Areva confirmed that a preliminary investigation, which follows a complaint by the CFE-CGC trade unions, was opened.

"We understand that some employee shareholders express their disappointment," Areva spokeswoman told Reuters.

"Nonetheless, Areva has always complied with the law in terms of financial communication," she added.

Areva launched its first employee shareholding programme for approximately 1.2 percent of the firm's share capital in 2013, according to its annual report. Areva said 36 percent of the employees in France, the United States and Germany participated in this operation.

At the end of 2014, the state-owned group said it would review its funding options and dropped its 2015-16 financial targets, blaming delays to its Olkiluoto reactor project in Finland, the slow restart of Japan's reactors and a lacklustre nuclear market.

"During the period of subscription to the employee shareholding programme in 2013, employees were informed that there was a risk of capital loss, inherent to any purchase and holding of shares," Areva said.

Le Parisien reported that investigators are trying to assess whether Areva management was aware of the upcoming problems as it invited employees to invest in the share purchase plan.

(Reporting by Julien Ponthus and Maya Nikolaeva; Editing by Geert De Clercq)