French utility GDF Suez SA (GSZ.FR) expects an extension of the operating lives of its three oldest nuclear reactors in Belgium to cost the company EUR1 billion, the company's chief executive said Wednesday.

However, CEO Gerard Mestrallet said on the sidelines of an energy conference that his company hasn't yet made a final decision if it will extend the reactors' lives, adding that this depends on whether the government will go through with the plan to increase a nuclear tax.