The sound of alarms during the evening of Oct. 26 at the Honeywell Metropolis Works alerted workers and those living closest to the Metropolis, Ill., site that something out of the ordinary had occurred.
The plant, a fixture in the southern Illinois river town since 1958, is the nation’s only uranium conversion plant. It converts raw uranium, or yellowcake, into uranium hexafluoride, or UF6, which is enriched at other facilities into fuel for commercial nuclear power plants.
That Sunday evening, Honeywell experienced a leak of UF6. The leak occurred in a cold trap inside the Feed Materials Building. (A cold trap is a large tank where raw UF6 accumulates so it can be cooled and solidified and later heated and drained during normal plant operations.) The leak occurred while the cold trap was heated and was being drained.
An operator put on a respirator and confirmed the leak at 7:24 p.m. local time. Plant emergency responders were dispatched to shut down operations and account for all personnel. Honeywell declared a “plant emergency,” but did not declare an “Alert,” the lowest NRC emergency classification for fuel facilities. Honeywell advised the NRC the cold trap was isolated, vacuum devices were being used to collect leakage, and the material in the device was cooling by 8:15 p.m.
No one was injured and Honeywell declared “all clear” status at 2:16 a.m. Monday.
People outside the plant reported that a cloud was visible, coming from the building even before mitigation spray towers were activated. Those towers generate gigantic streams of water and water vapor into the air inside and outside the plant, and contributed to clouds that were seen after they were activated.
The NRC quickly dispatched a senior fuel facilities inspector to Honeywell to independently assess what had occurred and how Honeywell had responded. Another inspector later travelled to the facility to gather more information.
After reviewing records, interviewing Honeywell employees and examining the affected areas, the NRC has reached a number of preliminary conclusions:
- The leaking UF6 vaporized and interacted with moisture inside the Feed Building, which converted it into UO2F2 (a solid form of uranyl fluoride, a yellow powder).
- The uranyl fluoride was contained within the Feed Building and settled within a few feet of the cold trap leak.
- The chemical conversion process also produced hydrogen fluoride gas, some of which was visible emanating from the building.
- The mitigating sprays outside the building were aimed at the windows in an effort to keep the hydrogen fluoride from getting offsite.
- Honeywell implemented their emergency plan, assessing the event and taking the actions spelled out in the plan, including stopping the leak.
- A potential violation was identified related to the emergency classification of the event and remains under agency review.
- The hydrogen fluoride gas that left the building had no health effects for workers or nearby residents.
Honeywell plant management has agreed to the terms of a Confirmatory Action Letter issued by the NRC to not restart the facility until the NRC is satisfied that the company has appropriately addressed the emergency classification issues raised during and after the event. Honeywell has also agreed to a number of corrective actions, including revised training and emergency procedures. The NRC is monitoring an emergency exercise at Honeywell to ensure that corrective measures are in place.
The NRC will remain vigilant and will closely inspect the corrective actions made by Honeywell this week under the CAL. As well, the NRC will consider potential enforcement actions stemming from the event.
Update: You can read the latest Preliminary Notification here.