Since 1986, when he gave the details of Israel's secretive nuclear weapons program to a British newspaper, Mordechai Vanunu has been a household name here--and a strangely spectral presence.

He has been demonized by the government as an enemy of Israeli security and lionized by some on the left as a brave whistle-blower and a man of principle. Practically everyone here knows the sensational story of how he was lured from England by a female Mossad agent, kidnapped and drugged by Israeli operatives in Rome and tried and convicted in Israel of espionage.

Despite his celebrity, Vanunu has been all but invisible. Having been tried in secret and sentenced to 18 years in prison, he has long been kept in solitary confinement and barred from giving interviews and making phone calls. His muffled words are heard only occasionally when his letters clear the prison censors.

So when the newspaper Yedioth Ahronoth published 11 pages of previously undisclosed court testimony and articles based on Vanunu's 1987 trial today, the impact here was sensational, even if the material was somewhat threadbare.

Yedioth, Israel's best-selling daily, published extracts of 1,200 pages of court documents with the state prosecutor's approval and the security establishment's acquiescence. Heavily edited by censors, the material contained few revelations and virtually nothing about Israel's nuclear arsenal that Vanunu had not already described for the Sunday Times of London 13 years ago. The published extracts deal mostly with the feelings and personal observations of Vanunu and the other main characters in the drama, including then-prime minister Shimon Peres.

But the fact that Israeli authorities, perhaps wishing to avoid a court battle, were willing to release so much material on Vanunu suggested a slight mellowing in their attitude toward what many consider the most serious security breach, by the most notorious spy, in the Jewish state's history.

"This by itself is a step forward toward lifting the policy of hush-hush" regarding Vanunu's case and Israel's nuclear arsenal, said Gideon Spiro, an Israeli anti-nuclear activist who has long been a champion of Vanunu's unpopular cause.

Although Israel's possession of nuclear weapons and the means to deliver them have been an open secret, particularly since Vanunu's revelations, the country has never acknowledged it openly. Successive governments have steadfastly maintained a policy of studied ambiguity, saying only that Israel will not be the first to introduce nuclear weapons into the Middle East.

Born to a Jewish family in Morocco, Vanunu immigrated to Israel at age 8 with his parents in 1962. He served in the army, attended university in Tel Aviv and in 1976 answered a newspaper advertisement seeking workers for the country's nuclear plant. While working there, he continued his studies and gradually developed friendships with Israeli Arab students.

In the excerpts from his court testimony, Vanunu, now 45, recounts the details of his youth and early career, tracing the path that led him in 1976 to a sensitive job as a technician in the nuclear facility, at Dimona in southern Israel's Negev Desert. He worked at Dimona for nearly 10 years until he was laid off in a budget cutback in 1985.

By that time, security agents were watching Vanunu carefully, wary of his contacts with Arabs, sympathy with Arab causes and left-wing political activities.

Vanunu left Israel and wound up in Sydney, where he converted to Christianity in the Anglican Church and met a journalist who persuaded him to sell his information on Dimona, as well as photographs he had taken in his workplace, to the Sunday Times. The paper flew Vanunu to London, checked his information and photos with experts and splashed the story across its pages on Oct. 5, 1986.

"I wanted to confirm what everyone knows," Vanunu testified that he told his mother, according to the materials published in Yedioth today. "I wanted [Israel's nuclear capacity] to be monitored in an orderly fashion. . . . Now Peres can no longer lie to [President] Reagan and tell him that we have no nuclear weapons. Now everyone knows."

In the Sunday Times, Vanunu described a secret six-level, underground facility at Dimona known as "the Tunnel," a chemical reprocessing plant handling highly radioactive materials. Among nuclear experts, including those at the CIA, the account was explosive. It revealed that Israel's ability to manufacture thermonuclear weapons was far more extensive and sophisticated than had been supposed, and laid bare the lengths to which the country had gone to conceal this from allies and foes alike.

Israeli officials, stunned and infuriated, discussed having Vanunu assassinated but settled on making an example of him. The damage he had done, Israeli officials believed, had alerted Israel's Arab enemies to the Jewish state's capabilities, leading those countries to intensify their own nuclear programs.

"The publication spurred some Arab countries to adopt difficult policies, undesirable for the state of Israel," Peres testified in Vanunu's trial.

Peres, minister without portfolio in the current government, and some other figures denounced the publication of Vanunu's testimony, but other officials saw it as essentially harmless.

"The fact that he claims that Israel has nuclear power is not new," said Dan Meridor, chairman of the defense and foreign affairs committee of the Knesset, Israel's parliament. "This is why he was put to trial and convicted to a long sentence."

CAPTION: Publication of testimony by convicted Israeli spy Mordechai Vanunu, shown in a 1998 photo, has caused a sensation, though it contains few revelations.