Source: Khan A (2021) Nuclear fusion: building a star on Earth is hard, which is why we need better materials. The Conversation.

Preface.  Of all the hundreds of obstacles fusion has yet to overcome, its death knell could be as simple as a shortage of the essential fuel it runs on: tritium. Yes, there are some kinds of imaginary fusion reactors that don’t need it, but they require a billion degrees Celsius (1.8 billion F) of heat. Tritium fusion reactors require a “mere” 150 million degrees Celsius (270 million Fahrenheit).

Today tritium for ITER is generated by 19 Canadian CANDU fission reactors, half of them slated to shut down. ITER will produce 30,000 tons of radioactive waste if it is ever is started up (Jassby 2018). But the odds are that will never happen. ITER was supposed to be up and running in 2016, and now it looks like 2025 will be the earliest test of plasma, and full fusion in 2035. Or later – their new schedule will be published by July supposedly. ITER is terribly mismanaged partly due to the enormous scope and number of countries involved.

Alice Friedemann  www.energyskeptic.com  Author of Life After Fossil Fuels: A Reality Check on Alternative Energy; When Trucks Stop Running: Energy and the Future of Transportation”, Barriers to Making Algal Biofuels, & “Crunch! Whole Grain Artisan Chips and Crackers”.  Women in ecology  Podcasts: WGBH, Financial Sense, Jore, Planet: Critical, Crazy Town, Collapse Chronicles, Derrick Jensen, Practical Prepping, Kunstler 253 &278, Peak Prosperity,  Index of best energyskeptic posts

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Clery D (2022) Out of gas. A shortage of tritium fuel may leave fusion energy with an empty tank. Science 376.

At $30,000 per gram, tritium is almost as precious as a diamond, but for fusion researchers the price is worth it. When tritium is combined at high temperatures with its sibling deuterium, the two gases can burn like the Sun.

But if ITER is ever completed, it will consume most of the world’s tritium, leaving little for reactors that come on line in 2055 or later, which also require tritium to jump start themselves.

Nor is more likely to be made in the future – quite the opposite. Today the world’s only commercial sources are the 19 Canada Deuterium Uranium (CANDU) nuclear reactors, each producing about half a kilogram a year as a waste product. Half of them will retire this decade.

This means the tritium stockpile of about 25 kilograms will peak by 2030 and begin a steady decline as it is sold off and decays.

Plan B would be to breed tritium, but for that you’d need an actual working fusion reactor. As mentioned in the preface above, ITER is a demonstration project to show that fusion is possible, but it won’t produce any power or breed more tritium even when it finally runs in 2035. Meanwhile it will be burning 1 kg of the precious 25 kg remaining.  Fusion scientists wishing to fire up reactors after that may find that ITER already drank their milkshake.

To compound the problem, some believe tritium breeding—which has never been tested in a fusion reactor—may not be up to the task. In a recent simulation, nuclear engineer Mohamed Abdou of the University of California, Los Angeles, and his colleagues found that in a best-case scenario, a power-producing reactor could only produce slightly more tritium than it needs to fuel itself. Tritium leakages or prolonged maintenance shutdowns will eat away at that narrow margin.

Scarce tritium is not the only challenge fusion faces, there are many other issues, such as fitful operations, turbulent bursts of plasma, and neutron damage. For Daniel Jassby, a plasma physicist retired from Princeton Plasma Physics Laboratory (PPPL) the tritium issue looms large. It could be fatal for the entire enterprise, he says. “This makes deuterium-tritium fusion reactors impossible.”

If not for CANDU reactors, D-T fusion would be an unattainable dream, since they produce tritium as a biproduct. If too much tritium builds up in the heavy water it can be a radiation hazard, so every so often operators send their heavy water to the utility company Ontario Power Generation (OPG) to filter out the tritium, selling about 100 grams of it a year, mostly as a medical radioisotope and for glow-in-the-dark watch dials and emergency signage.

But the supply will decline as the CANDUs, many of them 50 years old or more, are retired. Researchers realized more than 20 years ago that fusion’s “tritium window” would eventually slam shut, and things have only got worse since then. ITER won’t burn D-T until 2035 at the earliest, when the tritium supply will have shriveled.  Once ITER finishes work in the 2050s – if there are no more delays — only 5 kilograms or less of tritium will remain.

