Weapons-grade uranium that for years has been transported north into Canada from the United States is now being quietly returned over the same roads in a more radioactive and potentially dangerous form.

The two biggest threats posed by the transport of the material are the catastrophe that could result from an accident or spill, and the interest that terrorist organizations may have in stealing it for use in weapons of mass destruction.

A confidential federal memo obtained by The Canadian Press through the Access to Information Act says at least one payload of spent, U.S.-origin, highly enriched uranium fuel has already been moved stateside under an agreement signed last year by Prime Minister Stephen Harper and U.S. President Barack Obama.

The deal was part of a broader international project by the Obama administration to consolidate highly enriched uranium at fewer, more secure sites around the world. The U.S. government says it wants to convert the uranium into a form that cannot be used to build nuclear weapons.

For the past two decades, the United States has been sending Canada highly enriched uranium to run the National Research Universal reactor at Chalk River, Ont., which remains one of the world’s top suppliers of nuclear isotopes.

On its trip north from the United States, it is in the form that is most desired by terrorists, said Norm Rubin, the director of nuclear research at Energy Probe, a public policy research institute. “It is totally stealable and bomb-useable on its way up.”

When it has served its purpose, the new agreement requires it to be returned to the United States.

“In the reactor it turns nasty. But nasty is better that nice, in a way because you don’t have to worry as much about thieves stealing the nasty stuff that will kill them immediately as you do about the stuff that you can carry in your hands,” Mr. Rubin said.

On the other hand, he said, there are likely to be terrorist groups that would still want it to make it into “dirty bombs” where the radioactivity, and not the explosion, would do the damage.

And any mishap on the highway with such highly radioactive material could have disastrous results, Mr. Rubin said.

The government says there has never been a significant transport accident involving nuclear materials anywhere in the world, and that such shipments occur regularly in Canada. But Mr. Rubin said that is not true. While Canada has been unscathed, he said, around the world “there have been lost and busted bombs, truck crashes, fires, etc.”

Canada maintains a large inventory of the highly enriched material and the continued shipments back to the United States are scheduled to take place until 2018. They are protected by intense security protocol, which means specifics such as routes, transportation method, quantities and schedules remain top-secret.

Claude Gravelle, the Natural Resources critic for the federal New Democrats, said it’s wrong to leave Canadians in the dark about the hazardous materials that are being transported in and around their communities.

“I can understand that it’s security,” Mr. Gravelle said, “but still they should have some public consultation.”

Requests for comment from the federal government went unanswered Tuesday.

A ministerial memorandum, classified as “Secret” that was obtained by The Canadian Press says the Canadian Nuclear Safety Commission considers it unnecessary to hold public sessions that would allow citizens to ask questions and comment on the shipments.

That same memorandum, dated Feb. 25, 2011, points out that recent hearings for another nuclear-shipment case generated intense public and media interest. The controversy has stalled the project to ship 16 generators from a Bruce Power nuclear plant through the Great Lakes, up the St. Lawrence River and on to Europe.