Construction of the International Thermonuclear Experimental Reactor (ITER) in southern France, is now 50 percent complete. The machine, which features nearly 10 million parts, is widely seen as the most complex science project in history.

Fusion energy—the same process that powers the sun—potentially offers near limitless clean energy. The energy is created by fusing two lighter atomic nuclei to form a heavier nucleus, which requires plasma that is hotter than the sun. Scientists have not yet been able to generate more power from the fusion reaction that takes place than it takes to create the reactions in the first place. However, if the ITER is successful, that will change, providing enough electricity for millions of years.

“The stakes are very high for ITER,” said Bernard Bigot, Ph.D., Director-General of ITER. “When we prove that fusion is a viable energy source, it will eventually replace burning fossil fuels, which are non-renewable and non-sustainable. Fusion will be complementary with wind, solar, and other renewable energies.”

According to Daily Mail, the project’s members – China, the European Union, India, Japan, South Korea, Russia and the United States – agreed on a design that uses a doughnut-shaped device called a tokamak, to trap hydrogen that’s been heated to 150 million degrees Celsius (270 million Fahrenheit) for long enough to allow atoms to fuse together. The tokamak is surrounded by 18 giant magnetic coils that confine and circulate the superheated, ionized plasma.

The magnets are supported by six carbon fiber composite rings fabricated by Spanish company CASA Espacio. When complete, the rings will each be able to withstand 7,000 tons, as well as temperatures hotter than the sun.

“We cannot use traditional materials such as metal, which expands or contracts with temperature and are conductors of electricity, so we have to design a special composite material that is durable and lightweight, non-conductive and does not change shape,” explained José Guillamón, commercial and strategy director at CASA Espacio.

More than 80 percent of the cost of ITER, about $22 billion, is being contributed in the form of components manufactured by the partners. The European Union is paying 45 percent of the cost, with China, India, Japan, Korea, Russia, and the United States each contributing 9 percent. All members share in ITER’s technology; they receive equal access to the intellectual property and innovation that comes from building ITER. According to officials close to the project, it will be ready for its first stage of operation in December 2025, with the first power plants up and running by 2040.