Radioisotope Thermoelectric Generators Research Articles

Modeling of Am-241 as an alternative fuel source in a radioisotope thermoelectric generator

Radioisotope thermoelectric generators (RTGs) have been utilized in the USA to power satellites and space exploration equipment for over half a century. In our preceding paper, Dustin and Borrelli (2021), we assessed the performance of 241Am as an alternative to 238Pu by modeling the specific decay-heat and gamma spectra of several potential radioisotopes in SCALE-ORIGEN, and comparing these with 238Pu An analysis of heat output, heat stability, safety, availability and required mass showed that of the isotopes reviewed, 241Am had the greatest potential for replacing 238Pu, with additional design considerations. In our current work, we have assessed radiation transport for two configurations of an 241Am powered Multi-Mission Radioisotope Thermoelectric Generator (MMRTG), due to the potential health effects for operators, fabricators, and others with the potential to be exposed to neutrons and lower energy gamma rays. We have modeled the use of americium as a fuel source replacement for plutonium in current MMRTG designs. The assessment has been performed using MCNP6, in terms of neutron and gamma emissions that might interfere with sensitive equipment and power system production. The radiation transport behavior in terms of dose rate of the 241Am fueled models is compared with that of a 238Pu fueled model. Additional work was performed to validate modeling simplifications with regards to thermocouple material configurations.

Thermal Properties and Behaviour of Am-Bearing Fuel in European Space Radioisotope Power Systems

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.

Thermoelectric properties and service stability of Ag-containing Cu2Se

In 1970s, great efforts had been spent to develop Ag-containing Cu2Se-based radioisotope thermoelectric generators (RTG), but unfortunately the activity was completely stopped in 1981 because the Ag-containing Cu2Se-based thermoelectric module demonstrated poor service stability. Since then, a prevailing mindset in thermoelectric society is that Cu2Se, and related Cu-based thermoelectric materials, can not be fabricated into stable modules. In this study, a series of Ag-containing Cu2Se samples are synthesized. Their thermoelectric properties and service stability under current and/or temperature gradient are systematically investigated. It is found that introducing Ag in Cu2Se has significant influence on the thermoelectric properties but little influence on the stability. The critical voltages for Ag-containing Cu2Se samples, which represent the threshold for thermodynamic stability under electric field and/or temperature gradient, are determined by the total content of Cu and Ag rather than just the Cu content. Likewise, electrical conductivity is proposed to be a simple indicator to rapidly predict the stability and thermoelectric performance of Cu2Se-based materials. Based on the measured critical voltages, the rational ranges of leg geometry are recommended to fabricate the stable and efficient Cu1.97Ag0.03Se-based thermoelectric modules.

Uncovering the Structure and Stability of Thermoelectric La3–xTe4–Ni Composites Using High-Resolution and In Situ TEM

Lanthanum telluride (La3–xTe4) is a high-performance, next-generation thermoelectric material with a thermoelectric dimensionless figure of merit (zT) of 1.1 at 1273 K (x = 0.23) and has potential applications in radioisotope thermoelectric generators (RTGs) to power the space missions conducted by National Aeronautics and Space Administration (NASA). It has been shown that the zT can be increased by 30% when nickel (Ni) nanoparticle inclusions are introduced to the La3–xTe4 matrix. The coherent interfaces between La3–xTe4 and Ni are likely a key factor determining the stability and performance of the La3–xTe4–Ni composites, but their role and nature are not well understood. It is important to determine the stability of the La3–xTe4/Ni interface in deep-space conditions, in addition to the effect of heat and oxygen on the mechanisms and kinetics of interface degradation. Here, we show the high-resolution structural characterization and the epitaxial crystallographic relationship of La3–xTe4–Ni thermoelectric composites and their interfaces at high-vacuum and ambient-temperature conditions using transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). We further demonstrate the La3–xTe4/Ni interface degradation and Ni diffusion over its operating temperature range (25–1000 °C), in the presence and absence of oxygen, in real time, using in situ TEM.

