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Since 2009, ESA has been conducting R&D activities leading towards the development of a European capability in radioisotope power systems for space. The programme is focused on the use of americium-241 as an innovative alternative to the plutonium-238 currently used by the USA and Russia. Two different technologies are being developed by the University of Leicester in the UK: radioisotope thermoelectric generators and radioisotope heater units. The development of the fuel containment architectures, known as heat sources, has recently evolved from the design stage to more representative prototypic systems. Multiple containment layers ensure that the heat sources can survive accident conditions from launch failures to Earth re-entry, minimising the risk of inadvertently releasing radioactive material into the environment. Validated accident models are necessary for the design iteration process of these systems, and for the creation of a safety case for their launch. The research project here presented was a collaboration, supported by ESA , between the University of Leicester in the UK and ArianeGroup in France. The goals of this activity were to perform the first-ever impact tests for the European Am-based heat sources, and to start developing computer models to simulate the behaviour of the containment systems under different impact conditions in accident scenarios. Tested samples were then characterized both at ESA/ESTEC and at the University of Leicester, in order to understand the behaviour of the fuel containment materials under the most relevant accident conditions.

Nuclear power sources can offer a potential solution to some of the challenges related to space applications, such as the use of photovoltaics in opaque atmospheres or in a dark environment for a long time. Thanks to their high energy densities, certain isotopes are capable of generating considerable amounts of heat for long periods of time, independently of insolation levels; this heat can then be converted into electrical power, or used to keep suitable temperatures inside the spacecraft. However, the presence of radioactive material implies new sets of requirements, to properly manage all the aspects related to safety (heat source design, risk assessments, launch approval process etc.). This paper provides an overview of the international agreements, the existing NASA/US and Russian policies for the management of space nuclear power systems, as well as some updates and recent considerations for the ESA Radioisotope Power Systems (RPS) programme, which aims to provide a European capability for the independent design, management and launch of RPS. A significant amount of programmatic effort in many domains and by different entities will be required in order to create a common European safety framework, but the long and wide-ranging experience of other countries in dealing with space nuclear power systems could be an appropriate starting point. Recommendations are also made which could lead to an international standard linked to key stages of the mission lifecycle.

Radioisotope power systems utilising americium-241 as a source of heat have been under development in Europe as part of a European Space Agency funded programme since 2009. The aim is to develop all of the building blocks that would enable Europe to launch and operate deep space and planetary missions in environments where use of solar power or alternative power generation technologies is challenging. Although some technical and policy work activity predate the ESA programme, the maturity of the technology has now reached a level that it can be incorporated in mission studies and roadmaps targeting the period from the mid 2020s onwards. This paper describes the state of the art in European radioisotope thermoelectric generators and radioisotope heater units. This paper includes: the evolution of the technical programme in detail; descriptions of the design; evolution of RTG and RHU devices from laboratory prototypes to more advanced fully functional systems; and experimental data obtained to date. This paper also outlines the technical challenges and multidisciplinary skills required to develop what is a world leading, original, significant and transformative technology solution for planetary science and exploration missions from the mid 2020s onwards.

International audience; European Space Agency (ESA) radioisotope power systems will use 241 Am as their heat source. The chemical form of the americium oxide has yet to be decided but an option that may be investigated in future research are certain la-3 AmO2-(x/2) phases. In a previous investigation, la-3 (C-type) Ce1-xNdxO2-(x/2) with x values between 0.5 and 0.7 were proposed as candidate surrogates (Watkinson et al., 2017). A continuous oxalate precipitation and calcination route for fabricating such oxides was presented and the means to target an x value was described (Watkinson et al., 2017). An initial estimate of the lattice parameter was provided in the previous study by fitting Gaussians to X-ray diffraction data peaks. Previous quantitative X-ray fluorescence analysis suggested an x-value of 0.61 had been achieved. Characterising the crystal structure and the oxygen-to-metal ratio, which corresponds to x, of an oxide powder will be essential data for future sintering studies where changes in these parameters will be investigated. In this study, a Rietveld refinement investigation is presented, which has enabled the lattice parameter of the material to be better constrained and estimated more precisely. The result is consistent with the literature; a lattice parameter-x value relationship in the literature was used to estimate the x value using this improved estimate and it confirmed an x value of 0.6 has been made. This investigation has thus further validated the previously presented synthesis method and characterised the crystal structure of the material more precisely in preparation for future sintering trials. (C) 2018 Elsevier B.V. All rights reserved.

