Radioisotope Thermoelectric Generators Research Articles

Subcritical space nuclear system without most movable control systems

This paper describes the design and analysis of advanced space nuclear reactor (ASNR) whose design combines the advantages of radioisotope thermoelectric generator (RTG) and space nuclear reactor (SNR). As opposed to current SNRs designs, ASNR is a subcritical system driven by 232U–Be neutron source to generate thermal power continuously. Most movable control systems in the SNR design are removed. The detailed neutronic calculations by MCNPX (Monte Carlo N-Particle eXtended), including keff, flux, burn-up, loss-ratio of neutron source and immersion reactivity, show that ASNR has higher criticality safety and more compact structure to bear the risk of immersion accident compared with the past SNRs, and the new system can provide more thermal power than RTG. Furthermore, the neutron source efficiency is optimized to improve the utilization of 232U–Be neutron source with the improvement of criticality safety. Compared with the past designs of space nuclear power, ASNR could provide enough thermal power and avoid the occurrence of serious immersion accident in the case of total control system failure. ASNR has potential for future deep space missions.

Mechanical properties and microstructure of spark plasma sintered nanostructured p-type SiGe thermoelectric alloys

Abstract SiGe based thermoelectric (TE) materials have been employed for the past four decades for power generation in radio-isotope thermoelectric generators (RTG). Recently “nanostructuring” has resulted in significantly increasing the figure-of-merit (ZT) of both n and p-type of SiGe and thus nanostructured Si 80 Ge 20 alloys are evolving as a potential replacement for their conventional bulk counterparts in designing efficient RTGs. However, apart from ZT, their mechanical properties are equally important for the long term reliability of their TE modules. Thus, we report the mechanical properties of p-type nanostructured Si 80 Ge 20 alloys, which were synthesized employing spark plasma sintering of mechanically alloyed nanopowders of its constituent elements with 1.2% boron doping. Nanostructured p-type Si 80 Ge 20 alloys exhibited a hardness of ~ 9 ± 0.1 GPa, an elastic modulus of ~ 135 ± 1.9 GPa, a compressive strength of 108 ± 0.2 MPa, and fracture toughness of ~ 1.66 ± 0.04 MPa√m with a thermal shock resistance value of 391 ± 21 Wm − 1 . This combination of good mechanical properties coupled with higher reported ZT of nanostructured p-type Si 80 Ge 20 alloys are rendered to be a potential material for power generation applications, compared to its bulk counterpart.

Production of Y-86 and other radiometals for research purposes using a solution target system

IntroductionDiagnostic radiometals are typically obtained from cyclotrons by irradiating solid targets or from radioisotope generators. These methods have the advantage of high production yields, but require additional solid target handling infrastructure that is not readily available to many cyclotron facilities. Herein, we provide an overview of our results regarding the production of various positron-emitting radiometals using a liquid target system installed on a 13MeV cyclotron at TRIUMF. Details about the production, purification and quality control of 89Zr, 68Ga and for the first time 86Y are discussed. MethodsAqueous solutions containing 1.35–1.65g/mL of natural-abundance zinc nitrate, yttrium nitrate, and strontium nitrate were irradiated on a 13MeV cyclotron using a standard liquid target. Different target body and foil materials were investigated for corrosion. Production yields were calculated using theoretical cross-sections from the EMPIRE code and compared with experimental results. The radioisotopes were extracted from irradiated target material using solid phase extraction methods adapted from previously reported methods, and used for radiolabelling experiments. ResultsWe demonstrated production quantities that are sufficient for chemical and biological studies for three separate radiometals, 89Zr (Asat=360MBq/μA and yield=3.17MBq/μA), 86Y (Asat=31MBq/μA and yield=1.44MBq/μA), and 68Ga (Asat=141MBq/μA and yield=64MBq/μA) from one hour long irradiations on a typical medical cyclotron. 68Ga yields were sufficient for potential clinical applications. In order to avoid corrosion of the target body and target foil, nitrate solutions were chosen as well as niobium as target-body material. An automatic loading system enabled up to three production runs per day. The separation efficiency ranged from 82 to 99%. Subsequently, 68Ga and 86Y were successfully used to radiolabel DOTA-based chelators while deferoxamine was used to coordinate 89Zr.

