DOE Seeks to End MOX Plutonium Disposal Program

The Trump Administration requested $220 million next year “to continue the orderly and safe closure of the Mixed Oxide (MOX) Fuel Fabrication Facility.”

The MOX Fuel Fabrication Facility was intended to eliminate excess weapons-grade plutonium by blending it with uranium oxide to produce a “mixed oxide” that is not suitable for nuclear weapons. The Administration proposes instead to pursue a “dilute and dispose” approach.

Termination of the MOX Facility in South Carolina had previously been proposed — but not approved — in budget requests for the last two years, due to mounting costs.

“Construction remains significantly over budget and behind schedule,” the Department of Energy said in a November 2017 report to Congress. “The MOX production objective was not met in 2015 or 2016 and will not be met in 2017.”

“Due to the increasing costs of constructing and operating the MOX facility, both the Department’s analysis and independent analyses of U.S. plutonium disposition strategies have consistently and repeatedly concluded that the MOX fuel strategy is more costly and requires more annual funding than the dilute and dispose approach,” the DOE report said. The report was released by DOE under the Freedom of Information Act.

Though disfavored by the Administration, the MOX program has a champion in South Carolina Senator Lindsay Graham. “I will fight like crazy” to preserve it unless he is convinced that a superior alternative exists, he said at a February 8 hearing of the Senate Armed Services Committee.

Detailed background on the MOX program can be found in Mixed-Oxide Fuel Fabrication Plant and Plutonium Disposition: Management and Policy Issues, Congressional Research Service, December 14, 2017.

The latest proposal to terminate the MOX program was reported in “Aiken County legislators unsurprised by Trump’s anti-MOX budget” by Colin Demarest, Aiken Standard, February 19.

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.

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.

Yucca Mountain: Questions and Concerns

mythbusters signsYucca Mountain raises a lot of controversy – let’s face it; if it didn’t then a 4-part series of blog postings would hardly be necessary. Part of the reason for the controversy is that there are a number of worries about the impact of spent fuel disposal on the environment and on the health of people living and working in the area and along the transportation routes. So to close this series out I’d like to tackle some of these concerns to see which hold water and which might be over-stated. One good place to find a number of these concerns is a website put together by the State of Nevada in 1998.

Radiation levels from the spent fuel will be dangerously high for millennia

This is true, but not really relevant to the question of safe disposal at Yucca Mountain because no person will ever come in contact with the spent fuel. As I mentioned in the last posting, the spent fuel will be locked away inside of heavy-duty casks that are designed to shield the radiation, reducing it to less than 10 mR/hr at a distance of 2 meters from the cask. As long as the fuel remains inside the casks the fact that the fuel itself is intensely radioactive doesn’t make a difference – nobody can be harmed by radiation to which they’re not exposed. And with regards to the spent fuel remaining inside the casks – remember that the casks are rugged; they’re designed to survive hits from speeding locomotives, and once they’re in place in the ground they’ll not even face that risk. Finally, while the fuel remains radioactive for millennia, the radiation levels fall off fairly quickly over time – after several decades (far less than the design lifetime for the waste site or for the casks) radiation dose rates are down to a fraction of the original levels.

Spent fuel contains intensely toxic plutonium

Also true, but again not as dire as it sounds. Yes – spent fuel contains plutonium because plutonium is created when uranium-238 atoms capture neutrons during nuclear fission. And yes – plutonium is a very toxic heavy metal. But plutonium is hardly the most toxic element known to man – a toxicologist I used to work with could name a dozen substances that are more toxic (including shellfish toxins and fungal toxins). In fact, plutonium was administered to humans to help puzzle out how it acts and moves within the body and those to whom it was administered remained alive and well (and yes, many of these tests would be considered unethical today and they have generated a ton of controversy – but that doesn’t change the fact that the testing did not harm those who were tested).

And let’s think for a moment about what has to happen for the plutonium in the fuel to reach a person who might be harmed by it. Groundwater would have to percolate down through the hundreds of feet of rock to reach the spent fuel containers. Then it would have to penetrate through the casks by corroding the metal and soaking through the concrete layers. Once inside the casks it would have to dissolve the fuel elements – including the highly insoluble plutonium – and would then have to escape again. Finally, it would have to carry the dissolved plutonium through another several hundred feet of rock to the water table and, once there, would have to carry it however many miles to the nearest human with a well sunk into the aquifer. Possible? Yes. Plausible – especially in time measured in millennia? Not really.

Geologic events – earthquakes or volcanic eruptions – can cause the casks to fail, speeding the release of radioactivity to the environment.

Let’s take the easy one first. Yucca Mountain is made of volcanic rocks and there have been volcanic eruptions in the American Southwest within the last several thousand years. So it is plausible to think that there might be more such eruptions in the next few tens of thousands of years. But there are two primary types of eruptions – those with lava and those without. The style of eruption in the American Southwest has historically been ashfalls rather lava – such an eruption would only serve to entomb the spent fuel even more deeply, and the ash itself is too cool to melt the spent fuel casks. Being immersed in lava is more likely to damage the casks, but the lava itself isn’t likely to flow as far as the Las Vegas suburbs. To expose people to elevated levels of radiation the lava would have to immerse the casks long enough to melt them and then continue flowing and carrying the fission products with it – and continue far enough to expose people. There have been lava flows that have covered hundreds of miles, but not within millions of years. So while it is plausible to think that volcanic eruptions might be able to release radioactivity to the environment, the debris or lava are more likely to bury the waste even further than they are to release it to the environment.