La technologie actuelle a transformé les fantasmes d’exploration spatiale du passé en réalité tangible. Pourtant, le combustible reste au coeur des préoccupations des chercheurs. Depuis un demi-siècle, le nucléaire fournit énergie et chaleur aux satellites et sondes spatiales. Pleine de promesses, la recherche autour de l’énergie nucléaire pourrait ouvrir de nouvelles frontières à l’exploration spatiale.

One of the most successful uses of nuclear energy, other than nuclear fission reactors, is the use of radioisotopes to provide a simple source for heat and electrical power in space applications. The alpha decay heat from suitable radioisotopes can be harnessed to heat instrumentation and generate electricity through thermoelectric generators. Within Europe, the feasibility of using americium (241Am) in such applications is being considered. Part of this study is to develop a way to access a stock of suitable material and the UK stockpile of separated civil plutonium dioxide, generated through reprocessing operations, is a rich source of americium. This paper summarises 5 years of development work performed to establish a way to separate americium from the plutonium dioxide in a safe and cost effective way, generating a high purity product with high recovery efficiency.

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European Space Agency radioisotope power systems will use americium oxide as the heat source in pellet or disc form. The oxide form is yet to be decided. Sintering trials with CeO2 and Nd2O3 as analogues for AmO2 and Am2O3 were conducted. Spark plasma sintering (SPS) and cold-press-and-sinter methods were compared. Different sintering parameters and particle characteristics were investigated with commercial and synthesised powders. The synthesised powders contained lath-shaped particles, and batches with different particle sizes and specific surface areas were made and sintered. This is the first study in the public literature to report the sintering of lath-shaped CeO2. The targeted density range of 85–90% was met using both techniques. No ball-milling was required. Cold-pressing-and-sintering CeO2 produced intact discs. Large cracking was prevalent in the SPS discs. Some powders pressed more successfully than others. Powder morphology had a significant effect on the result but it was not possible to fully quantify the effects in this study. The cold-pressed-and-sintered CeO2 discs had comparable Vickers hardness values to a nuclear ceramic (UO2). The hardness values were greater than the spark plasma sintered CeO2 sample. Efforts to SPS near-net shaped pellets using CeO2 and Nd2O3 are reported. A follow on investigation was conducted to assess how the 85–90% T.D. target could be achieved. The aspect ratio impacts the sintering parameters and behaviour. The Vickers hardness of Nd2O3 is reported for the first time and compared to the results of sintered CeO2.

The European Space Agency (ESA) is sponsoring a research programme on the development of americium oxides for radioisotope generators and heater units. Cubic AmO2-(x/2) with an O/Am ratio between 1.65 and 1.75 is a potentially suitable compound for pellet sintering. C-type (Ia-3) Ce1-xNdxO2-(x/2) oxides with 0.5

Space nuclear power is the most viable energy source for some space missions, such as the exploration of outer planets or the exploration of a planetary surface with long day/night cycles. Thanks to high energy densities, certain isotopes can generate considerable amounts of heat for long time periods, independently of insolation levels; this heat can be converted into electrical power, or used to keep suitable temperatures inside the spacecraft. However, the presence of radioactive material implies new requirements, to properly manage all the aspects related to safety. A common European safety framework is required for Europe and ESA to independently manage radioisotope power systems: a significant amount of programmatic effort in many domains and by different entities will therefore be required, but the long and wide-ranging experience of other countries in dealing with space nuclear power systems could be an appropriate starting point.

