241Am is produced during irradiation of uranium, which goes through neutron capture during irradiation to 241Pu, which is one of the isotopes that degrades the usefulness of weapons-grade 239Pu. It decays in a very short time (14 day half-life) into 241Am, which is not useful as weapons-grade material, but is still useful in nuclear reactors. According to Ed Pheil, the CEO of Elysium Industries, the Monju sodium cooled fast reactor in Japan faced a restart problem because the 241Pu in the reactor’s fuel decayed before restarting the reactor, causing a reactivity deficit and delaying startup.

The US also looked into 241Am RHU fuel. A recent report for the DOE’s Nuclear Energy University Programs by Sudarshan Loyalka at University of Missouri – Columbia, looked into the possibility of an American supply from the US weapons Pu stockpile. It is available here:

241Am decays through a 5.64 MeV alpha emission into 237Np, which in turn goes through a 4.95 MeV alpha decay with a half-life of 2.14 x 10^6 years. This means that the daughter is effectively radiologically stable. With its longer half-life of 433 years (compared to 88 years for 238Pu), 241Am won’t put out as much energy at any given time compared to 238Pu fuel, but the difference in power output as the mission continues will allow for a more steady power supply for the spacecraft. A comparison of beginning of life power to 20 year end of life power for a 238Pu RTG shows a reduction in power output of about 15%, compared to only about 3.5% for 241Am. This allows for more consistent power availability, and for extremely long range or long duration missions a greater amount of power available. However, in the ESA design documentation, this extreme longevity is not something that is examined, a curious omission on first inspection. This could be explained by two factors, however: mission component lifetime, which could be influenced by multiple factors independent of power supply, and the continuing high cost of maintaining a mission control team and science complement to support the probe.

Depending on what power level is needed (more on that in the next section about the ESA RTG design), and how long the mission is, the longer half-life could make 241Am superior in terms of useful energy released compared to 238Pu, and is one of the reasons ESA started looking at 241Am as the main focus of their RTG efforts.

The EU reprocesses their fuel, unlike the US, and use the Pu to create mixed-oxide (MOX) fuel. If the Pu is chemically separated from the irradiated fuel pellets, then allowed to decay, the much shorter half-life of 241Pu compared to all of the others will lead to the ability to chemically separate the 241Am from the Pu fuel for the MOX. This could, in theory, allow for a steady supply of 241Am for European space missions. As to how much 241Am that would be available through reprocessing, this is a complex question, and one that I have not been able to explore sufficiently to give a good answer to how much 241Am would be available through reprocessing. Jaro Franta was kind enough to provide a pressurized water reactor spent fuel composition table, which provides a vague baseline:

However, MOX fuel generally undergoes higher burnup, and according to several experts the Pu is quickly integrated into fuel as part of the reprocessing of spent fuel. This could be to ensure weapons material is not lying around in Le Hague, but also prevents enough of a decay time to separate the 241Am – plus, as we see in 238Pu production, where the materials are fabricated in one place, separated in another, and made into fuel in a third, Le Hague and the Cumbria Laboratory are not only in different locations but different countries, and after this process the Pu is even more useful for weapons, this bureaucratic requirement makes the process of using spent nuclear fuel for 241Am production an iffy proposition at best. However, according to Summerer and Stephenson (referenced in one of the papers, but theirs is behind a paywall) the economical separation of 241Am from spent civilian fuel can be economical (I’m assuming due to the short half-life of 241Pu), so it seems like the problem is systemic, not technical.

241Am is used as RHU fuel in the form of Am2O3. This allows for very good chemical stability, as well as reasonable thermal transfer properties (for an oxide). This fuel is encased in a “multilayer containment structure similar to that of the general-purpose heat source (GPHS) system,” with thermal and structural trade-offs made to account for the different thermal profile and power level of the ESA RTG (which, as far as I can tell, doesn’t have a catchy name like “GPHS RTG” or “MMRTG” yet). Neptunium oxide most often takes the form of NpO2, meaning that a deficit of oxygen will occur over time in the fuel pellet. The implications of this are something that I am completely unable to answer, and something that is definitely distinct from the use of 238Pu, which then becomes 234U, both of which have two oxygen atoms in their most common oxide state. However, considering there’s a stoichiometric mismatch between the initial material, the partially-decayed material, and the final, fully-degenerate state of the fuel element. I know just enough to know that this is far from ideal, and will change a whole host of properties, from thermal conductivity to chemical reactivity with the clad, so there will be (potentially insignificant) other factors that have impacts on fuel element life from the chemical point of view rather than the nuclear one.

More information about the use of 241Am fuel for RTGs will be added as I find more information, and as ESA continues to develop their RTG design.

References and Further Reading

Isotope information for 241Am,

Advanced Radioisotope Heat Source and Propulsion Systems for Planetary Exploration, O’Brien et al INL 2010

Nuclear Power Sources for Space Applications – a key enabling technology (slideshow), Summerer et al, ESA 2012

Space Nuclear Power Systems: Update on Activities and Programmes in the UK, Ambrosi (University of Leicester) and Tinsley (National Nuclear Laboratory), 2015

SPACE NUCLEAR POWER SYSTEMS: ENABLING INNOVATIVE SPACE SCIENCE & EXPLORATION MISSIONS (Am surrogates for manufacturing testing of FEs – CeNdO), Watkinson thesis 2017

Am-241 Nuclear Safety and Environmental Interactions, Loyalka, University of Missouri – Columbia, 2018

Space Nuclear Power Systems: Update on Activities and Programmes in the UK, Ambrosi (University of Leicester) and Tinsley (National Nuclear Laboratory), 2015

Am-241 Nuclear Safety and Environmental Interactions, Loyalka, University of Missouri – Columbia, 2018