Houston – we need some plutonium

Pu-238 glowing with the heat of alpha radiation decay

Pu-238 glowing with the heat of alpha radiation

The outer Solar System is a dark and lonely place – solar energy drops off with the inverse square of distance to the Sun so a spaceship in orbit around Jupiter (5.5 times as far from the Sun as the Earth) receives only about 3% as much solar energy as one orbiting Earth. Solar panels do a great job of powering spacecraft out about as far as Mars but anything sent to the outer reaches of the Solar System needs to find some other source of power. For most spacecraft this means using plutonium – specifically the isotope Pu-238. And according to some recent reports, we might be running out this particular flavor of plutonium. Since we can’t visit the outer solar system on solar power and batteries have a limited lifespan, if we want to go past the asteroid belt we’ve got to go nuclear with either radioisotope thermoelectric generators (RTGs) or reactors. And according to a NASA scientist (quoted in the story linked to above) we are running out of Pu-238 – if we don’t take steps to either replenish our stocks or to develop an alternative then our deep space exploration might grind to a halt. But before getting into that, let’s take a quick look at why Pu-238 is such a good power source.

As with any other element, plutonium has a number of isotopes – Pu-239 is the one that fissions nicely enough to be used in nuclear weapons, and the slightly heavier version (Pu-240) also fissions nicely. These heavier plutonium isotopes are both produced in nuclear reactors when U-238 captures a neutron or two – any operating reactor produces them and, for that matter, fissioning these plutonium isotopes produces a significant amount of energy in any nuclear reactor. Pu-238 is also produced in reactors, but through a slightly more convoluted pathway. The bottom line is that useable quantities of plutonium – fissionable or non – are produced in reactors.

What makes Pu-238 valuable is that it decays away quite nicely and produces a boatload of energy when it decays – it has a long enough half-life (just a tad less than 88 years) to last for decades and it gives off a high-energy alpha particle (for those who are interested, the alpha energy is over 5.5 MeV).

So let’s look at how this is turned into energy. Plutonium-238 has a half-life of 87.7 years and a decay constant (a measure of the fraction of Pu-238 atoms that will decay in a year) of 0.0079. To get a bit geekish, if we can calculate the number of atoms in a kg of Pu-238 then we can multiply the number of atoms by the decay constant to figure out how many decays will occur in a given period of time. A kg of Pu-238 has about 2.5×1023 atoms – multiply this by the decay constant and we find that there should be about 2×1022 atoms decaying every year; a year has about 3.1×107 seconds so this will give a decay rate of about 6.4×1014 atoms every second. And since each decay carries with it about 5.5 million electron volts (MeV), 1 kg of Pu-238 produces 3.5×1015 MeV every second. Doing some unit conversions gives us an energy production of about 550 joules per second – one J/sec is 1 watt, so each kilogram of Pu-238 produces 550 watts of power. A 5-kg RTG (like the one that’s powering the Curiosity rover on Mars) will put out nearly 3 kW of thermal power. This is enough heat that a sufficiently large mass of Pu-238 will glow red-hot; captured, it can be transformed into electricity to power the spacecraft – with a 5% conversion efficiency from thermal to electrical energy, this 10 kg of Pu will produce about 150 watts of electrical power. There are more efficient ways of turning heat into electricity, but they all have their limitations or are untried technologies.

This is where the Pu-238 half-life comes into play – it will take 87.7 years for 50% of the Pu-238 (and for power production to drop by half), so power will drop by only about 0.8% in a year. The Pu-238 half life is short enough to make for a furious decay rate – enough to produce the power needed to run a spaceship – but long enough to last for the decades needed to reach Pluto (the destination of the New Horizons ship) or to linger in orbit around Jupiter and Saturn (a la Galileo and Cassini). Without RTGs powered by Pu-238 we can’t explore much beyond the asteroid belt. This is why the possible exhaustion of our stocks of this nuclide so alarms Adams. According to Adams, NASA has already delayed or cancelled a number of planned missions to the outer Solar System, including a mission to study Europa, whose oceans are considered a prime candidate as an abode for life outside of Earth. The Department of Energy estimates that an annual outlay of $20 million or less would be enough to supply NASA’s Pu-238 needs, but this amount has not been forthcoming.

