Washington (CNN) -- After three Canadian employees were exposed to yellowcake uranium last month when a lid blew off a pressurized 55-gallon drum, a uranium mining company has informed U.S. nuclear regulatory officials it has found additional drums possibly susceptible to the same problem, CNN has learned.
Canadian-based Uranium One sent a letter Monday to the Nuclear Regulatory Commission after the NRC demanded the company take action after the June 23 incident.
The NRC and Canadian nuclear officials are investigating drums shipped from a Willow Creek, Wyoming, mining facility operated by Uranium One to an Ontario processing plant where workers opened them, including the one that ejected the powder. The three employees were overcome by a cloud of yellowcake uranium that had unexpectedly become pressurized.
The worker closest to the drum and two others in the area, who were not wearing respirators, were exposed to airborne uranium, according to the NRC.
"Any adverse health effects to the workers would likely be caused by chemical, rather than radiological effects," the NRC said in a letter to Uranium One.
Yellowcake is the byproduct of uranium ore that is mined, crushed and milled until concentrated. It is a key component in manufacturing uranium fuel for nuclear reactors.
NRC officials gave Uranium One until this week to identify whether any other drums had become pressurized during manufacture or shipment.
A letter from Uranium One's president, Donna Wichers, to the NRC, and obtained by CNN, said the company had inspected all remaining drums and found nine that needed to have holes drilled in them to release pressurization. Wichers said six drums appeared to show "very minor" pressurization and have been isolated for further investigation.
"It should be noted that Uranium One personnel inspected all drums prior to shipment ... and no drums were observed to exhibit any obvious signs of pressurization," Wichers said.
Other drums shipped to the Canadian facility containing yellowcake were found to be bulging from internal pressure, the NRC said.
Yellowcake uranium can be highly toxic but does not contain significant radiation, according to the World Nuclear Association. It is transported from mines to conversion plants in drums within normal shipping containers, and the World Nuclear Association said no radiation protection is required to prevent exposure.
The Canadian uranium processor has stopped opening containers of yellowcake from Uranium One's Wyoming facility until officials have determined they are safe. The NRC said that Uranium One has suspended all shipping from its Willow Creek facility until it determines how the drums became pressurized.