Yet large test reactors planned by China, South Korea, and the USA could need several kilograms each, and even more to start up the successor to ITER, which will actually generate power if all obstacles are overcome by ITER.  The successor, DEMO will require between 5 kilograms and 14 kilograms of tritium to begin—more than is likely to be available when the reactor is expected to fire up in the 2050s.

Even if DEMO and other fusion reactor designers can cut the need for tritium, fusion will have no future if breeding tritium doesn’t work.  These future reactors will need huge amounts of tritium – one capable of generating 3 gigawatts of electricity would need to burn 167 kilograms of tritium per year, more than what’s produced by hundreds of CANDU reactors.

Breeding is a challenge because fusion doesn’t produce enough neutrons, unlike fission, where the chain reaction releases an exponentially growing number. With fusion, each D-T reaction only produces a single neutron, which can breed a single tritium nucleus. Because breeding systems can’t catch all these neutrons, they need help from a neutron multiplier, a material that, when struck by a neutron, gives out two in return. Engineers plan to mix lithium with multiplier materials such as beryllium or lead in blankets that line the walls of the reactors.

ITER will be the first fusion reactor to experiment with breeding blankets, so expensive only 4 of 600 square meters of the reactor interior will have them, but as much of the reactor as possible needs to be covered for any hope of getting enough tritium.  The blankets also must absorb gigawatts of power from the neutrons and turn it into heat where pipes with water will pick it up to produce steam to drive turbines to generate electricity.  All of these components somehow surviving in an ultrahigh vacuum and magnetic field, bombarded by neutrons.

Some scientists think this may be impossible. Their analysis found that with current technology, largely defined by ITER, breeding blankets could, at best, produce 15% more tritium than a reactor consumes. But the study concluded the figure is more likely to be 5%—a worrisomely small margin.

One critical factor the authors identified is reactor downtime, when tritium breeding stops but the isotope continues to decay. Sustainability can only be guaranteed if the reactor runs more than 50% of the time, a virtual impossibility for an experimental reactor like ITER and difficult for prototypes such as DEMO that require downtime for tweaks to optimize performance. If existing tokamaks are any guide, Abdou says, time between failures is likely to be hours or days, and repairs will take months. He says future reactors could struggle to run more than 5% of the time.

There are many reasons why breakdowns might occur in a fusion power plant, here are just a few:

1. The reactor’s powerful magnetic fields need to stop and start which generates mechanical stresses that could tear it apart.
2. To get even an hour long run, high pressures are required, but that can push the plasma out of control and damage the reactor.
3. Outbursts of turbulent plasma can scour metal off the vessel’s inner wall and threaten its integrity, as well as poison the plasma. So far methods to stop this don’t always work.
4. High-energy neutrons are also a threat. They can penetrate the reactor walls and lodge in surrounding steel structures, knocking atoms out of position and weakening them. Nuclei in the structures sometimes absorb the neutrons, creating radioactive isotopes that do further damage.
5. Neutron bombardment can turn the nickel in many steel alloys into a form that gives off helium, causing the steel to swell perceptibly and turn into a sponge.
6. Finding tougher materials is a challenge because there’s no fusion reactor to test materials in
7. Fixing damaged and weakened reactor parts takes time due to the hostile radioactive environment which only robots can repair

To make breeding sustainable, operators will also need to control tritium leaks. For Jassby, this is the real killer. Tritium is notorious for permeating the metal walls of a reactor and escaping through tiny gaps. Abdou’s analysis assumed a loss rate of 0.1%. “I don’t think that’s realistic,” Jassby says. “Think of all the places tritium has to go” as it moves through the complex reactor and reprocessing system. “You can’t afford to lose any tritium.”

To get around these problems, two private fusion projects will use deuterium and helium-3 rather than tritium. But that’s problematic too, they’ll need to run at far higher temperatures, 1 billion degrees rather than 150 million degrees Celsius. And helium-3, although stable, is nearly as rare and hard to acquire as tritium. Most commercial sources of it depend on the decay of tritium, typically from military stockpiles.  Developers are hoping that by adding extra deuterium they can breed helium-3.

