Express and Europa Orbiter) under consideration by NASA. The OSC system design studies demonstrated the critical importance of the thermal insulation subsystem, and culminated in a design in which the eight AMTEC cells on each end of the heat source stack are embedded in Min-K fibrous insulation, and the Min-K and the GPHS modules are surrounded by graded-length Mo multifoil insulation. The present paper depicts the OSC-recommended AMTEC cell and generator designs, and identifies the need for an electrically heated (scaled-down but otherwise prototypic) test assembly for the experimental validation of the generator’s system performance predictions. It then describes the design of an OSC-recommended test assembly consisting of an electrical heater enclosed in a graphite box to simulate the radioisotope heat source, four series-connected prototypic AMTEC cells of the OSC-recommended configuration, and a prototypic hybrid insulation package consisting of Min-K and graded-length Mo multifoils. Finally, the paper describes and illustrates an OSC-recommended detailed fabrication sequence and procedure for the above cell and test assembly. That fabrication procedure is being implemented by AMPS, Inc. with the support of DOE’s Oak Ridge and Mound Laboratories, and the Air Force Phillips Laboratory (AFPL) will test the performance of the assembly over a range of input thermal powers and output voltages. The experimentally measured performance will be compared with the results of OSC analyses of the same insulated test assembly over the same range of operating parameters.

The Next-Generation Radioisotope Thermoelectric Generator (RTG) Study was carried out in 2016-2017 to determine the characteristics of options for a new RTG that would best fulfill NASA Planetary Science Division (PSD) mission needs, considering applicability to different targets and mission types. The NASA Radioisotope Power Systems (RPS) Program Mission Analysis Team gathered information on past and recent RPS mission concept studies by various organizations in the United States to support the Next-Generation RTG Study in crystalizing an understanding of mission "pull" that is crucial to define requirements for a potential next generation of RTGs. This paper describes the top-down processes and findings that helped establish a reasonable set of draft requirements for a potential Next-Generation RTG, which are documented in the Next-Generation RTG Study final report. To perform the required mission analyses, and to combine data and present the results in a unique way, an extensive relational database was developed. The database includes detailed data for 63 targets, six generic potential destinations (comet, Trojan, Centaur, etc.), 142 spacecraft concepts (65 launched and 77 studied), and 249 missions associated with these spacecraft concepts. In total, 77 RPS-powered spacecraft and associated mission concept studies were gathered, evaluated and assessed for the Next-Generation RTG Study. A number of spacecraft are associated with a number of mission concept types/subtypes at different destinations; thus, in this report, a mission is defined as a unique vector of spacecraft, mission concept type (e.g., lander) /subtype (e.g., rover), and target. For example, Cassini conducted flybys of Enceladus and 2685 Masursky, and orbited Saturn, which is considered three missions under this definition. Furthermore, there are two categories of spacecraft concepts: launched missions (e.g., Cassini, Mars Science Laboratory [MSL]) and concepts studied for possible future missions.

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 power system, and resolving any logistical issues associated with the actual shipment of the selected radioisotopic power system. This paper presents a conceptual summary of the logistical considerations associated with shipping the selected radioisotopic power system using the USA/9904/B(U)F-85, Radioisotope Thermoelectric Generator Transportation System (RTGTS).

The alkali metal thermal to electric converter (AMTEC) is a device for the direct conversion of heat to electrical power. Operating hot-side temperatures of AMTEC cells are well compatible with those of solid oxide fuel cells (SOFC), so that the stacks of the SOFC can be directly coupled to hot-side of the AMTEC cells. The AMTEC cells would produce electrical power from released heat of the SOFC, as well as control the SOFC operating temperatures by modulating current of the AMTEC cell. A new honeycomb type AMTEC cell is discussed in this paper. The cell uses advanced beta"-alumina solid electrolyte (BASE) configurations to improve the device performance over the conventional tube BASE systems. Expected power densities and efficiencies were calculated. The influence of temperature differences between heat source and anode side electrode is also discussed for vaper-fed type and vaper-vaper-fed type cells.

The use of Radioisotope Thermoelectric Generators (RTGs) has been well established for deep space mission applications. The success of the Voyager, Galileo, Cassini and numerous other missions proved the efficacy of these technologies in deep space. Future deep space missions may also require Advanced Radioisotope Power System (ARPS) technologies to accomplish their goals. In the Exploration of the Solar System (ESS) theme, several missions are in the planning stages or under study that would be enabled by ARPS technology. Two ESS missions in the planning stage may employ ARPS. Currently planned for launch in 2006, the Europa Orbiter mission (EO) will perform a detailed orbital exploration of Jupiter’s moon Europa to determine the presence of liquid water under the icy surface. An ARPS based upon Stirling engine technology is currently baselined for this mission. The Pluto Kuiper Express mission (PKE), planned for launch in 2004 to study Pluto, its moon Charon, and the Kuiper belt, is baselined to use a n...

