During the first refueling outage at the Grand Gulf Nuclear Station in Mississippi, the plant’s owner contracted Westinghouse to come in and perform work on the main turbine. The main turbine is a massive metal object consisting of metal fan-shaped blades up to 12-feet long attached to a metal shaft as thick as a large tree trunk. During operation, steam produced in the reactor spins the turbine at 1800 revolutions per minute or 30 spins every second. When the plant is not operating, an electric motor spins the long turbine shaft at low speed to prevent it from bowing and warping as the metal cools down.
Some of Westinghouse’s work required the turbine to be stopped. The turbine shaft can be stopped, but only for short periods of time. Westinghouse personnel pre-staged their equipment and tools to be ready to go as soon as the turbine shaft stopped rolling. Like the finely rehearsed team, the Westinghouse crew worked rapidly. Their tasks were quickly completed and the turbine reassembled within the short time window.
An operator confirmed that Westinghouse was done and hit the pushbutton to restart the electric motor that spins the turbine shaft. Nothing happened. The operator rushed a maintenance crew out to fix the electric motor. They found the motor to be in perfect order and running, but the turbine shaft was not moving.
With the clock running out of time, the operator called out several equipment operators. In case the electric motor fails, the turbine shaft can be manually rolled using long levers and lots of muscle. This time, however, four or five husky equipment operators failed to budge the shaft.
The problem was escalated to upper management. Management ordered the turbine building’s overhead crane to be connected to the turbine shaft and used to roll it. That unusual connection was made within minutes and, sure enough, the turbine shaft was made to spin slowly.
As the shaft rolled, it made loud noises. Closer examination revealed deep scratches on the turbine shaft. The use of the overhead crane was immediately discontinued. A turbine bearing compartment was opened. Workers found a large socket, about ten inches in diameter and six inches long, wedged between the bearing’s surface and the 30-inch diameter turbine shaft. The workers had accidentally left the socket behind after completing their work on the turbine. It acted like a door stop, preventing the electric motor and the equipment operators from rolling the turbine shaft.
The turbine shaft had to be removed, placed on a special lathe, and re-machined to smooth out the gouges caused by the socket or, rather, by the unusual decision to use the overhead crane to make the turbine shaft roll.
In this case, a sincere effort to protect the turbine shaft from damage actually damaged it. It was well-known that the turbine shaft cannot remain motionless for long before being damaged by warping and sagging. But the decision to force it to move ultimately damaged the shaft.
This case is yet another reminder that in order to solve a problem, its cause must first be known. The turbine shaft would not spin using either the normal electric motor or the standard backup practice of manual rotation. Before figuring out why those two reliable methods failed, management opted for Plan C. Hooking up the stubborn shaft to the overhead crane did indeed force it to spin, but also inflicted damage the step sought to prevent.
“Fission Stories” is a weekly feature by Dave Lochbaum. For more information on nuclear power safety, see the nuclear safety section of UCS’s website and our interactive map, the Nuclear Power Information Tracker.