Radioisotope Thermoelectric Generators Research Articles

Design study of molten-salt-type reactor for powering space probes and its automated start-up

Space probes exploring the deep space need a noticeable amount of electricity for powering their instrumentation (5–8 kWe). The solar battery, however, is not able to afford this because of the extended distance between the probe and the sun. Furthermore, the radioisotope thermoelectric generator (RTG) may be extraordinarily sizable for this purpose. On the contrary, installation of nuclear reactor on the probes seems to be promising for the reasons such as its small mass, stability, and its high power density. Especially, the deep-space sample return mission has an essential demand for a nuclear reactor typically with the following specifications: 10-year reactor employment time, total system mass less than 500 kg, and electricity output more than 8 kWe. High power density is the key to realize these requirements. The thermal electricity conversion efficiency of a space reactor increases in accordance with the rise of the core temperature. Thus, in the present study, we propose a use of the molten-salt fuel considering that this type of fuel can achieve high core temperature over 1000 K. In addition to the space reactor for the space exploration, autonomic reactor control is extremely desirable because real-time control from the earth is difficult because of the long time lag of mutual communication. The molten-salt space reactor proposed in the present paper satisfies all the above essential demands primarily owing to the high-temperature operation availability. In addition, we concluded that the automatic reactor start-up in its orbit is feasible by virtue of introduction of novel reactivity-control devices proposed by Kambe (Kambe, Sato, Tsunoda. Space Nuclear Conference; 2007 June 24–28; Boston, MA. p. 36–45).

Studies on separation of Cs/Ba and Zn/Cu binary mixtures on zirconium antimonate as ion exchanger

Zirconium antimonate (ZrSb) ion exchanger was prepared. Distribution coefficients of metal ions on ZrSb ion exchanger in solutions of various acids (hydrochloric, nitric, acetic, citric, ascorbic) and salts (sodium chloride and sodium nitrate) were determined to evaluate the ion exchange behavior and selectivity to specific ions. ZrSb, as compared to other inorganic ion exchangers, showed increased thermal and chemical stability. ZrSb is an efficient and promising ion exchanger for preparation of 137Cs/137mBa and 62Zn/62Cu radioisotope generators.

Multi-layer stripper foils containing boron, PLD and CVD produced diamond-like carbon

The Applied Technology Group at TRIUMF operates three accelerators, TR30-1, TR30-2 and CP42 that use 30 MeV proton beams in the generation of commercial radioisotopes. These cyclotrons typically use diamond-like carbon (DLC) stripper foils produced in house of between 2.0 and 3.0 lm to extract particle beams from these cyclotrons. Micromatter (formerly the TRIUMF Carbon Foil Laboratory) manufactures DLC foils in a range of thicknesses from 10 nm to *10 lm using pulsed laser deposition. Continuing our previous work '(Zeisler and Jaggi Nucl Instrum Methods Phys Res A 613:434, 2010)' we further investigated the presence of boron in multilayer DLC foils, specifically DLC/boron/DLC hybrid foils. In this work, we investigated the mechanical properties and lifetime of these stripper foils in cyclotrons with varying thicknesses and boron content as well as a special DLC layer deposited by chemical vapor deposition and find an improvement in flexibility, mechanical strength and lifetime.

Generation of Radioisotopes with Accelerator Neutrons by Deuterons

A new system proposed for the generation of radioisotopes with accelerator neutrons by deuterons (GRAND) is described by mainly discussing the production of 99Mo used for nuclear medicine diagnosis. A prototype facility of this system consists of a cyclotron to produce intense accelerator neutrons from the natC(\(d\),\(n\)) reaction with 40 MeV 2 mA deuteron beams, and a sublimation system to separate 99mTc from an irradiated 100MoO3 sample. About 8.1 TBq/week of 99Mo is produced by repeating irradiation on an enriched 100Mo sample (251 g) with accelerator neutrons for two days three times. It meets about 10% of the 99Mo demand in Japan. The characteristic feature of the system lies in its capability to reliably produce a wide variety of high-quality, carrier-free, carrier-added radioisotopes with a minimum level of radioactive waste without using uranium. The system is compact in size, and easy to operate; therefore it could be used worldwide to produce radioisotopes for medical, research, and industrial...

Characteristics of Nuclear Spacecraft in Nano-Gravity for the Deep Space Explorer

Nuclear power is investigated for its use in space travel beyond Mars' orbit. Considered from the aspects of economy and safety, two kinds of spacecraft are compared: radioisotope thermoelectric generator (RTG)-powered and photovoltaic-battery powered. Optimized mass for RTG spacecraft is obtained for measuring nuclear power expenditure in space, required for finding the break-even distance for RTG-powered compared to photovoltaic-battery–powered spacecraft. It is found that the lighter mass of the RTG system makes it the more economical of the two systems considered. The safety assessment of the RTG system also shows greater safety of operation where the nonlinear algorithm is described by the system dynamics algorithm. Therefore, it is concluded that, of the two systems, nuclear power in space holds the most promise for both safety and economy.

