The Paper Reporter Sen Ning
In 2020, the Canadian Nuclear Laboratory delivered 5 steel barrels to the large-scale nuclear fusion reactor "Joint European Torus (JET)" in the United Kingdom. The barrels were lined with softwood to absorb impact. Inside each bucket is a Coke can-sized cylinder containing a wisp of hydrogen gas — just 10 grams, which weighs as much as a few sheets of paper.
This is not ordinary hydrogen, but hydrogen's rare radioactive isotope tritium. The nucleus of tritium is composed of two neutrons and one proton. At $30,000 for 1 gram of tritium, it's almost as expensive as diamonds, but for fusion supporters, it's worth every penny. Tritium combines with its isotope brother deuterium at high temperatures to burn like the sun. As long as people can find an effective way to stimulate nuclear fusion, this reaction can provide endless clean energy.
In 2021, tritium from Canada provided fuel for a JET experiment that proved that human nuclear fusion reactions are approaching an important critical value: the energy produced by the fusion reactor exceeds the energy invested in the reaction. Usually, people use the Q value (ratio of energy output and input) to express it. Only when the Q value is greater than 1, the fusion reaction device can be used to generate electricity. JET has reached about 0.67.
This achievement also provides guarantee for the International Thermonuclear Experimental Reactor (ITER) to achieve a breakthrough of Q value 1 in the next ten years. ITER is a fusion reactor similar to JET under construction in France, with a volume of Twice that of JET.
However, a recent article on the website "Science" (www.science.com) analyzed that this victory may not be worth the gain. Because by then, ITER will have consumed most of the tritium currently in human hands, leaving very little tritium for subsequent fusion reactors. ITER's initial experiments will use hydrogen and deuterium and produce no net energy. However, once it begins deuterium-tritium combustion that produces net energy, the reactor will consume up to 1 kilogram of tritium per year.
Proponents of nuclear fusion have long claimed that fusion reactor fuel is cheap and plentiful. For deuterium, this is undeniable: About one in every 5,000 hydrogen atoms in the ocean is deuterium, and it sells for about $13 per gram. However, the half-life of tritium is 12.3 years. Natural tritium is a product of cosmic ray bombardment and only exists in trace amounts in the upper atmosphere of the earth. The chain reactor also produces small amounts of tritium, but it is rarely collected.
Most fusion researchers dismiss this idea, believing that future nuclear fusion reactors can produce the needed tritium. If the inner wall of the reactor is lined with metallic lithium, the high-energy neutrons released in the fusion reaction can split the lithium into helium and tritium, and lithium resources are relatively abundant on the earth.
But the problem is that in order to breed tritium, one needs a functioning fusion reactor, and the first generation of nuclear fusion power plants will most likely not have enough tritium to start. Currently, the only commercial source of tritium worldwide is the 19 Canadian deuterium-uranium nuclear reactors (CANDU, also known as CANDU reactors, a pressurized heavy water reactor design), each producing about 0.5 kilograms of tritium per year, but these nuclear reactors Half of will be retired within ten years. According to ITER's 2018 projections, stocks of available tritium will peak within a decade and then decline steadily as tritium is sold and decays. The current global stock of tritium is approximately 25 kilograms.
Figure 1: Tritium supply trends
To make matters worse, some believe that tritium multiplication may not be truly achievable. Tritium breeding has never been tested in a fusion reactor, and in a recent simulation, UCLA nuclear engineer Mohamed Abdou and his colleagues found that, in the best-case scenario, a A fusion reactor that produces net energy produces only slightly more tritium than it needs for its own fuel. A tritium leak or a longer reactor shutdown for maintenance would eat away at this small bonus. The scarcity of tritium is not the only challenge facing fusion reactors. Operators must also learn to deal with issues such as turbulent bursts of plasma and neutron damage.But according to plasma physicist Daniel Jassby, a former staff member at the Princeton Plasma Physics Laboratory (PPPL), the problem of tritium shortage is imminent. “This could be a fatal blow to the entire nuclear fusion industry,” Jasby told Science.
The only commercial source of tritium - CANDU reactor faces decommissioning
Without CANDU reactors, deuterium-tritium fusion will be an unattainable dream. "The most fortunate thing for nuclear fusion reactors around the world is that they can use the by-product tritium produced by the CANDU reactor." Abdul said.