Fifty years after the Apollo programme, humanity prepares to return to the surface of the Moon with greater ambition to stay longer, explore wider and eventually use the Moon as a staging post to further destinations. The short Apollo missions could rely on primary batteries to power the lunar module, but longer robotic and crewed missions will require other power system solutions. The plans of ESA to enable European roles in the frame of an international human lunar exploration campaign are described, along with the power and energy needs of the foreseen mission concepts. The performance characteristics of solar generators, secondary batteries and nuclear power sources presented in a way specifically tailored for lunar surface missions. It is shown that the characteristics of a long-term lunar surface mission lead to an optimum power system architecture comprising a radioisotope power system as well as a solar generator and secondary battery. New European radioisotope power technology based on americium-241 is advancing in maturity and will complement the established U.S. and Russian capability in plutonium-238 systems, thus enhancing access to these mission-enabling but scarce power sources. The ESA-led international collaborative lunar mission HERACLES will be the first lunar surface mission that is required to survive and operate over multiple lunar days and nights. This achievement will be enabled by the first use of European space nuclear power sources, in the form of Am-241 radioisotope heater units (RHUs).

Within the framework of the ESA radioisotope power system (RPS) program, the University of Leicester is currently developing radioisotope heater unit (RHU) and radioisotope thermoelectric generator (RTG) systems for future space missions, with americium-241 as radioactive fuel. An important aspect of the overall program is safety, and this involves ensuring that the design of these systems, in particular of the heat source (i.e. fuel and containment layers), meets a set of stringent requirements: it is fundamental to properly design both the RTG and RHU in order to avoid inadvertently releasing radioactive material into the environment in the event of an accident. The inner containment, or cladding, made of a platinum-rhodium alloy, is the first line of defense surrounding the americium-based fuel; additional layers of carbon-based insulators and carbon-carbon composites for the aeroshell ensure that the heat source can survive all possible accident conditions, from launch failures to Earth re-entry. Validated heat source accident models are necessary to inform the design iteration of the RHU and RTG heat sources, and to construct a safety case for their launch. The goal of the activity here described, performed in collaboration with ArianeGroup in France and ESA, is to start the process of understanding the behavior of the fuel containment systems under the most relevant accident conditions by computer modelling, to validate them experimentally given the infrastructure, test means and expertise of ArianeGroup in this field, and to characterize the different materials at ESTEC. The data obtained will help to iterate and improve the design of the European RPS heat sources by focusing on the fuel containment.

Radioisotope heater units (RHU) and radioisotope thermoelectric generators (RTG) are currently being developed for the ESA radioisotope power system program. The state-of-the-art for the USA and Russian systems is to use plutonium-238 as the radioisotope fuel; however, for the ESA applications americium-241 has been selected due to its availability and relatively cost-effective production in the European context. The proposed designs implement a multi-layer containment approach for safety reasons, with a platinum-rhodium alloy for the inner containment of the fuel and carbon-based materials for the outer layers. The Am-fueled RHU provides 3 W of thermal power, and makes this design competitive with existing models in relation to specific power. The heat source for the RTG has a 6-side polygonal shape, with a distributed 3-fuel pellet architecture: this configuration allows to maximize the specific power of the RTG, since Am-based fuels have a lower power density than Pu-based fuels. The heat supplied by the fuel is 200 W, with an expected electrical power output of 10 W provided by six Bi-Te thermoelectric modules. Finite element structural and thermal analyses have been performed to assess the theoretical feasibility of the components as initially conceived. Mechanical and electrically-heated prototypes for the systems have already been tested in a representative lab environment at the University of Leicester; these tests have provided initial estimates for the efficiency of the systems. Both the RHU and RTG architectures are currently undergoing a new design iteration process. This paper reports on the overall architecture and design of the Am-fueled RTG and RHU, the modelling results and the experimental data obtained so far.

