Where does the plutonium come from?

new_horizonsLast week I wrote about how the shortage of Pu-238 might impact the exploration of the outer Solar System, but I didn’t much get into where the plutonium comes from. After all, while there are trace amounts of natural plutonium, there certainly isn’t nearly enough to fuel a space probe. So this week it seemed as though it might be worth going over where we get our plutonium, if only to understand why NASA (or DOE) needs tens of millions of dollars to produce it.

On the Periodic Table plutonium is two spots above uranium – uranium has an atomic number of 92 (that is, it has 92 protons) and plutonium is at 94. To make plutonium we somehow have to add two protons to a uranium atom. The way this happens is sort of cool – and there are different routes depending on the plutonium isotope that’s being produced.

To make Pu-239, the nuclide used in nuclear weapons, it’s a fairly simple process. Natural uranium is over 99% U-238, which doesn’t fission all that well. Put the U-238 (which makes up a minimum of 95% of the reactor fuel) into the middle of a reactor, which is seething with neutrons from uranium fission, and it will capture a neutron and turn into U-239. The U-239, in turn, decays by emitting a beta particle to neptunium-239, which gives off another beta particle. Since each beta decay turns a neutron into a proton, these two beta decays suffice to turn a uranium atom into one of plutonium. Thus, a single U-238 atom absorbing a single neutron and being allowed to sit long enough to undergo two beta decays (a few weeks or so) will turn into a single atom of Pu-239. Making heavier plutonium nuclides is just as easy – when Pu-239 captures additional neutrons it turns into Pu-240, Pu-241, Pu-242, and more. Not only is it fairly easy, but it happens all the time in any operating nuclear reactor.

OK – so we can see how simple neutron capture and patience can give us plutonium nuclides heavier than U-238, but this really doesn’t help us to make the Pu-238 needed to power a spacecraft. Making the lighter nuclide is a little more roundabout.

Remember that, through neutron capture, a reactor produces Pu-241. It turns out that Pu-241 also decays by beta emission, creating Am-241 – the stuff that’s used in smoke detectors (among other things). Am-241 is an alpha emitter and it decays to a lighter variety of neptunium (Np-237) which, when subjected to neutron irradiation, captures a neutron to become Np-238. One final transformation – a last beta decay – is the last step to producing Pu-238. This is the reason why Pu-238 is so expensive – making it requires two bouts of irradiation (the first long enough to produce the Pu-241), enough time for all of the radioactive decays to transform plutonium into americium and the americium into neptunium, and several steps of chemical processing to isolate the various elements of interest that are formed.

Although it sounds convoluted (well, I guess it is convoluted), making Pu-238 is fairly straight-forward. The science and engineering are both well-known and well-established, and its production certainly breaks no new scientific or technical ground. But the politics…that’s another matter altogether.

As I mentioned last week, the American Pu-238 production line shut down over two decades ago. Since then we’ve been buying it from the Russians, but they’ve got their own space program and have limited stocks to boot. So this option is not going to work for much longer, regardless of the future of US-Russian international relations.

A recent blog posting by Nuclear Watch suggested that the US might be able to meet its Pu-238 needs by dismantling nuclear weapons and by digging into its inventory of scrap Pu-238 – it notes that the Los Alamos National Laboratory (LANL) documents indicate that over 2000 RTGs’ worth of the nuclide can be recovered from nuclear weapons alone. But I’m not sure if I can accept this assertion, primarily because putting this nuclide into a nuclear weapon makes absolutely no sense. I can’t comment on the “scraps” of Pu-238 that LANL is said to have lying around, and unfortunately Nuclear Watch didn’t provide a link to the LANL documents they cited, making it difficult to check or to comment further. But if there is a Pu-238 stockpile at LANL it would certainly be nice to tap it for space exploration – not to mention the savings in disposal costs.

Yet another way to make Pu-238 is in a liquid fluoride thorium reactor (LFTR) – a reactor that uses naturally occurring thorium (Th-232) to breed U-233, which fissions quite nicely. Additional neutron captures can turn U-233 into Pu-238, which can be chemically separated from the fuel. There’s a lot more to the topic than this, but I covered the topic of thorium reactors fairly thoroughly last year (the first of these posts is at this URL, and there are three others in the same series) and it’s also covered on the Thorium Energy Alliance’s website. There are a lot of nice things about thorium reactors in addition to their being able to produce Pu-238, and it’s a technology that’s been worked out and tested – but the US shows no sign of building any of them anytime soon. India and China might develop extensive thorium reactor systems – but what these nations might do a decade or two in the future won’t do much for NASA in the next few years. The bottom line is that, however promising they might be for future needs, thorium reactors aren’t likely to help us send more spacecraft to the outer Solar System anytime soon.

So here’s where we stand. The US stopped producing the Pu-238 needed to run our deep-space probes and we’ve pretty much used up our stocks of the material. In the intervening years we’ve been buying Russian Pu-238, but that won’t be available for much longer, leaving us high and dry. There may be scraps of the material – possibly even stockpiles – at various DOE facilities, but dismantling nuclear weapons is probably not going to do the job. Over the long run thorium-cycle reactors might be a great way to make it, but these reactors aren’t operating anywhere in the world today and there are no American plans to build any of them anytime soon. That would seem to leave us with only three options – re-start our Pu-238 production line, find another way to make (or obtain) the material, or confine ourselves to the inner Solar System. As I mentioned last week, I sincerely hope we don’t go the last route. So let’s see what we can come up with – and let’s hope we don’t leave the solution (and decisions) too long.

The post Where does the plutonium come from? appears on ScienceWonk, FAS’s blog for opinions from guest experts and leaders.

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.

The post Houston – we need some plutonium appears on ScienceWonk, FAS’s blog for opinions from guest experts and leaders.