Many nuclear reactors use ordinary water to cool the core, moderate chain reactions, and slow down neutrons so that they can more easily initiate fission. But CANDU reactors use heavy water, with deuterium instead of hydrogen, because it absorbs fewer neutrons and leaves more neutrons for fission. But occasionally, a deuterium nucleus captures a neutron and turns it into tritium.
Radiation hazards may occur if too much tritium accumulates in heavy water, so operators often send heavy water to Canada's Ontario Power Generation Company (OPG) for "degradation." Ontario Power Generation Company filters out the tritium and sells it, about 100 grams per year, primarily as a medical radioisotope or in luminous watch dials and emergency signs.
Fusion reactors will significantly increase the demand for tritium. Jason Van Wart, vice-president of Ontario Power Generation Co., expects tritium shipments to reach 2 kilograms per year from 2030 onwards, when ITER and other fusion startups begin burning tritium.
But as CANDUs, many of which have been operating for 50 years or more, are retired, the supply of tritium will decline, and the "tritium window" of fusion reactors may eventually slam shut. ITER was originally planned to launch around 2010 and begin burning deuterium-tritium within a decade. But its start-up has been delayed until 2025, and may be delayed again due to the coronavirus pandemic and safety inspections required by France's nuclear regulator. Therefore, ITER may not burn deuterium- tritium until 2035 at the earliest, when the supply of tritium will be exhausted.
According to ITER predictions, once ITER ends its work after 2050, there will be only 5 kilograms or less of tritium left in the world. The worst-case scenario could be that "there won't be enough tritium to meet the needs of fusion reactors after ITER," said Gianfranco Federici, director of fusion technology at the European Agency for Nuclear Fusion Research (EuroFusion).
Figure 2: ITER fusion reactor device in France
Some private companies are designing smaller fusion reactors that can use less tritium in the early stages of operation. Commonwealth Fusion Systems, a startup in Massachusetts, said it has secured a supply of tritium for its compact prototype and early demonstration reactors and expects to require less than 1 kilogram of tritium during development. .
However, each of the large experimental fusion reactors planned by the Chinese, Korean and US governments may require several kilograms of tritium per year. According to the European Nuclear Fusion Research Agency, ITER's successor DEMO will need more tritium. DEMO will be 50% larger than ITER and will serve as a demonstration power station to provide 500 megawatts of power to the grid.
Fusion reactors typically require large amounts of tritium to start because fusion only occurs in the hottest parts of the ionized gas plasma. This means that very little tritium is burned in the donut-shaped tokamak, a ring-shaped vessel that uses magnetic confinement to achieve controlled nuclear fusion. Researchers expect ITER to burn less than 1% of the tritium injected into it; the rest will diffuse to the edges of the tokamak and be swept into the recovery system.
To this end, the designers of DEMO are studying some methods to reduce the need for tritium when starting the fusion reaction. One approach is to launch frozen fuel pellets into the reactor's deep combustion zone, where they will burn more efficiently. Another approach would shorten the recovery time to 20 minutes, using metal foil as a filter to quickly remove impurities and sending the tritium directly back into the machine rather than separating them.
But Abdul said the appetite for DEMO may still be huge.He and his colleagues simulated the deuterium-tritium fuel cycle in reactors including DEMO and concluded that DEMO alone would require between 5 kilograms and 14 kilograms of tritium to start up, more than is expected to be available when the reactor starts up after 2050. Tritium amount.
Achieving tritium breeding using nuclear fusion itself is fraught with challenges
However, even if the DEMO team and other reactor designers can reduce their need for tritium, if tritium breeding cannot be successfully achieved, nuclear fusion will have no future. According to Abdo, a commercial fusion plant producing 3 gigawatts of electricity per year would burn 167 kilograms of tritium per year - equivalent to the tritium production of hundreds of CANDU reactors.
The challenge with tritium multiplication is that nuclear fusion cannot produce enough neutrons, unlike fission, which is a chain reaction and the number of neutrons released increases exponentially. In nuclear fusion, each deuterium-tritium reaction produces only one neutron, forming a tritium nucleus. Therefore, the reactor's breeding system requires the help of neutron multiplier materials. When hit by one neutron, the multiplier material will produce two neutrons. Engineers plan to mix lithium with a multiplying material, such as beryllium or lead, in a "blanket" of reactor walls.