Thermoelectric energy convertors in the form of solid state modules are utilised in space nuclear power systems such as a radioisotope thermoelectric generator (RTG). However, to ensure that implemented thermoelectric modules are reliable, efficient, and capable of delivering power and energy over a required lifespan, Standardised, accurate and repeatable high-throughput measurement systems are needed. Recently, Impedance spectroscopy has shown promise as a tool to parametrically characterise thermoelectric modules with one simple measurement, showcasing itself as a potentially key enabling technology. This paper investigates the use of impedance spectroscopy as a measurement system for assessing the health state of practical thermoelectric modules from batch production runs. Particularly for the case of candidate thermoelectric modules which are required to operate for long durations within an RTG system. The complex impedance response of non-ideal modules, in the form of Nyquist spectra, is reported. The characteristic high frequency response is here linked with a new parametric model which considered the influences of the copper contacts as well as the thermal contact resistance between the semiconductor-copper region. From these results it was found that the thermally capacitive nature of the copper contacts, as well as an unavoidable thermal contact resistance, was highly necessary for an accurate fitting of the mid-high frequency response of Nyquist spectra.

Radioisotope thermoelectric generators (RTG) and heater units (RHU) systems are being developed in Europe as part of a European Space Agency (ESA) funded program. Aimed at enabling or significantly enhancing space science and exploration missions, these systems rely on the cost-effective production of americium-241 for the fuel. The use of an iterative approach and the application of lean methodologies for the development these systems have been the focus of this technology program. Isotope containment architectures and, in the case of RTG systems, bismuth telluride based thermoelectric generators are under development. At the small end of the scale, the RHU configuration is based on a 3 W thermal power output. The first version of this system has been designed and analysed. Electrically-heated and mechanical models have been produced and tested. The RTG heat source configuration is designed to deliver 200 W of thermal power output while minimizing the volume occupied by the fuel. A 5% total system conversion efficiency and a modular scalable design imply that electrical power output can range between 10 W and 50 W. Each RTG system could house up to 5 heat sources. An electrically-heated RTG system based on the 200 W heat source architecture has been designed, analysed and tested. The advancement in the design of the heat source for both RTGs and RHUs is currently the focus of the programme with the aim of advancing the technology readiness level of the containment structures. The most recent results of the programme will be presented.

The European Space Agency funded programme into the research and development of European radioisotope power systems (RPSs) began in 2008. Three RPS technologies are under development, namely, radioisotope heater units, radioisotope thermoelectric generators, and Stirling generators. Americium (241Am) was selected as the ‘fuel’, which provides radiogenic heat to the RPSs. An essential aspect of the programme is the ability to create an americium oxide fuel form, namely discs or pellets, that meet a range of requirements e.g. intact bodies with relatively high relative densities that allow for He-outgassing. Research with surrogates for americium oxides is essential for investigating the range of variables that influence the ability to achieve this whilst limiting the research with the highly radioactive material. In this study, americium oxide surrogates (e.g. Nd 2 O 3 ) have been created using two different techniques (continuous oxalate precipitation and calcination, and sol-gel and calcination) with the objective of creating particles with differing morphology. Owing to the polymorphism of Nd 2 O 3 , X-ray diffraction is conducted to assess crystal structure phase changes in the powder material to inform sintering studies. The surrogate powders are cold-pressed and sintered to assess the impact on pellet properties e.g. density and integrity. The surrogate fuel study highlights the importance of assessing the impact of particle shape and crystal structure on the ability to meet fuel form requirements, and will inform future research with the americium oxide fuel.

