NASA's worries about running out of plutonium by the middle of the next decade are premature, according to a senior U.S. Department of Energy (DOE) official.

 

The U.S. space agency has three missions planned between 2009 and 2017 that call for long-lasting spacecraft batteries that convert heat from decaying plutonium-238 into electricity that is used to power instruments and other electronics. The batteries, known as radioisotope power systems, are considered critical for space missions bound for the outer planets of the solar system where sunlight is insufficient for solar arrays to provide a spacecraft with sufficient power.

 

NASA Administrator Mike Griffin told the House Appropriations commerce, justice, science subcommittee in March that the United States could find its outer planets program hamstrung by the middle of the next decade unless the Department of Energy resumes production of plutonium-238.

 

"Looking ahead, plutonium is in short supply," he said.

 

Dennis Miotla, DOE deputy assistant secretary for nuclear power development, does not dispute that there is only so much plutonium to go around. But he said in a March 27 interview that the United States could wait until as late as 2012 to begin bringing a plutonium-238 production capability back on line and still be able to meet NASA's projected future demand for the material.

 

"I don't think it an imminent danger that we will run out in their time frame," Miotla said. "They're looking at their most optimistic budget projections and their most optimistic mission deployment timelines."

 

NASA Already Feeling Shortage

 

The dwindling U.S. plutonium-238 inventory already has put a crimp in NASA's plans. To ensure there is enough of the stuff available for the 2009 Mars Science Laboratory mission, a power-hungry outer planets probe planned for 2016 or 2017, and a still-undefined demonstration flight of a new, more efficient radioisotope power system planned for 2013 or 2014, NASA already has ruled out using radioisotope power for a competed New Frontiers planetary mission slated to launch around the same time.

 

That limitation is expected to put scientists wishing to propose missions to Jupiter and Saturn for the 2016 opportunity at a decided disadvantage compared to scientists pushing more sun-drenched destinations.

 

DOE's 2009 budget request, sent to Congress in February, included no funding to restart plutonium-238 production. That omission has some NASA officials privately grumbling about what they see as foot dragging on the part of White House budget officials unwilling to request money for a restart.

 

The United States stopped producing plutonium-238 in the late 1980s when it shut down reactors at the Department of Energy's Savannah River Site in South Carolina for safety reasons. U.S. nuclear laboratories still are able to process and package the material for use in radioisotope power systems, but they have been meeting NASA's demand for the past two decades from a finite stockpile supplemented with periodic purchases from Russia. By agreement, the Russian material can only be used for civil space applications.

 

The Energy Department estimated in 2005 that restarting would cost $250 million, but that figure assumed that all plutonium-238-related activities would be consolidated in a single location — the Idaho National Laboratory — for security reasons. Miotla said closer study found that the cost of consolidation outweighed the security advantages, and that it made more sense to leave processing and packaging activities where they are today and simply restart production at a still-to-be-determined lab. That would cost around $150 million, he said.

 

Reconciling with Reality

 

Leonard Dudzinski, NASA's program officer for radioisotope power capability, told the Outer Planets Assessment Group during a March 31 meeting in Boulder, Colo., that even if the Department of Energy received funding in 2009 to restart production, the first kilograms of plutonium-238 would not start coming out the other end of the line until around 2014 or 2015. "Even that would not be enough to meet the ambitious demands that we have," he said. "Our plans for using [plutonium] in the future are going to have to be reconciled with reality."

 

Miotla said he understood NASA's concern, even if he does not share the agency's same sense of urgency.

 

"They are looking ahead and saying, 'suppose there is a windfall and we get a lot of money and get to do all these missions, we won't have the material,'" Miotla said. "But the likelihood that all those stars align ... is just a low probability event."

 

Taking inventory

 

Miotla declined to say exactly how much plutonium-238 the United States has in inventory today, or how long it is expected to last. In 2005, the Department of Energy reported the inventory stood at 39.5 kilograms, with NASA and U.S. national security customers expected to consume all but 6.5 kilograms by 2010. Since those numbers were published, the Department of Energy has taken delivery of another 5 kilograms of plutonium from Russia with another 4.9 kilograms due to arrive this year.

 

Miotla said that among the Department of Energy's customers for plutonium-238, only NASA was complaining about a looming shortfall.

 

"You might notice that you've heard nothing from any national security source," Miotla said, "I don't have any knowledge of them having issue with the supply and requirements, so I think that at a high level it is reasonable to assume that they're taken care of. They would, of course, be the first priority."

