A research center near Tripoli stocks uranium and other material that could be used to make a nuclear "dirty bomb" and Libya's rebels will need to secure it, a former senior UN inspector said on Wednesday.

Seeking to mend ties with the West, Libya's Muammar Gadhafi agreed in 2003 to abandon efforts to acquire nuclear, chemical and biological weapons - a move that brought him in from the cold and helped end decades of isolation.

Olli Heinonen, head of nuclear safeguards inspections worldwide for the UN atomic watchdog until mid-2010 and now at Harvard University, said Libya's uranium enrichment program was subsequently taken apart.

Sensitive material and documentation ranging from nuclear weapons design information to centrifuge components were also confiscated, Heinonen said in an online commentary.

Libya's highly-enriched uranium, which was used to fuel the Tajoura research reactor on Tripoli's outskirts, took longer to remove but the last consignment of spent fuel was flown out of Libya in late 2009.

But "nuclear security concerns still linger", said Heinonen, a former deputy director general of the Vienna-based International Atomic Energy Agency (IAEA).

Tajoura continues to stock large quantities of radioisotopes, radioactive waste and low-enriched uranium fuel after three decades of nuclear research and radioisotope production, he said.

"While we can be thankful that the highly enriched uranium stocks are no longer in Libya, the remaining material in Tajoura could, if it ended up in the wrong hands, be used as ingredients for dirty bombs. The situation at Tajoura today is unclear."

A so-called dirty bomb can combine conventional explosives such as dynamite with radioactive material.

Experts describe the threat of a crude fissile nuclear bomb, which is technically difficult to manufacture and requires hard-to-obtain bomb-grade uranium or plutonium, as a "low
probability, high consequence act" - unlikely but with the potential to cause large-scale harm to life and property.

On the other hand, a "dirty bomb", where conventional explosives are used to disperse radiation from a radioactive source, is a "high probability, low consequence act" with more
potential to terrorize than cause large loss of life.

After the fall of Iraq's Saddam Hussein in 2003, looting of nuclear and radioactive material storage took place at the Tuwaitha nuclear research center near Baghdad, Heinonen said.

"Most likely due to pure luck, the story did not end in a radiological disaster," Heinonen wrote, adding the rebel Transitional National Council would need to be aware of the
material sitting around Tajoura.

Once a transition of power takes place, "it should assure the world that it accepts its responsibility and will take the necessary steps to secure these potentially dangerous
radioactive sources", he said.