Plutonium-238 has been used as a power source for spacecraft since the early days of space exploration. It has proven to be an effective source of power where the use of solar generated power is impractical. Historically, Europe has relied on collaborations with the USA or Russia to access these nuclear power sources. During 2009, the European Space Agency (ESA) funded a project to examine the cost and practicality of establishing a European source of material suitable for Radioisotope Power Systems (RPS) and concluded that 241Am was the most suitable choice for European based production. This takes what would otherwise be a waste material from the nuclear industry and uses it to power future science exploration missions in outer space. This is also very much cheaper than the development of a European supply of 238Pu, and the material has much greater availability opening up the potential for many more missions. The preferred European alternative of 241Am for use in future RPS and the issues that will need to be addressed has continued with the development and underpinning of a conceptual flowsheet to be used for production of 241Am. The National Nuclear Laboratory has assessed the feasibility and costs associated with installing within its existing facilities a European Radioisotope Production Facility to produce 241Am for use by the European Space Agency in radioisotope power systems for space missions. Work has also been completed on validating the flowsheet, along with the production of a quantity of separated 241Am for analysis. This has included using aged plutonium in NNL's PuMA laboratory and the separation of 241Am from this material. Scale up of the process to produce quantities of material suitable for use in RPS heater units is ready, housed within the NNL's Central Laboratory in an existing facility designed for plutonium active operations. The required product is americium oxide powder in a package suitable for temporary storage pending fabrication into RPSs. As part of a consortium, the National Nuclear Laboratory has also assessed the feasibility and design required for an Am 2 O 3 fueled pellet that is consistent with conventional RTG and RHU configurations. With confirmation of the flowsheet performance, and the development of the costed design for a suitable production plant, the next phase of work will see prototypic pellets manufactured for testing, leading to a demonstration RPS heat system being produced. Provision of RPSs to future missions would bring significant benefit to the range of science in space exploration that is able to be achieved. The paper will outline the reasons behind the choice of 241Am, the development work that has taken place so far, and the expected route forward towards a flight ready system.

Fifty years after the first human step on the Moon, many challenges for its exploration have yet to be overcome. Among them, the survival of the crew and/or lunar assets during the lunar night is mandatory for long duration missions. The environmental conditions of the lunar surface and its day-night cycle, with long periods of darkness, make the provision of energy a critical challenge. Several approaches have recently been considered to store and provide energy in the surface of the Moon by means of ISRU (In-Situ Resource Utilisation). We present a trade-off analysis of the options identified for an ISRU-based system to store heat and generate electricity for lunar missions with both robotic and human activities. A critical review of the energy requirements for a mission scenario consisting of long duration stays on the lunar surface has been carried out. Technologies potentially suitable for system components have been identified. These technologies are related to solar energy collection, heat transport, heat storage, heat-to-electricity conversion, and heat rejection. The outcome of the trade-off analysis provides a selection of the most suitable technologies to use in an ISRU-based heat storage and electricity generation system.

Electrical power sources used in outer planet missions are a key enabling technology for data acquisition and communications. State–of-the-art power sources generate electricity from alpha decay of 238Pu via thermoelectric conversion. However, production of 238Pu requires specialist facilities including a nuclear reactor, a source of 237Np for target irradiation and hotcells to chemically separate neptunium and plutonium within the irradiated targets. These specialist facilities are expensive to build and operate, so naturally, a more economical alternative is attractive to the space industry. Within Europe 241Am is considered a promising alternative heat source for radioisotope thermoelectric generators (RTGs) and radioisotope heating units (RHUs) since, as a daughter product of 241Pu decay, 241Am exists in 1000 kgs quantities within the UK civil plutonium stockpile. A chemical separation process is required to extract the 241Am in a pure form and this paper describes the AMPPEX process (Americium and Plutonium Purification by Extraction), successfully developed over the past five years to isolate 241Am in high yield (> 99%) and to a high purity (> 99%). The process starts by dissolving plutonium dioxide in nitric acid with the aid of a silver(II) catalyst, which is generated electrochemically. The solution is then conditioned and fed to a PUREX type solvent extraction process, where the plutonium is separated from the americium and silver. The plutonium is converted back to plutonium dioxide and the americium is fed forward to a second solvent extraction step. Here the americium is selectively extracted leaving the silver in the aqueous phase. The americium is stripped from the solvent and recovered from solution as americium oxalate, which is calcined to give americium dioxide as the final product. This paper describes the development of the AMPPEX process over a series of six solvent extraction separation trials using centrifugal contactors. The technical underpinning reported here has allowed the design of a facility capable of producing 25-50 g of americium per day.

