Sweden's environment minister is to meet with the country's nuclear energy regulator and power plant operators on Wednesday after activists on Tuesday were able to break into two nuclear plants.

"I have asked the authority responsible ... and owners Vattenfall and Eon to come see me (on Wednesday) to explain themselves," Environment Minister Lena Ek told news agency TT.

Sweden's nuclear energy sector faced further embarrassment on Wednesday following reports that four activists remained inside the Ringhals power plant after 16 Greenpeace activists were arrested there on Tuesday.

"We slept here undisturbed. I'm about 75 metres (yards) from Reactor 1," activist Isadora Wronski told Swedish Radio. The activists had been able to log on to social networking site Twitter and had spent the past hour talking to the media, she added.

On Tuesday 67 Greenpeace activists were arrested inside the restricted area surrounding Ringhals on Sweden's west coast and at Forsmark, on the east coast.

"Today Greenpeace activists stress test the Swedish nuclear power plants to alert the public, the nuclear industry and minister Lena Ek on the serious safety deficiencies," the organisation said on its website on Tuesday.

Results of the European Union's stress tests of European nuclear reactors were released last week, showing that immediate safety upgrades costing billions of euros (dollars) are needed in power plants "nearly everywhere" in Europe.

Many of the EU's 132 reactors failed to meet international safety standards, according to the report, which was ordered in the aftermath of Japan's Fukushima disaster in March last year.