Earthquakes are a little more problematic – the concerns here are that an earthquake could open up new fractures, speeding the flow of water from the surface to the casks and from the casks to the water table. Another concern would be that an earthquake could rupture the casks and release radioactivity. Both of these are plausible – we know that earthquakes fracture rock and can disrupt groundwater flows and they can certainly do that in Yucca Mountain. And we know that they can fracture rock, so they can certainly fracture spent fuel casks. So it is plausible to think that an earthquake could cause radionuclides to be released from the spent fuel casks. But we also have to think about the odds that an earthquake will open a fracture that passes through the very rock – the exact part of the rock – that the casks are sitting in. It’s certainly possible, but the odds are against it.

Plutonium might leak out of the canisters and accumulate in a critical mass in the environment and explode

This is one of my favorites. Not only do we have to get the plutonium out of the casks (water leaking into the waste repository, penetrating into the casks, dissolving plutonium, making its way into the environment), but then enough of the plutonium has to precipitate out of solution in the same place – and under the correct conditions – to form a critical mass. And it’s important to understand that a critical mass is not something that will explode but simply something that will sustain a fission chain reaction under the right circumstances. Going through the steps to even get plutonium into the environment is challenging enough and not likely to happen. Precipitating the plutonium out of solution in a critical mass adds to the unlikelihood. And putting together something that could blow up is well-nigh impossible.

Putting all of the spent fuel – which contains plutonium – in one spot makes a tempting target for terrorists and is a proliferation risk

Putting all of the spent fuel in one place certainly increases the amount of plutonium in this one location. On the other hand, we also have to wonder if it’s better to have only one location at risk or the 50+ that exist today. We can make a good argument that it’s easier to guard and make impregnable a single location than to try to secure every reactor facility in the nation.

With regards to non-proliferation, anyone trying to make a nuclear weapon would first have to get to the spent fuel casks and would then have to either steal some very large and heavy casks or would have to open them up at the waste site and remove the fuel from them – actions that would be hampered by high radiation levels anytime in the next several decades. And did I mention getting the spent fuel offsite and out of the country? Then the putative terrorists (or infiltrators from a prospective nuclear power) would have to remove the fuel, chop it up and dissolve it in acid, and chemically process it to remove the plutonium. The bottom line is that no terrorist group has the resources to pull this off, and neither do most nations. So…possible? Well – maybe, from the standpoint that winning the lottery by buying a single ticket is possible. Plausible – nope. This is another one that just doesn’t pan out.

——————

I could go on and on – there have been tons of arguments raised about why spent reactor fuel shouldn’t be disposed of at Yucca Mountain. Some of these arguments – the one about plutonium leaking out, forming a critical mass, and going boom comes to mind – are either deliberately specious or are a sort of worst-case wishful thinking; they will certainly not happen in the real world. Others – the possibility of an earthquake rupturing the spent fuel casks is one – are plausible, but the odds are very much against their happening. But here’s what it comes down to – we will be able to come up with lengthy lists of arguments both for and against putting spent reactor fuel just about anywhere. At some point we have to say one of three things:

  • We’re happy with the current situation and are going to stick with it forever,
  • We’re going to suck it up and put the waste in a location where our best science tells us it will be safe from any reasonable set of circumstances, or
  • We’re going to give up nuclear power and find some other way to produce 20% of our electrical needs.

The bottom line is that the entire nation benefits from the use of nuclear energy – again, 20% of our power is nuclear. There are ample places where the waste from these reactors can be stored with minimum risk to the environment or to people. We may never find a single location that we can certify as being “best” and the nit-pickers among us will always be able to find arguments – however irrational, specious, or ill-informed – that seem to mitigate against any particular site. But at some point the nation will need to find a place that, while perhaps not perfect, is good enough to meet our needs because it meets all reasonable criteria for waste disposal in the real world.

At some point, whether the nation is going to continue using nuclear energy or not, we are going to have to find a spot to put the spent reactor fuel that has accumulated and that is currently being stored across the nation. It makes sense that this location be someplace that is dry and under-populated, that is convenient to major transportation routes, and that is geologically and hydrogeologically suitable for isolating the waste while it is dangerous. These sites exist and Yucca Mountain is one of them. I would suggest that the technical problems of long-term radioactive waste disposal are relatively minor – the natural nuclear reactor at Oklo has shown us that even wet and fractured rock can retain radioactive waste for eons – it is the political problems that have thus far proven insurmountable. But let’s not deceive ourselves – the seemingly scientific objections to Yucca Mountain are pretexts for the underlying political objections. It is politics, not science or engineering that’s holding up our waste disposal solution. And until we can resolve these political problems we will continue to store our waste in a host of vulnerable locations scattered around the US.

The post Yucca Mountain: Questions and Concerns appears on ScienceWonk, FAS’s blog for opinions from guest experts and leaders.