Thermoelectric devices have potential energy conversion applications ranging from space exploration through to mass-market products. Standardised, accurate and repeatable high-throughput measurement of their properties is a key enabling technology. Impedance spectroscopy has shown promise as a tool to parametrically characterise thermoelectric modules with one simple measurement. However, previously published models which attempt to characterise fundamental properties of a thermoelectric module have been found to rely on heavily simplified assumptions, leaving its validity in question. In this paper a new comprehensive impedance model is mathematically developed. The new model integrates all relevant transport phenomena: thermal convection, radiation, and spreading-constriction at junction interfaces. Additionally, non-adiabatic internal surface boundary conditions are introduced for the first time. These phenomena were found to significantly alter the low and high frequency response of Nyquist spectra, showing their necessity for accurate characterisation. To validate the model, impedance spectra of a commercial thermoelectric module was experimentally measured and parametrically fitted. Technique precision is investigated using a Monte-Carlo residual resampling approach. A complete characterisation of all key thermoelectric properties as a function of temperature is validated with material property data provided by the module manufacturer. Additionally, by firstly characterising the module in vacuum, the ability to characterise a heat transfer coefficient for free and forced convection is demonstrated. The model developed in this study is therefore a critical enabler to potentially allow impedance spectroscopy to characterise and monitor manufacturing and operational defects in practical thermoelectric modules across multiple sectors, as well as promote new sensor technologies.

This work was performed under contract to the European Space Agency under the Thermoelectric Converter for Small-Scale RTG programme, 23026/10/NL/AT.

Plutonium as the isotope 238 has been used as a power source for spacecraft since the early days of space exploration. It has proven to be an effective source of power where the use of solar generated power is impractical. Historically, Europe has relied on collaborations with the USA or Russia to access these nuclear power sources. During 2009, the European Space Agency (ESA) funded a project to examine the cost and practicality of establishing a European source of material suitable for Radioisotope Power Systems (RPS) and concluded that 241Am was the most suitable choice for European based production. This takes what would otherwise be a waste material from the nuclear industry and uses it to power future science exploration missions in outer space. This is also very much cheaper than the development of a European supply of 238Pu, and the material has much greater availability opening up the potential for many more missions. The preferred European alternative of 241Am for use in future RPS and the issues that will need to be addressed has continued with the development and underpinning of a conceptual flowsheet to be used for production of 241Am. The National Nuclear Laboratory has assessed the feasibility and costs associated with installing within its existing facilities a European Radioisotope Production Facility to produce 241Am for use by the European Space Agency in radioisotope power systems for space missions. Work has also been completed on validating the flowsheet, along with the production of a quantity of separated 241Am for analysis. This has included using aged plutonium in NNL's PuMA laboratory and the separation of 241Am from this material. The full scale process is planned to be housed within the NNL's Central Laboratory in an existing facility designed for plutonium active operations. The scope of the overall project is “store to store” — the starting point is aged plutonium currently housed in existing Sellafield stores and the end point is the storage of separated plutonium and americium powders on the Sellafield site. The minimum 241Am content of the feed will be 3.5% (w/w HM). The required product is americium oxide powder in a package suitable for temporary storage pending fabrication into RPSs. The design basis throughput for the capability is for a 15 year operating lifetime, producing on average 8.4kg Am/year. As part of a consortium, the National Nuclear Laboratory has also assessed the feasibility and design required for an Am2O3 fuelled pellet that is consistent with conventional RTG and RHU configurations. With confirmation of the flowsheet performance, and the development of the costed design for a suitable production plant, the next phase of work is focus on the optimization of the process to achieve a particular particle size range / morphology coupled with pellet forming studies. Provision of RPSs to future mission would bring significant benefit to the range of science in space exploration that is able to be achieved. The paper will outline the reasons behind the choice of 241Am, the development work that has taken place so far, and the expected route forward towards a flight ready system.