Development of Kabila rocket: A radioisotope heated thermionic plasma rocket engine

A new type of plasma rocket engine, the Kabila rocket, using a radioisotope heated thermionic heating chamber instead of a conventional combustion chamber or catalyst bed is introduced and it achieves specific impulses similar to the ones of conventional solid and bipropellant rockets. Curium-244 is chosen as a radioisotope heat source and a thermal reductive layer is also used to obtain precise thermionic emissions. The self-sufficiency principle is applied by simultaneously heating up the emitting material with the radioisotope decay heat and by powering the different valves of the plasma rocket engine with the same radioisotope decay heat using a radioisotope thermoelectric generator. This rocket engine is then benchmarked against a 1N hydrazine thruster configuration operated on one of the Pleiades-HR-1 constellation spacecraft. A maximal specific impulse and power saving of respectively 529s and 32% are achieved with helium as propellant. Its advantages are its power saving capability, high specific impulses and simultaneous ease of storage and restart. It can however be extremely voluminous and potentially hazardous. The Kabila rocket is found to bring great benefits to the existing spacecraft and further research should optimize its geometric characteristics and investigate the physical principals of its operation.

Probing the mechanical properties and microstructure of WSi2/SixGe1−x multiphase thermoelectric material by nanoindentation, electron and focused ion beam microscopy methods

Thermoelectric (TE) materials such as silicon germanium (SiGe) alloys have been traditionally used in radioisotope thermoelectric generators (RTG) NASA applications. Beyond traditional RTG applications, we are exploring other applications in the energy harvesting arena. There is still a need to increment the TE figure of merit (ZT) of SiGe based TE alloys and we have been working on ways to improve it by incorporating tungsten di-silicide (WSi2) phases into the matrix by directional solidification (DS) process. Considerable efforts have been focused until now in microstructural engineering methods that lead to ZT improvement by microstructure optimization of TE materials. Although critical for the previous mentioned applications, work pertinent to the mechanical integrity of this type of WSi2/SiGe based TE materials is lacking. In this work, we explored for the first time the local mechanical properties and microstructure of WSi2/SixGe1−x multiphase thermoelectric material by nanoindentation, scanning electron microscopy (SEM), focused ion beam (FIB) and transmission electron microscopy (TEM) methods. We report hardness (H), modulus (E) and fracture toughness (kc) data for all phases. We obtained average H (and E) values (in GPa) of 12.94 (464.95) for the WSi2 phase, 19.49 (214.52) for the matrix, 14.95 (142.84) for the Si rich phase, and 13.98 (138.56) for the Ge rich phase respectively; while average kc values (in MPam0.5) were 1.37 for theWSi2, 0.52 for the matrix, 0.36 for the Si rich and 0.24 for the Ge rich phases respectively. FIB serial sectioning and cross-sectional TEM analysis is also included which provided insights on the deformation process below the nanoindentation area.

Atomic Batteries: Energy from Radioactivity

With alternate, sustainable, natural sources of energy being sought after, there is new interest in energy from radioactivity, including natural and waste radioactive materials. A study of various atomic batteries is presented with perspectives of development and comparisons of performance parameters and cost. We discuss radioisotope thermal generators, indirect conversion batteries, direct conversion batteries, and direct charge batteries. We qualitatively describe their principles of operation and their applications. We project possible market trends through our comparative cost analysis. We also explore a future direction for certain atomic batteries by using nanomaterials to improve their performance.

Medical Isotope Production using High Intensity Accelerator Neutrons

We proposed aprototype facility for the generation of radioisotopes with accelerator neutrons by deuterons. The neutrons are producedbynatC(d,n) with 40MeV 2mA deuteron beams, and about 8.1 TBq/week of 99Mois produced by irradiating an enriched 100Mo sample with the neutrons.High-quality 99mTc can be separatedfrom an irradiated 100MoO3 sample by thermo-chromatographic separation.In this contribution we present the system to produce medical radioisotopes, such as 99Mo, 90Y, and 67Cu, and experimental studies on 99Mo and 67Cu produced by using accelerator neutrons.

Increasing the Efficiency of the Multi-mission Radioisotope Thermoelectric Generator

The National Aeronautics and Space Administration’s Mars Science Laboratory terrestrial rover, Curiosity, has recently completed its first Martian year (687 Earth days) during which it has provided a wealth of information and insight into the red planet’s atmosphere and geology. The success of this mission was made possible in part by the reliable electrical power provided by its onboard thermoelectric power source—the multi-mission radioisotope thermoelectric generator (MMRTG). In an effort to increase the output power and efficiency of these generators, a newly designed enhanced MMRTG (eMMRTG) that will utilize the more efficient skutterudite-based thermoelectric materials has been conceptualized and modeled, and is now being developed. A discussion of the motivations, modeling results and key design factors are presented and discussed.

Improving the Overall Efficiency of Radioisotope Thermoelectric Generators

Radioisotope thermoelectric generators (RTG) convert the decay energy of a radioisotope ( 238 Pu) into heat then into electricity. RTGs have been used to power space exploration missions. This review paper studies several crucial features of past and present Static RTGs, for instance the advantages of Static RTGs. In addition it reviews Static RTGs limitations such as the shortage in the amount of 238 Pu in (U.S.A). Furthermore it compares the future Dynamic RTGs with Static RTGs. It indicates the future Dynamic RTGs, which include a thermodynamic cycle, use 2-4 times less amount of 238 Pu. In spite of that, their efficiency is almost 4 times greater than past and present RTGs.

