Could nuclear waste power our way to the stars?

The European Space Agency certainly does. It has funded a program to produce americium-241 – the end product of the decay of plutonium isotopes found in the fuel of civil nuclear reactors.

Tim Tinsley, of the National Nuclear Laboratory in the UK, explains why this is an important development.

All spacecraft share the same power source at their hearts, he says, that provides the heat and power essential for them to survive and operate in the cold, dark vacuum of space.

Each derives its power from a few kilograms of red-hot plutonium-238. Without it, the spacecraft would not survive. Solar energy is too weak. The heat from the plutonium is able to keep essential systems warm and to also be converted to electricity using the thermoelectric Seebeck effect. With a half-life of 87.7 years, the plutonium has the potential to provide heat and electricity for well over a century.

But making plutonium-238 is expensive and difficult and stocks are running out.

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.

But americium-241 could be a good alternative. To the nuclear industry it is a waste product that must be removed before plutonium can be reused in nuclear fuel. It is a potentially plentiful and reliable source of americium-241 that would otherwise be disposed of as waste.

But how would it perform in space?

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.

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