Belgium's nuclear watchdog is mulling the permanent closure of two of the country's seven reactors producing electricity due to safety concerns, according to an internal document seen by AFP Wednesday.

The move comes after possible cracks were detected in the Doel 3 reactor after it was shut down in June for routine checks. The vessel of the second reactor, Tihange 2, was produced by the same manufacturer, the document said.

Electrabel, a subsidiary of France's GDF-Suez, operates the nuclear plants.

The AFCN, Belgium's federal agency for nuclear control, said the Doel 3 reactor, located 25 kilometres (20 miles) north of Antwerp, would be closed at least until August 31.

It would only be restarted if further checks cleared it convincingly.

The agency's director general Willy de Roovere said the vessel of the second reactor, situated in the country's south near Liege, was also produced by Dutch firm Rotterdam Drydocks and could also be closed "in the worst case".

The company, which is no longer in existence, also manufactured 21 vessels for nuclear units worldwide, he said.

Europe has been reviewing nuclear energy after last year's accident in Fukushima in Japan. Belgium recently said it would exit nuclear energy in phases between 2016 and 2025, with Doel 3 originally set to shut for good in 2022.