Science

At the heart of a nuclear fusion reactor is an ultra-powerful superconducting magnet, operating at temperatures near absolute zero and under immense magnetic stress. For decades, scientists from around the world have struggled to find materials that simultaneously endure such extreme cold and extreme force.

Chinese scientists have detailed how they created CHSN01 (China high-strength low-temperature steel No 1), deployed it this year in the construction of world’s first fusion nuclear power generation reactor and put China in a leading position in materials science.

It was a decade-long journey marked by setbacks, doubt and ultimate triumph.

In 2011, the International Thermonuclear Experimental Reactor (ITER), which is under construction in southern France, faced a critical material challenge. Testing revealed that the cryogenic steel prepared had become brittle and lost its ductility.

ITER, the world’s largest fusion experiment, was launched in 2006 from a collaboration between seven members, including China.

At the core of the fusion device, superconducting magnets are armoured with cryogenic steel, like a jacket engineered to endure ultra-low temperatures. This material must withstand both liquid helium’s −269 degrees Celsius (−516 Fahrenheit) cryogenic environment and the massive Lorentz forces generated by intense magnetic fields.

Global ITER teams investigated the cause and refined production protocols. In 2011, China’s team developed the first viable solution but Li Laifeng, a researcher at the Chinese Academy of Sciences’ (CAS) Technical Institute of Physics and Chemistry in Beijing, still had qualms.

In a China Science Daily article, Li wrote: “While ITER’s maximum 11.8 Tesla field design is enough for itself, future higher-field magnets will require advanced materials”. He added that ITER could not generate electricity, but China’s own reactor would.

The challenge in 2011 pushed China to spend more than a decade developing the proprietary cryogenic steel. Li’s team initially experimented with nitrogen-enhanced N50 stainless steel, which improved yield strength but failed to enhance cryogenic toughness.

In 2017, Li went to the United States to take part in the International Cryogenic Materials Conference, where he introduced his new material.

However, foreign experts were sceptical, believing the existing technological route was “absolutely impossible” to produce better cryogenic steel, according to the Science Daily report. They had considered the existing ITER conductor jackets which used 316LN austenitic stainless steel – a specialised alloy designed for extreme conditions – to be sufficient.

In 2017, subsequent trials, in which the Chinese team incorporated vanadium while controlling carbon/nitrogen ratios, achieved a better strength-toughness balance. But the new steel still did not perform at the level needed.

Progress stalled until 2020, when physicist Zhao Zhongxian joined the team’s meetings.

Zhao is a Chinese Academy of Sciences academician and a world-leading expert in cryogenic physics who won China’s top national science award in 2017 for his superconducting material research.

He had long emphasised the critical role of materials in superconducting technology applications across multiple fields, and actively asked to take part in Li’s team meetings.

“Do not blindly trust foreign authorities. This matter is worth pursuing.” he was quoted by the Science Daily report as saying to the researchers.

In 2021, the Institute of Plasma Physics, CAS, in Hefei province, established core engineering specifications for China’s fusion programme: 1,500-megapascal (MPa) yield strength and over 25 per cent elongation at cryogenic temperatures.

At the time, magnetic confinement fusion expert Li Jiangang described the stakes: “Developing next-gen cryogenic steel isn’t optional – it’s essential for the success of China’s compact fusion energy experimental devices”.

By late 2021, the High-Strength Steel Research Alliance was formed, uniting four institutes, 13 enterprises and four welding specialists under Li Laifeng’s leadership, sharing its technological advances with the industry and carrying a goal of developing a new type of domestic cryogenic steel.

Their biweekly technical forums and a “racehorse” development model – in which blind samples underwent independent evaluation at the Institute of Physics and Chemistry – accelerated progress.

In August 2023, experts confirmed that the new CHSN01 steel had met the engineering benchmarks that had been set. The material could withstand 20 Tesla magnetic fields and 1,300MPa stresses while showing superior fatigue resistance compared to traditional alloys.

It has since been deployed in a Chinese fusion reactor project, and the authors wrote about the 12-year process to develop the material in a paper published in Applied Sciences in May.

On May 1, China’s Best (Burning Plasma Experimental Superconducting Tokamak) entered its assembly phase, with an expected completion date in 2027.

Of the more than 6,000 tonnes of components assembled on-site, the straight segments of the superconducting conductor jackets comprise 500 tonnes. These core components are all made from domestically produced CHSN01 steel.

The country is looking to capitalise on the cryogenic steel development beyond the reactor.

“In addition to its applications in superconductivity, this steel can also be used in other related areas,” Zhao said.