The space program is controversial and has been controversial for a half-century. Some decried the spending on Apollo, in spite of the fact that it gave us humanity’s first steps on another world. The Shuttle program also came under fire for a number of reasons, as has the International Space Station. And unmanned programs have been criticized as well. The common thread in most of this criticism is a matter of money – asking why in the world we should spend billions of dollars to do something that doesn’t provide any tangible benefit to those of us on Earth. Those making this argument are those who are reluctant to spend (or waste, as they’d put it) a few tens of millions of dollars annually to power the spacecraft that could help us learn more about our cosmic neighborhood.

The economic argument is hard to refute on economic grounds – there’s no denying that close-up photos of Saturn’s rings or Titan’s hydrocarbon seas haven’t fed a single hungry person here at home. And for that matter, even finding life on Mars (or Europa) will not feed the hungry here on Earth. But there has got to be more to life than simple economics – if not then there would be no need for art, for music, for sports, or for any of the other things we do when we’re not working, eating, sleeping, or attending to personal hygiene.

Discussing the relative merits of “pure” science is beyond the scope of this post (although I did discuss it in an earlier post in this blog). But I think it’s worth pointing out that the public showed a genuine interest in the exploits of the Voyager probe, the Galileo mission, and the Cassini craft – not to mention the missions to Mars, Venus, and elsewhere. I’d like to think that the deep space program is worth another few tens of millions of dollars a year for the entertainment value alone – especially given the vast sums that are spent on movies and TV shows that are watched by fewer people and that provide little in the way of enlightenment or uplifted spirits.

One other point that’s worth considering is that NASA’s outer Solar System missions are billion-plus dollar missions and the cost of plutonium is a small fraction of this amount. While not a major part of the nation’s economy, NASA programs employ a lot of people throughout the US to design and build the machines and the rockets that loft them into space, not to mention everyone who works to collect and analyze the data as it comes to Earth. That our deep-space capacity and those who keep it running might grind to a halt for lack of a few tens of millions of dollars of plutonium is a shame. The loss of everything else that goes along with our space program – the influx of new knowledge, the cool pictures, the sense of pride that we can send a working spacecraft so far and can keep it working so long, and the sense of wonder that comes from considering (even if only for a short time) our place in the universe – losing this for want of a little plutonium would be a crime.

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5 Responses to “Houston – we need some plutonium”

  1. James Greenidge September 27, 2013 at 7:12 AM #

    Re: “The space program is controversial and has been controversial for a half-century. Some decried the spending on Apollo, in spite of the fact that it gave us humanity’s first steps on another world.”

    I’m a black American and vividly recall the black community protests about “wasting money” in space, the biggest argument being that if we just stop going to the moon we can build paradise here on earth. Anyone seen paradise anywhere in the last forty years?

    We don’t have to justify positive investigative and exploratory science. It’s its own social nurishing justification.

    James Greenidge
    Queens NY

  2. Physco219 September 27, 2013 at 3:46 PM #

    After reading this I feel like we are so much on the same page. It would be and is such a shame that we are not furthering the scientific discoveries and out puts. It is a sad day when we can no longer use our nation’s treasure to further our world and be the standing beacon for everywhere else. While I agree money needs to be spent wisely and we should spend it for socially economical reasons first there is good reasons for funding NASA and other space programs. I do think that placing it in the perspective of movies, sports, and other leisure gives it more of something that would allow everyone to say, yeah, lets pitch in for this. I even think crowd sourcing would be a viable option if it could and would reach enough people. I mean what would be $0.50 per person? However I am sure that we will figure something out and maybe, the privet sector will come up with either the new technology or help progress whats been already proven, or will drive down costs of it as well as the cost of Pu-238. I realize this could be slightly dangerous in some ways but the potential may mitigate it to a viable degree. I would also like (yes I know Liberal thoughts) the uber wealthy in our society to kick in some maybe even a little more, Warren Buffet can, why cant an actor or actress chip in a few million when they have made say $50 million for the year? How many people do we have in this country that make more then, I dont know say more then $25 million? Could that number times say $1000 each fund maybe over fund this program and maybe make it better? It would be for the benefit for all of creation. Lets try, why not and who knows maybe all this will some day benefit someone hungry or make someone more rich, just think of mining that diamond planet, or discovering the asteroid weighted in the tons with gold. Who knows maybe even exo-planet real estate will be a driving force someday. In all, what I think is we should all get behind this for humanity’s sake, we should all fund dreams that we believe in no matter how big nor how small. If the number of people that enjoy our ventures via our space program stood behind it as they do with movies, or other luxuries (not needed things soda-pop to another iPhone for Johnny) I dont believe that we would be in this or for that matter any decline, and if corporations paid their “fair” share as they should (not what the law dictates and not the wheels and deals they work out with the govt and each other) then this would be a much better place over all.

