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.

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.

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 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.

The job ahead is enormous, complicated by digging out radioactive “hot zones” entombed in concrete during Saddam’s rule, said an IAEA official who spoke on condition of anonymity because the decontamination plan has not yet been publicly announced.

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.