By Inti Landauro 

PARIS--French nuclear engineering firm Areva SA discovered flawed steel in a crucial part of the nuclear reactor it is building in Flamanville, northern France, the country's nuclear safety regulator said Tuesday.

The announcement is a further blow for Areva as it struggles to stay afloat after reporting four consecutive annual losses caused by bad investment decisions, costly delays in the construction of reactors such as the one at Flamanville and another in Finland.

The company has also been hurt by changing international attitudes toward nuclear power, especially following the 2011 Fukushima disaster in Japan.

Areva will carry out further tests on the reactor this month after finding that steel in its vessel, which is designed to contain the nuclear reactor, showed high carbon content, leading to lower mechanical strength. The vessel has already been installed on the site, according to a spokesman for state-controlled power utility Electricité de France SA, which commissioned the reactor.

The results of those tests will be released in October, French Energy Minister Segolene Royal said.

The French government, which controls more than 85% of Areva, is considering merging parts of the company with its main client, the power utility Electricite de France SA.

Areva is building the Flamanville reactor for EDF. It had already suffered a series of incidents that led the two companies to delay its completion by more than three years to 2017.

EDF recently estimated that its construction costs had skyrocketed to EUR8 billion ($8.7 billion) from an original estimate of EUR3.3 billion.

The French regulator said it would forward the report on the vessel's flaws to countries with similar projects.

Areva is currently building a reactor of the same design in Olkiluoto, Finland. That too is also suffering massive delays and cost overruns.

An Areva spokeswoman said the vessel is installed in the Finland project and that the country's regulator has approved the parts there.

Write to Inti Landauro at inti.landauro@wsj.com

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