Ceramics, squared: J. Carlos Diez and colleagues discuss thermoelectric ceramics and the challenge of optimizing their properties

Thermoelectric materials can directly convert a temperature gradient into an electric voltage thanks to the Seebeck effect. Since its discovery in 1823, this characteristic has allowed the use of metallic junctions to control temperatures. These thermocouples possess a low electrical resistivity and Seebeck coefficient with high thermal conductivity. The thermoelectric performances of these materials are evaluated by the dimensionless figure-of-merit (ZT = SsT/k, where S is the Seebeck coefficient, T is the absolute temperature, s is the electrical conductivity, and k is the thermal conductivity) which is very small, indicating that metallic materials possess poor thermoelectric behavior. The introduction of intermetallic semiconductor materials, with higher thermoelectric performances than their metallic counterparts, has resulted in their practical application in thermoelectric modules. These modules can be found as Peltier devices, when used for refrigeration purposes, or Seebeck devices when used for electrical energy generation. Current applications of this kind of material include the environmentally friendly recoveryof industrial and automobilewasteheat, and inthe production of electrical energy in radioisotope thermoelectric generators installed on spacecrafts, or in lighthouses located in isolated regions. Nevertheless, the application of semiconductor materials for energy generation has been limited due to their relatively low thermal stability under air, which can result in the release of heavy and/or toxic elements, and degradation and/or oxidation processes at high temperatures which can diminish their performances. Nowadays, research effort is being focused on ceramic materials with relatively high thermoelectric performances which can work at higher temperatures than the intermetallic ones. Some of these ceramics are based on Co-oxides, with a high Seebeck coefficient and low electrical resistivity and thermal conductivity. Moreover, they are mostly composed of abundant, less expensive and more environmental friendly elements than the intermetallic compounds. Their main drawback can be found in their relatively low (compared with the intermetallic materials) ZT values, meaning that raising the figure-or merit is the most pressing task to be performed, so that they may be used in practical power generation applications. One of these Co-oxides is [Bi0.87SrO2]2[CoO2]1.82, described as a misfit layer compound with monoclinic symmetry, composed – in turn – of two subsystems exhibiting incommensurate periodicities. The structure of the misfit layer crystal can be described as an alternation along the c-axis of distorted rock-salt-type slabs, formed from [BiO] and [SrO] layers (the first subsystem), and of [CoO2] layers (the second subsystem and the electrical conducting one) displaying a distorted CdI2-type structure [1]. In order to improve the performance of these layered materials texturing techniques can be applied.

Problems of designing radioisotope thermoelectric power generators with a service life of decades for use in outer space exploration vehicles

The present work deals with the feasibility of developing a radioisotope thermoelectric power generator (RTPG) capable of operating unattended in outer space over a period of several decades, among other things, on the basis of chemical compounds that occur in meteoric matter. The possibilities for solving problems related to the production of three-dimensional materials and to thermoelement interconnections are discussed. The implementation of nanotechnology will allow one to achieve an increase in the efficiency of a RTPG by 15% and higher.

Synthesis and characterization of zirconyl molybdopyrophosphate and its application for preparing 137Cs/137m Ba radioisotope generator

Semicrystalline zirconyl molybdopyrophosphate (ZMPP) ion exchanger was synthesized and characterized by thermal analysis, IR spectroscopy, and X-ray diffraction and was used for preparing 137Cs/137m Ba radioisotope generator. The interaction of 134Cs radionuclides in HCl solutions was studied by batch equilibration method. The corresponding distribution coefficients were determined in relation to the acid concentration. Cesium-137 was loaded onto 1 g of zirconyl molybdopyrophosphate packed in 0.6 cm i.d. column. Elution of the generated 137m Ba was examined in relation to the chemical composition and flow rate of the eluent. Barium-137m was obtained with high and reproducible elution yield and high chemical and radionuclidic purity.

Single-element spark plasma sintering of silicon germanium

Various compositions of the thermoelectric material, silicon germanium (SiGe), were synthesized directly from single elements (SE) via the spark plasma sintering (SPS) process. Through proper choice of commercially available powders, sintering conditions and stoichiometry, n-type and p-type samples with thermoelectric properties comparable with those of the Si80Ge20 alloys used in radioisotope thermoelectric generators for space missions were generated using the SE SPS process. The SE SPS technique is a rapid process requiring only 1 h to synthesize the desired material. This new synthesis technique is a viable alternative to traditional synthesis methods and provides the potential to discover novel avenues for improving the thermoelectric dimensionless figure of merit of SiGe alloys.

