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KAI RYSSDAL: How much do you suppose it costs to capture the power of the sun? A rhetorical question, I'll grant you. But one that does have an actual answer: $12 billion for an initial downpayment. Today in Paris the U.S. and the European Union, along with Russia, China and Japan, signed a deal to develop and run the first world's nuclear fusion reactor. If it works, it will solve the world's energy crisis in one stroke. If it works. From the Marketplace European Desk in London Stephen Beard reports.

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STEPHEN BEARD: Blink and you'd miss it. That was nuclear fusion at a small nuclear facility outside London. It lasted half a second. The first full-scale fusion plant will attempt a sustained reaction. Jerome Pamela is a member of the multibillion-dollar project launched in Paris today:

JEROME PAMELA: What is at stake is huge because, if we succeed with fusion, we will have an almost inexhaustible source of energy which can be used everywhere in the world.

Fusing atoms rather than splitting them will, it's believed, generate safe and non-polluting energy. But you can hardly call it cheap. So far the world has spent $30 billion on fusion. The reactor in the south of France will cost a further $6 billion and take eight years to build.

There's little hope of it generating much electricity before the middle of the century. And, says Clive Cookson of the Financial Times, there is another problem:

CLIVE COOKSON: There is a risk that it really — for scientific and technical reasons that haven't yet been discovered or understood — there is a risk that it won't work.

The EU will pay half the cost. The U.S. and the other parties will pay 10 percent each. Skeptics say we'd all be better off if they put the money into proven, renewable energy sources like wind, wave and solar power.

In London, this is Stephen Beard for Marketplace.