The European Space Agency is funding the research and development of 241Am-bearing oxide-fuelled radioisotope power systems (RPSs) including radioisotope thermoelectric generators (RTGs) and European Large Heat Sources (ELHSs). The RPSs’ requirements include that the fuel’s maximum temperature, Tmax, must remain below its melting temperature. The current prospected fuel is (Am0.80U0.12Np0.06Pu0.02)O1.8. The fuel’s experimental heat capacity, Cp, is determined between 20 K and 1786 K based on direct low temperature heat capacity measurements and high temperature drop calorimetry measurements. The recommended high temperature equation is Cp(T/K) = 55.1189 + 3.46216 × 102 T − 4.58312 × 105 T−2 (valid up to 1786 K). The RTG/ELHS Tmax is estimated as a function of the fuel thermal conductivity, k, and the clad’s inner surface temperature, Ti cl, using a new analytical thermal model. Estimated bounds, based on conduction-only and radiation-only conditions between the fuel and clad, are established. Estimates for k (80–100% T.D.) are made using Cp, and estimates of thermal diffusivity and thermal expansion estimates of americium/uranium oxides. The lowest melting temperature of americium/uranium oxides is assumed. The lowest k estimates are assumed (80% T.D.). The highest estimated Tmax for a ‘standard operating’ RTG is 1120 K. A hypothetical scenario is investigated: an ELHS Ti cl = 1973K-the RPSs’ requirements’ maximum permitted temperature. Fuel melting will not occur.

Design, Analyses, and Fabrication Procedure of Amtec Cell, Test Assembly, and Radioisotope Power System for Outer-Planet Missions

The Department of Energy (DOE) has selected Free-Piston Stirling Convertors as a technology for future advanced radioisotope space power systems. In August 2000, DOE awarded competitive Phase I, Stirling Radioisotope Generator (SRG) power system integration contracts to three major aerospace contractors, resulting in SRG conceptual designs in February 2001. All three contractors based their designs on the Technology Demonstration Convertor (TDC) developed by Stirling Technology Company (STC) for DOE. The contract award to a single system integration contractor for Phases II and III of the SRG program is anticipated in late 2001. The first potential SRG mission is targeted for a Mars rover. This paper provides a description of the Flight Prototype (FP) Stirling convertor design as compared to the previous TDC design. The initial flight prototype units are already undergoing performance tuning at STC. The new design will be hermetically scaled and will provide a weight reduction from approximately 4.8 kg to...

Radioisotope heater units (RHU) and radioisotope thermoelectric generators (RTG) are currently being developed for the ESA radioisotope power system program. The state-of-the-art for the USA and Russian systems is to use plutonium-238 as the radioisotope fuel; however, for the ESA applications americium-241 has been selected due to its availability and relatively cost-effective production in the European context. The proposed designs implement a multi-layer containment approach for safety reasons, with a platinum-rhodium alloy for the inner containment of the fuel and carbon-based materials for the outer layers. The Am-fueled RHU provides 3 W of thermal power, and makes this design competitive with existing models in relation to specific power. The heat source for the RTG has a 6-side polygonal shape, with a distributed 3-fuel pellet architecture: this configuration allows to maximize the specific power of the RTG, since Am-based fuels have a lower power density than Pu-based fuels. The heat supplied by the fuel is 200 W, with an expected electrical power output of 10 W provided by six Bi-Te thermoelectric modules. Finite element structural and thermal analyses have been performed to assess the theoretical feasibility of the components as initially conceived. Mechanical and electrically-heated prototypes for the systems have already been tested in a representative lab environment at the University of Leicester; these tests have provided initial estimates for the efficiency of the systems. Both the RHU and RTG architectures are currently undergoing a new design iteration process. This paper reports on the overall architecture and design of the Am-fueled RTG and RHU, the modelling results and the experimental data obtained so far.

