Seattle-area company Helion Energy gets $425 million from investors including Sam Altman and Softbank to pursue clean energy's "Holy Grail."

A US start-up whose backers include tech billionaires Sam Altman and Peter Thiel has raised $425mn to keep it on track to achieve its target of producing electricity from nuclear fusion in 2028. Helion has the most ambitious timeline among start-ups racing to develop nuclear fusion,

They were joined by several existing backers including Sam Altman, who invested $375 million in Helion four years ago. The company is now valued at $5.245 billion. Nuclear reactors generate energy through a process called fission,