Viewpoint: Can americium replace plutonium in space missions?
Work by the UK's National Nuclear Laboratory (NNL) to recover americium-241 from the UK's ageing civil plutonium stockpile under a contract with the European Space Agency (ESA) raises some interesting issues, writes Ian Hore-Lacy.
ESA said that it wanted the stuff for radioisotope thermoelectric generators (RTGs) – the standard western power source in satellites and other space vehicles such as the Mars rover Curiosity now rambling around that planet. This was interesting, since virtually all RTGs so far are powered by plutonium-238, which has a high heat yield (560 W/kg) and low gamma radioactivity.
But ESA said that Am-241 was more attractive because it's cheaper, even though it produces only one quarter of the power (115 W/kg, but the Pu-238 is apparently only 85% pure). Also Pu-238 is virtually unobtainable at the moment - the USA stopped producing it about 1988 and the Russians have stopped selling theirs.
For space vehicles such as Cassini, Voyager and Galileo, RTGs with up to 33 kg of Pu-238 provide their power. These would need more Am-241 to do the same job, though RTG efficiency has improved greatly over recent decades. Am-241 is quite gamma-radioactive - it has gamma effect of 8.48 mSv/hr/MBq at one metre, much more than Pu-238. In the Mars rover Curiosity, which is much bigger than most satellites using RTGs but has the latest and most efficient RTGs, one would need 15-20 kg of Am-241 instead of the 4.8 kg of Pu-238 now powering it, producing 2kW thermal.
The amount of Am-241 for these space applications is huge compared with current usage, where it is in most household smoke detectors in microgram quantities – about 0.3 micrograms in each. One gram of americium oxide provides enough active material for more than three million household smoke detectors. Its cost is about $1500 per gram. And with a half-life of 432 years, those smoke detectors should outlast the homes they protect, at least in the ionization department, though after 30 years about 5% of the Am-241 has turned into neptunium-237.
Apparently the ESA is only seeking a few grams of americium at this point, and how the trade-off between lower cost than Pu-238 and higher gamma levels works out remains to be seen. NNL says that the longer half-life of Am-241 is helpful – Pu-238 is 88 years, and that they can manage the gamma activity OK.
But the other interesting aspect of all this is that much of the British plutonium stockpile is ageing. Civil plutonium contains 10-14% Pu-241, and this has a half-life of 14 years, producing Am-241 by beta decay. This can be separated from the plutonium, and obviously the older the plutonium the more Am-241 it will have. (Military plutonium has very little Pu-241.) But the chemical separation process is not trivial.
Of much more significance, if stockpiled plutonium has too much Am-241 in it, it may be too radioactive to feed through a standard MOX plant, which has very little in the way of shielding for gamma radiation (though some for neutrons from Pu-240). The Sellafield MOX plant for instance could not use plutonium more than six years old (ie that had been separated for more than six years), as it then contained more than 3% Am-241. It was then past its use-by date. The French, of course, dispatch their separated civil plutonium fairly promptly to the Melox plant where it soon becomes fresh fuel, and Japan plans the same.
The UK's hassles with MOX fuel fabrication plans however have meant that the national stockpile of civil plutonium is static and growing, and the Pu-241 portion is gradually turning into americium, a silent invasion. This raises questions of how it will be used. One option is simply to treat it chemically so as to remove the americium, as it becomes due for use. Another might be to build a MOX plant which is shielded to cope with the high gamma levels, and to find ways of handling 'hot' fresh fuel. This is approaching what is envisaged for Prism fast reactor fuel, where all transuranics will be part of the fuel and burned in the fast neutron environment, yielding useful energy. The fuel will be recycled after removal of only fission products in electrometallurgical reprocessing. Part of the rationale for using Prism in UK is in fact initially to make the plutonium highly radioactive, to 'spent fuel standard' with up to three months irradiation. Once the whole stockpile has been thus zapped and then stored in air-cooled silos, the reactors would turn to using it for power generation over the next 55 years or so.
It is not yet clear how the UK will deal with its infected plutonium, but meanwhile NNL is making hay by extracting at least some small amounts to power up European space ambitions. A happy spin-off!
Ian Hore-Lacy
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Ian Hore-Lacy is a Senior Research Analyst with the World Nuclear Association. One of the WNA's longest serving staffers, Ian is the author of the organisation's Information Library.
