He said the mishap did not involve any nuclear aspects of the 1,000-megawatt reactor, one of two on the site, which went on line in 2000 following safety upgrades by the US Westinghouse corporation.

The breakdown followed two leaks of radioactive water from the block's cooling system last year, the latest only three weeks ago.

Located just 60 kilometres (40 miles) from the Austrian border, the Temelin plant has been a source of controversy ever since its launch in 2000, with nuclear-free Austria campaigning for its closure.

"The repeated breakdowns are arousing fear among the people who live in the surroundings of Temelin," said Dana Kuchtova, a spokesman for the ecology group Mothers of south Bohemia.