PARIS — The international ITER project to build a prototype nuclear fusion reactor will be delayed by more than a decade and faces another 4 billion euros of cost overruns, its director told the French daily Les Echos.

ITER chief Bernard Bigot said the experimental fusion reactor under construction in Cadarache, France, would not see the first test of its super-heated plasma before 2025 and its first full-power fusion not before 2035.

"The previous planning, which foresaw first plasma by 2020 and full fusion by 2023, was totally unrealistic," said Bigot, who succeeded Japan's Osamu Motojima at the head of ITER early last year.

Bigot, the former head of the French nuclear agency CEA, also said he expected the new delay to add 4 billion euros ($4.6 billion) of cost overruns to the 14 billion to 15 billion euros estimated so far.

The International Thermonuclear Experimental Reactor project's seven partners — Europe, the US, China, India, Japan, Russia, and South Korea — launched it 10 years ago with a cost estimated at 5 billion euros.

In June an ITER board meeting is set to review the new deadlines, which Bigot said were considered ambitious by independent experts.

Unlike existing fission reactors, which produce energy by splitting atoms, ITER would generate power by combining atoms, but it remains unclear whether the technology will work and whether it will eventually be commercially viable.