Radioisotope Thermoelectric Generators Research Articles

In Situ Growth of a Yb2O3 Layer for Sublimation Suppression for Yb14MnSb11 Thermoelectric Material for Space Power Applications

The compound Yb14MnSb11 is a p-type thermoelectric material of interest to the National Aeronautics and Space Administration (NASA) as a candidate replacement for the state-of-the-art Si-Ge used in current radioisotope thermoelectric generators (RTGs). Ideally, the hot end of this leg would operate at 1000°C in the vacuum of space. Although Yb14MnSb11 shows the potential to double the value of the thermoelectric figure of merit (zT) over that of Si-Ge at 1000°C, it suffers from a high sublimation rate at elevated temperatures and would require a coating in order to survive the required RTG lifetime of 14 years. The purpose of the present work is to measure the sublimation rate of Yb14MnSb11 and to investigate sublimation suppression for this material. This paper reports on the sublimation rate of Yb14MnSb11 at 1000°C (∼3 × 10−3 g/cm2 h) and efforts to reduce the sublimation rate with an in situ grown Yb2O3 layer. Despite the success in forming thin, dense, continuous, and adherent oxide scales on Yb14MnSb11, the scales did not prove to be sublimation barriers.

A study of the ion-exchange behavior of Pb and Bi as homologs of elements 114 and 115, respectively, in aqueous and aqueous-methanol solutions of HNO3

The ion-exchange behavior of Pb and Bi as homologs of elements 114 and 115, respectively, in aqueous and aqueous-methanol solutions of HNO3 was studied. The distribution coefficients of Bi, Pb, Ra, and some actinides between the ion exchangers and HNO3 solutions were determined in relation to the acid and methanol concentrations in the solution and to the kind of the ion exchanger. The distribution coefficients of Bi and other elements under consideration in sorption on the cation and anion exchangers from mixed aqueousmethanol HNO3 solutions differ considerably. The optimum conditions were found for the isolation of carrier-free short-lived Pb and Bi isotopes and for their separation from actinides and other elements using the ion exchangers and aqueous-methanol HNO3 solutions. The possibility of repeated isolation of short-lived Bi isotopes with their simultaneous separation from the long-lived parent radionuclides (Ac, Ra, Pb) on the cation exchanger (radioisotope generators) using mixed HNO3/CH3OH solution and of on-line preconcentration of these isotopes on the anion exchanger was demonstrated.

US planetary sciences to focus on smaller missions, research, and technology development

Heeding community advice, NASA’s planetary science division seeks to scale back its flagship missions while retaining strong international partnerships.

Milliwatt-Power Radioisotope Thermoelectric Generator (RTG) Based on Plutonium-238

Results of design and experimental studies aimed at developing a milliwatt-power radioisotope thermoelectric generator (RTG) based on plutonium-238 for space power are considered and analyzed. Milliwatt-power RTGs based on plutonium-238 are completely self-contained electric power sources offering a long proven service lifetime (>15 years) and high reliability. Such electric power sources find application both for space exploration and in terrestrial equipment, especially with the advance of microsystem technologies.

Separation of lanthanum, hafnium, barium and radiotracers yttrium-88 and barium-133 using crystalline zirconium phosphate and phosphonate compounds as prospective materials for a Ra-223 radioisotope generator

Crystalline hybrid organic/inorganic ion exchangers based on zirconium phosphate and phosphonate compounds were evaluated for application in radium-223 generator for radiopharmaceutical applications. Various compositions were synthesized and the selectivity of these materials was determined for inactive lanthanum, hafnium and barium, and radiotracers yttrium-88 and barium-133. The hybrid materials show very efficient lanthanum/barium separation; the response for zirconium phosphate was even better. A small-scale column loaded with pelletized zirconium phosphate compound demonstrated excellent retention of (88)Y and release of (133)Ba.

Technology-based design and scaling for RTGs for space exploration in the 100 W range

This paper presents the results of a study on design considerations for a 100W radioisotope thermo-electric generator (RTG). Special emphasis has been put on designing a modular, multi-purpose system with high overall TRL levels and making full use of the extensive Russian heritage in the design of radioisotope power systems. The modular approach allowed insight into the scaling of such RTGs covering the electric power range from 50 to 200We (EoL). The retained concept is based on a modular thermal block structure, a radiative inner-RTG heat transfer and using a two-stage thermo-electric conversion system.

