Equipment and materials used in uranium casting operations at the Y-12 National Security Complex fell several feet from a furnace in Building 9212 last month after someone accidentally bumped a control lever, a federal safety board said in a recently released report.
The lever controls the movement of what is known as the stack assembly, which includes a crucible, mold, and uranium “charge.”
The loaded stack assembly had recently been cast, and it fell several feet from the body of an induction furnace to the base of the furnace, where loading and unloading occur, the Defense Nuclear Facilities Safety Board said in a report for the week ending April 19.
“Normally, operators watch carefully while the stack assembly is being lowered to ensure that it does not catch on the furnace interior and tip over,” the DNFSB report said. “However, in this instance, the lever that controls the movement of the stack assembly was inadvertently bumped into the down position during the casting run. This caused the stack assembly to be lowered without any operator oversight, catch on the furnace interior, and fall.”
The report said operators gathered and removed the spilled parts under the direction of a criticality safety engineer. While doing so, they noticed that someone had unintentionally moved the control lever for a different furnace to the up position.
“The stack assembly in this furnace had also completed its casting run and, in accordance with criticality safety requirements, operators had installed a cover on the crucible to prevent water intrusion,” the DNFSB report said. “This cover, which is not designed to be present while the stack assembly is in the raised position, damaged the brick lining of the furnace.”
It said production managers are considering corrective actions to prevent unintentional changes to the positions of the levers that control movements of the stack assemblies.
Afterward, criticality safety engineers questioned whether safety evaluations address a scenario in which a loaded stack assembly falls and damages a fluid line, resulting in the entire mass of fissile material collecting in an uncontrolled configuration at the base of the furnace while it is being inundated with hydraulic fluid, the report said.
All induction furnace casting operations were then paused while that question was resolved, although it wasn’t immediately clear Tuesday if the operations are still stopped.
There was a separate issue with casting reported by the DNFSB in an April 12 report.
Site representatives William Linzau and Rory Rauch said operators were running a production microwave for casting in Building 9212 when the “power supply to one of the magnetrons (and, in turn, the system temperature) continued to increase without any operator input.
“The operators attempted to correct the condition (as allowed per procedure) by decreasing the power set point using the touch screen that provides the human/machine interface for the furnace,” the report said. “The power decreased in response to these commands, but would begin to increase again when operator input ceased. The operators eventually removed the power supply to the magnetrons using the emergency stop button and notified their supervisor, who immediately suspended work.”
The shift manager consulted with production management and engineering and gave the operators direction to re-enter the procedure and provided the specific steps needed to place the caster in a safe and stable configuration.
“The responsible instrumentation and control engineer, after recreating this condition in a simulated environment, has preliminarily concluded that the condition resulted from a communication loss between the touch screen and magnetron controller,” the report said.
Y-12 plans to correct this condition and two others that could be affected by the same communication loss before running the production microwave caster again, the report said.
“The production microwave caster contains a credited design feature that prevents the charge from reaching temperatures that could result in a safety consequence to the worker,” it said.