Radioisotope Thermoelectric Generators Research Articles

Comprehensive modeling and characterization of the General-Purpose Heat Source Radioisotope Thermoelectric Generator for solar system missions

The radioisotope thermoelectric generator relies solely on radioactive decay for its energy and generates power through the thermoelectric effect. The General-Purpose Heat Source Radioisotope Thermoelectric Generator (GPHS-RTG) represents the RTG with the largest power and highest conversion efficiency ever built, and its energy supply supported four daring interplanetary missions (Galileo, Ulysses, Cassini, New Horizons). To explore the use potential of the GPHS-RTG in the solar system, its conservative application range and performance should be determined. In this work, a comprehensive model of the GPHS-RTG was developed, and its calculated results agreed well with Lockheed Martin’s test report (temperature relative error < 1 %). The study demonstrates that within the Earth orbit, the distance of the mission area from the sun and the angle of incidence sunlight significantly affect the thermoelectric performance and hot-junctions’ temperature. Moreover, conservative application ranges and their extending methods (changing coating material, thermal loading, and operational voltage) for GPHS-RTG at different attitudes in the solar system were determined for the first time by comparing the proportion of the hot-junction over-temperature area. The temperature distribution and its cause of formation in the axial and circumferential directions of the GPHS-RTG during solar system missions were revealed in the end. This work is valuable for future RTG design and mission planning.

Exploring the structural, mechanical, magneto-electronic and thermophysical properties of f electron based XNpO3 perovskites (X = Na, Cs, Ca, Ra).

Here, we present systematic investigation of the structural and mechanical stability, electronic profile and thermophysical properties of f-electron based XNPO3 (X = Na, Cs, Ca, Ra) perovskites by first principles calculations. The structural optimization, tolerance factor criteria depicts the cubic structural stability of these alloys. Further, the stability of these materials is also determined by the cohesive and formation energy calculations along with mechanical stability criteria. The electronic structure is explored by calculating band structure and density of states which reveal the well-known half-metallic nature of the materials. Further, we have calculated different thermodynamic parameters including specific heat capacity, thermal expansion, Gruneisen parameter and their variation with temperature and pressure. The thermoelectric effectiveness of these materials is predicted in terms of Seebeck coefficient, electrical conductivity and power factor. All-inclusive we can say that calculated properties of these half-metallic materials extend their route in spintronics, thermoelectric and radioisotope generators device applications.

Radiopharmaceuticals: Production, Physics, and Clinical Applications in Nuclear Medicine: A Short Review

Introduction: The use of radioactive atoms in medicine is growing, particularly in nuclear medicine, where radiation is emitted from the body. Radiopharmaceuticals or radiotracers are artificial short-living radioisotopes labeled with special pharmaceuticals. There are around 1800 radioisotopes, but only around 200 are suitable for general applications. There are four methods for generating radioisotopes: reactor-producing, neutron activation, charged acceleration, and radioisotope generators. Cyclotrons are used to produce many other radioisotopes for medical applications. Physical and biomedical characteristics are crucial for radiopharmaceuticals for clinical use. Physical aspects include the type and energy of radiation, mother and daughter radioactive elements, purity, and half-life of radioactives. Biomedical considerations include easy adhesion to biomolecules, a dynamic time course in the body, toxicity, and high tissue targeting. Radiopharmaceuticals used in diagnosis differ from those used in therapy, with positron emission tomography (PET) using radioisotopes, and gamma-emitting radioisotope-labeled radiopharmaceuticals suitable for SPECT imaging. Over 90% of radiopharmaceuticals are used for diagnostic purposes. Conclusion: Nuclear medicine, as an inevitable part of modern medicine, follows different methods to create more effective diagnostic and therapeutic radiopharmaceuticals with respect to several physical and biological considerations.

Thermal and Electrical Performance Analyses of a 100-W Radioisotope Thermoelectric Generator for Space Exploration

A 100-W radioisotope thermoelectric generator (RTG) is by far the most suitable power supply for long-term deep space exploration where solar power would not be feasible. Understanding the thermal performance and electrical performance of the RTG under operational conditions is paramount for its nominal and safety performance during the space mission. In this paper, modeling and experimental studies on the thermal behavior and electrical performance of the 100-W RTG have been conducted. The RTG uses high conversion efficiency skutterudite-based thermoelectric convertor (TEC) arrays thermally coupled with a radioisotope heat unit (RHU) to generate electricity. A comprehensive finite element model and an electrical heating prototype of the 100-W RTG have been built to assess the performance of the RTG designs. Critical temperature, generated power, and energy conversion efficiency were evaluated. The simulation results show that the maximum output power of the RTG can reach about 120 W(electric); the temperature of the hot end of the TECs is about 853 K, and the temperature of the cold end is about 473 K, making a temperature difference of about 380 K. The RTG prototype with Bi2Te3 TECs generated about 60 W(electric) of electrical power in the first experimental research stage. These research results have significant reference for extension of the RTG prototype to the actual power source of the RHU and allow for future research and development improvements of the 100-W RTG.