Enabling interstellar probe

The scientific community has advocated a scientific probe to the interstellar medium for over 30 years. While the Voyager spacecraft have passed through the termination shock of the solar wind, they have limited lifetimes as their radioisotope power supplies decay. It remains unclear whether they can reach the heliopause, the boundary between shocked solar wind and interstellar plasmas, and, in any case, they will not reach the undisturbed interstellar medium. As with most exploratory space missions, their ongoing observations continue to raise even more questions about the nature of the interaction of our heliosphere and the interstellar medium. Scientific questions including:1.What is the nature of the nearby interstellar medium?2.How do the Sun and galaxy affect the dynamics of the heliosphere?3.What is the structure of the heliosphere?4.How did matter in the solar system and interstellar medium originate and evolve?can only be answered by an “interstellar precursor” probe. Such a mission is required to make in situ measurements in the interaction region and interstellar medium itself at distances far from the Sun, but in a finite mission lifetime. By launching a probe toward the incoming “interstellar wind,” whose direction is known, the distance to be traveled can be minimized but is still large. The current consensus is that a scientifically compelling mission must function to at least a distance of 200 astronomical units (AU) from the Sun and return a reasonable stream of data during the voyage. The central problem is that of providing a means of propulsion to accelerate a probe from the Solar System. Even with a low-mass payload and spacecraft, achieving the high speeds needed, even with gravity assists, have remained problematic. Voyager 1, the fastest object ever to leave the system is now traveling ∼3.6AU/yr, and a credible probe must reach at least 2–3 times this speed. The use of an Ares V is an approach for enabling a fast interstellar precursor mission. Maximum capability uses the combination of an Ares V, two-engine Centaur upper stage, close fly-by of Jupiter, and radioisotope electric propulsion (REP). Deletion of any of these pieces does not disable the mission, but does increase the flyout time to a given distance. This approach is more robust and provides a faster probe than an earlier alternative, designed for launch by a Delta IV 4050H plus twin Star 48A upper stages.

Production of carrier free 188Re radioisotope generator based on aluminum tungstate matrix

Improved radionuclide generator include a substantially insoluble salt of a radioactive parent which may be directly packed in column for subsequent elution of the daughter radionuclide. An improved 188Re generator was prepared by reacting a radioactive tungsten (188W) as parent radionuclide incorporated with aluminum chloride to obtain an insoluble radioactive aluminum tungstate matrix. The investigated matrix was characterized on the basis of the chemical composition, IR, thermal analysis and mechanical stabilities. The factors affecting the elution performance were studied such as influence of pH, molar ratio and drying temperature. From the obtained data, the molar ratio W:Al was 1.5:1 at pH = 4, the matrix dried at 105 °C for 2 h. Chromatographic and multichannel analysis has been currently used to investigate the radiochemical and radionuclidic purity respectively on eluted 188Re. An elution yield more than 80%, with radiochemical purity 10−4% of the column. The Al+3 and W contents value were about 2 and 3 μg/mL eluate. The obtained data approved the stability of the prepared generator and its suitability for medical application.

Nanostructured Silicon-based Composites for High Temperature Thermoelectric Applications

AbstractRecently nanostructured bulk silicon and silicon-germanium have achieved large increases in the thermoelectric figure of merit (ZT). The ZT enhancement is attributed to a significant reduction in the lattice thermal conductivity while maintaining relatively high carrier mobility. Silicon-based thermoelectric devices are attractive due to their low-toxicity, thermal stability, low density, relative abundance and low cost of production. Although significant enhancements in ZT have been achieved using the nanostructuring route, additional decoupling of the thermal and electric transport terms is still necessary in order for silicon-based materials to be viable for thermoelectric applications such as waste heat recovery or radioisotope thermoelectric generators. It is theorized that additional increases in ZT could be achieved by forming composites with nanostructured inert inclusions to further scatter the heat-carrying phonons. Here we present the impact of insulating and conductive nanoparticle composites on ZT. The nanostructured composites are formed via ball milling and high pressure sintering of the nanoparticles. The thermoelectric properties and microstructure of the silicon-based composites are discussed.

