The main threat from uranium hexafluoride, the gas that leaked at a Honeywell plant in Metropolis IL on Sunday night, does not derive from its radioactivity, but from its chemical toxicity, according to studies of people who have been exposed accidentally and animals who have been exposed intentionally.
“The carcinogenic hazard from radiation exposure is negligible compared with the chemical toxicity from acute inhalation exposure to UF6,” as uranium hexafluoride is commonly known, according to a 2004 report on the “Acute Exposure Guideline Levels for Selected Airborne Chemicals” prepared by the National Research Council of the National Academies of Science.
When exposed to water vapor, UF6 breaks down into hydrofluoric acid (HF) and uranyl fluoride (UO2F2), both of which are highly toxic. UF6 emits alpha, beta and gamma radiation, but radiation damage has not been observed in people who have been exposed. Instead:
“At high concentrations, death from HF-induced pulmonary edema is observed. Severe ocular injury; skin burns; and ocular, mucous membrane, and respiratory irritation are also attributable to HF. Kidney damage attributable to UO2F2, was also suggested from urinalysis data.”
This means anyone exposed to high concentrations at the Honeywell Plant on Sunday would already know it, and it tends to support the statement by Honeywell officials that the leak was contained to the plant’s “operations area.” At least one eyewitness report casts some doubt on that.
Honeywell officials say the leak and the noxious haze were contained within the operations area in about a half hour. UF6 is contained by spraying water vapor. The uranyl fluoride, a solid, drops out of the air as particulate matter.
“Uranyl fluoride is a heavy, solid material that immediately falls to the floor near the equipment. The plant has confirmed that none of this material was found beyond the immediate area of the leak,” said Plant Manager Jim Pritchett in a letter to employees.
Plant officials say a haze seen drifting outside the plant—and displayed on a video posted to a Facebook page supporting union workers who are locked out of the plant—consisted of water vapor sprayed in the air as a cautionary measure. Had any UF6 or its derivatives escaped the building, Pritchett said, it would have triggered alarms and the area would have been evacuated.
One alarm can be heard in the video, and the Nuclear Regulatory Commission incident report suggests a different scenario:
“Members of the public outside the plant reported a cloud emanating from the building for five minutes before the mitigation spray towers were activated by Honeywell staff,” according to the NRC report, which also notes that the incident was initially reported to the NRC by a member of the public.
Nonetheless, no injuries have been reported, and there is no evidence to dispute Honeywell’s claim that the leak was contained to its operations area.
The NRC dispatched an inspector to review the event, assess Honeywell’s response, and monitor the recovery and cleanup. The plant remains closed pending the outcome of the investigation.
A uranium hexafluoride leak killed two workers at the Philadelphia Naval Shipyard in 1944 when a weld ruptured on a cylinder containing the gas. The cylinder launched like a rocket, traveling 164 meters, tearing out pipes and equipment in its path, and exposing 20 workers to the gas.
In addition to the two killed, “three people were seriously injured, 12 were hospitalized for observation, and three were without symptoms,” according to the National Research Council report.
“The seriously injured individuals experienced chemical conjunctivitis with edema, chemical erosion of the cornea (resulting in temporary blindness), first-, second-, and third-degree chemical burns, nausea and vomiting, chemical bronchitis, pulmonary edema, and/or shock,” the report states. “The seriously injured workers completely recovered within 3 weeks of the accident.”
A follow-up exam on two of the seriously injured workers found no apparent damage from radiation exposure after 38 years.
Uranium hexafluoride also killed one worker when an overloaded cylinder containing the gas ruptured at a uranium conversion facility in Gore, Oklahoma in 1986. He died within a few hours from pulmonary edema—fluid accumulation in the lungs—after inhaling hydrofluoric acid.
The Metropolis Honeywell plant uses a fluorine chemical process to convert raw uranium into a nuclear fuel precursor.