A gram of tritium costs $30,000 (about 890,000 New Taiwan dollars), but it’s worth it to nuclear fusion and its supporters. When tritium combines with its isotope sibling deuterium at high temperatures, it burns like the sun. As long as people find an efficient way to accomplish nuclear fusion, this reaction can provide endless amounts of green energy.

According to China’s “The Paper”, the Canadian nuclear laboratory delivered five steel drums, each about the size of a Coke-can, to the “Joint European Torrus (JET)” (JET), a large-scale nuclear fusion reactor in the United Kingdom in 2020. Cylinders containing tritium, a rare radioactive isotope of hydrogen whose nucleus contains two neutrons and one proton.

In 2021, tritium from Canada fueled a JET experiment that demonstrated that human fusion reactions are approaching a critical limit: fusion reactors are producing more energy than is being put into the reaction. Generally, people use the Q value (the ratio of energy output to input) to express that only if the Q value is greater than 1, the nuclear fusion reactor can be used for power generation. JET reached about 0.67.

This achievement provides a guarantee for the International Thermonuclear Experimental Reactor (ITR), a JET-like fusion reactor under construction in France, to achieve a Q value of 1 within the next ten years.

However, a recent article in the journal “Science” analyzed that this success may outweigh the benefits. Because by then, ITER will have used up most of the tritium currently in human hands, leaving very little tritium for subsequent nuclear fusion reactors. Initial experiments at ITER will use hydrogen and deuterium, which will not produce green energy. However, once the net energy of deuterium (tritium burning) starts to be produced, the reactor will consume up to 1 kg of tritium per year.

Fusion advocates have long claimed that reactor fuel is cheap and plentiful. For deuterium, it’s undeniable: Deuterium makes up one of the 5,000 hydrogen atoms in the ocean, and it sells for about $13 a gram. But tritium has a half-life of 12.3 years, natural tritium is a product of cosmic ray bombardment, and only exists in small amounts in Earth’s upper atmosphere. Chain reactors also produce small amounts of tritium, but this is rarely collected.

Most fusion researchers avoid this, arguing that future fusion reactors can produce enough tritium. If the inner walls of the reactor are lined with metallic lithium, the high-energy neutrons released in the fusion reaction can split the lithium into helium and tritium, which are relatively abundant on Earth.

But to grow tritium, one must have a working nuclear fusion reactor, and the problem is that the first generation of nuclear fusion power plants didn’t have enough tritium to begin with. Currently, the only commercial source of tritium worldwide is the 19 Canadian heavy water uranium reactors (CANDU, a pressurized heavy water reactor design), each producing 0.5 kg of tritium per year, but half of these nuclear reactors will be retired within this period. According to ITER In 2018, stocks of available tritium hit a decade high and have been steadily declining as tritium is sold and destroyed. At present, the global stock of tritium is 25 kg.

To make matters worse, some believe that tritium multiplication is not actually possible. Tritium multiplication has never been tested in nuclear fusion reactors, and in a recent simulation, UCLA nuclear engineer Mohamed Abdu and his colleagues found that, in the best case scenario, a reactor producing green energy produces only slightly more tritium than it needs. the fuel A leak of tritium or a prolonged shutdown of the reactor for maintenance can wipe out this small profit margin.

The shortage of tritium isn’t the only challenge facing fusion reactors, but operators have to learn to deal with turbulent bursts of plasma and neutron damage. But for plasma physicist Daniel Jasby, a former staff member of the Princeton Plasma Physics Laboratory (PPPL), the tritium shortage is dire. “This would be a fatal blow to the entire nuclear fusion business,” Jasby told Science.

The CANDU reactor, the only commercial source of tritium, faces decommissioning, and without the CANDU reactor, deuterium-tritium fusion would be an unattainable dream. “For the global nuclear fusion reactor, the most fortunate thing is that the byproduct tritium from the CANDU reactor can be used.” Abdul said.

When ITER and other nuclear fusion startups start burning tritium starting in 2030, annual tritium exports could reach 2 kilograms, estimates Jason Wart, vice-president of Ontario Power Generation.

But as many of the CANDUs that have been in operation for 50 years or more are decommissioned, the supply of tritium will dwindle, and the “tritium windows” of fusion reactors may eventually close. ITER was originally planned to launch in 2010 and begin burning deuterium-tritium within a decade. But its launch has been delayed until 2025, and could be delayed again due to the coronavirus pandemic and safety checks required by France’s nuclear regulator. As a result, ITER may not burn deuterium-tritium until 2035, at which point the supply of tritium will dwindle.

When tritium combines with its isotope sibling deuterium at high temperatures, it burns like the sun. As long as people find an efficient way to accomplish nuclear fusion, this reaction can provide endless amounts of green energy.

1 gram of tritium, worth up to US$30,000 (equivalent to about 890,000 New Taiwan dollars), faces a depletion crisis.