Figure 3: The internal structure of a fusion reactor using a "breeder blanket"
ITER will be the first fusion reactor to test a "breeder blanket". Testing will include a liquid blanket (a molten mixture of lithium and lead) as well as a solid "pebble bed" (ceramic balls containing lithium mixed with beryllium balls). Due to cost cutting, ITER will lay out only 4 square meters of breeding material inside the 600 square meter reactor. Fusion reactors after ITER will need to cover as much surface as possible to have any chance of meeting the demand for tritium.
However, Abdo and his colleagues analyzed and found that with current technology and based on ITER estimates, the "breeder blanket" can only produce at most 15% more tritium than the reactor consumes, and this number is even higher. Maybe 5%.
Another factor affecting tritium breeding is reactor downtime. When the tritium multiplication stops, the isotope continues to decay. Sustainability can only be guaranteed if the reactor operates more than 50% of the time.
Whether it is for an experimental reactor like ITER or a prototype reactor like DEMO that needs to be shut down for adjustments to optimize performance, such a running time is difficult to guarantee. Abdul said that if existing tokamak devices are any guide, the time between failures could be hours or days, and repairs would take months. Future fusion reactors may struggle to operate more than 5 percent of the time, he said.
In addition, in order for tritium valorization to be sustainable, operators also need to control tritium leakage. Tritium escapes from the reactor's metal reactor walls through tiny gaps. Abdo's analysis assumes a loss rate of 0.1%. But Jasby thinks there's more to it than that. As tritium goes through complex reactors and reprocessing systems, "think of all the places it's going to go," Jasby said.
Is it okay to give up tritium fuel in fusion reactors?
Under such circumstances, two private fusion companies have decided to abandon tritium fuel altogether.
California-based startup TAE Technologies plans to use ordinary hydrogen and boron, while Washington state-based startup Helion will fuse deuterium and helium-3 - a A rare helium isotope. These reactions require higher temperatures than deuterium-tritium, but the companies believe it's a price worth paying to avoid the trouble that tritium brings.
These alternative fusion reactors have the added attraction of producing fewer or no neutrons, which avoids the material damage and radioactivity caused by deuterium-tritium fusion. Michl Binderbauer, TAE's chief executive, said TAE's reactor - a stable rotating plasma ring - could last for 40 years without neutrons. But the main challenge for this model of reactor comes from temperature: deuterium-tritium fusion requires 150 million degrees Celsius, while ordinary hydrogen and boron require 1 billion degrees Celsius.
Helion's fuel is tritium and helium-3, which burns at a temperature of 200 million degrees Celsius. It uses a plasma ring similar to TAE, but compressed by a magnetic field.However, although helium-3 is stable, it is almost as rare as tritium and difficult to obtain. Most commercial sources of it rely on the decay of tritium. Helion CEO David Kirtley said that by adding additional deuterium to the fuel mixture, a deuterium-deuterium fusion reaction can occur to produce helium-3.
Nonetheless, traditional deuterium-tritium fusion advocates believe that tritium supplies can be expanded by building more fission reactors. Currently, some countries with nuclear weapons have specially built or modified commercial nuclear reactors and established their own tritium inventories. The U.S. Department of Energy provided tritium to the Princeton Plasma Physics Laboratory in the 1980s and 1990s, which at the time had a deuterium-tritium combustion reactor. But Federici does not believe the agency or militaries around the world will be in the business of selling tritium. "It is unlikely that defense stockpiles of tritium will be shared," he said.
Or maybe, the world could see a renaissance in CANDU technology. South Korea has four CANDU reactors and a plant that extracts tritium, but they do not sell them commercially. Romania has two and is building a tritium facility. China has several CANDU reactors. They can increase tritium production by adding lithium rods to the core, or by doping heavy water moderators with lithium. However, this approach may compromise the safety of the reactor, and tritium itself is also dangerous.
In decades of research on nuclear fusion, plasma physicists have single-mindedly pursued breakthroughs in the Q value of fusion reactors, hoping that the reactors could produce excess energy. Jasby said they view other issues, such as getting enough tritium, as "trivial" projects. But as the reactors get closer to criticality, it's time to start worrying about engineering details that are far from trivial. “Putting the issue off until later is just a big mistake,” Abdul said.
Editor in charge: Kang Yimei
Proofreader: Zhang Liangliang
