A Reactor That Cannot Melt Down Is Being Built at Commercial Scale

Construction has begun on the nuclear island of a high-temperature gas-cooled reactor at Xuwei in Jiangsu — part of a plant that will supply both electricity and industrial steam. The reactor, called Chiron-1, is the commercial scale-up of a design China has been developing since 2006.

What makes the design unusual is what happens when things go wrong. The fuel is not a bundle of rods but hundreds of thousands of spherical pebbles, each with a kernel of uranium wrapped in layers of silicon carbide and graphite. Those layers hold fission products in at temperatures around 1,620°C. If cooling is lost entirely, the core sheds heat through its own surfaces and settles without melting.

That is a stronger claim than “passive safety”, the phrase used for reactors that need no pumps for 72 hours. Here the physics does the work: as the core heats towards roughly 1,600°C, the chain reaction slows by itself — partly through Doppler broadening, the effect that makes uranium absorb more neutrons as it warms — and that temperature stays below what the fuel can survive. A meltdown is suppressed by the material, not by an operator or a backup system.

The coolant is helium, which does not become radioactive and does not react. Because it leaves the core at about 750°C, the plant can make steam at 541°C and still reach efficiencies above 40% on an ordinary steam cycle — but more importantly, it can supply heat, not just electricity. That matters for industries that need high-temperature steam: oil refining, coal chemicals, heavy-oil recovery, and eventually hydrogen.

The reference plant is at Shidaowan in Shandong, which went commercial in December 2023 as the world's first operating fourth-generation reactor. Its equipment was 93.4% made in China, and the project produced 2,201 items that had never been built before.

Xuwei takes a different approach. It pairs two Hualong One pressurised water reactors with one gas-cooled unit, 3,076 megawatts in total: the Hualong One units make saturated steam, and the gas-cooled reactor reheats it to a higher quality, so the site runs as combined heat and power. The first unit is due in 2030 and the whole plant in 2031, with annual output of 32.5 million tonnes of industrial steam and more than 11.5 terawatt-hours of electricity.

The fourth-generation label covers several very different machines. The Generation IV International Forum picked six concepts in 2002 — sodium-cooled, lead-cooled, gas-cooled and supercritical water reactors, molten salt, and the very-high-temperature gas-cooled design. China is pursuing more than one, and the gas-cooled line is the one now furthest along.