Titanium tritide radioisotope heat source development: Palladium-coated titanium hydriding kinetics and tritium loading tests

For applications requiring 5–20mW electrical power for 10–20years, tritium-based radioisotope thermoelectric generators may be an alternative to Pu-238 based devices. Tritium can be stored compactly on a titanium bed. However, one of the main challenges then becomes loading the heat source at temperatures compatible with existing bismuth telluride thermoelectric module technology (<300°C). We find that a 180nm palladium coating enables titanium to be loaded with hydrogen isotopes without the typical 400–500°C vacuum activation step. Further, we observe that the hydriding kinetics of Pd coated and vacuum activated Ti are similar; both of which can be described by the Mintz–Bloch adherent film model, where the rate of hydrogen absorption is controlled by diffusion through an adherent metal-hydride layer. Finally, we design a prototype heat source vessel and demonstrate that it can be loaded completely, at temperatures below 300°C, in less than 10h.

Mars Science Laboratory Mission and Science Investigation

Scheduled to land in August of 2012, the Mars Science Laboratory (MSL) Mission was initiated to explore the habitability of Mars. This includes both modern environments as well as ancient environments recorded by the stratigraphic rock record preserved at the Gale crater landing site. The Curiosity rover has a designed lifetime of at least one Mars year (∼23 months), and drive capability of at least 20 km. Curiosity’s science payload was specifically assembled to assess habitability and includes a gas chromatograph-mass spectrometer and gas analyzer that will search for organic carbon in rocks, regolith fines, and the atmosphere (SAM instrument); an x-ray diffractometer that will determine mineralogical diversity (CheMin instrument); focusable cameras that can image landscapes and rock/regolith textures in natural color (MAHLI, MARDI, and Mastcam instruments); an alpha-particle x-ray spectrometer for in situ determination of rock and soil chemistry (APXS instrument); a laser-induced breakdown spectrometer to remotely sense the chemical composition of rocks and minerals (ChemCam instrument); an active neutron spectrometer designed to search for water in rocks/regolith (DAN instrument); a weather station to measure modern-day environmental variables (REMS instrument); and a sensor designed for continuous monitoring of background solar and cosmic radiation (RAD instrument). The various payload elements will work together to detect and study potential sampling targets with remote and in situ measurements; to acquire samples of rock, soil, and atmosphere and analyze them in onboard analytical instruments; and to observe the environment around the rover. The 155-km diameter Gale crater was chosen as Curiosity’s field site based on several attributes: an interior mountain of ancient flat-lying strata extending almost 5 km above the elevation of the landing site; the lower few hundred meters of the mountain show a progression with relative age from clay-bearing to sulfate-bearing strata, separated by an unconformity from overlying likely anhydrous strata; the landing ellipse is characterized by a mixture of alluvial fan and high thermal inertia/high albedo stratified deposits; and a number of stratigraphically/geomorphically distinct fluvial features. Samples of the crater wall and rim rock, and more recent to currently active surface materials also may be studied. Gale has a well-defined regional context and strong evidence for a progression through multiple potentially habitable environments. These environments are represented by a stratigraphic record of extraordinary extent, and insure preservation of a rich record of the environmental history of early Mars. The interior mountain of Gale Crater has been informally designated at Mount Sharp, in honor of the pioneering planetary scientist Robert Sharp. The major subsystems of the MSL Project consist of a single rover (with science payload), a Multi-Mission Radioisotope Thermoelectric Generator, an Earth-Mars cruise stage, an entry, descent, and landing system, a launch vehicle, and the mission operations and ground data systems. The primary communication path for downlink is relay through the Mars Reconnaissance Orbiter. The primary path for uplink to the rover is Direct-from-Earth. The secondary paths for downlink are Direct-to-Earth and relay through the Mars Odyssey orbiter. Curiosity is a scaled version of the 6-wheel drive, 4-wheel steering, rocker bogie system from the Mars Exploration Rovers (MER) Spirit and Opportunity and the Mars Pathfinder Sojourner. Like Spirit and Opportunity, Curiosity offers three primary modes of navigation: blind-drive, visual odometry, and visual odometry with hazard avoidance. Creation of terrain maps based on HiRISE (High Resolution Imaging Science Experiment) and other remote sensing data were used to conduct simulated driving with Curiosity in these various modes, and allowed selection of the Gale crater landing site which requires climbing the base of a mountain to achieve its primary science goals. The Sample Acquisition, Processing, and Handling (SA/SPaH) subsystem is responsible for the acquisition of rock and soil samples from the Martian surface and the processing of these samples into fine particles that are then distributed to the analytical science instruments. The SA/SPaH subsystem is also responsible for the placement of the two contact instruments (APXS, MAHLI) on rock and soil targets. SA/SPaH consists of a robotic arm and turret-mounted devices on the end of the arm, which include a drill, brush, soil scoop, sample processing device, and the mechanical and electrical interfaces to the two contact science instruments. SA/SPaH also includes drill bit boxes, the organic check material, and an observation tray, which are all mounted on the front of the rover, and inlet cover mechanisms that are placed over the SAM and CheMin solid sample inlet tubes on the rover top deck.