Radioisotope research and development at the Linear Accelerator of the Institute for Nuclear Research of RAS

Radioisotope research at the linear accelerator of INR RAS has been developing since 1989. In 1992, a facility for the irradiation of isotope targets and radioisotope production was built, operating on a diverted proton beam with an energy of 160 MeV and a current of more than 120 μA, which was the most productive facility in the world at that time. The facility is used for both fundamental and applied research. The main focus is the study of the processes of the generation and recovery of medical radioisotopes, as well as the development of technology for their production. The main radionuclides produced now are strontium-82 and actinium-225. A Russian 82 Sr/82 Rb generator for PET-diagnostics has been developed by INR RAS and is being introduced for medicine. New types of 225Ac/213Bi generators are also being developed for the treatment of oncological diseases. In addition, the study of the process for the production and chemical recovery of medical radionuclides 117mSn, 68 Ge, 72Se, 64, 67Cu, 223Ra, 230 Pa, and 230U is being carried out, as is the study of some generators with some of these isotopes. The technology developed at INR is used in Russia and abroad. Physical processes of proton interaction with different nuclei are also investigated in the laboratory of the radioisotope complex of INR RAS. The produced 22Na, 83Rb, 109Cd, and radioactive sources with these radionuclides are used in physical research and technology.

Plutonium-238: The Fuel Crisis

Plutonium-238 is currently still the best fuel to power satellites to be sent to deep space in regions where the solar panels can no longer efficiently receive the sunlight. For 50 years, the National Aeronautics and Space Administration (NASA) has used this radioisotope as a fuel in radioisotope thermoelectric generators (RTGs) installed on satellites such as Pioneer 10 and 11, Voyager 1 and 2, Cassini-Huygens and New Horizons, as well as the various rovers sent to the Moon and to Mars, among others. Plutonium-238 is not a naturally occurring isotope on the planet, it was produced in greater quantity during the Cold War period as a by-product of the production of Plutonium-239 used for nuclear bombs. However, after the shutting down of the Savannah River reactors in 1988 and the ending of the Soviet Union in 1991, the United States stock of Plutonium-238 has been increasingly reduced, which threatens NASA's future space projects. This paper presents a brief bibliographic review about the subject, as well as commentaries on the options available to the United States, from restarting the production of this fuel, to possible alternatives for a new type of fuel or equipment that may supply the spacecrafts.

Mission analysis of a Neptune detector with a 10 kWe nuclear reactor power generator

<p indent=0mm>Based on a comparative analysis of the advantages and disadvantages of radioisotope and nuclear reactor power generators in space applications, this paper determines that the space reactor power generator is the energy scheme of the Neptune exploration mission, proposes four kinds of major scientific problems involved in Neptune exploration, and introduces preliminary scheme ideas, such as the function module, fight orbit, and design constraints of the mission. Then, focusing on the reactor, the thermal-electric conversion device, the technical route, nuclear safety and radiation protection, the study elaborates the technical scheme of a 10 kWe space reactor power generator. The Neptune detector bus scheme, including TT&C communication, electric propulsion, independent operation and management, high-reliability and long-life design, payload, and microsatellite probe, can provide reference for the demonstration and implementation of Neptune exploration.

Accelerator Production Method in 140Nd/140Pr Radioisotope Generator Based on Electron Capture

140Nd/140Pr radioisotope generator has importance for cancer diagnosis purposes and promises as an in vivo generator in positron emission tomography (PET). 140Nd/140Pr radioisotope generator is based on physicochemical processes, such as electron capture (EC), between parent (140Nd) and daughter (140Pr) radioisotopes. The production of the parent 140Nd radioisotope was performed by particle accelerators using charged particle induced reaction processes for energy region of Eparticle = 100 → 1 MeV in irradiation time of 1 h with constant beam current of 1 µA. In addition to the production of the parent 140Nd radioisotope, we also considered the direct production of 140Pr radioisotope via particle accelerators at the same conditions because 140Pr radioisotope in low-budget facilities can be directly used in PET scans such as in hospitals due to its short half-life. Hence, the cross-section and integral yield curves of reaction processes were calculated as dependent on projectile energies, and activities and yields of products were simulated. The obtained results were compared with the experimental data in the literature and provided new nuclear data to the literature which lead to the experimentalist.