Assessment of architectural options for surface power generation and energy storage on human Mars missions

The provision of power for human Mars surface exploration is generally assumed to be achieved using nuclear fission power systems, particularly if in-situ production of part or all of the Earth return propellant is considered. This paper provides a comprehensive analysis of surface power generation and energy storage architectures for human Mars surface missions, including tracking and non-tracking photovoltaic power generation, nuclear fission power generation, dynamic radioisotope power generation, and battery and regenerative fuel cell energy storage. The quantitative analysis is carried out on the basis of equal energy provision to the power system user over the course of one Martian day (including day and night periods); this means that the total amount of energy available to the user will be the same in all cases, but the power profile over the course of the day may be different from concept to concept. The analysis results indicate that photovoltaic power systems based on non-tracking, thin-film roll-out arrays with either secondary batteries or regenerative fuel cells for nighttime energy storage achieve comparable levels of performance as systems based on nuclear fission power across the entire range of average power levels investigated (up to 100 kW of average usable power over the course of the Martian day). Given the significant policy and sustainability advantages of solar power compared to nuclear fission power generation, as well as the significant development and performance increase for thin-film photovoltaic arrays and energy storage technologies that is anticipated over the coming decades, solar power as the primary source for human Mars surface power generation should be seriously considered as an alternative to traditional nuclear fission based power generation approaches.

New powerful thermal modelling for high-precision gravity missions with application to Pioneer 10/11

The evaluation of about 25 years of Doppler data has shown an anomalous constant deceleration of the deep space probes Pioneer 10 and 11. This observation became known as the Pioneer anomaly (PA) and has been confirmed independently by several groups. Many disturbing effects that could cause a constant deceleration of the craft have been excluded as possible source of the PA. However, a potential asymmetric heat dissipation of the spacecraft surface leading to a resulting acceleration still remains to be analysed in detail. We developed a method to calculate this force with very high precision by means of finite element (FE) modelling and ray tracing algorithms. The elaborated method is divided into two separate parts. The first part consists of the modelling of the spacecraft geometry in FE and the generation of a steady state temperature surface map of the craft. In the second part, this thermal map is used to compute the force with a ray-tracing algorithm, which gives the total momentum generated by the radiation emitted from the spacecraft surface. The modelling steps and the force computation are presented for a simplified geometry of the Pioneer 10/11 spacecraft including radioisotope thermoelectric generators (RTG), equipment/experiment section and the high gain antenna. Analysis results how that the magnitude of the forces to be expected are non-negligible with respect to the PA and that more detailed investigations are necessary. The method worked out here for the first time is not restricted to the modelling of the Pioneer spacecraft but can be used for many future fundamental physics (in particular gravitational physics) and geodesy missions like LISA, LISA Pathfinder or MICROSCOPE for which an exact disturbance modelling is crucial.

Radioisotope electric propulsion (REP): A near-term approach to nuclear propulsion

Studies over the last decade have shown radioisotope-based nuclear electric propulsion to be enhancing and, in some cases, enabling for many potential robotic science missions. Also known as radioisotope electric propulsion (REP), the technology offers the performance advantages of traditional reactor-powered electric propulsion (i.e., high specific impulse propulsion at large distances from the Sun), but with much smaller, affordable spacecraft. Future use of REP requires development of radioisotope power sources with system specific powers well above that of current systems. The US Department of Energy and NASA have developed an advanced Stirling radioisotope generator (ASRG) engineering unit, which was subjected to rigorous flight qualification-level tests in 2008, and began extended lifetime testing later that year. This advancement, along with recent work on small ion thrusters and life extension technology for Hall thrusters, could enable missions using REP sometime during the next decade.

