The country has a four-stage security risk scale for the plants, with four representing the highest security alert.
Ringhals was rated at one — situation "normal" — but after the incident, the nuclear authority said it raised the security alert by a notch to 2, on a four-grade scale, meaning that security has been increased at all the plants.
In 1980, the Nordic country decided to phase out the use of nuclear energy after Swedes voted for that in a referendum. But two years ago, the center-right government overturned that decision, citing the lack of viable long-term environmentally friendly alternatives, a move that paved the way for old reactors to be replaced by new ones.
Ringhals is Sweden's largest power plant, producing 28 Terawatt-Hours a year, or supplying around 20 percent of the country's electricity.
Associated Press writer Matti Huuhtanen in Helsinki, Finland, contributed to this report.