Talk like a nuclear engineer

Clean energy transition — the global shift away from fossil fuels to clean energy sources

Low-temperature plasma — ionised gas in which the smaller, negatively-charged electrons have higher energy than the larger, positively-charged ions, allowing reactions to occur at lower temperatures

Metal foam — a solid metal with many gas-filled pores, like a sponge made of metal

Nuclear radiation — the energy released by the nuclei of unstable atoms in the form of particles or electromagnetic waves

Nuclear reactor — a device used to initiate and control a self-sustaining nuclear reaction

Photon — a particle representing the smallest possible unit of light

Thermionic energy conversion (TEC) — a method of directly converting heat into electrical energy without intermediate conversions, such as movement

In most of our energy generation systems, heat is created and converted into electricity. For example, burning fossil fuels, such as coal and natural gas, generates heat, as do the nuclear reactions that take place in nuclear power plants. After this heat is created, both systems work on the same principles: the heat causes water to evaporate, creating high-pressure steam which spins turbines, creating mechanical energy that is converted to electrical energy by generators. But these intermediate steps – evaporating water and spinning turbines – require a lot of parts, space and ongoing maintenance.

While the fossil fuel industry is expected to wind down over the coming decades – because of its negative impacts on the climate – the nuclear industry could become a major source of clean energy in the future. Current nuclear power plants are massive and take a long time to plan and build, but Dr Austin Lo, Chief Research Officer at GenAlpha Nuclear Technologies, believes it is possible to harness a different, much more compact system to convert nuclear heat into electricity. Such a system would open up a huge range of new applications for nuclear energy – not least as a power source for the spacecraft of the future.

Thermionic energy conversion: cutting out the middleman

Thermionic energy conversion (TEC) is a method of converting heat into electricity without any moving parts. “TEC involves heating up a special ‘emitter’ metal to very high temperatures, at which point it begins to lose electrons,” explains Austin. “These electrons travel across a very small gap to a ‘collector’ metal, creating an electrical current.”

In principle, this process works well, but in practice, it has some major efficiency limitations. For example, as electrons are released, they can build up around the surface of the emitter, creating an electric field that blocks any further electrons from being emitted. “This is known as the space-charge effect, and most current thermionic energy converters counteract it using a low-temperature plasma to help conduct the flow of electrons between the emitter and collector,” explains Austin. “Most designs create this plasma using the energy from the emitted electrons, which means there’s much less energy available to be converted to electricity.” Given the extremely high temperatures involved, further energy is lost as heat radiation. Current TEC systems have a typical efficiency of below 7%, and have few applications beyond research laboratories.

SPACE-TEC: uniting nuclear radiation and thermionic energy conversion

Austin’s ground-breaking project, SPACE-TEC, employs innovative solutions to address these inefficiencies. In particular, SPACE-TEC uses nuclear radiation to heat the emitter and create the low-temperature plasma needed for TEC to operate. While other nuclear-based TEC systems also use nuclear radiation as a heat source, they still rely on energy from emitted electrons to create the plasma.

“In contrast, the highly-pressurised gases used in SPACE-TEC create an environment in which the nuclear radiation naturally knocks electrons off atoms in the emitter, collector and other structural components, creating the low-temperature plasma,” says Austin. “This means that more energy is available to be received by the collector, rather than being used to create the plasma.”

SPACE-TEC also uses specialised metal foams as the emitter and collector. These are essentially highly-porous metal ‘sponges’, full of tiny holes and channels that give them a huge overall surface area. “This means more places for electrons to escape from the emitter, travel through the plasma, and be collected at the collector,” says Austin.

Metal foams also help address the issue of heat loss. In a conventional TEC system, heat is lost through the emission of photons, which radiate out of the system. However, in SPACE-TEC, photons have a much higher chance of being reabsorbed thanks to the high surface area of the porous metal foam. This means that the system can operate efficiently at lower temperatures.

Nuclear-powered futures: even the sky’s not the limit

The qualities of SPACE-TEC technology give it a range of exciting applications. “SPACE-TEC could make nuclear power plants much smaller, simpler and cheaper,” says Austin. “Since it doesn’t involve big turbines and complicated equipment, we can build power systems that are easier to transport and quicker to set up.” This could make nuclear power a much more practical, efficient and economical option for the clean energy transition – not to mention a viable means of exploring the final frontier. “In space, it’s ideal to have a lightweight, compact power source with no moving parts,” explains Austin. “Our system could help power future space stations, rovers, or even settlements on the Moon or Mars.”

But before these applications can become reality, Austin needs to test and refine the technology. “We’re currently testing the system in a research nuclear reactor,” says Austin. “Our next steps include improving the materials, making the system more efficient, and running longer tests to make sure it’s safe and reliable. After that, we’ll work on scaling it up and designing versions for different uses – both on Earth and in space.”

Dr Austin Lo
Chief Research Officer, GenAlpha Nuclear Technologies, USA

Fields of research: Nuclear engineering, thermionic energy conversion, low-temperature plasma physics

Research project: SPACE-TEC (Structured Plasma Cell- Thermionic Energy Conversion): developing a groundbreaking nuclear fuel technology that can directly convert nuclear heat into electricity

Funder: US Advanced Research Projects Agency – Energy (ARPA-E)

The contents are solely the responsibility of the authors and do not necessarily represent the official views of ARPA-E.