(Adds details on suspects, new background paras 9, 18-20)

STOCKHOLM, May 21 (Reuters) - Swedish police detained two men on suspicion of planning to sabotage a nuclear power station on Wednesday after one of them was discovered entering it with small amounts of a highly explosive material.

"Two men who were taken in for questioning this morning have now been detained on suspicion of preparing for sabotage," said Kalmar County Police spokesman Sven-Erik Karlsson.

Police were alerted shortly before 8 a.m. by the Oskarshamn nuclear power plant on the southeast coast of Sweden. Initially, police only said they were interrogating one man.

"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," Karlsson said.

TATP, or triacetone triperoxide, is extremely unstable, especially when subjected to heat, friction and shock.

The compound can be prepared in a home laboratory from easily available household chemicals. It has been used by suicide bombers in Israel and by Richard Reid, the thwarted British "shoebomber" who attempted to blow up a transatlantic airliner in 2001.

Police did not initially treat the men as criminal suspects.

"They were only being questioned in order to gather information," Karlsson said.

He said both were contract workers and one of them was previously known to police. He had no other details other than the years in which they were born, 1955 and 1962.

SUSPECT BAG

Police sealed off a 300-meter (330-yard) area around the substance and called in explosives technicians from Malmo, the nearest large city.

Oskarshamn, jointly owned by Germany's E.ON

EONG.DE

and Finland's Fortum

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

An E.ON spokesman said the material had been found on or inside the first man's bag. "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," E.ON spokesman Johan Aspegren said.

An official at the plant said the men had been at one of the plant's three reactors, which had been shut for maintenance.

Professor Hans Michels, an explosives expert at Imperial College London, said TATP was mainly used as an initiator or "trigger explosive" to detonate a larger main charge.

He said four men who tried unsuccessfully to set off bombs on London transport in July 2005 had used detonators with 5-10 grams (0.18 to 0.35 oz) of TATP but failed to ignite the main charge of their devices.

Michels said TATP could also be used as a main charge, in which case he estimated that more than 100 grams (3.5 oz) of it would be needed to blow a hole in a heavy structure with an inch (2.5 cm) or more of high-quality steel.

"Normal explosive experts shun (TATP) because it's very unstable, it's dangerous and it's not very pure. It tends to decompose," Michels said.

An experienced British investigator, who asked not to be named, said it was possible for small traces of household products such as hair bleach to trigger positive readings when picked up by explosive-screening devices. Hair bleach commonly contains hydrogen peroxide, an ingredient in TATP.

Oskarshamn is one of three nuclear plants in Sweden that meet half the country's power needs. Sweden's nuclear industry has been hit by a series of mishaps in recent years, prompting the United Nations nuclear watchdog to call for safety measures.

The Swedish nuclear regulator said there has never been an incident involving sabotage of a Swedish nuclear plant, although last year a bomb threat was received at one facility and turned out to be false.