Citing security concerns, Israel's Justice Ministry last month renewed its ban on travel by Vanunu, a former technician at the Dimona atomic reactor who all but blew away the country's cherished nuclear secrecy with a 1986 newspaper interview.

The ban is subject to annual review. Senior Israeli security sources said they expected it to be extended indefinitely.

Israeli officials accuse Vanunu, a 51-year-old Jewish convert to Christianity who has repudiated the Jewish state, of having more military secrets to spill.

He denies it but has won few friends in Israel by pursuing a strident campaign to expose Dimona.

"Vanunu's behavior both vexes and perplexes the security establishment," said Michael Karpin, author of "The Bomb in the Basement," a study of Israel's nuclear capability. "Their thinking is: 'Why should we let him go and hope for the best?"'

Keen to deter foes but eager to avoid an arms race, Israel neither confirms nor denies having the Middle East's only atomic arsenal under a policy of "strategic ambiguity." The monopoly has long aggrieved Arabs and arch-foe Iran, which is now developing its own nuclear program -- for energy, it says.

Police charges were filed against Vanunu last year after he violated restrictions on contacts with foreign journalists.

The indictment quoted him as telling U.S., British, Australian and French media that Israel assembled hydrogen and neutron bombs at Dimona and was annually producing 40 kg (88 pounds) of plutonium, enough to make 10 atom bombs, at the facility.

Vanunu's supporters noted that there was little new here -- a retread of disclosures made to Britain's Sunday Times in the interview for which he was abducted in Rome by Israeli agents and sentenced to 18 years in prison.

But such details may not be what worries Israel now.

Security sources said that, during his nine years at Dimona, Vanunu was exposed to something -- perhaps equipment, technical data, procedures or personnel -- of major significance to Israel, though he remains unaware of its true value.

"Let's just say he doesn't know what he knows, but that an expert debriefer could get it out of him if given the chance," a security source said without elaborating.

Vanunu's lawyer, Michael Sfard, rejected such suggestions.

"This is a claim that, by definition, Mordechai cannot be expected to address fairly," he said.

THE LIMITS OF PRE-EMPTION

Israel's Supreme Court, which has limited the security forces in areas such as interrogations and punitive counter-terrorism measures, upheld the travel ban on Vanunu.

Sfard said that while Israel, like many countries, acts pre-emptively against recidivists -- people repeatedly arrested for criminal behavior -- Vanunu should not be so targeted.

"What sort of democracy limits a man's liberty on the pure assumption that he could break the law again?" he said.

Frank Barnaby, a nuclear proliferation expert who questioned Vanunu about Dimona over a period of several days on behalf of the Sunday Times, said he doubted whether the whistleblower could produce any further information of use.

"I believe he told us everything he knew," Barnaby said by telephone from Britain.

A former senior official from Israel's Mossad intelligence agency said more time would be needed to scour Vanunu's memories of Dimona.

By all accounts, they are extensive. Israel's Justice Ministry cited a scrapbook that Vanunu kept in prison in which he drew extensive sketches of the reactor. He has confirmed its existence but described it as an innocent memory exercise.

"We learned while handling defectors during the Cold War that sometimes it takes many months to fully debrief someone who was exposed to something of value," said the Mossad veteran, who declined to be named due to the sensitivity of the subject.

Barnaby said such thinking should be taken in context.

"So much will have changed at Dimona by now that any knowledge he (Vanunu) has would have become obsolete," he said.

Vanunu, who lives in a church hostel in Jerusalem on handouts from his fans, says that by refusing international inspections Israel risks a nuclear disaster and inflames regional tensions.

He says he went public on Dimona to stop a "second Holocaust." He has also questioned the Jewish state's right to exist.

When Vanunu was freed from jail in 2004, a former Mossad chief expressed concern the whistleblower could invent details about Israel's nuclear capability and fuel calls for it to be curbed.

While some of Vanunu's comments may have bolstered this theory -- in one interview, he alleged that Israel engineered the 1963 assassination of U.S. President John F. Kennedy to stop him probing Dimona -- there is now a question of credibility.

"So few people are interested in hearing what he has to say these days, that even when it comes to Israel's image abroad Vanunu is not much of a threat," Karpin said.