A Novel Optimization Method for TEG System Performance Based on Temperature and Heat Flux

The thermoelectric generator (TEG) can convert heat to electricity and is widely used. To overcome the limitation for low efficiency of thermoelectric materials, optimizing the performance of TEG in given system is necessary. This study proposes a method to match individual TEG components to the system environment based on two boundary conditions—temperature and heat flux. With this method, TEG geometrical parameters can be determined to optimize the system performance by varying temperature as an independent variable. Using this method, the TEG in an undersea Radioisotope thermoelectric generator (u‐RTG) is optimized. The TEG system exhibits an output power of 14.10 W when powered by a 200 W heat source. The simulate result in the comprehensive system model of u‐RTG is 14.04 W, which has 0.45% deviation from the TEG optimization result. It is proved that the TEG optimized by this method matches the system, the optimization results are accurate.

Solution-based 3D printed thermoelectric composite for custom-shaped thermoelectric generator applications

To fulfill the ever-increasing power demands of deep space exploration, the output performance of radioisotope thermoelectric generators, the only accessible power source, must be enhanced in several aspects, including thermoelectric properties of materials, geometry, welding, etc. In this study, we present our results on the development of a novel thermoelectric slurry suitable for the 3D printing of thermoelectric generators with optimized leg geometry. The rheological properties of the slurry are optimized by combining proper amounts of organic solvent, binder, and bismuth telluride-based thermoelectric powder. The addition of Cu to the slurry as a conductive additive are also investigated. The homogeneous dispersion of Cu within the material not only increases the electrical conductivity of the final thermoelectric leg significantly but also promotes the crystallization of the thermoelectric particles during the sintering process. These effects result in 3D-printed thermoelectric composites exhibiting ZT values up to 0.91 and extended optimum operating temperature range. Thermoelectric modules composed of 3D-printed thermoelectric legs show excellent output performance and structural strength. This work could also produce other special shaped thermoelectric devices to match irregularly shaped heat sources to reduce contact heat loss and improve output performance.

Preliminary Design of a Climate Controlled Environmental Test and Measurement

Climate chambers play an important role in the design and testing process. Several different chambers have been built over the years, specialising in different areas. It means there is a wide choice on the market and as a consequence, parameters necessary for us limit when choosing the ideal test chamber. Our instruments might be subjected to several environmental effects which require a lot of time and effort to be tested. An example is the Mars rover. NASA's Perseverance Mars rover was equipped with 2 pieces of Li-ion battery packs and the old solar charging was replaced by RTG (Radioisotope Thermoelectric Generator) thus a generator feeds the electricity consumers of the rover. Temperature on the surface of Mars varies from -150 C° to 20 C°. In addition, there are huge storms at regular interval lasting for month sometimes. These storms cause great difficulties because, on the one hand, the main power source on older types was the solar panels, and storms can cover them with Martian dust, limiting or eliminating their power supply. On the other hand, large sandstorms can completely cover the rover, and once the sandstorm is over, the Mars rover has to free itself from its trap. In addition, there were many other factors that engineers had to consider in the design. It is a well-known fact that we are not able to consider all the possible environmental conditions in design, and it is also known that some measurements may be inaccurate, so the prototypes produced have to be tested on the ground. The ground tests must ensure that the operating conditions on Mars are maintained. Special test chambers have been developed for this purpose. [1] The aim of the authors is to design and build a prototype of a climate chamber of their own design. An important component of the chamber is a measurement data acquisition system, which allows the collection of measured data and their preliminary processing.

