However, making plutonium-238 is expensive and difficult. You need a reactor with the right neutron flux and a supply of neptunium-237 feedstock to produce the plutonium. You also need a small nuclear reprocessing plant to separate the plutonium chemically from the highly radioactive fuel. Over the years, plutonium-238 has been produced by a number of countries including the USA, Russia, and the UK. Historically, some material has even been used to provide the electrical power in heart pacemakers.

In the case of plutonium for space applications, stocks of the material are now running low. The USA is restarting production, but the current stocks and production rate in the near term are unlikely to be high enough to support the broad range of space missions that the US science community might wish to target. In Europe, without the neptunium-237 feedstock and necessary processing facilities, the production of plutonium-238 is considered too expensive. As a consequence, Europe has decided to focus on an accessible alternative material that could power future spacecraft: americium-241. This isotope - in very small 1 microcurie quantities - provides the ionizing alpha radiation that makes smoke detectors in all our homes work. In larger quantities, this same alpha radiation generates heat in a similar way to plutonium-238.

A member of the transuranic series, americium is a waste produced when uranium nuclear fuel is irradiated by neutrons in a reactor. A ton of used nuclear fuel can contain 100 grams of americium. It is possible to extract the americium from used nuclear fuel, but it is difficult, expensive, and has a relatively low yield. The other big problem is that this americium is not pure americium-241; it contains many other isotopes of americium, rendering this route for the production of americium-241 unsuitable.

An alternative route is to exploit the beta decay of plutonium-241. When nuclear fuel is reprocessed, the plutonium is separated from the uranium and fission products and stored for reuse as fuel in civil nuclear reactors. Nuclear fuel that has been in a civil reactor will contain a range of plutonium isotopes including 241, which has a decay half-life of 14 years to americium-241. The long-term storage of civil-separated plutonium will therefore produce very isotopically pure americium-241 via this beta decay. Like the americium in used nuclear fuel, this americium is also considered by the nuclear industry to be a waste product that needs to be removed before the plutonium can be reused in nuclear fuel. In the UK, more than 100 tons of plutonium have been separated and stored, some for several decades, resulting in a potentially plentiful and reliable source of americium-241 that would otherwise be disposed of as waste. This americium-241 can be harvested from the stored plutonium by using a relatively simple chemical and a cost effective process that does not require working with highly radioactive used nuclear fuel.

Compared to plutonium-238, which has a half-life of 87.7 years and heat output of 0.4 watts per gram, americium-241 lasts longer with a half-life of 432 years but has a lower heat output of 0.1 watts per gram. The longer half-life means that the heat, and therefore the power output, reduce more slowly through time when compared to systems of equal power output. In addition, the higher isotopic purity of the americium-241 (greater than 99%) partially compensates for the reduced heat and power output. At present the European programme has focused on developing scalable Radioisotope Thermoelectric Generators (RTGs) in the 10- to 50-watt electrical power range and with a specific power of between 1 and 2 watts per kilogram. For comparison, current plutonium-238 based RTGs produce roughly 120 watts with a specific power of approximately 3 watts per kilogram.