Katwala A (2022) Nuclear Fusion is already facing a fuel crisis. It doesn’t even work yet, but nuclear fusion has encountered a shortage of tritium, the key fuel source for the most prominent experimental reactors. Wired.  https://www.wired.com/story/nuclear-fusion-is-already-facing-a-fuel-crisis/

In the south of France, ITER is inching towards completion. When it’s finally fully switched on in 2035, the International Thermonuclear Experimental Reactor will be the largest device of its kind ever built, and the flag-bearer for nuclear fusion. Inside a donut-shaped reaction chamber called a tokamak, two types of hydrogen, called deuterium and tritium, will be smashed together until they fuse in a roiling plasma hotter than the surface of the sun, releasing enough clean energy to power tens of thousands of homes—a limitless source of electricity lifted straight from science fiction.

Or at least, that’s the plan. The problem—the white elephant in the room—is that by the time ITER is ready, there might not be enough fuel left to run it.

Like many of the most prominent experimental nuclear fusion reactors, ITER relies on a steady supply of both deuterium and tritium for its experiments. Deuterium can be extracted from seawater, but tritium—a radioactive isotope of hydrogen—is incredibly rare.

Atmospheric levels peaked in the 1960s, before the ban on testing nuclear weapons, and according to the latest estimates there is less than 20 kg (44 pounds) of tritium on Earth right now. And as ITER drags on, years behind schedule and billions over budget, our best sources of tritium to fuel it and other experimental fusion reactors are slowly disappearing.

Right now, the tritium used in fusion experiments like ITER, and the smaller JET tokamak in the UK, comes from a very specific type of nuclear fission reactor called a heavy-water moderated reactor. But many of these reactors are reaching the end of their working life, and there are fewer than 30 left in operation worldwide—20 in Canada, four in South Korea, and two in Romania, each producing about 100 grams of tritium a year. (India has plans to build more, but it is unlikely to make its tritium available to fusion researchers.)

But this is not a viable long-term solution—the whole point of nuclear fusion is to provide a cleaner and safer alternative to traditional nuclear fission power. “It would be an absurdity to use dirty fission reactors to fuel ‘clean’ fusion reactors,” says Ernesto Mazzucato, a retired physicist who has been an outspoken critic of ITER, and nuclear fusion more generally, despite spending much of his working life studying tokamaks.

The second problem with tritium is that it decays quickly. It has a half-life of 12.3 years, which means that when ITER is ready to start deuterium-tritium operations (in, as it happens, about 12.3 years), half of the tritium available today will have decayed into helium-3. The problem will only get worse after ITER is switched on, when several more deuterium-tritium (D-T) successors are planned.

These twin forces have helped turn tritium from an unwanted byproduct of nuclear fission that had to be carefully disposed of into, by some estimates, the most expensive substance on Earth. It costs $30,000 per gram, and it’s estimated that working fusion reactors will need up to 200 kg of it a year. To make matters worse, tritium is also coveted by nuclear weapons programs, because it helps makes bombs more powerful—although militaries tend to make it themselves, because Canada, which has the bulk of the world’s tritium production capacity, refuses to sell it for nonpeaceful purposes.

In 1999, Paul Rutherford, a researcher at Princeton’s Plasma Physics Laboratory, published a paper predicting this problem and describing the “tritium window”—a sweet spot where tritium supplies would peak before declining as heavy-water-moderated reactors were switched off. We’re in that sweet spot right now, but ITER—running almost a decade behind schedule—isn’t ready to take advantage of it. “If ITER had been doing deuterium-tritium plasma like we planned about three years ago, everything kind of would have worked out fine,” says Scott Willms, fuel cycle division leader at ITER. “We’re hitting the peak of this tritium window roughly now.”

Scientists have known about this potential stumbling block for decades, and they developed a neat way around it: a plan to use nuclear fusion reactors to “breed” tritium, so that they end up replenishing their own fuel at the same time as they burn it. Breeder technology aims to work by surrounding the fusion reactor with a “blanket” of lithium-6.