Neutron sources for in-situ planetary science applications

There are a number of future European Space Agency (ESA) and NASA planetary science missions that are in the planning or initial study phases, where the scientific objectives include determining the surface composition, measuring planetary surface heat flow and constraining planetary chronology. The University of Leicester is developing instrumentation for geophysical applications that include γ-ray spectroscopy, γ-ray densitometry and radiometric dating. This paper describes the modelling of a geophysical package, with the Monte Carlo code MCNPX, in order to determine the impact that a neutron source would have on in-situ composition measurements, radiometric dating and, in particular, trace element detection. The suitability of ∅ 2.54×2.54 cm LaBr 3(Ce) detectors in the geophysical package for in-situ missions was examined. 252Cf, Am–Be and Pu–Be neutron sources were compared in a trade-off study to determine mission suitability, potential for thermal and electric power production, mass and shielding requirements. This study is linked to a parallel examination of the suitability of radioisotope thermal generators for in-situ planetary science applications. The aim of the modelling was to optimise the source type and detector geometry in order to measure the elemental peaks of interest with a precision of 10% or better based on the Poisson statistics of the detected counts above background.

Thermal dissipation force modeling with preliminary results for Pioneer 10/11

The dissipation of thermal energy can produce disturbance forces on spacecraft surfaces if the energy is not dissipated in a symmetric pattern. This force can be computed as the quotient of the radiated power and the speed of light for a plate surface element. Depending on mission and spacecraft design the resulting surface forces have to be included into the disturbance budget. At ZARM (Center of Applied Space Technology and Microgravity) a raytracing algorithm was developed that allows the computation of the resulting force for complex spacecraft geometries. The method is based on the modeling of the spacecraft geometry in finite elements (FEs). Using an FE-solver the surface temperatures of the satellite can be derived with geometry and material parameters using heat sources/sinks as constraints. The outgoing radiation force is computed including reflectivity and absorption between all elements of the model. As an example for the method a test case model of the radio isotope thermal generators (RTGs) of Pioneer 11 is processed with this force computing method. The results show that detailed thermal modeling for the whole craft is necessary as the simplified test case results in a force that is non-negligible with respect to the pioneer anomaly.

Thermal Expansion Studies of Selected High-Temperature Thermoelectric Materials

Radioisotope thermoelectric generators (RTGs) generate electrical power by converting the heat released from the nuclear decay of radioactive isotopes (typically plutonium-238) into electricity using a thermoelectric converter. RTGs have been successfully used to power a number of space missions and have demonstrated their reliability over an extended period of time (tens of years) and are compact, rugged, radiation resistant, scalable, and produce no noise, vibration or torque during operation. System conversion efficiency for state-of-practice RTGs is about 6% and specific power ≤ 5.1 W/kg. A higher specific power would result in more onboard power for the same RTG mass, or less RTG mass for the same onboard power. The Jet Propulsion Laboratory has been leading, under the advanced thermoelectric converter (ATEC) project, the development of new high-temperature thermoelectric materials and components for integration into advanced, more efficient RTGs. Thermoelectric materials investigated to date include skutterudites, the Yb 14 MnSb 11 compound, and SiGe alloys. The development of long-lived thermoelectric couples based on some of these materials has been initiated and is assisted by a thermomechanical stress analysis to ensure that all stresses under both fabrication and operation conditions will be within yield limits for those materials. Several physical parameters are needed as input to this analysis. Among those parameters, the coefficient of thermal expansion (CTE) is critically important. Thermal expansion coefficient measurements of several thermoelectric materials under consideration for ATEC are described in this paper. The stress response at the interfaces in material stacks subjected to changes in temperature is discussed, drawing on work from the literature and project-specific tools developed here. The degree of CTE mismatch and the associated effect on the formation of stress is highlighted.

Searching for modifications to the exponential radioactive decay law with the Cassini spacecraft

Data from the power output of the radioisotope thermoelectric generators aboard the Cassini spacecraft are used to test the conjecture that small deviations observed in terrestrial measurements of the exponential radioactive decay law are correlated with the Earth–Sun distance. No significant deviations from exponential decay are observed over a range of 0.7–1.6 A.U. A 90% CL upper limit of 0.84 × 10 - 4 is set on a term in the decay rate of 238 Pu proportional to 1 / R 2 and 0.99 × 10 - 4 for a term proportional to 1 / R . The terrestrially measured Earth–Sun distance correlation is ∼ ( 3 × 10 - 2 ) / R 2 .

