The car park of ITER's headquarters in St Paul-lez-Durance, France, is neatly divided by a green metal fence. On one side is the temporary building that houses the ITER Organization, the body overseeing the international fusion-energy project. On the other lies a roughly one-kilometre by half-kilometre earthen platform on which the fusion reactor will be constructed. Yet members of ITER's central team cannot visit it without an escort. The site is under the tight control of the CEA, the French Atomic Energy Commission.

This is the realpolitik of the planet's most expensive science experiment. In 2006, when ITER was approved by the European Union, China, India, Japan, Russia, South Korea and the United States, the members were careful to ensure that their own agencies had strong influence on the project. Through ITER's governing council, domestic nuclear agencies have ultimate control over everything from the appointment of the central organization's senior staff to the project's scope and schedule. The same agencies will be the ones letting contracts to industry and overseeing production of most of the parts for the machine.

The original cost estimate for ITER was €5 billion (US$7 billion) for construction and the same amount again to operate it for 20 years. But costs are rising and the schedule is growing ever more drawn out (see page 488). Educated guesses now put the construction costs at roughly €10 billion.

These guesses might be the closest that anyone will get to knowing ITER's true cost. The national agencies running the project are under no obligation to tell the central organization how much they are paying their industrial contractors for each piece of the reactor. Some countries might disclose the cost of the components they build, but others might well wish to keep their budgets secret — both to protect their nation's industry and to shield themselves from potential embarrassment. As long as the pieces are technically satisfactory, the central team must accept them, no questions asked.

This set-up may be good for the nuclear agencies running ITER, but it is deeply unfair to those who will ultimately pay for the project — around half of the world's taxpayers. These citizens have a right to know how much their countries are paying in the quest for fusion energy. The seven members of ITER should explicitly commit themselves to providing cost estimates for their in-kind contributions to the project. The central organization should collect and review those estimates, and it should then make them available to the public, both individually and as a collective price tag.

If any members of ITER's council are unwilling to provide those data, then politicians should take up the issue. ITER is run by agencies that have to answer to political bodies such as parliaments. Where necessary, these groups should intercede to demand the publication of financial information about ITER.

A full financial disclosure could be painful. It is likely to show that the reactor is costing far more than originally promised and that some countries are paying more than others for the same components. Those revelations could lead the public to ask numerous questions — including whether fusion can deliver affordable electricity on a timescale that anyone could deem germane.

Such questions are difficult to answer, but they must be asked of every energy technology if humanity is to tackle the climate and energy challenges ahead. The only way to decide fusion's role in resolving these challenges is via an honest public debate. And that debate cannot take place without a transparent price tag for the world's first fusion reactor capable of producing net energy.