When a neutron escapes the reactor and hits a lithium-6 molecule, it should produce tritium, which can then be extracted and fed back into the reaction. “Calculations suggest that a suitably designed breeding blanket would be capable of providing enough tritium for the power plant to be self-sufficient in fuel, with a little extra to start up new power plants,” says Stuart White, a spokesperson for the UK Atomic Energy Authority, which hosts the JET fusion project.

Tritium breeding was originally going to be tested as part of ITER, but as costs ballooned from an initial $6 billion to more than $25 billion it was quietly dropped. Willms’ job at ITER is to manage smaller-scale tests. Instead of a full blanket of lithium surrounding the fusion reaction, ITER will use suitcase-sized samples of differently presented lithium inserted into “ports” around the tokamak: ceramic pebble beds, liquid lithium, lead lithium.

Even Willms admits that this technology is a long way from being ready to use, however, and a full-scale test of tritium breeding will have to wait until the next generation of reactors, which some argue might be too late. “After 2035 we have to construct a new machine that will take another 20 or 30 years for testing a crucial task like how to produce the tritium, so how are we going to block and stop global warming with fusion reactors if we will not be ready until the end of this century?” says Mazzucato.

There are other ways of creating tritium—actively inserting breeding material into nuclear fission reactors, or firing neutrons at helium-3 using a linear accelerator—but these techniques are too expensive to be used for the quantities required, and they will likely remain the reserve of nuclear weapons programs. In a perfect world, there would be a more ambitious program developing the breeding technology in parallel to ITER, Willms says, so that by the time ITER has perfected the fusion reactor there’s still a fuel source to run it. “We don’t want to get the car built and then run out of gas,” he says.

The tritium problem is fueling skepticism of ITER, and D-T fusion projects more generally. These two elements were initially chosen because they fuse at a relatively low temperature—they’re the easiest things to work with, and it made sense in the early days of fusion. Back then, everything else seemed impossible.

But now, with the help of AI-controlled magnets to help confine the fusion reaction, and advances in materials science, some companies are exploring alternatives. California-based TAE Technologies is attempting to build a fusion reactor that uses hydrogen and boron, which it says will be a cleaner and more practical alternative to D-T fusion.

It’s aiming to reach a net energy gain—where a fusion reaction creates more power than it consumes—by 2025. Boron can be extracted from seawater by the metric ton, and it has the added benefit of not irradiating the machine as D-T fusion does. TAE Technologies CEO Michl Binderbauer says it’s a more commercially viable route to scalable fusion power.

But the mainstream fusion community is still pinning its hopes on ITER, despite the potential supply problems for its key fuel. “Fusion is really, really difficult, and anything other than deuterium-tritium is going to be 100 times more difficult,” says Willms. “A century from now maybe we can talk about something else.”

Jassby D (2018) ITER is a showcase … for the drawbacks of fusion energy. Bulletin of the Atomic Scientists. https://thebulletin.org/2018/02/iter-is-a-showcase-for-the-drawbacks-of-fusion-energy/

Tritium tribulations. The most reactive fusion fuel is a 50/50 mixture of the hydrogen isotopes deuterium and tritium; this fuel (often written as “D-T”) has a fusion neutron output 100 times that of deuterium alone, and a spectacular increase in radiation consequences.

Deuterium is abundant in ordinary water, but there is no natural supply of tritium, a radioactive nuclide with a half-life of only 12.3 years. The ITER website states that the tritium fuel will be “taken from the global tritium inventory.” That inventory consists of tritium extracted from the heavy water of CANDU nuclear reactors, located mainly in Ontario, Canada, and secondarily in South Korea, with a potential future source from Romania. Today’s “global inventory” is approximately 25 kilograms, and increases by only about one-half kilogram per year, notes Muyi Ni and his co-authors in their 2013 journal article, “Tritium Supply Assessment for ITER,” in Fusion Engineering and Design. The inventory is expected to peak before 2030.

