Pluto Flyby Mission Powered by Plutonium

News of the Earth these days is such that one welcomes news from elsewhere, especially when it concerns a prospect as spectacular as the impending flyby of Pluto by the NASA spacecraft New Horizons that will take place on July 14.

In reality, of course, New Horizons also represents news from Earth, having been built by humans and launched from Cape Canaveral in January 2006. Moreover, the New Horizons probe is not simply a technological artifact; it is the result of a political process and a policy debate. At issue were not only the parameters of the mission — its scope, timing, budget, and so on — but also the fact it uses a nuclear power source fueled, appropriately if controversially, by plutonium.

The plutonium-238 isotope used by New Horizons is an exceptionally hazardous material that is dangerous to produce, manufacture into suitable form, handle, transport and launch. The hazards are sufficient, in the eyes of some, to preclude its use altogether.

NASA and Department of Energy engineers did not dismiss public concerns about the safety of plutonium-fueled power sources, but they argued that the risks could be mitigated to an acceptably low level by proper design.

“Safety was the principal design driver for the [plutonium-fueled General-Purpose Heat Source used aboard New Horizons],” according to a 2006 retrospective account of its development. “The main safety objective was to keep the fuel contained or immobilized to prevent inhalation or ingestion by humans.” See “Mission of Daring: The General-Purpose Heath Source Radioisotope Thermoelectric Generator” by Gary L. Bennett, et al.

In effect, the design of the plutonium power source was predicated on the assumption that a launch accident or other mishap would occur, and that any resulting health and safety impact had to be minimized. Simulations were performed to validate the design, but fortunately no real-world test of the safety of the device under extreme conditions ever came to pass.

The GPHS plutonium power source has been used successfully on some of the boldest and most productive missions of space exploration ever undertaken, including Galileo, Ulysses, Cassini, and New Horizons.

For the most part, these missions were conducted with commendable openness, especially in earlier years. When one young critic raised questions about the use of plutonium power sources and the hazards of high-velocity Earth flybys in the Galileo mission prior to its 1989 launch, project manager John Casani of NASA Jet Propulsion Laboratory forthrightly invited him to come inspect the spacecraft in its clean room at JPL and to discuss the alternatives.

“Pluto is going to change us,” wrote analyst Dwayne Day last month, anticipating the possible consequences of the New Horizons mission for science, art, culture, politics, and space policy. See “Deep in space, corner of No and Where,” The Space Review, June 15, 2015.

NASA Releases Space Nuclear Power Study

NASA has released a long-awaited Nuclear Power Assessment Study that examines the prospects for the use of nuclear power in civilian space missions over the next 20 years.

The Study concludes that there is a continuing demand for radioisotope power systems, which have been used in deep space exploration for decades, but that there is no imminent requirement for a new fission reactor program.

The 177-page Study, prepared for NASA by Johns Hopkins University Applied Physics Laboratory, had been completed several months ago but was withheld from public release due to unspecified “security concerns,” according to Space News. Those concerns may have involved the discussion of the proposed use of highly enriched uranium as fuel for a space reactor, or the handling of plutonium-238 for radioisotope power sources.

Nuclear power can be enabling for a variety of space missions because it offers high power density in compact, rugged form. Radioisotope power sources (in which the natural heat of decay is converted into electricity) have contributed to some of the U.S. space program’s greatest achievements, including the Voyager I and II probes to the outer solar system and beyond. But development of nuclear reactor technology for use in space has been dogged by a repeated series of false starts in which anticipated mission requirements failed to materialize.

“The United States has spent billions of dollars on space reactor programs, which have resulted in only one flight of an FPS [fission power source],” the new NASA report noted. That was the 1965 launch of the SNAP 10-A reactor on the SNAPSHOT mission. It had an electrical failure after a month’s operation and “it remains in a 1300-km altitude, ‘nuclear-safe’ orbit, although debris-shedding events of some level may have occurred,” the report said.

The development and use of space nuclear power raises potential environmental safety and public health issues. As a result, the NASA report said, “it may be prudent to build in more time in the development schedule for the first launch of a new space reactor. Public interest would likely be large, and it is possible that opposition could be substantial.”

In any case, specific presidential approval is required for the launch of a nuclear power source into space, pursuant to Presidential Directive 25 of 1977.

“For any U.S. space mission involving the use of RPS [radioisotope power sources], radioisotope heating units, nuclear reactors, or a major nuclear source, launch approval must be obtained from the Office of the President,” the report noted.

Wanted: Astronomer with Top Secret Clearance

NASA’s orbiting James Webb Space Telescope will be “the premier observatory of the next decade, serving thousands of astronomers worldwide, and studying every phase in the history of our Universe, ranging from the first luminous glows after the Big Bang, to the formation of solar systems capable of supporting life on planets like Earth, to the evolution of our own Solar System.”

So why does its Director need to have a Top Secret/SCI security clearance, as specified in the job description posted last month on USA Jobs?

Clearly, the secrets of the universe do not lend themselves to, or require, national security classification controls, let alone non-disclosure agreements or polygraph testing.

But in practice, the civilian space program intersects the national security space program at multiple points, and former CIA analyst Allen Thomson suggested that the future Webb Director might need a Top Secret intelligence clearance in order to engage with the National Reconnaissance Office on space technology and operations, for example.

The Webb Space Telescope “will complement and extend the discoveries of the Hubble Space Telescope, with longer wavelength coverage and greatly improved sensitivity,” according to NASA. “The longer wavelengths enable the Webb telescope to look much closer to the beginning of time and to hunt for the unobserved formation of the first galaxies, as well as to look inside dust clouds where stars and planetary systems are forming today.”

The Webb Telescope has a projected launch date in 2018.

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.

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