The ‘Plutonium Problem’

Hello Readers!  Following the exciting news of the Chang’e 3 successfully landing on the moon, I thought I’d focus a little more on what is powering the little rover as it makes its way over the lunar surface.

Chang’e 3 is powered by the same fuel source as the Voyager, Cassini, New Horizons and Pioneer Spacecraft as well as the Curiosity Rover on Mars and even some of the instruments left on the surface of the moon from the Apollo missions. They are all powered by a few kilograms of Plutonium-238. But why use radioactivity for energy? And why plutonium in particular?

Solar energy is a perfectly useable source of energy for most spacecraft. Those that orbit Earth, the Sun or the Moon are all powered by solar cells that convert sunlight into energy. However, some missions cannot get the required power from the sun and have to rely on radioactivity. The Voyager, Cassini and Pioneer probes are too far from the sun to efficiently convert solar energy. If they were to rely on the Sun for power, the solar panels would have to be many times larger than those used on spacecraft orbiting Earth in order to gather the same amount of energy. In the case of Chang’e 3 and the lunar instruments, they have to endure the long lunar nights – equivalent to 14 Earth days – in temperatures of -170°C with no source of energy. Batteries could not power a rover for that length of time and they also need to stay warm so the electronics stay functional. This led scientists and engineers to design and create an alternative power source that can provide electrical energy and heat energy at a constant rate.

The answer was to use radioactive isotopes. As they decay, they release large amounts of heat that can be channelled throughout the craft and be converted to electricity via the use of a Radioisotope Thermoelectric Generator. Radioactive isotopes are also a very reliable source of energy as they decay at a constant rate and so release a set amount of energy per second. However, every radioactive isotope has a different rate of decay; those that decay too fast release all of their energy over a short time and are not suitable for lengthy missions. The isotopes with a very long decay rate (some have half-lives of billions of years) do not release enough energy per second to power the spacecraft. Plutonium-238 (238Pu) was decided to be the fuel of choice as its half-life is around 88 years and provides around 500W of energy per kilogram. It decays via alpha decay (so does not require heavy radiation shielding as alpha particles are stopped by a few centimetres of air) and was one of the waste products from building nuclear warheads so there was plenty of it 40 years ago.

The problem is that we are running out of Plutonium-238 to power future space missions. No 238Pu has been produced since 1988 when the last of the nuclear warheads were produced and it is a very costly and lengthy process to create more. A specific type of reactor has to be designed and built and production of 238Pu takes at least one year after the reactor is fired up. To date, there is only around 10kg of 238Pu remaining to power these missions and each spacecraft requires between 1.5 and 2kg of plutonium.

So what are the answers to the ‘Plutonium Problem’?

One option is to use a different type of radioactive isotope as the fuel source. Plutonium-238 is by far the most effective radioisotope but there are other possible candidates: Americium-241 and Strontium-90 being the most favourable as they have comparable half-lives but do not release as much energy per decay as plutonium. This means that the efficiency of the generator is greatly reduced so more fuel is required which makes the craft weigh heavier etc. so not the most ideal option.

Another answer is to try to use the plutonium we already have to power the spacecraft. The Sellafield Nuclear site in Cumbria, UK is the most radioactive place in Britain and is currently storing over 110 tonnes of plutonium in storage facilities. While only around 3% of this is the useful 238Pu isotope (most is 239Pu or 240Pu), this could be separated in a series of gas centrifuges to obtain around 3 tonnes of pure 238Pu. While transportation and use of a centrifuge is expensive and highly inefficient, 238Pu currently sells at over $4000 per gram! That means lots of money to be made (and future missions to be powered of course!) should an investor be looking for a completely open market to conquer.

Finally, NASA has pledged time and money into the development of a more efficient generator, called the Advanced Stirling Radioisotope Generator (ASRG). The ASRG is around 4x more efficient than current generators so much less 238Pu will be used per mission, thereby lengthening the lifetime of current 238Pu stocks. The main concern with this design is that it uses a similar set-up to a Stirling Engine and so has moving parts. This may not operate as well under microgravity conditions and if anything does fail, repair will be almost impossible once the spacecraft has launched. Regardless of all of this, NASA has recently announced that it has cancelled investment into this program anyway!

So to conclude, there is a serious global shortage of 238Pu to power future space missions and no current plan to create more or to use an alternative fuel. So what are the different space agencies going to do about it? That, ladies and gentlemen, is the real Plutonium Problem! If something isn’t done soon to tackle the problem, future missions to the outer solar system or to the surfaces of the Moon and Mars that rely on this power will be sparse or even non-existent by 2020 so let’s hope the problem is solved soon!

About Dayl Martin

I'm currently a first-year PhD student at the University of Manchester studying lunar meteorites and minerals using mid-infrared light. Particular interests of mine are lunar rocks and minerals, geological mapping and spectroscopy of planetary surfaces and the formation and evolution of the Moon. If you have any questions, please don't hesitate to contact me via e-mail. Happy reading!
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