The head of Belgium's federal agency for nuclear safety AFCN said on Friday he was "sceptical" that an ageing reactor closed over fears of cracks could be restarted.

"I'm fairly sceptical for the moment," Willy de Roovere told RTBF public radio, even if "the possibility remains that I am wrong."

According to French-language daily Le Soir, a crack of between 15 and 20 millimetres (0.6 and 0.8 inches) was discovered during a test in June. There has been no denial of this report.

According to the agency, repairs are "practically impossible" and are "not an option" for fear of creating new tensions "which we must avoid at all costs."

Installing a replacement meanwhile has never been attempted anywhere because of the problem of high radiation levels.

The AFCN revealed on Wednesday that the Doel 3 reactor, located 25 kilometres (20 miles) north of Antwerp, would remain closed at least until August 31 after the discovery of possible cracks in the protective vessel surrounding the core during routine June testing.

The agency is also mulling the permanent closure "in the worst case" of a second reactor in the country's south near Liege.

The tests showed "faults in the steel base material" on which the reactor vessel is mounted, the AFCN said.

The Dutch firm, Rotterdam Drydocks, that made the vessels is out of business, which has amplified concerns about others it delivered in Europe and in the Americas.

Spain has indicated it has two reactors in the same bracket, Switzerland and Sweden one each.

The firm supplied one to the Netherlands, but had not manufactured it. The government in The Hague said it has still to decide whether to test its nuclear facilities.

The German government said reactors supplied by the defunct company were no longer in service.

Representatives of nuclear safety bodies from all the countries involved will meet in Brussels on August 16 to "exchange information," the AFCN said.