"Two Swedish men were arrested ... The prosecutor suspects them of sabotage," Sven-Erik Karlsson of the Kalmar county police told AFP, adding that both men were contractors who had been working at the Oskarshamn nuclear plant "for some time."
One of the contractors was arrested on Wednesday when traces of highly explosive material were found on him as he was about to enter a nuclear power plant in southern Sweden, police and the plant said.
"At 8am we received a call from the nuclear plant at Oskarshamn. They told us one worker was stopped in the control. He had explosive material in his bags," Sven-Erik Karlsson of the Kalmar county police told AFP.
The company that operates the Oskarshamn plant, OKG, meanwhile said the man's bags contained "no visible illegal substances" but routine tests at the entrance to the plant "detected traces of explosives."
"We can see that our security routines functioned properly," the managing director of OKG, Lars Thuring, said in a statement, adding that the plant was collaborating with police.
Karlsson said the man, who was being interrogated by police, was a welder hired for temporary purposes, but could provide no further details on his age nor his background.
"The explosive material has been taken care of by ... police and apparently it is highly explosive, probably TATP," Karlsson said.
TATP is relatively easy to make and has surfaced in a number of recent terrorism investigations, including bombings in the Middle East and the London bombings in July 2005.
It was the same type of explosive that Al-Qaeda "shoe bomber" Richard Reid tried to detonate on a Miami-bound flight in December 2001, three months after the September 11 attacks in New York and Washington that killed some 3,000.
Although the recipe for TATP is complex, its ingredients can be found in simple household goods: sulfuric acid -- found in drain cleaner -- hydrogen peroxide, and acetone, often a constituent of nail polish remover.
The Oskarshamn plant, which is owned by German energy giant EON, has three boiling water reactors, in service since 1972, 1974, and 1985. The three reactors produce about 10 percent of Sweden's electricity, according to the plant.
Two of the three reactors were running as normal on Wednesday, while the third reactor had already been shut down for maintenance, according to the TT news agency.
"Our joint assessment is that the security of the reactors was never threatened," OKG's Thuring said.
"We are however taking all necessary measures to verify this, of course" he added.
Nuclear power accounts for nearly half of all electricity production in Sweden, which has 10 working nuclear reactors.