The ITER site: ready for buildings. Credit: ITER

Construction at the site of ITER — the multibillion-euro project to prove controlled nuclear fusion — has been at a standstill since April, Nature has learned.

The stoppage comes as European contributors negotiate how to pay for their share of ITER, a collaboration between Europe, Japan, South Korea, Russia, the United States, China and India. The European Union (EU) is by far the largest participant, providing some 45% of construction costs, including the buildings that will eventually house the giant machine in St Paul lez Durance, in the south of France.

Excavations for the buildings, slated to begin this autumn, will not start until spring 2010 — roughly a year after site preparations were completed.

European officials say that the reasons for the delay are technical rather than political, and that they will be able to meet the 2018 deadline for completing construction. "The project is not on standby," says Catherine Ray, a spokeswoman for research for the European Commission in Brussels.

But some researchers are concerned that the political impasse could push back ITER's start date. "I'm worried that whatever we lose now could delay the project's completion," says Günther Hasinger, scientific director of the Max Planck Institute of Plasma Physics in Garching, Germany.

When completed, the machine will heat and compress hydrogen isotopes until they fuse into helium, releasing energy.

In 2006, ITER was slated to cost around €5 billion (US$7.4 billion) to construct and another €5 billion to operate over a 20-year period. But following an extensive design review, the construction costs are now expected to at least double (see Nature 459, 488; 2009).

Ray says that the EU had budgeted nearly €10 billion for construction, operation and decommissioning over a 35-year period. But now the 27 EU member states, plus Switzerland, must come up with additional commitments to cover the cost increase. Hasinger says that until they provide a plan for funding, construction is unlikely to begin. "The problem in the European situation is that they need the whole commitment for construction before they can award the contracts," he says.

Sources close to the negotiations say that a number of options are being considered. One would be to secure additional commitments from member states. Another would be a promise from the European Investment Bank, headquartered in Kirchberg, Luxembourg, to provide loans for any additional funds needed.

Such a loan scheme would not be unprecedented: in 2002 CERN, Europe's high-energy physics laboratory near Geneva, Switzerland, took out a €300-million loan to pay for construction costs for the Large Hadron Collider particle accelerator.

Officials say that the situation is under control. Although the budget delays could cause problems down the line, "I am not limited by the amount of money for right now," says Didier Gambier, the director of Fusion for Energy in Barcelona, Spain, which oversees the contracts for ITER's buildings.

Gambier says that the decision to delay contracts was purely technical, and driven in part by the fact that the design of ITER is still being finalized. "We still need to have more information from the other parties," he says. Nevertheless, he says, "we are pushing as hard as we can".

ITER is expected to be completed in 2018 and to conduct its first power-producing experiments in 2026.