Radioisotope generators of short-lived α-emitting radionuclides promising for use in nuclear medicine

α-Emitting radionuclides open unique possibilities for nuclear medicine owing to short range of α-particles and high linear energy transfer values. The use of radioactive generators is the simplest and the most efficient method for production of short-lived α-emitting radionuclides. The designs of radioisotope generators and generator systems used for repeated production of short-lived α-emitting radionuclides 225Ac, 213Bi, 211Pb/211Bi, 212Pb/212Bi, 223Ra, and 224Ra, promising for targeted radiotherapy, are described in detail. Particular attention is paid to the methods for production and purification of short-lived α-emitting radionuclides and for their isolation in the form suitable for use in medicobiological studies. Other procedures for producing alternative α-emitting radionuclides (211At, 227Th, 230U, 253Es, 255Fm, 149Tb) and possibilities of their use are also considered. The objectives and prospects of radiochemical studies in the field of production and isolation of α-emitting radionuclides meeting the requirements of nuclear medicine are discussed.

Microstructure and mechanical properties of thermoelectric nanostructured n-type silicon-germanium alloys synthesized employing spark plasma sintering

Owing to their high thermoelectric (TE) figure-of-merit, nanostructured Si80Ge20 alloys are evolving as a potential replacement for their bulk counterparts in designing efficient radio-isotope TE generators. However, as the mechanical properties of these alloys are equally important in order to avoid in-service catastrophic failure of their TE modules, we report the strength, hardness, fracture toughness, and thermal shock resistance of nanostructured n-type Si80Ge20 alloys synthesized employing spark plasma sintering of mechanically alloyed nanopowders of its constituent elements. These mechanical properties show a significant enhancement, which has been correlated with the microstructural features at nano-scale, delineated by transmission electron microscopy.

Are the makings of a dirty bomb in your neighborhood?

Although regulators say that current protections are adequate, the US government continues to pay for extra security features for facilities that hold radiological materials.

Use of a GM counter to measure the half-life of Ba-137m generated by using an isotope generator

A Cs-137/Ba-137m isotope generator is used to demonstrate the properties of radioactive decay. In this experiment, we investigate and verify the random behavior of radioactive decay and determine the half-life of a radioactive isotope. The half-life of a radioisotope is measured in this experiment. The purpose of this experiment is to determine the half-life of Ba-137m by using several methods and to determine the percent error for each determination. Ba-137m decays by gamma emission (662 keV) with a half-life of 2.6 minutes to the stable Ba-137 element. During elution, Ba-137m is selectively “milked” from the generator, leaving behind the Cs-137 parent. Each generator is supplied with 250 mL of an eluting solution (0.9% NaCl in 0.04M HCl). A Geiger counter interfaced with a computer is used to acquire and record the activity at set time intervals. The half-life of the radioactive isotope Ba-137m is determined by measuring the activity of a sample as it decays. The half-life of Ba-137m, which has been extracted from the radioisotope Cs-137, is detected by using AktivLab. The theoretical half-life of Ba-137m is approximately 2.56 minutes, and the result from the experiment is 2.6 minutes. In summary, a radioisotope generator containing Cs-137 produces Ba-137m, which is extracted in a solution. The Ba-137m has a half-life of about 2.6 minutes as measured during this research.

Understanding Icy Worlds to Maximize Science Return on Future Missions

Missions to the outer planets are challenging. Far beyond the asteroid belt, the Sun is a weak source of energy—radioisotope thermoelectric generators (RTGs) tend to be the power system of choice over solar panels. The radiation environments can be extreme. Yet these icy worlds call to us. Some have icy plumes erupting into space; some are laden with organic material. All of the ingredients for life are there—water, chemistry, and energy—meaning that these planets could be livable environments.

Hesperides: Solar/nuclear missions to the Sun׳s inner gravity focus

The Sun׳s gravity focus at >550AU is of interest to astrophysicists including SETI scientists, researchers seeking to image extra-solar planets and others. One method for an extra-solar probe to reach the Sun׳s inner gravity focus within a human working lifetime (less than 50 years) is to combine solar and nuclear propulsion techniques. Here, we present a non-optimized probe concept including state-of-the-art solar-sail, radioisotope-electric propulsion and giant-planet gravity assists. Application of radioisotope propulsion allows some cross range capability during and after the powered and cruise phases of the flight to >600AU. Such a capability is likely necessary to fully utilize the solar gravitational lens effect for SETI and astrophysical observations.

