The executive in charge of the world’s biggest fusion-energy experiment is trying to rehire retired engineers, who possess knowledge that’s critical to advancing an unfinished reactor in southern France.

The 35-nation International Thermonuclear Experimental Reactor, or ITER, is seeking to reboot the fusion project after its supply chains were disrupted by war and pandemic. Delays mean ITER’s efforts to harness the mechanics of the Sun’s clean energy on Earth could be overtaken by more nimble startups.

“What it takes to integrate a facility like ITER and design it from scratch has been lost,” said Pietro Barabaschi, ITER’s director general. “The knowledge is available somewhere but it is not consolidated. We have to get some retired people on board again.”

ITER revealed the knowledge gap Monday at an International Atomic Energy Agency conference in London, where hundreds of scientists and engineers are convening to assess the state of an industry drawing investment from billionaires including Jeff Bezos and Bill Gates. It’s likely to pile more pressure on the star-crossed government project, now facing competition from dozens of privately funded startups.

ITER was originally scheduled to cost about $5 billion and begin testing in 2020. The budget has ballooned past $22 billion, with no date set for trials. The dizzyingly complex machine pieces together more than a million parts sourced from around the world and even the slightest measurement anomalies can cause months or years of delay.

“Fusion needs to combine a number of different technologies that have never been seen,” Kyoto Fusioneering Ltd. Chief Executive Officer Satoshi Konishi said. “The plasma physicists dream is the engineers nightmare.”

ITER’s Barabaschi said fusion scientists have done a better job than engineers at managing knowledge, partly because they tend to publish and share discoveries in peer-reviewed journals. That kind of culture isn’t as strong in the engineering sciences, but that needs to change with the next generation, he said.

Engineers who know the science and can operate the machinery of fusion are “like gold dust,” said Ian Chapman, who heads the UK Atomic Energy Authority.