Full production has resumed at the Metropolis uranium conversion plant in Illinois after it was closed earlier this month following a uranium hexafluoride (UF6) leak. Metropolis, the only operating uranium conversion plant in the USA, is owned and operated by Honeywell. The plant converts uranium oxide powder into gaseous UF6 suitable for enrichment, which is marketed by Converdyn, a joint venture of Honeywell and General Atomics.

Honeywell said that it had completed its root-cause investigation of the leak, which was caused by over-pressurisation of a system that was undergoing maintenance. The results were submitted to the US Nuclear Regulatory Commission (NRC) which has reviewed the immediate corrective actions that were taken to prevent such an incident occurring in future, a company statement said.

The leak resulted in the release of 12.04 lbs of UF6, which converted to 2.41 lbs of hydrofluoric acid when it was exposed to air. The state of Illinois only requires plants to report releases over 100 pounds of hydrofluoric acid. Honeywell said the company is "committed to the safe operation of the plant" and continues to "review the incident and make improvements to the operations and its response as needed."

There have been three UF6 leaks at the plant since 2003, including one last October, caused by a mechanical equipment failure. The NRC determined Honeywell did not classify the October leak properly. Plant personnel classified the leak as a plant emergency rather than a more serious plant alert, which requires the company to notify the NRC of the incident sooner.Production resumed in mid-November after the NRC approved the steps taken by the company to enhance emergency procedures and drills.

A scheduled, extended three-month maintenance shutdown took place earlier this year. UF6 production was restarted in April, but targets have been aligned with reduced customer demand. Honeywell has said it plans to invest $17.5m in capital and safety improvements at the plant during 2015, including a new liquid hydrogen storage facility enabling it to eliminate the use of anhydrous ammonia on site.