Canada's nuclear safety agency says cylinders carrying radioactive material fell during unloading in Halifax because one end of a shipping container they were in was not properly secured to a crane.

Four steel cylinders encased in concrete and each weighing 4.5 tonnes fell into the cargo hold of a ship during unloading at the Cerescorp container terminal on March 13.

Andre Regimbald, the director general of the Canadian Nuclear Safety Commission, says it is not clear yet whether the problem attaching the container to the crane was a human or mechanical error.

The container pivoted downwards from the crane and the bottom of the container plummeted about seven metres into the cargo hold with the cylinders still bolted to it.

The commission has said there were no spills or radiation leakage when the cylinders containing granular uranium hexafluoride fell.

Regimbald, who gave a presentation to the commission in Ottawa on the incident, says the regulatory system worked as expected because the cylinders were properly packaged and withstood the fall.