Assessment of the possibility for large-scale 238Pu production in a VVER-1000 power reactor

The paper presents the estimates for the possibility for large-scale production of 238Pu in the core of a VVER-1000 power reactor. The Np-fraction of minor actinides extracted from transuranic radioactive waste is proposed to be used as the starting material. The irradiation device with NpO2 fuel elements is installed at the reactor core center. The NpO2 fuel lattice pitch is varied and the irradiation device is surrounded by a heavy moderator layer to create the best possible spectral conditions for large-scale production (~ 3 kg/year) of conditioned plutonium with the required isotopic composition (not less than 85% of 238Pu and not more than 2 ppm of 236Pu). Plutonium with such isotopic composition can be used as the thermal source in thermoelectric radioisotope generators and in cardiac pacemakers. It has been demonstrated that the estimated scale of the 238Pu production in a VVER-type power reactor exceeds considerably the existing scale of its production in research reactors.

Thermoelectricity: from the iron arc of Alessandro Volta to radioisotope thermoelectric generators

We start with an overview of the fascinating history of thermoelectricity, which included such famous scientists as Luigi Galvani, Alessandro Volta, Thomas Johann Seebeck, Jean Charles Athanase Peltier, and even the philosopher Georg Wilhelm Friedrich Hegel. Then we move on to the basic concepts for describing thermoelectric phenomena and how, as our understanding of nature improves, thermoelectricity finds more and more applications. We formulate the requirements for the materials necessary for the wide practical use of thermoelectricity and will mention those that exist, and those that Nature has forgotten to create but will be discovered/invented by scientists. Finally, we enumerate several more significant applications of thermoelectricity.

Combined energy supply and management of self-powered wireless sensors based on radioisotope thermoelectric generator for multiple scenarios

Combined energy supply and management of self-powered wireless sensors based on radioisotope thermoelectric generator for multiple scenarios

Bayesian Reliability Analysis of the Enhanced Multimission Radioisotope Thermoelectric Generator

Radioisotope thermoelectric generators (RTGs) have been used as power sources for several space missions, including the multimission RTG (MMRTG) in NASA’s recent Mars Curiosity and Perseverance rovers. The enhanced MMRTG (eMMRTG) design is an MMRTG retrofitted with advanced skutterudite thermoelectric couples to improve efficiency and power output over time. The eMMRTG’s predicted reliability over the mission duration is influenced by the uncertainty in the physics parameters governing its performance. We use a Bayesian approach to account for two types of uncertainty: epistemic and aleatory. The reliability analysis has two steps: 1) uncertainty quantification from experimental data for key physics parameters, and 2) uncertainty propagation through a computational RTG life performance prediction. In particular, we use hierarchical models to separate specimen-to-specimen uncertainty and also to model the separate manufacturing batches of thermoelectric couples loaded into the eMMRTG. We use a secondary probability method to predict reliability with epistemic uncertainty intervals for the eMMRTG system at 10,000 h for which recent data are available. We show how reliability and uncertainty intervals depend on hypothetical power requirements at 10,000 h. The approach is generally applicable when experimental data and computer simulation are used to predict the reliability of a new technology.

Progress and perspectives in thermoelectric generators for waste-heat recovery and space applications

The direct conversion of thermal energy into electrical current via thermoelectric (TE) effects relies on the successful integration of efficient TE materials into thermoelectric generators (TEGs) with optimized characteristics to ensure either optimum output power density or conversion efficiency. Successfully employed for powering deep-space probes and extraterrestrial rovers since the 1960s, the development of this technology for waste-heat-harvesting applications faces several key issues related to the high temperatures and oxidizing conditions these devices are subjected to. This Perspective provides a brief overview of some prospective thermoelectric materials/technologies for use in radioisotope thermoelectric generators utilized in space missions and highlights the progress made in the field over the last years in the fabrication of TEGs. In particular, we emphasize recent developments that enable to achieve increased power densities, thereby opening up novel research directions for mid-range-temperature applications. In addition to showing how using lower quantities of TE materials may be achieved without sacrificing device performance, we provide an outlook of the challenges and open questions that remain to be addressed to make this technology economically and technologically viable in everyday-life environments.