Since the launch of the first radioisotope power system (RPS) on Transit 4A in 1961, numerous research and development activities have been performed centered on increasing the overall thermal to electrical conversion efficiency of the selected nuclear fueled power system. The latest U.S. fielded RPS (MMRTG - Multi-Mission Radioisotope Thermoelectric Generator) contains eight GPHS (General Purpose Heat Source) modules which nominally yield ∼2000W Th from the thirty-two 238PU5O 2 ceramic fuel pellets. MMRTG's PbTe/TAGS-85 based thermoelectric couples have a thermal to electrical conversion efficiency of ∼5.5% thus yielding ∼110W e at launch. This paper centers on the discussion of a conceptual idea that entails employing a second set of thermoelectrics on an MMRTG. These would be employed for possibly converting a portion of the excess “waste heat” (∼1750 W th ) into additional electrical mission power. First-order experiments and calculations employing bismuth telluride (Bi 2 Te 3 ) based thermoelectric modules being considered for a European RPS by the University of Leicester indicate that an improvement in the efficiency of an MMRTG could be achieved by integrating them with the PbTe/TAGS-85 thermoelectrics being utilized in the MMRTG in a “dual” or “cascaded” arrangement. This arrangement of two different integral thermoelectric materials suggests the intriguing possibility of the additional harvesting of a portion of the MMRTG's currently unutilized “waste heat”. This appears to be feasible since the cold side temperature of the PbTe/TAGS-85 in an MMRTG is ∼200°C which corresponds to typical hot-side operating temperatures of the Bi 2 Te 3 thermoelectric modules. It is recognized that extensive thermo-mechanical-electrical design, modeling, and analysis are required to fully investigate the cascaded thermoelectric concept. In addition, materials compatibility and assembly aspects will need to be fully addressed in the future. The present work indicates that system-level performance gains could be achieved via a “cascaded” or cMMRTG which could result in electrical power increases at BOM (Beginning of Mission) of up to ∼25%, and perhaps more significantly, gains in EODL (End of Design Life) power of up to ∼40% which could be utilized on a future space mission.

The Office of Space and Defense Power Systems of the Department of Energy (DOE) provides Radioisotope Power Systems (RPS) for applications where conventional power systems are not feasible. For example, radioisotope thermoelectric generators were supplied by the DOE to the National Aeronautics and Space Administration for deep space missions including the Cassini Mission launched in October of 1997 to study the planet Saturn. For the Cassini Mission, ORNL produced carbon-bonded carbon fiber (CBCF) insulator sets, iridium alloy blanks and foil, and clad vent sets (CVS) used in the generators. The Oak Ridge National Laboratory (ORNL) has been involved in developing materials and technology and producing components for the DOE for more than three decades. This report reflects program guidance from the Office of Space and Defense Power Systems for fiscal year (FY) 2004. Production and production maintenance activities for flight quality (FQ) CBCF insulator sets, iridium alloy blanks and foil, and CVS are summarized in this report. In all three cases, production maintenance is assured by the manufacture of limited quantities of FQ components. Technology activities are also reported that were conducted to improve the manufacturing processes, characterize materials, or to develop information for new radioisotope power systems.

The Office of Space and Defense Power Systems of the Department of Energy (DOE) provides Radioisotope Power Systems (RPS) for applications where conventional power systems are not feasible. For example, radioisotope thermoelectric generators were supplied by the DOE to the National Aeronautics and Space Administration for deep space missions including the Cassini Mission launched in October of 1997 to study the planet Saturn. The Oak Ridge National Laboratory (ORNL) has been involved in developing materials and technology and producing components for the DOE for more than three decades. For the Cassini Mission, for example, ORNL was involved in the production of carbon-bonded carbon fiber (CBCF) insulator sets, iridium alloy blanks and foil, and clad vent sets (CVS). This report has been divided into three sections to reflect program guidance from the Office of Space and Defense Power Systems for fiscal year (FY) 2002. The first section deals primarily with maintenance of the capability to produce flight quality (FQ) CBCF insulator sets, iridium alloy blanks and foil, and CVS. In all three cases, production maintenance is assured by the manufacture of limited quantities of FQ components. The second section deals with several technology activities to improve the manufacturing processes, characterize materials, or to develop technologies for new radioisotope power systems. The last section is dedicated to studies related to the production of {sup 238}Pu.

The Office of Space and Defense Power Systems of the Department of Energy (DOE) provides Radioisotope Power Systems (RPS) for applications where conventional power systems are not feasible. For example, radioisotope thermoelectric generators were supplied by the DOE to the National Aeronautics and Space Administration for deep space missions including the Cassini Mission launched in October of 1997 to study the planet Saturn. For the Cassini Mission, ORNL produced carbon-bonded carbon fiber (CBCF) insulator sets, iridium alloy blanks and foil, and clad vent sets (CVS) used in the generators. The Oak Ridge National Laboratory (ORNL) has been involved in developing materials and technology and producing components for the DOE for more than three decades. This report reflects program guidance from the Office of Space and Defense Power Systems for fiscal year (FY) 2005. Production activities for prime quality (prime) CBCF insulator sets, iridium alloy blanks and foil, and CVS are summarized in this report. Technology activities are also reported that were conducted to improve the manufacturing processes, characterize materials, or to develop information for new radioisotope power systems.