Mechanical test of thermoelectric device in RTG prototype

Since the United States launched the first space radioisotope thermoelectric generator (RTG), the United States has launched a total of 28 spacecraft with 43 RTGs. There are still more than 30 RTGs in orbit, and the longest working life has exceeded 30 years. During the launching process of RTG, the thermoelectric energy converter needs to withstand the impact at start up and high-frequency vibration during flight, and it faces the harsh service conditions that heat, force, electricity and other external fields are used and coupled together. For this reason, this paper has carried out environmental test verification on the designed RTG prototype through vibration and shock tests. By applying shocks of the same magnitude as the launch and vibrations of the same magnitude during flight, it is found that the thermoelectric devices loaded inside are in the experiment. There is no obvious change in the internal resistance before and after, indicating that the installation method can withstand the complex mechanical environment test during launch, which provides a reference for RTG’s thermoelectric device loading method.

Mechanical simulation optimization of RTG prototype

The thermocouple energy converter in the radioisotope thermoelectric genera tor (RTG) requires a long time work under high temperature and large temperature diff erence, and needs to withstand the harsh service conditions of heat, force, electricity an d other external fields during the launch process. In order to understand the dynamic p urchase response and dynamic strain of various parts of the structure when the thermoe lectric module is under dynamic load under different conditions, this paper uses softwa re to simulate the vibration and shock scenarios of the thermoelectric module in the RT G prototype. According to the existing conditions, a three-direction impact spectrum a nalysis was carried out. It was found that in the three-direction random vibration, from the root mean square stress result, the risk of damage is small; while during the impac t, the shell and the thermoelectric The displacement in the in-plane direction between t he module furnishing surfaces will cause the thermoelectric material in the thermoelect ric module to produce a large shear stress, which will cause the risk of shear damage to the structure. The imitation result provides a reference for the further optimization des ign of RTG prototype.

Persephone: A Pluto-system Orbiter and Kuiper Belt Explorer

Abstract Persephone is a NASA concept mission study that addresses key questions raised by New Horizons’ encounters with Kuiper Belt objects (KBOs), with arguably the most important being, “Does Pluto have a subsurface ocean?” More broadly, Persephone would answer four significant science questions: (1) What are the internal structures of Pluto and Charon? (2) How have the surfaces and atmospheres in the Pluto system evolved? (3) How has the KBO population evolved? (4) What are the particles and magnetic field environments of the Kuiper Belt? To answer these questions, Persephone has a comprehensive payload, and it would both orbit within the Pluto system and encounter other KBOs. The nominal mission is 30.7 yr long, with launch in 2031 on a Space Launch System Block 2 rocket with a Centaur kick stage, followed by a 27.6 yr cruise powered by existing radioisotope electric propulsion and a Jupiter gravity assist to reach Pluto in 2058. En route to Pluto, Persephone would have one 50–100 km class KBO encounter before starting a 3.1-Earth-year orbital campaign of the Pluto system. The mission also includes the potential for an 8 yr extended mission, which would enable the exploration of another KBO in the 100–150 km size class. The mission payload includes 11 instruments: Panchromatic and Color High-Resolution Imager, Low-Light Camera, Ultra-Violet Spectrometer, Near-Infrared (IR) Spectrometer, Thermal IR Camera, Radio Frequency Spectrometer, Mass Spectrometer, Altimeter, Sounding Radar, Magnetometer, and Plasma Spectrometer. The nominal cost of this mission is $3.0 billion, making it a large strategic science mission.

Purification of rad-waste arising from irradiated natural tin target towards tellurium-125m/antimony-125 radioisotope generator elaboration

The present study introduces a new approach based on an irradiated tin target's impurity to produce a125Sb/125mTe radioisotope generator. A prepared generator gel matrix of zirconium-silico-tungstate was loaded with 6660 MBq (180 mCi) activity of the separated antimony. The loading and elution method has been studied as a function of the concentration of H2SO4 acid solution. The effect of mild oxidant (ascorbic acid) in the loading and complexing agent (acetic acid) in elution was investigated to improve the generator's elution profile. A mixture of (0.5 M) ascorbic acid and (0.5 M) H2SO4 acid solution as a loading solution resulted in a maximum distribution of 125Sb. In comparison, using (0.4 M) acetic acid as an eluent solution 125mTe yield reached 88.7% and minimised percentage of 125Sb breakthrough was obtained (<0.01%).

