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A Stirling solution and a plutonium problem

msl msl msl.jpgA new generation of nuclear generators could power space missions with unprecedented efficiency, but a critical shortage of the element needed to fuel them is fast approaching.


Many past space missions have used electricity stemming from nuclear power. Rather than using nuclear fission, these have generally relied on converting the heat from nuclear decay of plutonium 238. The Mars Science Laboratory (pictured) is hopefully to become the latest mission to use such a power source, when the rover launches in November and attempts to reach the Red Planet in 2012.


But as some researchers work on an entirely new design for nuclear power conversion, others are dealing with another problem. The US has lost the capacity to make the plutonium fuel these electricity generators require, and in the future, it may not be able to rely on buying the element from abroad.


Stephen Johnson, of the Space Nuclear Systems and Technology Division at Idaho National Laboratory, says reliability, low maintenance requirements and good power-to-weight ratios make nuclear power sources ideal for space missions.


“They are a very effective way of providing power in remote or hostile environments,” he told the American Chemical Society meeting in Denver. Beyond Mars, the weakening of the Sun’s rays makes nuclear the preferred option. Only one NASA mission beyond Mars – the recent Juno mission to Jupiter – has relied on solar power.


Current models operate via the Seebeck effect – where temperature differences between two different metals, in this case usually silicon and germanium, produce electricity. These designs have been in operation since the 1960s, and have demonstrated their resilience through a number of unplanned crashes, even appearing on the Moon during the Apollo programme years. But they have very low efficiency and convert only 7% or so of the decay energy into useful electricity.


Now researchers are working on a nuclear powered Stirling Generator, which in basic terms mechanically converts heat into work by heating a gas, which then expands to drive a piston. This could increase efficiency to around 30%. Although using a Stirling system has the additional problem of moving parts, Johnson says that in his field many people are “really betting everything on the Stirling generator right now”.


Although the technology still needs to be demonstrated as reliable and has not yet passed a final design review, it is hoped that a Stirling based system could fly on the 12th of NASA’s Discovery class missions in 2016 (three have been shortlisted of which two would likely use Stirlings).


But there is a fundamental stumbling block to space missions using plutonium power much past this date.

“What we don’t have right now is the ability to make plutonium 238 for deep space missions,” Robert Wham, of the Oak Ridge National Laboratory, told the Denver meeting.

Plutonium 238 is made by putting Neptunium oxide into a nuclear reactor. In the past, the ‘K Reactor’ at Savannah River was used, but this is no longer in operation. Wham is working on getting two research reactors – the Advanced Test Rector and High Flux Reactor – used instead.

His aim is to get the United States in a position where it can produce around 2 kilogrammes of plutonium 238 per year. America currently has a sizeable stockpile of Neptunium but work needs to be done to prove that this can be turned into oxide ‘targets’ suitable for introduction into the two reactors and that plutonium 238 of a sufficient quality can be recovered afterwards.

America has previously purchased some plutonium 238 from Russia and may be able to buy some more to tide itself over, but this is a finite supply too, says Wham. A “safe bet” to get production up and running in the United States would be about 5 years, he adds.

Back in 2009 the National Research Council recommended that domestic plutonium 238 production be restarted, but little progress has been made. Switching to Stirling based systems – which use less Plutonium to generate the same power – “would stretch the current supplies”, says Johnson.

Ralph McNutt, of Johns Hopkins University, could not attend the meeting due to Hurricane Irene. But in a series of slides presented by Johnson, McNutt pointed out that there has been no domestic production of plutonium 238 since 1988 and the world stockpile of this element is estimated at about 30kg. By 2020, this stockpile will be exhausted, he says.

If the US were to take immediate steps to restart production, there would be no gap between the current supplies running out and new supplies coming on line. But this would require immediate action, which may be unlikely in the current financial climate.

McNutt’s last slide features an image of the grim reaper. “The future is at risk,” it warns.

Image: NASA via wikipedia.


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