Tuesday, September 19, 2017

Avoiding Costly Damage from ‘Foreign Objects’

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EPRI Model to Help Nuclear Plant Operators Determine Best Course of Action

On first glance, it may not seem like a big deal: A bolt, gasket, or other small part gets stuck in a steam generator tube bundle in a nuclear power plant.

But decades of operating experience have shown that such foreign objects can lead to costly damage. When subjected to a rapid water flow, an object’s repeated motion can wear a tube’s surface, causing a leak. Because steam generator tubes serve as a pressure boundary between the plant’s primary (radioactive) and secondary (nonradioactive) water systems, a leak would result in radioactive contamination of secondary water. At a prescribed rate of leaking, operators must shut down the plant to fix it, possibly costing $500,000 to $1 million for maintenance and lost production revenue. Each year, the U.S. nuclear fleet plugs 50 tubes on average to address such damage, with eight tube leaks documented since 2000.

EPRI is developing a model to help operators predict tube wear rates related to foreign objects and determine if immediate removal is needed or if it’s reasonable to defer the task to a refueling outage. The model’s predictions are based on the object’s location and size, the tube’s position in the steam generator tube bundle, the rate of water or steam flow between tubes, and other factors.

“Validation of the model must demonstrate conservative tube wear predictions so that tube structural integrity is always maintained,” said EPRI Technical Executive Jim Benson.

Foreign objects can enter the steam generator tube bundle during maintenance or as a result of degradation of component materials. During scheduled outages, inspectors look for foreign objects, either by guiding cameras between tubes or by passing eddy current probes inside tubes. Foreign objects vary in shape and size and often require tools and removal methods tailored to the situation.

In developing the model, EPRI and Polytechnique Montreal used a 3-D printer to recreate more than a hundred foreign objects based on objects found in nuclear plants. Researchers printed versions of these objects with various dimensions and, in the laboratory, placed them in an experimental tube array and subjected them to water flowing at various rates. From this, they determined the force that the objects imparted on the tubes.

“We used the results to obtain the parameters for calculating wear rates,” said Benson. “This formed the basis of our model.”

EPRI has asked nuclear operators in the United States for size, shape, location, and other data on foreign objects identified at their plants. EPRI will use these data to validate the model, using at least 20 objects to compare the predicted wear rates and those observed at the plants based on eddy current inspection.

If the model is proved accurate, EPRI plans to develop software that operators can use to predict wear rates as well as potential impacts related to foreign objects.

This video shows the changes in water velocity and direction as it flows past a foreign object in a steam generator tube bundle. Red and yellow indicate higher velocities, and green and blue represent lower velocities.

Key EPRI Technical Experts:

Jim Benson
For more information, contact techexpert@eprijournal.com.