Five years ago theoretical research on a new form of nuclear reactor that might have applications for space propulsion was widely reported by the press. The concept for the new nuclear reactor was designed to use americium 242 (Am-242) – a unique fuel, different from the conventional uranium and plutonium used in most nuclear reactors today. Americium is a synthetic heavy element discovered in 1944 at the wartime Metallurgical Laboratory at the University of Chicago (now Argonne National Lab). Although Am-241 can be produced relatively easily and in large quantities, its rarer isotope, Am-242, is difficult to manufacture and so far has had very few applications. Over the last couple of years, scientists at the Ben Gurion University of the Negev in Israel led by Professor Yigal Ronen, dean of the Faculty of Engineering Sciences and a prominent nuclear scientist, have worked to create an advanced design for an Am-242-based nuclear battery.
The new battery concept benefits from the distinctive properties of Am-242m (the most stable of the eight meta states of Americium), enabling design of a critical reactor with ultra-thin fuel elements on the order of one micron. Such thin fuel enables the fission products to escape from the fuel and to be used for direct conversion of their kinetic energy into electricity. One of the major advantages of the improved nuclear battery is its simplicity. With no moving parts, the battery is inherently more reliable than any nuclear power source with dynamic power conversion. Furthermore, since the surface of the battery serves as a radiator to reject the residual heat, no active cooling is required for battery operation.
The battery concept described in a paper published by Ronen’s team includes three hollow concentric beryllium spheres that were chosen for their excellent neutron moderation properties, low density, and good electric and thermal conductivities. The center sphere is plated with a metallic coating of Am-242 on both sides and serves as a cathode emitting positively-charged high energy fission products. The two other spheres serve as anodes, thus creating a functional battery.
According to calculations by Ronen’s team, a 175 cm (69 inch) 10.5 ton Am-242 battery could produce 140 kW of electricity for a period of up to 80 days, after which its 3.2 kg (7 lb.) Am-242 “core” would require replacement.
Q: What are the advantages of the americium 242 over existing nuclear fuels such as plutonium and uranium 235?
A: Americium 242 is the best known nuclear fissile material; it makes possible building nuclear reactors with nuclear fuel less then one micron thick.
Q: What is the benefit of such thin nuclear fuel?
A: In the nuclear battery concept, the use of an ultra-thin fuel allows the highly charged energetic fission products to escape from the fuel and “catch” onto the cathode, thus allowing the production of electrical current without the need of a generator.
Q: Is americium 242 radioactive and does this pose any danger to the people working next to a device containing this material?
A: Americium 242 is radioactive, but this should not pose any special problem as we have already been working with materials much more radioactive than americium 242 in close environments for many years. (Nuclear subs are the most obvious example).
Q: How is americium 242 currently being manufactured?
A: Americium 242 is created when americium 241 absorbs a neutron. This process is currently done in the most effective way in the kind of nuclear reactors know as “fast reactors”. Americium 241 exists in large quantities as a by-product of regular power reactors. The process that turns it into americium 242 is still expensive today.
Q: What could be the possible applications of the americium 242 battery?
A: One of the possible applications of the battery will be as a power source for the international space station (ISS). In the more distant future it might also be used as a power source for a manned mission to Mars.
Q: The americium 242 battery will only be able to provide 140 kW for a period of 80 days. Will this be enough for the ISS and how is this better than the existing solar power cells that the station is using?
A: 140 kW is currently enough for the space station. In the future it will be possible to supply more power if needed using larger and higher power americium batteries or simply more of them. After 80 days, the americium battery will need to be refueled.The space station is currently powered by solar panels that have much lower energy density compared to the americium battery. As a result, the solar panels need vast collectors.
Q: Did you present the battery concept to NASA and what was their reaction?
A: We presented the americium battery concept in a conference sponsored by NASA. There was interest in the concept and we are continuing the research.
Q: What’s aspects of your recent battery concept are new relative to your previous work?
A: Unlike our previous battery concept, the current design does not include any moving parts and thus requires much lower maintenance for its operation.
Q: Could you estimate how long will it take to bring your current americium battery concept into production?
A: Development depends on the funds allocated for the project. We estimate that with proper funding we can have a battery in 5-10 years.