CADARACHE, France (Reuters) - Construction of an experimental nuclear fusion reactor in southern France is in full swing as the cost estimate has ballooned to nearly four times the original estimate, but the ITER project’s new head says new forecasts are realistic.

Technicians work at the construction site of the International Thermonuclear Experimental Reactor (ITER) in Saint-Paul-lez-Durance,Southern France, October 6, 2016. REUTERS/Jean-Paul Pelissier

The seven partners in the International Thermonuclear Experimental Reactor (ITER) - Europe, United States, China, India, Japan, Russia and South Korea - launched the project 10 years ago with a 5 billion euro ($5.6 billion) cost estimate and plans to heat the first plasma by 2020 and achieve full fusion by 2023. By 2011, the budget forecast had swollen to about 16 billion euros.

In May, new ITER chief Bernard Bigot - former head of French nuclear state agency CEA – told a French newspaper ITER would be delayed by more than a decade and incur another 4 billion euros of cost overruns, with the first test of its super-heated plasma not before 2025 and its first full-power fusion not before 2035.

Unlike existing fission reactors, which produce energy by splitting atoms, ITER would generate power by combining atoms in a process similar to the nuclear fusion that produces the energy of the sun.

“We expect first plasma in December 2025 and full power by 2035. For sure, that schedule is still challenging but it is the best technically achievable schedule, taking into account the financial constraints,” Bigot told reporters during a visit to the ITER site in rural Cadarache.

Bigot estimates the overall cost until commissioning will be of the order of 18 billion euros. Compared to the 2010 baseline, the cost increase is about 4 billion euros, he said.

“For the first time, we have a reliable estimate ... In the past there was no realistic schedule, no detailed appreciation of the cost ... It was much underestimated,” said Bigot, who succeeded Japan’s Osamu Motojima as ITER head early last year.

He said that giving a precise estimate is difficult as partner countries contribute most of their shares in the project in kind, by producing components.

“Many domestic agencies do not want to disclose their exact costs,” he said.

Slideshow ( 7 images )

He said that only the contribution of Europe, which funds 45 percent of ITER, and that of the United States are released to the public.

Bigot said the running cost of the ITER organization plus the domestic agencies in the partner countries is about 200 million euros per year. Any delay to the project automatically increases the cost by that much.

Laban Coblentz, ITER head of communication, said that since partner countries contribute about 80 percent of the value of the project in kind, it is difficult to give precise cost estimates. “This is the source of the inaccuracy when we try to compile the total number,” he said.

Coblentz said ITER estimates the total project cost at between 18 to 22 billion euros.

“If all partner countries had European levels of cost and bureaucracy and you extrapolate based on European costs, it would be at the higher end of the range ... Cost could be up to 22 billion euros at the maximum,” he said.

Construction at the ITER site in rural Cadarache got under way in 2013-14, but has accelerated from April-May 2015 onwards.

“We have seen more progress in the last six months than in the last three years,” Coblentz said.

Laurent Schmieder, head of construction at ITER, said by 2019 the building that will house the so-called tokamak fusion reactor will be complete. The cost of the buildings alone at the complex will be about 2 billion euros, he said.

The challenges of replicating the fusion process on earth are enormous and critics say it remains unclear whether the technology will work and eventually be commercially viable.

($1 = 0.8964 euros)