Analysis of neptunium oxides produced through modified direct denitration

Production of neptunium-237 (237Np) target materials for plutonium-238 (238Pu) radioisotope thermoelectric generators (RTGs) for deep space exploration requires advanced chemistry and engineering development. Currently, the domestic Pu-238 Supply Program at Oak Ridge National Laboratory produces neptunium dioxide (NpO2) for target material using a modified direct denitration (MDD) flowsheet. Although the chemistry, reaction mechanisms, and product characteristics of MDD are well understood for uranium, corresponding studies of the neptunium system are still needed to continue optimization of target material properties, production equipment design, and production flowsheets. The objective of this work is to characterize crystalline phases, morphology, surface texture, and particle size of NpO2 produced via MDD reactions. Solid-phase characterization techniques, including powder X-ray diffraction (pXRD) and scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS), were employed to achieve this objective. Subsequent data processing using the Morphological Analysis for Material Attribution (MAMA) software was performed to analyze particle morphology and size. Broadly, the powders were found to contain a mixture of NpO2 and Np2O5 after denitration with a variety of morphologies. After high-firing, the product was found to be NpO2 with a typical polycrystalline oxide morphology and a grain size ranging from 0.72 to 0.94 µm. These analyses provide knowledge on the reaction pathway for a non-traditional NpO2 synthesis method and offer additional unique insight into production-scale environments for transuranic materials.

A skutterudite thermoelectric module with high aspect ratio applied to milliwatt radioisotope thermoelectric generator

This study focuses on the design and optimization of a high aspect ratio skutterudite (SKD) thermoelectric module (TEM) for a milliwatt Radioisotope Thermoelectric Generator (RTG). The RTG is a solid-state energy conversion device that utilizes the heat generated by radioactive isotope decay to generate electrical energy through the Seebeck effect. The aim of this work is to enhance the output performance of the RTG by investigating the influence of TEM structure on temperature distribution and overall performance using thermal resistance network and finite element analysis (FEA) methods. In this work, the RTG's Radioisotope Heater Unit (RHU) and diameter are kept constant as 4 W and 70 mm, while the effects of leg length (L), number of legs (n), and cross-sectional area (A) on voltage, output power, and conversion efficiency are studied. It is observed that increasing L, selecting an appropriate A, and reducing n result in a larger temperature gradient within the TEM, ultimately improving the output power and conversion efficiency of the milliwatt RTG. For A = 0.64 mm2, L = 25 mm, and n = 36, the milliwatt RTG generates maximum output power (Pmax) of 86.95 mW and maximum conversion efficiency (ηrmax) of 2.17%. To optimize the output power and conversion efficiency of the milliwatt RTG while simultaneously assuring the large voltage and manufacturability, a SKD TEM with 64 legs, each with dimensions of 0.8 × 0.8 × 25 mm3, is chosen. The milliwatt RTG installed with this SKD TEM will provide a voltage output (Vout) of 1.13 V, maximum output power (Pmax) of 63.30 mW, and maximum conversion efficiency (ηrmax) of 1.58%. Furthermore, it exhibits excellent environmental adaptability, capable of operating in ambient temperatures (T∞) up to 538 K. The thermal stress distribution in the high aspect ratio SKD TEM is also analyzed. Due to the varying coefficients of thermal expansion (CTE) between the SKD material and the adhesive, the maximum stress within the SKD TEM occurs inside and around the p-SKD legs. Finally, a SKD TEM of 9.9 × 9.9 × 25 mm3 containing 64 legs with each leg's dimension of 0.8 × 0.8 × 25 mm3 is fabricated. It shows an open circuit voltage (Voc) of 3.51 V, a maximum output power (Pmax) of 194.45 mW, and a maximum conversion efficiency (ηtmax) of 3.4% at the hot side temperature (Th) of 723 K and cold side temperature (Tc) of 296 K.

Feasibility of power beaming through the Venus atmosphere

Despite great interest by the planetary science community in the robotic exploration of Venus, solutions for operating in this extreme environment present significant challenges to mission architects. A particular challenge in designing any in situ Venus mission of reasonable duration is the selection of a suitable power source. State-of-the-art space power technologies comprising solar arrays, batteries and radioisotope thermoelectric generators are not capable of operating on the surface of Venus, limited by the high temperatures, high pressures and corrosive environment. Despite its proximity to the Sun, the solar resource on the surface is limited by the dense cloud layers. One potential approach for circumventing this limitation includes collecting the abundant visible solar energy above the Venus atmosphere, and wirelessly transferring this power to the surface at a wavelength that can more effectively penetrate the dense cloud layer. The feasibility of such an approach and other related mission concepts are discussed herein from a perspective of atmospheric absorption and scattering of the beamed energy.

