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, said Thursday during a presentation in Ottawa that two connectors on the crane let go and the container then swung down and snapped off.
"What happened is two of the lifting points on one side ... had not been properly anchored so all of 18,000 kilograms of weight was being supported by only one side," Regimbald told the commission.
"Due to this tremendous weight ... the side eventually sheared off as the flatrack swung downward in a pendulum motion."
The container plummeted about seven metres into the cargo hold with the cylinders still bolted to it.
Calvin Whidden, the vice-president of Cerescorp, the operator of the Fairview Cove terminal, said the company is still investigating what went wrong.
Whidden said Cerescorp has ruled out a problem with the crane itself, leaving open the possibility that the container's attachment points were deformed or the crane operator made an error.
He said the company is consulting with engineers to see how changes to the attachment systems might prevent a similar incident in the future.
"We're going to look at some additional modification to the crane to prevent what took place here," he said.
"There are different engineering solutions we haven't finalized."
A spokesman for Transport Canada said in an email that it is also looking into what happened.
"A complete review of the incident is presently underway for compliance with regulations regarding the loading and securing of dangerous goods," said Steve Bone.
The commission has said there were no spills or radiation leakage when the cylinders containing granular uranium hexafluoride fell.
Regimbald said the regulatory system worked as expected because the cylinders met standards for packaging and withstood the fall.
He added that the cylinders are certified to withstand a fall of nine metres.
Commissioner Dan Tolgyesi said during the hearing he believes the commission should review how often cylinders containing uranium hexafluoride are hoisted more than nine metres.
"I think we should look at that," he said.
The four cylinders were being transported from an enrichment facility in the United Kingdom to their destination in Columbia, S.C., by the shipping company RSB Logistics of Saskatoon. At the time of the incident the container was about to be transferred to a train to complete its journey.
Uranium hexafluoride is a chemical compound used in the gas centrifuge process to enrich uranium.
Michael Rinker, a director at the commission, said the material isn't flammable at normal temperatures but if it gets wet it can become extremely corrosive.
"If a container were to break, it could be cleaned up," Rinker said. "If it gets wet, it's a little more difficult."
Commission member Sandy McEwan said he was concerned that an expert from the commission didn't arrive in Halifax until the afternoon after the incident was reported.
Regimbald said the situation was under control and there were no safety risks, and the first flight available to Halifax was at 1 p.m. on the day after the incident.