(AP) - The last major remnant of Saddam Hussein's nuclear program - a huge stockpile of concentrated natural uranium - reached a Canadian port Saturday to complete a secret U.S. operation that included a two-week airlift from Baghdad and a ship voyage crossing two oceans.

The removal of 550 metric tons of "yellowcake" - the seed material for higher-grade nuclear enrichment - was a significant step toward closing the books on Saddam's nuclear legacy. It also brought relief to U.S. and Iraqi authorities who had worried the cache would reach insurgents or smugglers crossing to Iran to aid its nuclear ambitions.

What's now left is the final and complicated push to clean up the remaining radioactive debris at the former Tuwaitha nuclear complex about 12 miles south of Baghdad - using teams that include Iraqi experts recently trained in the Chernobyl fallout zone in Ukraine.

"Everyone is very happy to have this safely out of Iraq," said a senior U.S. official who outlined the nearly three-month operation to The Associated Press. The official spoke on condition of anonymity because of the sensitivity of the subject.

While yellowcake alone is not considered potent enough for a so-called "dirty bomb" - a conventional explosive that disperses radioactive material - it could stir widespread panic if incorporated in a blast. Yellowcake also can be enriched for use in reactors and, at higher levels, nuclear weapons using sophisticated equipment.

The Iraqi government sold the yellowcake to a Canadian uranium producer, Cameco Corp., in a transaction the official described as worth "tens of millions of dollars." A Cameco spokesman, Lyle Krahn, declined to discuss the price, but said the yellowcake will be processed at facilities in Ontario for use in energy-producing reactors.

"We are pleased ... that we have taken (the yellowcake) from a volatile region into a stable area to produce clean electricity," he said.

The deal culminated more than a year of intense diplomatic and military initiatives - kept hushed in fear of ambushes or attacks once the convoys were under way: first carrying 3,500 barrels by road to Baghdad, then on 37 military flights to the Indian Ocean atoll of Diego Garcia and finally aboard a U.S.-flagged ship for a 8,500-mile trip to Montreal.

And, in a symbolic way, the mission linked the current attempts to stabilize Iraq with some of the high-profile claims about Saddam's weapons capabilities in the buildup to the 2003 invasion.

Accusations that Saddam had tried to purchase more yellowcake from the African nation of Niger - and an article by a former U.S. ambassador refuting the claims - led to a wide-ranging probe into Washington leaks that reached high into the Bush administration.

Tuwaitha and an adjacent research facility were well known for decades as the centerpiece of Saddam's nuclear efforts.

Israeli warplanes bombed a reactor project at the site in 1981. Later, U.N. inspectors documented and safeguarded the yellowcake, which had been stored in aging drums and containers since before the 1991 Gulf War. There was no evidence of any yellowcake dating from after 1991, the official said.

U.S. and Iraqi forces have guarded the 23,000-acre site - surrounded by huge sand berms - following a wave of looting after Saddam's fall that included villagers toting away yellowcake storage barrels for use as drinking water cisterns.

Yellowcake is obtained by using various solutions to leach out uranium from raw ore and can have a corn meal-like color and consistency. It poses no severe risk if stored and sealed properly. But exposure carries well-documented health concerns associated with heavy metals such as damage to internal organs, experts say.

"The big problem comes with any inhalation of any of the yellowcake dust," said Doug Brugge, a professor of public health issues at the Tufts University School of Medicine.

Moving the yellowcake faced numerous hurdles.

Diplomats and military leaders first weighed the idea of shipping the yellowcake overland to Kuwait's port on the Persian Gulf. Such a route, however, would pass through Iraq's Shiite heartland and within easy range of extremist factions, including some that Washington claims are aided by Iran. The ship also would need to clear the narrow Strait of Hormuz at the mouth of the Gulf, where U.S. and Iranian ships often come in close contact.

Kuwaiti authorities, too, were reluctant to open their borders to the shipment despite top-level lobbying from Washington.

An alternative plan took shape: shipping out the yellowcake on cargo planes.

But the yellowcake still needed a final destination. Iraqi government officials sought buyers on the commercial market, where uranium prices spiked at about $120 per pound last year. It's currently selling for about half that. The Cameco deal was reached earlier this year, the official said.

At that point, U.S.-led crews began removing the yellowcake from the Saddam-era containers - some leaking or weakened by corrosion - and reloading the material into about 3,500 secure barrels.

In April, truck convoys started moving the yellowcake from Tuwaitha to Baghdad's international airport, the official said. Then, for two weeks in May, it was ferried in 37 flights to Diego Garcia, a speck of British territory in the Indian Ocean where the U.S. military maintains a base.

On June 3, an American ship left the island for Montreal, said the official, who declined to give further details about the operation.

The yellowcake wasn't the only dangerous item removed from Tuwaitha.

Earlier this year, the military withdrew four devices for controlled radiation exposure from the former nuclear complex. The lead-enclosed irradiation units, used to decontaminate food and other items, contain elements of high radioactivity that could potentially be used in a weapon, according to the official. Their Ottawa-based manufacturer, MDS Nordion, took them back for free, the official said.

The yellowcake was the last major stockpile from Saddam's nuclear efforts, but years of final cleanup is ahead for Tuwaitha and other smaller sites.

The U.N.'s International Atomic Energy Agency plans to offer technical expertise.

Last month, a team of Iraqi nuclear experts completed training in the Ukrainian ghost town of Pripyat, which once housed the Chernobyl workers before the deadly meltdown in 1986, said an IAEA official who spoke on condition of anonymity because the decontamination plan has not yet been publicly announced.

But the job ahead is enormous, complicated by digging out radioactive "hot zones" entombed in concrete during Saddam's rule, said the IAEA official. Last year, an IAEA safety expert, Dennis Reisenweaver, predicted the cleanup could take "many years."

The yellowcake issue also is one of the many troubling footnotes of the war for Washington.

A CIA officer, Valerie Plame, claimed her identity was leaked to journalists to retaliate against her husband, former Ambassador Joe Wilson, who wrote that he had found no evidence to support assertions that Iraq tried to buy additional yellowcake from Niger.

A federal investigation led to the conviction of I. Lewis "Scooter" Libby, Vice President Dick Cheney's chief of staff, on charges of perjury and obstruction of justice.