Neutron-irradiated effect on the thermoelectric properties of Bi2Te3-based thermoelectric leg

Thermoelectric (TE) materials working in radioisotope thermoelectric generators are irradiated by neutrons throughout its service; thus, investigating the neutron irradiation stability of TE devices is necessary. Herein, the influence of neutron irradiation with fluences of 4.56 × 1010 and 1 × 1013 n/cm2 by pulsed neutron reactor on the electrical and thermal transport properties of n-type Bi2Te2.7Se0.3 and p-type Bi0.5Sb1.5Te3 thermoelectric alloys prepared by cold-pressing and molding is investigated. After neutron irradiation, the properties of thermoelectric materials fluctuate, which is related to the material type and irradiation fluence. Different from p-type thermoelectric materials, neutron irradiation has a positive effect on n-type Bi2Te2.7Se0.3 materials. This result might be due to the increase of carrier mobility and the optimization of electrical conductivity. Afterward, the effects of p-type and n-type TE devices with different treatments on the output performance of TE devices are further discussed. The positive and negative effects caused by irradiation can cancel each other to a certain extent. For TE devices paired with p-type Bi0.5Sb1.5Te3 and n-type Bi2Te2.7Se0.3 thermoelectric legs, the generated power and conversion efficiency are stable after neutron irradiation.

Development of the generation III (ATR generation I) Plutonium-238 production target design

A domestic supply chain to produce 238Pu fuel for radioisotope thermoelectric generators is critical for enabling future space exploration. A multi-laboratory effort has worked to establish a common target design to efficiently produce 238Pu in two research reactors. This approach ensures that the annual production goals set forth by NASA are met, while also establishing redundant production capabilities. This paper describes the effort to develop the common target design as well as considerations for future applications for the irradiation platform.

Synthesis, Characterization, and Stability of Two Americium Vanadates, AmVO3 and AmVO4.

In search for chemically stable americium compounds with high power densities for radioisotope sources for space applications, AmVO3 and AmVO4 were prepared by a solid-state reaction. We present here their crystal structure at room temperature solved by powder X-ray diffraction combined with Rietveld refinement. Their thermal and self-irradiation stabilities have been studied. The oxidation states of americium were confirmed by the Am M5 edge high-resolution X-ray absorption near-edge structure (HR-XANES) technique. Such ceramics are investigated as potential power sources for space applications like radioisotope thermoelectric generators, and they have to endure extreme conditions including vacuum, high or low temperatures, and internal irradiation. Thus, their stability under self-irradiation and heat treatment in inert and oxidizing atmospheres was tested and discussed relative to other compounds with a high content of americium.

Application of Np–Am Mixture in Production of 238Pu in a VVER-1000 Reactor and the Reactivity Effect Caused by Loss-of-Coolant Accident in the Central Np–Am Fuel Assembly

This paper presents the results obtained from numerical evaluations for the possibility of large-scale 238Pu production in the light-water VVER-1000 reactor and the reactivity effect caused by the loss-of-coolant accident in the central fuel assembly of the reactor core. This fuel assembly containing the Np–Am-component of minor actinides was placed in the center of the reactor core and intended for intense production of 238Pu. Optimal conditions were found for large-scale production of plutonium with an isotope composition suitable for application in radioisotope thermoelectric generators. The reactivity effect from the loss-of-coolant accident in the central Np–Am fuel assembly was evaluated, and the perturbation theory was used to determine the contributions of some neutron processes (leakage, absorption, and moderation) to the total variation of the effective neutron multiplication factor.

A Micro-Nuclear Power Generator for Space Missions

The significance of reliable energy storage systems in spacecraft applications cannot be overstated, since they play a vital role in ensuring continuous power supply and prolonged mission durations. This research deals with the modeling of a hybrid multi-mission radioisotope thermoelectric generator (MMRTG)-lithium-ion (Li-ion) battery integrated energy storage system for spacecraft applications to combine the RTGs’ long lifespan and reliability benefits alongside the Li-ion battery’s rechargeability and high energy density to achieve a single energy unit. The investigation’s main problem was exploring a power unit that improves the limitations of MMRTG and Li-ion batteries to achieve a highly efficient and reliable power supply for autonomous systems, such as a spacecraft. The proposed hybrid system comprises a 110 W/32 V RTG and a 3.6 V/43 Ah Li-ion battery connected to a DC motor through power converters. Results demonstrate the potential of the adopted hybrid energy system in improving the efficiency, reliability, and mission duration of spacecraft missions. The assessment of the hybrid energy system under various load conditions shows that the highest power peak of 3500 W was achieved at a load resistance of 1 Ω. Furthermore, the results show that the hybrid energy system output voltage at temperatures of 253 °K and 293 °K are relatively equal. However, the power cycle was wider and required a long time before dropping.