While fusioneers blithely talk about fusing deuterium and tritium, they are in fact intensely afraid of using tritium for two reasons: First, it is somewhat radioactive, so there are safety concerns connected with its potential release to the environment. Second, there is unavoidable production of radioactive materials as D-T fusion neutrons bombard the reactor vessel, requiring enhanced shielding that greatly impedes access for maintenance and introducing radioactive waste disposal issues.

In 65 years of research involving hundreds of facilities, only two magnetic confinement systems have ever used tritium: the Tokamak Fusion Test Reactor at my old stomping grounds at the Princeton Plasma Physics Lab, and the Joint European Tokamak (JET) at Culham, UK, way back in the 1990s.

ITER’s present plans call for the acquisition and consumption of at least 1 kilogram of tritium annually. Assuming that the ITER project is able to acquire an adequate supply of tritium and is brave enough to use it, will 500 MW of fusion power actually be achieved? Nobody knows.

“First plasma” at ITER is supposed to occur in 2025. That will be followed by a relatively subdued 10 years of continued machine assembly and periodic plasma operations with hydrogen and helium. These gases produce no fusion neutrons, and thereby permit the resolution of shakedown problems and optimization of plasma performance with minimal radiation hazards. Plasma instabilities must be kept at bay to ensure adequate energy confinement, so the reacting plasma can be heated and maintained at high temperature. Influxes of non-hydrogenic atoms must be curtailed.

ITER’s schedule calls for deuterium and tritium use beginning in the late 2030s. But there’s no guarantee of hitting the 500 MW target; generating fusion power in large quantities depends, among other things, on developing the optimal recipe of deuterium and tritium injection by frozen pellets, particle beams, gas puffing, and recycling. During the unavoidable teething stage through the early 2040s, it’s likely that ITER’s fusion power will be only a fraction of 500 MW, and that more injected tritium will be lost by non-recovery than burned (i.e., fused with deuterium).

Analyses of D-T operation in ITER indicate that only 2 percent of the injected tritium will be burned, so 98 percent of the injected tritium will exit the reacting plasma unscathed. While a high proportion simply flows out with the plasma exhaust, much tritium must be continually scavenged from the surfaces of the reaction vessel, beam injectors, pumping ducts, and other appendages for processing and re-use. During their several dozen traverses of the Tritium Trail of Tears around the plasma, vacuum, reprocessing and fueling systems, some tritium atoms will be permanently trapped in the vessel wall and in-vessel components, and in plasma diagnostic and heating systems.

The permeation of tritium at high temperature in many materials is not understood to this day, as R. A. Causey and his co-authors explained in “Tritium barriers and tritium diffusion in fusion reactors.” The deeper migration of some small fraction of the trapped tritium into the walls and then into liquid and gaseous coolant channels will be unpreventable. Most implanted tritium will eventually decay, but there will be inevitable releases into the environment via circulating cooling water.

Designers of future tokamak reactors commonly assume that all the burned tritium will be replaced by absorbing the fusion neutrons in lithium completely surrounding the reacting plasma. But even that fantasy totally ignores the tritium that’s permanently lost in its globetrotting through reactor subsystems. As ITER will demonstrate, the aggregate of unrecovered tritium may rival the amount burned and can be replaced only by the costly purchase of tritium produced in fission reactors.

Radiation and radioactive waste from fusion. As noted earlier, ITER’s anticipated 500 MW of thermal fusion power is not electric power. But what fusion proponents are loathe to tell you is that this fusion power is not some benign solar-like radiation but consists primarily (80 percent) of streams of energetic neutrons whose only apparent function in ITER is to produce huge volumes of radioactive waste as they bombard the walls of the reactor vessel

References

Alley WM et al (2013) Too Hot to Touch: The Problem of High-Level Nuclear Waste. Cambridge University Press.

Clery D, et al. May 6, 2016. More delays for ITER, as partners balk at costs. Science 352: 636-637

Jassby D (2018) ITER is a showcase … for the drawbacks of fusion energy. Bulletin of the Atomic Scientists.

Stone R (2016a) Spent fuel fire on U.S. soil could dwarf impact of Fukushima. Science Magazine.

Stone R (2016b) Near miss at Fukushima is a warning for U.S., panel says. Science Magazine.