Introduction We sought to determine if lymph node harvesting and survival for CRC were comparable between laparoscopic and open resections in a community hospital setting. Methods A retrospective chart review of patients at two community hospitals who underwent open or laparoscopic resection for CRC between January 2008 and September 2013 was performed. Results Three hundred seventy-one patients had open and 110 had laparoscopic resections. There was no difference between open (17.85) and laparoscopic (18.91) approaches (p = 0.171) in the number of lymph nodes harvested. Patients who had more nodes removed tended toward improved survival, independent of stage (p = 0.052), an effect that was more pronounced in the open resection group (p = 0.031). There was no difference in survival between the open and laparoscopic groups overall (HR 1.52, p = 0.208). Discussion No survival advantage was found between the open and laparoscopic resection groups, affirming that the choice of operative approach for CRC does not affect the quality of the oncologic procedure in a community hospital setting. Patients who had more lymph nodes removed tended toward improved survival. The explanation for this effect remains unclear.

The European Space Agency is currently supporting the research and development of advanced radioisotope power systems utilising thermoelectric modules. The performance of thermoelectric modules following exposure to neutron radiation is of significant interest due to the likely application of radioisotope thermoelectric generators in deep space exploration or planetary landers requiring prolonged periods of operation. This study utilises impedance spectroscopy to characterise the effects of neutron irradiation on the performance of complete thermoelectric modules, as opposed to standalone material. For a 50 We americium-241 radioisotope thermoelectric generator design, it is estimated that the TE modules could be exposed to a total integrated flux of approximately 5 × 1013 neutrons cm-2 (>1 MeV). In this study, an equivalent neutron dose was simulated experimentally via an acute 2-hour exposure in a research pool reactor. Bi2Te3-based thermoelectric modules with different leg aspect ratios and microstructures were investigated. Gamma-ray spectroscopy was initially used to identify activated radionuclides and hence quantify irradiation induced transmutation doping. To evaluate the thermoelectric properties pre- and post-irradiation, impedance spectroscopy characterization was employed. Isochronal thermal annealing of defects imparted by the irradiation process, revealed that polycrystalline based modules required significantly higher temperature than those with a monolithic microstructure. Whilst this may indicate a greater susceptibility to neutron irradiation, all tested modules demonstrated sufficient radiation hardness for use within an americium-241 radioisotope thermoelectric generator. Furthermore, the work reported demonstrates that impedance spectroscopy is a highly capably diagnostic tool for characterising the in-service degradation of complete thermoelectric devices.The European Space Agency is currently supporting the research and development of advanced radioisotope power systems utilising thermoelectric modules. The performance of thermoelectric modules following exposure to neutron radiation is of significant interest due to the likely application of radioisotope thermoelectric generators in deep space exploration or planetary landers requiring prolonged periods of operation. This study utilises impedance spectroscopy to characterise the effects of neutron irradiation on the performance of complete thermoelectric modules, as opposed to standalone material. For a 50 We americium-241 radioisotope thermoelectric generator design, it is estimated that the TE modules could be exposed to a total integrated flux of approximately 5 × 1013 neutrons cm-2 (>1 MeV). In this study, an equivalent neutron dose was simulated experimentally via an acute 2-hour exposure in a research pool reactor. Bi2Te3-based thermoelectric modules with different leg aspect ratios and microstruct...