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 .
Three Canadian workers are exposed to yellowcake when drum lid blows off . NRC has found other drums may have same pressure problem . Problem creates `` chemical , rather than radiological effects , '' NRC says .