Use the water: In-situ resource technology for icy-surface landers

Robotic landers serve vital reconnaissance roles in the exploration of planetary surfaces, but are constrained by deliverable payload size and environment survivability. Although the Mars exploration rovers (MER) have shown incredible survivability, their solar power source limits the science output per sol. Future landers will be larger, and will incorporate more sophisticated data-collection and analysis packages, which will likely bring with them an increased demand for power. Anticipating this demand, we propose an innovative hybrid power system combining a primary radioisotope thermoelectric generator (RTG) with a secondary alkaline fuel cell. This combination provides the opportunity to utilize more effectively the energy produced by the RTG, to produce and store O 2 and H 2 via electrolysis of melted ice, and use this obtained O 2 and H 2 in a variety of ways, including as fuel for a regenerative fuel cell. This hybrid system has applications ranging from planetary rovers and deep-space probes to human habitats.

Critical review of thermoelectrics in modern power generation applications

The thermoelectric complementary effects have been discovered in the nineteenth century. However, their role in engineering applications has been very limited until the first half of the twentieth century, the beginning of space exploration era. Radioisotope thermoelectric generators have been the actual motive for the research community to develop efficient, reliable and advanced thermoelectrics. The efficiency of thermoelectric materials has been doubled several times during the past three decades. Nevertheless, there are numerous challenges to be resolved in order to develop thermoelectric systems for our modern applications. This paper discusses the recent advances in thermoelectric power systems and sheds the light on the main problematic concerns which confront contemporary research efforts in that field.

Helium solubility and behaviour in uranium dioxide

A set of devices was developed in order to infuse UO2 disks with helium, at high temperature and pressure, to measure the helium infused quantity and from these data to calculate the helium solubility in the UO2 matrix. Samples of UO2 single crystal and UO2 polycrystal were infused at a temperature of 1473 and 1743K in a helium atmosphere ranging between 50 and 100MPa. These samples were then annealed and the helium released was measured with a mass spectrometer. From the obtained spectra it was possible to give an interpretation of the helium release mechanism and to calculate its solubility in the UO2 lattice in these specific thermodynamic conditions. Additionally to the helium solubility measurement from infused samples, a 37 years old sample of 238PuO2, retrieved from an old 242Cm radioisotope thermoelectric generator (RTG), containing radiogenic helium, was also measured to widen perspectives of this kind of measurements to damaged sample more representative of spent fuel.

The New Horizons Spacecraft

The New Horizons spacecraft was launched on 19 January 2006. The spacecraft was designed to provide a platform for seven instruments designated by the science team to collect and return data from Pluto in 2015. The design meets the requirements established by the National Aeronautics and Space Administration (NASA) Announcement of Opportunity AO-OSS-01. The design drew on heritage from previous missions developed at The Johns Hopkins University Applied Physics Laboratory (APL) and other missions such as Ulysses. The trajectory design imposed constraints on mass and structural strength to meet the high launch acceleration consistent with meeting the AO requirement of returning data prior to the year 2020. The spacecraft subsystems were designed to meet tight resource allocations (mass and power) yet provide the necessary control and data handling finesse to support data collection and return when the one-way light time during the Pluto fly-by is 4.5 hours. Missions to the outer regions of the solar system (where the solar irradiance is 1/1000 of the level near the Earth) require a radioisotope thermoelectric generator (RTG) to supply electrical power. One RTG was available for use by New Horizons. To accommodate this constraint, the spacecraft electronics were designed to operate on approximately 200 W. The travel time to Pluto put additional demands on system reliability. Only after a flight time of approximately 10 years would the desired data be collected and returned to Earth. This represents the longest flight duration prior to the return of primary science data for any mission by NASA. The spacecraft system architecture provides sufficient redundancy to meet this requirement with a probability of mission success of greater than 0.85. The spacecraft is now on its way to Pluto, with an arrival date of 14 July 2015. Initial in-flight tests have verified that the spacecraft will meet the design requirements.