While drilling a pilot hole for sampling of a Tc-99 cylinder, an unanticipated condition was identified. The Industrial Hygiene Technician monitoring the work activity detected abnormally high levels of hydrogen fluoride (HF) near the drill hole. The crew was performing a sampling activity per an approved Work Package with a Job Specific Hazard Analysis (JHA). During development of the work package JHA a non-conservative assumption was made by the JHA team that the cylinders had been previously vented and would contain no pressure. The result of this assumption was an approved JHA that did not identify or mitigate the hazard of a pressurized cylinder. The engineering control for the cylinder breach was local ventilation with the exhaust duct set up at the cylinder breach point. Exhaust velocity at the duct was measured in excess of 3,000 fpm. Cylinder pressure, though small, was enough to create a stream from the breach point adequate to bypass the local ventilation and release HF into the work area.
During the task pre-evolution brief, actions to vent the cylinder were discussed. The cylinder had a valve connected to the inlet of the cylinder with a tubing stub and plug on the downstream end of the valve. These actions were not identified in the work package. Prior to breaching the cylinder, an attempt was made to remove to tubing plug by hand. The plug could not be removed by hand and the task to breach the cylinder with a drill continued resulting in the uncontrolled HF release.
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