BRUSSELS (Reuters) - An investigation into possible cracks in the core tank of Belgian nuclear power station Doel 3 will close the unit until the end of August at least, Belgian regulator FANC said, casting doubt over the government's plan to keep it open until 2022.

The 1,006 MW Doel 3 reactor, operated by GDF Suez unit Electrabel, is scheduled to close in ten years according to the nuclear exit plan the Belgian government adopted in July.

"FANC will only give a permit for further operation if convincing arguments can be made. The aim is to guarantee safety," the agency said in a statement.

The regulator said the 1,008 MW Tihange 2 reactor in the south of the country would also be closed for inspection in September, with inspections scheduled at the country's other reactors in 2013.

GDF Suez is expected to significantly trim its 47-year old nuclear business now that its only showroom, Belgium, is gradually wrapping up its reliance on the energy form.

Belgium has long considered a complete exit, but the move depends on it having enough alternative sources of energy in time.

(Reporting by Robert-Jan Bartunek; Editing by Mark Potter)