May 21 (Bloomberg) -- Swedish police detained two people for questioning on suspicion of planning sabotage against the Oskarshamn nuclear power plant in the country's southeast.

Two people are in custody, Kalmar county police spokesman Sven-Erik Karlsson said today in a telephone interview. A male contract welder was caught in a security check after traces of explosives were found on a carrying bag, said operator OKG AB, controlled by Germany's E.ON AG. The other person is also a contract employee at the plant, Karlsson said.

One of the two, who are both men, one aged 44 and the other 55, is already known to police, Swedish television program Rapport reported tonight, citing police officials. They are colleagues from the same welding company and had been contracted by OKG to carry out isolation work at one of three reactors at the site, Rapport said, citing Karlsson.

The reactor where they worked, which has been closed for maintenance, is being searched by bomb technicians today. Karlsson declined to tell Rapport why one of the men is known to police.

``As an extra safety measure because of the incident in the check for explosives in the entrance of Oskarshamn-1 and 2, a shut down of Oskarshamn-1 is planned,'' E.ON said today in a statement to the Nord Pool ASA exchange. The halt will start at 10 p.m. local time, two hours sooner than announced in an earlier statement, and no duration was given.

Status Update

An update on the status of the 467-megawatt reactor will be given tomorrow, the Nord Pool statement said.

The Oskarshamn plant, located 334 kilometers (207 miles) south of Stockholm, has enough capacity to meet about 10 percent of the largest Nordic country's electricity needs. Today's detentions are the second major incident at a Swedish nuclear plant in as many years after a power failure led to an emergency shut down of the Forsmark reactor in 2006.

Police are investigating the traces, which according to OKG contained triacetone triperoxide, or TATP, Karlsson said today by phone. ``OKG has told us they think it's TATP, but until our bomb technicians get there and analyze the substance we don't know for sure.''

OKG alerted police shortly before 8 a.m. local time after three tests showed traces of a high-explosive substance. Bomb specialists from Malmoe in southern Sweden have been called in to aid the investigation.

Three Reactors

OKG is majority owned by E.ON and part owned by Finland's Fortum Oyj. The plant's three reactors, one of which is offline for maintenance, have a capacity of 2,215 megawatts.

The substance was detected on the handle of a plastic bag the contractor was carrying. There was no bomb threat prior to today's incident, said plant spokesman Roger Bergman.

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.

Sweden has 10 nuclear reactors that account for about half of its generation capacity.

Largest Utility

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 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 to as long as 2025, according to the World Nuclear Association.

To contact the reporters on this story: Janina Pfalzer in Stockholm at jpfalzer@bloomberg.net; Niklas Magnusson in Stockholm at nmagnusson1@bloomberg.net

Last Updated: May 21, 2008 15:37 EDT