Thermoelectric materials for space applications

Solid-state energy conversion through thermoelectric effects remains the technology of choice for space applications for which, their low energy conversion efficiency is largely outweighed by the reliability and technical requirements of the mission. Radioisotope thermoelectric generators (RTGs) enable the direct conversion of the heat released by nuclear fuel into the electrical power required to energize the scientific instruments. The optimization of the conversion efficiency is intimately connected to the performances of the thermoelectric materials integrated which are governed by the transport properties of these materials. Recent advances in the design of highly efficient thermoelectric materials raise interesting prospects to further enhance the performances of RTGs for future exploratory missions in the Solar system. Here, we briefly review the knowledge acquired over the last years on several families of thermoelectric materials, the performances of which are close or even higher than those conventionally used in RTGs to date. Issues that remain to be solved are further discussed.

High-power RTG simulation prototype based on skutterudite

In this paper, a simulation prototype of a high-power radioisotope thermoelectric generator (RTG) simulation prototype based on skutterudite is designed. By replacing the isotope heat source with an electrically heated heat source, its output power is 40 W under the condition of 1600 W electric power heating, and the thermoelectric conversion efficiency can reach 2.5%. The simulation prototype contains a total of 8 thermoelectric modules, and each module contains 16 pairs of thermoelectric materials. Its maximum theoretical output power can reach 66 W under the temperature of 550 °C. This research provides a leading guide and reference point for the temperature difference structure of RTGs at the hundred watt level.

Transmutation of americium and neptunium in the single-fluid double-zone thorium-based molten salt reactor

In the current work, we introduce a new approach compared to solid-fuel reactors to load the minor actinides (MAs) into the Single-fluid Double-zone Thorium-based Molten Salt Reactor (SD-TMSR). The proposed approach merges the advantages of both homogeneous and heterogeneous approaches. Among MAs nuclides, 241Am and 237Np are selected for transmutation due to their long half-life. We simulate two separate tanks; Pu + U tank and FPs tank. In this study, a tank is a right cylinder with a volume of 1.0 m3. The Pu + U tank is used to store Pu and U isotopes extracted from the central channel of the SD-TMSR. However, the FPs tank is used to store all fission products (FPs) produced from the transmutation process in the central channel. The overall change in the actinides and FPs mass during the irradiation has been calculated using direct SERPENT-2 calculations. The results show that the transmutation ratio of 241Am and 237Np reaches 98.5% and 93.2%, respectively after 1500 days of irradiation. We notice that the major isotope in the Pu + U tank is 238Pu. Under 241Am irradiation, our proposed approach offers ≈0.3kg of 238Pu after 1 year of operation. However, under 237Np irradiation, 2.5 times more 238Pu can be extracted after the same period of operation. The produced 238Pu can be used in the radioisotope thermoelectric generators (RTG, RITEG) and radioisotope heater units.

Study of the Nuclear Structure for Some Target Nuclei Used in the Production of Beta-Emitting Radioactive Isotopes for the Fabrication of Nuclear Batteries

The nuclear structure of 40Ar, 112Cd, 133Cs, 151Eu, 154Sm, and 226Ra target nuclei used in nuclear battery technology was investigated. These nuclei are widely used for the radioisotope thermo-electric generator space studies and for betavoltaic battery microelectronic systems. For this purpose, some nuclear static properties were calculated. In particular, the single particle radial nuclear density distribution, the corresponding root mean square radii, neutron skin thicknesses, and binding energies were calculated within the framework of Hartree-Fock approximation with Skyrme interaction. The bremsstrahlung spectra produced by the absorption of beta particles through the betavoltaic process and backscattered photons spectrum were also studied. All the obtained results were compared with available experimental data.

Development of Engineering Qualification Model of a Small ETG for a Launch Environmental Test

According to the development plan for the Korea Space Launch Vehicle, a performance verification satellite will be installed in the launch vehicle in 2021. In addition, three payloads, including a small electrically heated thermoelectric generator (ETG), will be developed and installed in the satellite. In particular, a small ETG has been developed by the Korea Atomic Energy Research Institute for the purpose of evaluating the characteristics of the ETG in the space environment prior to the development of a radioisotope thermoelectric generator for lunar missions. In this study, shock and vibration tests were carried out to check whether the ETG can endure the launch environment of the spacecraft. In addition, a thermal cycle test and a thermal vacuum test were conducted to check whether the ETG maintains its performance, even at drastic temperature changes, which can be applied to the ETG in space. Finally, a periodic performance test was carried out to measure the degradation level of the small ETG.

