UPDATE 1-Belgian atomic watchdog says tough to prove Doel 3 safe

* EU energy commissioner expects stress tests completed in Oct

* Stress tests will include regulators' findings on Doel 3 (Adds background, quotes, context)

By Ben Deighton

BRUSSELS, Aug 16 (Reuters) - GDF Suez's Belgian unit, Electrabel, will find it difficult to prove its Doel 3 nuclear reactor is safe enough to operate, Belgian regulator FANC said on Thursday, raising the prospect that the 30-year-old unit could stay shut for good.

Belgium has halted the 1,006 megawatt Doel 3 reactor until at least the end of August after the discovery of suspected cracks in the core tank.

Speaking after a meeting of regulators, FANC Director General Willy De Roovere said it was always hard for a company to meet a requirement to prove a nuclear plant is safe.

He told a news conference Electrabel would have to show that "in a period of the remaining lifetime there is no single risk, there is no risk at all that cracks can go (on to produce leaks)".

A spokeswoman for Electrabel said: "Is it safe or not to continue the production of Doel 3 ... that's what we will have to prove to the FANC."

The component was built by now defunct Dutch company Rotterdamsche Droogdok Maatschappij, which also constructed the core tank for another Belgian unit, Tihange 2, as well as parts for nuclear plants throughout Europe and in the Americas.

Beyond Belgium, Rotterdamsche Droogdok was responsible for two units in Germany that are no longer operating, two in the Netherlands, two in Spain, one in Sweden, two in Switzerland, 10 in the United States and one in Argentina, the Paris-based Nuclear Energy Agency said.

De Roovere said it was doubtful the plant could reopen before the end of September and that regulators would meet again in October to discuss the issue.

STRESS TESTS

The Doel 3 reactor had been scheduled to close in 10 years' time, according to a nuclear exit plan the Belgian government adopted in July.

GDF Suez is expected to trim significantly its 47-year-old nuclear business now that Belgium, the only nation where it operates nuclear plants, is phasing out its reliance on atomic power.

Belgium has long considered a complete exit from nuclear energy, but that will depend on its having enough alternative sources of energy in place.

EU member states are each responsible for determining policy on nuclear power and on the energy mix in general.

However, the European Commission has initiated a series of voluntary stress tests as part of efforts to ensure safety following Japan's Fukushima nuclear disaster.

They were meant to be completed before the Commission's August summer break, but governments have been given extra time for further assessments.

Speaking in Essen, Germany, EU Energy Commissioner Guenther Oettinger said he expected the stress tests to be completed in October and that they would include the assessment from regulators about risks associated with the possible cracks in the Belgian unit.

"In the coming weeks, we expect clear results from the Belgium regulators about possible risks," he said. (Additional reporting by Tom Kaekenhoff in Essen and Barbara Lewis in Brussels; editing by Jane Baird)