Development of a Radioisotope Heated Hollow Cathode

A new type of hollow cathode using a radioisotope heat source instead of a conventional sheathed heater was introduced and it achieved thermionic emission performances similar to the ones of conventional hollow cathodes. Strontium-90, Plutonium-238 and Curium-244 were chosen as radioisotope heat sources and a thermal reductive layer was also used to obtain precise thermionic emissions. A new system design methodology called the Self-Sufficiency Principle was introduced and was applied by powering the keeper electrode with the radioisotope decay heat using a radioisotope thermoelectric generator (RTG). The heater supply of the hollow cathode power configuration was replaced with a RTG supply and the mode of operation of the device was modified because radioisotope heat sources cannot be switched off. This hollow cathode was then benchmarked against two ion thruster configurations and a maximal overall power saving of 3% was achieved. Its advantages are its power saving capability and scalability but it can however be voluminous, heavy and potentially hazardous. Further research in this field ought to explore the range of applications of this new power-free electron emission technology.

Design and analysis of nuclear battery driven by the external neutron source

Based on the theory of ADS (Accelerator Driven Subcritical reactor), a new type of nuclear battery was investigated, which was composed of a subcritical fission module and an isotope neutron source, called NBDEx (Nuclear Battery Driven by External neutron source). According to the structure of GPHS-RTG (General Purpose Heat Source Radioisotope Thermoelectric Generator), the fuel cell model and fuel assembly model of NBDEx were set up, and then their performances were analyzed with MCNP code. From these results, it was found that the power and power density of NBDEx were almost six times higher than the RTG’s. For fully demonstrating the advantage of NBDEx, the analysis of its impact factors was performed with MCNP code, and its lifetime was also calculated using the Origen code. These results verified that NBDEx was more suitable for the space missions than RTG.

Use of a charge-injection technique to improve performance of the Soft X-ray Imager aboard ASTRO-H

We are developing the Soft X-ray Imager (SXI), a charge-coupled device (CCD) camera system to be deployed onboard the ASTRO-H satellite. Using an engineering model system in which design specifications were the same as those of the flight model, we measured charge transfer inefficiency (CTI) and the effects of charge trailing. The CCD was irradiated with monochromatic X-rays produced by a radio isotope (55Fe) and X-ray generator using alpha particles from 241Am. We used four targets for the X-ray generator: (C2F4)n, SiO2, Ti, and Ge. Since CTI degrades energy resolution, we adopted the charge-injection technique to the SXI. With this technique, injected charges fill traps, and subsequent signal charges are transferred with less loss of charge. However, the charge-injection technique can cause positional variations in gain on the CCD chip. Thus, we constructed a method for correcting CTI. We also evaluated the charge trailing effect and tested a method for correcting its effects. After applying these corrections to charge injection, variations in gain improved from 0.5% to 0.1% over the CCD chip, and the energy resolution (FWHM) improved from ~220eV to ~180eV at 5.9keV.

Thermoelectric properties of WSi2–SixGe1−x composites

Thermoelectric properties of the W/Si/Ge alloy system have been investigated with varying concentration levels of germanium and tungsten. The alloys were fabricated by directional solidification with the Bridgman method using boron nitride and fused silica crucibles. The effect of crucible contamination was investigated and found to result in doping the system to suitable levels for thermoelectric applications. The system has been demonstrated as a suitable high temperature p-type thermoelectric material exhibiting high power factors, >3000μW/mK2. Seebeck coefficients of the system are on the order of +300μV/K and electrical conductivities of 2.8×104S/m at the optimum operating temperature. The best composition, 0.9at% W/9.3at% Ge, achieved a figure of merit comparable to RTG values over the temperature range of interest. The results suggest that W addition can reduce the use of expensive Ge component of the alloy. Reported are the details of processing conditions, microstructure development, and temperature dependent thermoelectric properties. The material system was stable at the temperatures required for NASA’s radioisotope thermoelectric generators.

방사성동위원소 열전 발전기 최적설계를 위한 차폐 및 열전달 해석

방사성동위원소 열전발전기는 장반감기 알파 혹은 베타 핵종에서 방출하는 하전입자를 차폐하여 방사선에너지를 열에너지로 전환하고 이때 발생하는 열전재료의 온도차를 이용하여 전력을 생산하는 시스템이다. 이 기술은 에너지 밀도가 높고 수명이 길며 신뢰성이 높아 우주개발, 국방 등 극한 환경에서 사용되는 장치, 센서 및 로봇 등의 에너지원으로 그 효용성이 매우 높다. 본 연구에서는 방사선 차폐해석 및 열전달 해석을 통하여 차폐체, 그리고 최대 온도구배를 가지는 열전재료의 형상과 배치를 결정하여 열전발전기 기초설계를 도출하였다.