Empirical Analysis of the Multi-Mission Radioisotope Thermoelectric Generator Qualification Unit Operated at a Low Thermal Inventory with Potential for Improved End-of-Life Power

Performance predictions for the first multi-mission radioisotope thermoelectric generator (MMRTG) flight unit and engineering unit were recently reported. Both units were produced and operated/tested within specifications [i.e., nominal thermal inventory = 2000 W(thermal)]. In an attempt to study the effect of a deep space cruise on an MMRTG that has been operational for 6.25 years (2.25 years storage + 4 years cruise), the qualification unit (QU) was placed on life test with a below-specification thermal inventory of 1904 W(thermal). Analysis indicates that loading an MMRTG with a lower thermal inventory may result in less power at the beginning-of-life but more power at the end-of-design-life (EODL). The lower thermal inventory in the QU produces a lower operating temperature, which appears to cause a significant reduction in the degradation rate of the thermoelectric couples. Preliminary calculations indicate that a thermal inventory of 1904 W(thermal) could result in a 9 W(electric) power boost at EODL [i.e., 84 W(electric)], which is a 12% improvement over the first MMRTG flight unit and engineering unit predictions. Preliminary degradation analysis suggests that a 1904 W(thermal) unit will begin to produce more power than a 2027 W(thermal) unit approximately 4 years after fueling. This suggests that missions with a primary power requirement more than 4 years after fueling would benefit from a lower thermal inventory. In addition, using a lower thermal inventory has significant benefits for 238Pu stockpile management and may allow for additional MMRTGs to be fueled from our current reserves. Conclusions and hypotheses presented here should be considered preliminary because the QU data set is very small and there are some uncertainties regarding how early-life QU data will translate into later-life performance. More QU testing at a thermal inventory of 1904 W(thermal) is needed to prove that the preliminary conclusions presented here are valid.

Analysis of producing 238Pu as a byproduct in an MSFR

238Pu can be used as the heat source in radioisotope thermoelectric generators for deep space exploration, while it is currently in shortage because of the suspension of 238Pu production in the USA and Russia for many years. Two possible scenarios for the production of 238Pu as a byproduct in a molten salt fast reactor (MSFR) are assessed in this work. For the first scenario, the MSFR is started with low-enriched uranium (LEU), and neptunium is extracted from the fuel salt and then fed into the fertile salt of thorium for high-purity 238Pu production. For the second scenario, the ignition material for the MSFR is 233U, and highly purified 238Pu is produced by directly extracting plutonium from the fuel salt. The simulated results indicate that on average 15.48 kg 238Pu with the quality (ratio of 238Pu mass to total mass of Pu isotopes) larger than 98.6% can be produced per year during a 60-year operation for scenario I. In parallel, 8.31 kg 238Pu with the quality larger than 91.9% can be produced annually for scenario II. The 238Pu production has a small impact on the neutronic performance of the core for the two scenarios except that bred 238U is reduced by 922 kg for scenario I and by 261 kg for scenario II during the 60-year lifetime. The 238Pu production has a very tiny effect on the radiotoxicity of discharged nuclear fuel for scenario I, while it can reduce the total radiotoxicity of discharged nuclear fuel by three times for scenario II.

MCNP modelling of radiation effects of the Dragonfly mission's RTG on Titan

The Dragonfly mission to Titan is proposed to use a radioisotope thermoelectric generator to supply electrical and thermal power to the spacecraft during flight and once it arrives at its destination. As no spacecraft continuously powered by an RTG has been flown down through Titan's atmosphere before, the effects of such a generator's radiation field on the surrounding environment are unknown. MCNP (Monte Carlo N-Particle) simulations were run on a simplified model of the generator in its protective hull section in a coarse-grained Titan environment with both neutron and photon transport modelled, the former to determine the degree to which the surface was likely to become activated, and the latter to determine the energy likely to be deposited in a surface covering of tholin-analogue hydrocarbons, and consequently the potential for damage to chemical bonds found therein. Tests on the model also provided data which allowed for a determination of the degree to which the insulation surrounding the generator spacecraft might be activated by stray neutrons, how much heating might be expected in the hull section surrounding the generator and how much attenuation the beam from the spacecraft's pulsed neutron generator is likely to experience due to Titan's thick nitrogen atmosphere.