Searches for solar-influenced radioactive decay anomalies using spacecraft RTGs

Experiments showing a seasonal variation of the nuclear decay rates of a number of different nuclei, and decay anomalies apparently related to solar flares and solar rotation, have suggested that the Sun may somehow be influencing nuclear decay processes. Recently, Cooper searched for such an effect in 238Pu nuclei contained in the radioisotope thermoelectric generators (RTGs) on board the Cassini spacecraft. In this paper we modify and extend Cooper’s analysis to obtain constraints on anomalous decays of 238Pu over a wider range of models, but these limits cannot be applied to other nuclei if the anomaly is composition-dependent. We also show that it may require very high sensitivity for terrestrial experiments to discriminate among some models if such a decay anomaly exists, motivating the consideration of future spacecraft experiments which would require less precision.

Optimized Characterization of Thermoelectric Generators for Automotive Application

New developments in the field of thermoelectric materials bring the prospect of consumer devices for recovery of some of the waste heat from internal combustion engines closer to reality. Efficiency improvements are expected due to the development of high-temperature thermoelectric generators (TEG). In contrast to already established radioisotope thermoelectric generators, the temperature difference in automotive systems is not constant, and this imposes a set of specific requirements on the TEG system components. In particular, the behavior of the TEGs and interface materials used to link the heat flow from the heat source through the TEG to the heat sink must be examined. Due to the usage patterns of automobiles, the TEG will be subject to cyclic thermal loads, which leads to module degradation. Additionally, the automotive TEG will be exposed to an inhomogeneous temperature distribution, leading to inhomogeneous mechanical loads and reduced system efficiency. Therefore, a characterization rig is required to allow determination of the electrical, thermal, and mechanical properties of such high-temperature TEG systems. This paper describes a measurement setup using controlled adjustment of cold-side and warm-side temperatures as well as controlled feed-in of electrical power for evaluation of TEGs for application in vehicles with combustion engines. The temperature profile in the setup can be varied to simulate any vehicle usage pattern, such as the European standard driving cycle, allowing the power yield of the TEGs to be evaluated for the chosen cycle. The spatially resolved temperature distribution of a TEG system can be examined by thermal imaging. Hotspots or cracks on thermocouples of the TEGs and the thermal resistance of thermal interface materials can also be examined using this technology. The construction of the setup is briefly explained, followed by detailed discussion of the experimental results.

Modeling a novel energy harvester working in Lame mode

A new square shaped piezoelectric bimorph structure for energy scavenging purposes has been proposed and simulated. It is derived from theoretical analysis that the output power of the structure is proportional to the value of the resonant frequency. The device working in Lame mode has a much higher resonant frequency of 2.39 MHz than devices working in ordinary bending modes, which is expected to significantly increase the energy output of radioisotope power generators (RPGs). The results of static analysis and dynamic response show that output voltage is linear with the applied load; the output power is quadratic with the applied load.

238Pu

Plutonium-238 (238Pu) has a half-life of about 87.7 y and thus a higher specific activity than 239Pu. It is used in radioisotope thermoelectric generators and is a substantial source of plutonium alpha-radiation in spent nuclear fuels. Early animal studies demonstrated differences in the biokinetics of inhaled oxides of 238Pu and 239Pu with 238Pu having a substantially more rapid translocation from the lungs to the systemic organs, particularly the skeleton. This resulted in the predominant occurrence of skeletal cancers in animals exposed to 238Pu oxides but lung cancers in those with exposures to 239Pu oxides. The anatomical distribution of osteogenic sarcomas seen in animal studies was similar to that observed with 239Pu and also in plutonium workers but differed from naturally occurring tumors. The in vivo "solubility" of 238Pu has been associated with the relative amounts of 238Pu/239Pu in the particles and calcination temperatures during the preparation of the dioxides. There is experimental evidence of in vivo 238Pu particle fragmentation attributed to nuclear recoil during radioactive decay. The resulting conversion of microparticles to nanoparticles may alter their interactions with macrophages and transport across epithelial barriers. There are few documented cases of human exposures, but the biokinetics appeared to depend on the chemical and physical nature of the aerosols. Robust human biokinetic and dosimetric models have not been developed, due in part to the lack of data. With the acceleration of nuclear technologies and the greater demand for reprocessing and/or disposal of spent nuclear fuels, the potential for human exposure to 238Pu will likely increase in the future.

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.