European radionuclide power source for space applications is now generating electricity

Electricity generation from a new radioisotope power source based on the radionuclide ²⁴¹Am has recently been tested for the first time. This brings us closer to a working model. Humanity should once again gain the ability to ensure the operation of probes and robotic craft beyond Jupiter, as well as in the conditions of the lunar night or Martian sandstorms and winter.

The joint efforts of scientists from the National Nuclear Laboratory (NNL) and the University of Leicester have achieved a significant step towards a working model of a radionuclide electricity source for space applications.

Existing radionuclide sources are based on plutonium ²³⁸Pu

The isotope plutonium ²³⁸Pu is used in radionuclide generators deployed to date. They have been used in probes sent to Jupiter and Saturn, such as Galileo and Cassini, as well as in the Curiosity Mars rover. This plutonium is produced by irradiating neptunium ²³⁷Np in special reactors. The isotope neptunium ²³⁷Np is found in large quantities in spent nuclear fuel, where it is isotopically pure. The other long-lived isotopes of neptunium are ²³⁶Np and ²³⁵Np, with half-lives of 150,000 years and 396 days respectively; all other isotopes have half-lives of no more than a few days and rapidly transform. However, uranium isotopes 235 and 236 decay through spontaneous fission and alpha decay, which do not produce neptunium 235 and 236. Chemical separation of neptunium can therefore yield large stocks of relatively highly isotopically pure neptunium 237.

High-neutron-flux reactors built to produce weapons-grade plutonium were used to irradiate neptunium ²³⁷Np and produce plutonium ²³⁸Pu. All of them were shut down in the 1980s and 1990s. Since then, stocks of plutonium ²³⁸Pu have declined dramatically. In recent years, NASA has been trying to restore production using research reactors. However, these can produce only very small and insufficient quantities. The issue of the shortage of plutonium 238 is described in detail here and here. More on the path towards nuclear power sources can be found here.

Radionuclide sources using americium ²⁴¹Am

This is why the European Space Agency (ESA) opted for the possibility of using americium ²⁴¹Am. It can be obtained from spent nuclear fuel. If plutonium is separated from it and left to "age" for a longer period, pure americium ²⁴¹Am accumulates in it. This is because only plutonium ²⁴¹Pu transforms into americium through beta decay. All other plutonium isotopes transform through other types of decay and cannot lead to the production of americium. Chemically, pure americium ²⁴¹Am can then be separated. Such separated stocks of old plutonium exist in large quantities at the Sellafield reprocessing plant in the UK. This is also why development of the European radionuclide source is taking place in the UK. Both isotopes have their advantages and disadvantages. The main advantage of plutonium ²³⁸Pu is that its half-life is sufficiently long for most planned missions, yet it is roughly five times shorter than that of americium ²⁴¹Am. Five times less radionuclide is needed for the same activity and thus thermal output. Another advantage is that plutonium ²³⁸Pu has almost no gamma activity, so the issue of gamma-radiation shielding does not need to be addressed. The main advantage of americium ²⁴¹Am is its easier extraction from spent nuclear fuel, without the need for irradiation in a special reactor. A more detailed analysis of the properties of both radionuclides and their comparison is available here.

Achievements and first test prototypes

NNL laboratories have succeeded in resolving the chemical separation of americium from old plutonium and in manufacturing pellets suitable for use in a radionuclide power source. Three types of source are being prepared: a small thermal source with an output of 3 Wt, a small thermoelectric source with an output of 5 – 20 We, and finally a larger electricity generator using a Stirling engine with an output of 100 We. A prototype thermoelectric generator with an output of 10 W, capable of lighting a bulb, is currently being tested. For now, the limitations are mainly the quantity of americium produced and the need to complete the development and preparation of the individual components.

Meeting another milestone shows that everything is on the right track for Europe to soon have the necessary nuclear power sources for its space ventures. This is very important for a return to the Moon, where heat and electricity must also be ensured during the two-week lunar night, for more intensive activity on Mars, and above all for studying the outer parts of the Solar System. For those interested, an older, more detailed article on nuclear power sources for space is available here, while a recent popular lecture presented at the Brno observatory is recorded here.

Written for oEnergetice, Osel and Kosmonautix.

Lead image: The New Horizons probe was able to explore Ultima Thule, the most distant object in the Solar System studied in detail so far, only thanks to the use of radionuclide electricity sources (source: NASA).