Three Cameco workers in Ontario were exposed to airborne uranium dust in an incident at the Saskatchewan company's Blind River refinery last month, federal regulators say.

The exposure happened June 23 when a worker loosened a ring clamp on a 208-litre drum of uranium oxide yellowcake. The lid blew off and about 26 kilograms of the material were ejected into the air.

The worker closest to the drum and two others in the area, who were not wearing respirators, were exposed to the dust.

The drum of yellowcake came from Uranium One's Willow Creek facility in Wyoming.

According to the U.S. government, several other Uranium One drums that had been shipped to Blind River were found to be bulging from internal pressure.

It says Cameco has stopped opening containers of yellowcake from the Willow Creek facility until they can develop a plan to safely do so.

Uranium One is investigating how the drums became pressurized.

The Canadian Nuclear Safety Commission said it wants Cameco's confirmation that appropriate interim corrective measures are taken, including mandatory use of respirators by workers. It also said it wants permanent measures taken to prevent a recurrence.

Cameco says it has has run tests, including urinalysis, on all three workers and the results suggest none of them breathed in a significant amount of uranium. Decontamination procedures were taken immediately after the accident, the company said.

"Obviously, we open thousands of drums like this from around the world at Blind River, and we've never had an incident like this before," Cameco spokesman Gord Struthers said.

According to the U.S. federal regulator, any adverse health effects to the workers would likely be caused by chemical rather than radiological effects.