Last 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.