Plant officials, however, where quick to downplay any potential danger and insist that the welder’s bag only had traces of explosives on it’s handle.

Two people are in custody, Kalmar county police spokesman Sven-Erik Karlsson said in an interview Wednesday. One of the people, a male contract welder, was caught in a security check at the plant after traces of explosives were found on a bag the welder was bringing with him to work, according to Oskarshamn plant operator OKG AB.

Karlsson declined to comment on the identity of the other person, also a contract employee, but said they were possibly operating in collusion to carry out an act of sabotage.

“Two Swedish men were arrested (…) The prosecutor suspects them of sabotage," said Karlsson, adding that both men were contractors who had been working at the Oskarshamn nuclear plant "for some time."

Police are investigating the substance, which may have carried traces of triacetone triperoxide, or TATP, according to OKG, Karlsson said. Part of the Oskarshamn plant — which is jointly owned by E.ON AG and Finland’s Fortum — was sealed off, without disrupting operations.

OKG contacted police shortly before 8 a.m. local time after three tests showed traces of a high-explosive substance, Karlsson said. He added the plant had called indicating that the welder had been stopped with a bag containing explosives.

Bomb specialists from Malmoe in southern Sweden have been called in to aid the investigation.

“OKG has told us they think it is TATP, but until our bomb technicians get there and analyze the substance we don’t know for sure,” Karlsson said.

Nils Bøhmer, Bellona’s nuclear physicist said the arrest of the two workers was a siren call for stronger security measures at the Oskarshamm Plant.

“The suspects were taken in a random check – something that shows that the two had the opportunity to bring in explosive materials with them earlier,” he said.

Plant officials play down police reaction
Plant officials and owners, said the detained welder was merely carrying a bag on which traces of explosives were found.

Karlsson said police received a call from the Oskarshamn plant at 7:58 a.m Wednesday morning Stockholm time.

"They told us a welder who was going to perform a job there had been stopped in a random security check. He had been carrying small amounts of the highly explosive material TATP (of triacetone triperoxide)" Karlsson said.

Oskarshamn plant operator OKG played down the incident, saying there was no threat to the safety of the plant, about 250 kilometers south of Stockholm.

Plant spokesman Anders Osterberg even contradicted the police, saying the bag the welder had been carrying contained no explosives, only that traces of an explosive substance had been found on the bag’s handle. Another plant spokesman, Rodger Bergman said the plant received no bomb threat prior to today’s incident.

A spokesman for Oskarshamn co-owner E.ON, Johan Aspegren, backed up Osterburg saying that only traces of explosives had been found on the man.

"What has happened is that a guy, a contractor, this morning came to the security check with a bag on which, or in which, there were traces of explosives," Aspegren said.

Oskarshamn said in a statement on its Web site that it believed the reactor’s safety was never threatened.

Explosives among the most dangerous — and popular
TATP has been linked to the London suicide bombings in 2005, which killed 52 commuters, after traces of the explosive were found in the suspects’ home. It has also been used by suicide bombers in Israel and as a detonator by Richard Reid, the man who hid a bomb in his shoe in a 2001 attempt to down a flight to Miami.

The substance is one of the most sensitive explosives in the world because of its reactivity to heat, impact and friction, according to research group GlobalSecurity.org. Its main ingredients, which include bleach and acetone, can be bought in pharmacies and hardware stores.

Oskarshamn is one of three nuclear plants in Sweden that meet half the country’s power needs. Joint owner Fortum was not immediately available for comment. The Oskarshamn plant itself has three reactors. The Oskarshamn plant, with its three reactors, makes up about 10 percent of Sweden’s energy consumptions.

Other incidents in Sweden’s nuclear industry
In July 2006, Sweden’s Forsmark plant reported a fault and had to be shut down when a back-up power failure led to an emergency shutdown of the reactor. Other plants were also forced to shut down for tests and improvements.

Vattenfall AB, the region’s largest utility, sent home six workers at Forsmark in 2006 because they had used drugs and alcohol. The incidents and the discovery of an inferior “safety culture” prompted Vattenfall to strengthen its nuclear safety policies. Saab AB, a Swedish defense company, is in the process of improving security at Oskarshamn.

While Sweden voted in a 1980 referendum to phase out nuclear power by 2010, a 1995 government investigation found that such a move would be impossible economically and environmentally.

The country instead opted to close down the Barsebaeck plant in southern Sweden and boost output at the other plants. They are likely to run until as 2025, according to the World Nuclear Association.