The incident, which Endesa labeled "minor," happened last November at a nuclear power plant in Asco, northwestern Spain, but wasn't detected until last month. Spanish media reports Monday said officials from the country's nuclear watchdog, CSNhad, found that the levels of radioactivity meant Asco was the fourth-gravest incident connected to nuclear energy in Spain.

CSN said in a statement it rated the leak as 2 on the International Nuclear Event Scale because of "inadequate control of nuclear material and for supplying incomplete and deficient information to the regulator." The watchdog is considering fining Endesa.

CSN also ordered medical tests for some 800 people working at the plant. Some 580 people have already been screened, without the tests having revealed health issues, the Spanish media said.

As a direct response to the affair, Endesa Wednesday sacked both the head of the plant and its top radiological protection official.