Spain's Nuclear Safety Council (CSN) criticized operators at the Asco I plant for failing to tell it about contamination from a leak which happened last November until April this year.
"I believe citizens have a right to be fully informed," Isabel Mellado, head of nuclear safety at the CSN, told an energy conference in Barcelona.
"So I think this could lead to measures taken in the safety of the installations themselves and in organizations," she added.
Plant operator Endesa said it had replaced the director of the power station and it's head of radiological protection.
Particles of radioactive materials including cobalt-60 were found outside the 1,000 MW reactor the northeast port of Tarragona after a spillage of contaminated cooling water during refueling in November.
The CSN is mulling sanctions against the plant operators but has said that the amount of radioactivity released was within legal limits. The watchdog said on Monday that none of 579 people examined, out of an estimated 700-800 who had passed through the plant since the leak, had been contaminated.
Environmental group Greenpeace has asked for the plant to be closed and protested that a school trip to Asco was allowed to go ahead after the leak. Greenpeace first made the leak public in April and it was confirmed shortly afterwards by the CSN, which sent inspectors to the plant.
News of the leak comes at a delicate time from Spain's nuclear industry. The recently re-elected Socialist government has pledged to phase out the country's eight nuclear plants and get more energy from renewable sources.
Spain is already a leading producer of wind power and solar energy and wind has met up to 24 percent of the country's demand for electricity.
Operating permits for seven of Spain's nuclear plants are due to expire between 2009 and 2011.
Together, Spain's nuclear plants produce about 7,500 MW of power, or some 10 percent of the country's installed capacity. They account for about 20 percent of output, however, as they work steadily through the year if there are no production problems or refueling outages, while wind and hydroelectric power depend on changing weather conditions.