 

Miotla also implied that the United States has a larger inventory that it has been willing to tell NASA or the public, suggesting the possibility that the "cupboard is actually more full than you might believe."

 

Miotla said that it was "not out of the question" that NASA might be able to use some of the plutonium-238 that has been set aside for national security uses.

 

"But it would never be done unless there was a very compelling reason and/or the national security requirements were more clear in the long run," Miotla said. "You always protect the national security inventories. Period."

 

Russian Roulette

 

While Russia's plutonium-238 stockpile is off limits to U.S. national security customers, NASA is free to use whatever the Energy Department can get its hands on. Under the current agreement, 5 kilograms a year is all Russia is willing to sell.

 

Dudzinski said NASA has allocated money to reimburse the Energy Department for the purchase of 10 kilograms of plutonium-238 from Russia for delivery in 2009 and 2010. According to Dudzinski, it could be the last plutonium-238 the United States is able to get from Russia.

 

"What the Russians have told us is that they have 10 more kilograms to sell us. After that they don't have any more plutonium to sell us," Dudzinski said. "They have carefully worded their statements ... so that doesn't mean that there's not more plutonium, it means they are only willing to sell us 10 more kilograms."

 

Miotla agreed it is not clear how much more plutonium Russia is willing and able to sell the United States.

 

"Every time we make a purchase they tell us it's the last one," he said. "And then there's another one. So frankly nobody knows."

Miotla said the positives outweigh the negatives associated with buying from Russia.

 

"We don't like buying their material, but it is good material and I'd rather have it in our labs than in theirs," he said. "So this is kind of a good deal and we will buy as long as they come up with it. And my guess is, personally, they are going to keep coming up with it because they've told us 'only 10 kilograms [are] left' for the last 10 years."

New Technology

 

Dudzinski, meanwhile, told the scientists gathered in Boulder that NASA was doing everything it could to ensure it has the plutonium it needs for its top priorities, which includes flight-testing a new type of space battery that is more fuel-efficient than the radioisotope thermoelectric generators it has been using for decades. That new battery, called an Advanced Stirling Radioisotope Generator (ASRG), is designed to generate as much power as the proven system with only a fourth as much plutonium.

 

Dudzinski said NASA plans to give the ASRG its first in-space workout in 2013 or 2014 as part of a Discovery-class mission yet to be selected. Two ASRGs, cranking out a combined 250 watts of electricity, will be built for that mission, he said.

While switching to the Stirling system could help stretch the current plutonium inventory, Miotla — whose office oversees Stirling development — said the new technology still has to prove itself. "We are quite a bit away from betting a multibillion-dollar mission on a Stirling generator," he said. "Although I think the day will come when that will happen, it's not in the next five years."

 

As a result, NASA's planned outer planets flagship mission, a multibillion-dollar project that will target the moons of Saturn or Jupiter, will not use ASRGs. Instead it will rely on the same type of radioisotope thermoelectric generator NASA is using for the Mars Science Laboratory mission due to launch in 2009. Supplying the 800 watts of power NASA officials believe they will need for the outer planets flagship would require around 30 kilograms of plutonium-238.

 

According to Dudzinski, the outer planets flagship threatens to be NASA's last nuclear-powered mission for a while unless Russia is bluffing about cutting the United States off, or the Department of Energy resumes production soon.

 

Miotla acknowledged that the Energy Department eventually would have to restart production to ensure a steady, reliable supply. But exactly when that needs to happen, he said, still is under study.

 

"Production is very expensive and we wouldn't want to necessarily jump into that before looking at a lot of other options," he said. "We've raised discussions with NASA about revisiting their power density requirements and we've also looked at more clever ways to produce the heat sources because right now we are using the exact same technology we've been using since day one."

Fred Flintstone Technology

 

Designing a new production capability around more up-to-date technologies than DOE used at Savannah River might mean construction would get off to a later start than if the department stuck with what Miotla called "Fred Flintstone technology." But a newer design might come together faster, he said, if the chosen approach did not involve enormous amounts of concrete and steel.

 

"That's the long lead-time item in creating the production capability," he said.

 

DOE has no fixed date for deciding whether to pursue such a capability.

 

And that is what has NASA worried.

 

"If we do not do anything else as a nation to acquire more plutonium — either produce it ourselves, acquire it from Russia or some other source — we would be out of plutonium after the flagship mission and would not be able to fly any other deep space missions, or anything that would require plutonium as a heat source or a power source," Dudzinski said.

Comments: bberger@space.com