(Adds details about investigation findings so far)

By Johan Ahlander

STOCKHOLM, May 22 (Reuters) - Swedish police said on Thursday they had released two men who had been held on suspicion of planning to sabotage a nuclear power station.

Police detained two contract workers on Wednesday after one of them was stopped in a security check at the Oskarshamn nuclear plant, on the southeast coast of Sweden, with traces of a highly explosive material thought to be TATP.

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

Police issued a brief statement saying the two would remain under suspicion until after technical experts had concluded an investigation.

"Both men have been cooperative but they deny any wrongdoing and waived the right to legal counsel," the statement said. "There was no legal ground to hold them any longer."

Police were alerted early on Wednesday by the Oskarshamn plant. Oskarshamn is jointly owned by Germany's E.ON <EONG.DE> and Finland's Fortum <FUM1V.HE>.

Authorities sealed off a 300-metre (330-yard) area, called in explosives experts and worked with plant officials to determine any security risks through the day.

Plant authorities have said investigators had not found evidence of additional explosive materials beyond what was discovered on Wednesday.

TATP 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 British "shoebomber" who attempted to blow up a transatlantic airliner in 2001.

The Oskarshamn plant has three nuclear reactors, one of which was shut for maintenance when the alarm was first raised. Work has now resumed, a plant spokesman said.

Authorities found that the two workers also had access to a second reactor. That reactor has been shut as a precautionary measure pending a more thorough investigation and is expected to be restarted on Saturday.

Police said one of the two men, born in 1955, was previously convicted of a minor crime. They have said the other was born in 1962 but have given no other details about their identities.

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 has 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. (Editing by Elizabeth Piper)

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