Curiosity's nuclear power source

GEOFF BRUMFIEL
Last updated 11:11 03/09/2012
Curiosity
NASA
The deck of the Curiosity rover.

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I'm as happy as anyone that the Curiosity rover got to Mars; it's hard not to root for all those Nasa geeks in their blue polo shirts. But before you get all sentimental about the achievement, there's something you should know: Curiosity runs on plutonium from a Soviet-era nuclear weapons plant.

Take a look at Curiosity's rear end. Other rovers have solar panels, but not Curiosity. Instead, there's a little white thing that looks cute, almost like a tail. Inside are eight boxes filled with pellets of nuclear fuel.

This stuff is hot, so hot that the boxes glow bright red, and will glow for years to come. Think of it as nuclear charcoal. The fuel will keep the rover toasty on cold Martian nights and supply it with electricity.

It's a neat trick, and one that Nasa has used before. Since the 1960s, the United States has been launching nuclear-powered spacecraft. The first were military satellites. That worked swell, except that when the mission ended, you had a radioactive pile of junk orbiting the planet. And every now and then, one would fail to launch or fall back to Earth. That was bad for PR.

These days, Nasa puts nuclear fuel on things that aren't coming back. The Voyager missions that left the solar system carried it, as did the first Martian missions, the Viking landers. It's particularly useful in places far from the sun - where solar panels don't work.

Curiosity's particular kind of fuel is called plutonium-238. It's the perfect stuff for the job: It's extremely radioactive, so it gives off plenty of heat, but the type of radioactive particles released by plutonium-238 can't even penetrate a sheet of paper. As long as you don't touch it or swallow it, plutonium-238 is safe, and with a half-life of 87.7 years, it decays so slowly a small supply can power a spacecraft for a decade or more.

But plutonium-238 isn't easy to come by. It doesn't exist in nature, and only two places in the world have made serious quantities of it. Both made something else: nuclear warheads.

You see, plutonium-238 is really a byproduct of the process for making another kind of plutonium, known as isotope 239.

Plutonium-239 is the real terror: Just a couple of kilos of the stuff can make a bomb as powerful as many kilotons of TNT.

Almost all modern warheads in the US arsenal use plutonium-239 as a trigger. When it explodes, it sets off an even larger thermonuclear device capable of flattening a midsized city. Russian warheads have even higher yields.

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In the 1960s, the US and Soviet Union were hungry for 239. They built secret reactors that irradiated uranium to create it. Then they dissolved the uranium-plutonium mix in acid and used a slew of toxic chemicals and solvents to isolate the plutonium. The work provided the plutonium-239 for thousands of tiny, high-efficiency warheads - many still atop missiles today.

Plutonium-238, the stuff in the rover, was an afterthought. Nasa asked the Atomic Energy Commission to get some for the agency's satellites in the 1950s, after falling behind in the space race.

The eggheads at the nuke plant came up with a clever way of producing it from unwanted isotopes they were just going to throw away. The Soviets had the same idea. Using a similar system of acids and solvents to dissolve their uranium fuel, the Soviets skimmed plutonium-238 off of their production operation at a secret Ural Mountain bomb factory. It went on for decades: In came uranium fuel, out went plutonium- 239 for bombs, plutonium-238 for spacecraft, and other isotopes for other needs.

The factories churned out something else, too: radioactive waste. At the US plant on the South Carolina-Georgia border, workers dumped tens of millions of litres of radioactive waste a year into open- air basins. The worst of the stuff, 140 million litres of radioactive sludge, salt and liquid waste, was put into underground storage tanks, where it sits to this day. The site, known as Savannah River, is still heavily contaminated, and clean-up operations have run to many billions of dollars.

In Russia, the situation is even grimmer. In true Soviet fashion, the bomb makers secretly dumped liquid waste into giant reservoirs around the plant. Nobody knows how much radioactive contamination is out there, but a single accident - the explosion of a waste tank in 1957 - is thought to have been Chernobyl-like in scale. As recently as the 1990s, the plant was spewing radioactive waste at a rate that makes the leaks of radioactive water from Fukushima look like a bubble bath. Residents near the plant have elevated rates of leukaemia and genetic mutations. Their children get cancer.

The US quit making plutonium in the late 1980s, after it became apparent both sides had stockpiled enough warheads to destroy civilisation.

At first, Nasa drew on the supply of plutonium-238 left over at Savannah River, but that soon ran out. So it turned to Russia. The first Russian shipment arrived in the 1990s, and to date, Nasa has received about 30 to 40 kilos of plutonium. Almost four kilos of Stalin's finest plutonium- 238 hitched a ride to Mars with Curiosity.

Even this supply is now running out. Russia has got out of the bomb-making business as well, and it's running low on plutonium- 238.

Nasa is looking for new ways of making it, and this year, it asked the US Congress for $10 million to investigate the possibility of restarting production at smaller research reactors near Savannah River and in Idaho.

This time around, scientists say they'll do things differently. They'll be working with smaller quantities in more modern facilities, they're going to try to find cleaner ways to chemically separate the fuel, and they'll be subject to environmental regulations - something the old bomb factories avoided in the name of national security.

The circumstances may have changed, but not the chemistry and physics: Making plutonium-238 is still a very sloppy, very radioactive business, and setting up new facilities won't be cheap. A review by the US National Academies of Science estimates it would cost hundreds of millions of dollars.

There's nothing wrong with oooh-ing and aaah-ing over Curiosity's photos. The project is an incredible achievement, and the science it produces will be amazing.

But remember this, too: That little rover on Mars has left a big mess back here on Earth.

- © Fairfax NZ News

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