Spain's nuclear watchdog agency said Tuesday it is checking about 800 people for contamination after finding that a leak at a nuclear plant in northeast Spain last year was bigger than previously reported.

None of the 579 people scanned so far have shown signs of contamination, the Nuclear Safety Council said.

In a statement on its Web site Tuesday, the agency said a leak at the Asco I nuclear power station on the Ebro River, 44 miles upstream from the Mediterranean Sea, was greater than initially reported by the controlling company.

"The radiological impact derived from the new data is still of very little significance to the population around the plant," the council said. It said it was checking people who had been at the plant since Nov. 28.

The environmental group Greenpeace first reported the leak on April 4. It is believed to have taken place in November.

The council said it would open an investigation and would demand that action be taken against those responsible.

It said it was changing the classification of the leak from Level I to Level 2, on a range of one to seven.

It accused the plant of "inadequate control of radioactive material and of providing incomplete and deficient information to the controlling body."

"From the sequence of events it is deduced that on April 9 the operator knew that the information on total activity spilled was not correct and it was not passed on to the CSN (council)," the statement said.

The council says the plant, operated by the utility company Endesa, now estimates a maximum of 84.95 million becquerels of radioactivity was spilled. The company initially reported 235,000 becquerels.

Greenpeace said radioactivity linked to cobalt, manganese and other elements had been detected on roofs, fences and other places around the Asco plant after the accident with cooling liquid. It said the leak had not been properly cleaned up.

The El Pais newspaper said the accident happened during a procedure to wash pipes through which nuclear fuel bars are passed.

The Ebro is, along with the Duero, Tagus and Guadalquivir, one of Spain's biggest and most important rivers.

Spain has seven operating nuclear plants. The Socialist government says it will let them run until their operating licenses expire, then decommission them.