  3. RickInDC September 30, 2013 at 9:23 AM #

    The US will spend $617 Billion to fund United Nations programs in 2014 (22% of their entire budget), much of which I view as a form of bribery to get other nations to like US. And our political leaders cannot find $20 million for NASA? http://cnsnews.com/news/article/senators-demand-transparency-us-taxpayer-funding-united-nations

  4. Jay Coghlan September 30, 2013 at 12:31 PM #

    Wired Magazine’s alarmist article “NASA’s Plutonium Problem Could End Deep-Space Exploration” argues that virgin production of plutonium-238 in nuclear reactors is needed, or U.S. space exploration is dead. Instead the nation’s future Pu-238 needs should be met through accelerated nuclear weapons dismantlements and recycling/scrap recovery efforts.

    Processing and encapsulation of Pu-238 currently takes place at the Los Alamos National Laboratory in northern New Mexico. [Having said that, all plutonium operations at the Lab have been shut down since the end of June because of nuclear criticality safety issues, which is a story in and of itself]. A Pu-238 scrap recovery line capable of recovering 2-8 kilograms per year was slated to start in 2005, but apparently has never become fully operational. In fact, LANL claimed in a 2008 site-wide environmental impact statement that it was capable of recycling/recovering up to 18 kilograms of Pu-238 per year, far more than needed to take care of the nation’s needs.

    LANL has a large existing inventory of Pu-238 scrap material. Moreover, the Pantex Plant was supposed to ship radioisotope thermoelectric generators (RTGs) from dismantled nuclear weapons to the Lab to harvest Pu-238. That hasn’t happened either, we conjecture because the LANL’s scrap recovery line hasn’t been properly working (or perhaps never really started in the Lab’s troubled Plutonium Facility-4). Indeed, the government estimated that approximately 3,200 RTGs would become available for recycling between 2009 and 2022 through nuclear weapons dismantlements. Significantly, increased dismantlements could also supply sufficient recycled tritium for existing nuclear weapons instead of current military production in civilian reactors, a big nonproliferation no-no. But unfortunately dismantlements at the Pantex Plant are substantially blocked by exorbitant “Life Extension Programs” that extend the service lives of existing nuclear weapons by three decades or more while giving them new military capabilities.

    Before the U.S. resumes virgin Pu-238 production, the government should make LANL straighten out its Pu-238 recovery operations. Safely that is, because Pu-238 is a very energetic gamma emitter and therefore very dangerous to handle. But the nation’s future Pu-238 needs should be met through accelerated nuclear weapons dismantlements (instead of Life Extension Programs) and recycling/scrap recovery efforts, not new virgin production in nuclear reactors.

    Jay Coghlan
    Nuclear Watch New Mexico
    http://www.nukewatch.org

    • Dr. Y September 30, 2013 at 8:30 PM #

      I appreciate your comments, but I’m not sure I can accept them at face value without some documentation. Can you provide a URL for the documents on Pu-238 scrap (and recovery) at LANL? Without being able to review this it’s hard to assess your statement that they’ve got so much Pu-238. I’m also curious about your claim that Pu-238 can be recovered from nuclear weapons dismantlement – as I noted in my posting, I can’t fathom a reason for having large quantities of Pu-238 in a nuclear weapon – can you tell me where it comes from, our point me in the direction of a document that explains how we can glean so much of this nuclide from nuclear weapons? I’m not saying that you’re wrong – just that I’m not sure I can buy into what you say without having a chance to see if the documentation is consistent with your statements. But if the Pu-238 is there for the taking (or recovery) then I agree it can at least be a stop-gap until we can line up another source.

      With respect to the potency of the Pu-238 gamma I’m afraid I don’t agree with your assessment. You’re correct that it emits a relatively high-energy gamma (766 keV), but the important factor is the gamma constant – the radiation dose rate per unit of activity. The gamma constant for Pu-238 is much lower than for many other radionuclides that we handle safely on a routine basis. Specifically, 1 Ci of Pu-238 has a dose rate of less than 0.1 R/hr at 1 meter. By comparison, Co-60 has a gamma constant of 1.3 (13 times higher), Cs-137 has a gamma constant of 0.3 (three times higher), and Ra-226 sources (with progeny nuclides in equilibrium) is on par with Co-60. The bottom line is that Pu-238 is neither exceptionally potent nor extraordinarily dangerous to handle. We work with stuff all the time that’s far more tricky and far more dangerous.

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