Analysis and Support for Nuclear Steam Generators

In pressurized-water-reactor plants, some of the most critical components are the large shell-and-tube heat exchangers that use the heat generated in the reactor to produce the steam that drives the turbines. Complex assemblies that are up to 70 feet tall and weigh hundreds of tons, steam generators have been subject to a variety of reliability problems since their initial use more than 30 years ago. These problems include widespread corrosion of the tubing material (a nickel-iron-chromium alloy) and other internal structures, decreased heat-transfer performance levels, and thermal-hydraulic instabilities—difficulties that have caused costly plant shutdowns and losses in revenue due to reduced power levels.

DEI has been actively supporting utilities in managing their steam generators since its founding in 1980, and currently provides analysis and expert consulting in a variety of areas, including:

Statistical analysis of tube degradation trends
Evaluation of non-destructive test results (eddy-current testing) and development of alternate tube repair criteria
Laboratory analysis of corrosion deposit samples (chemical and physical property testing)
Evaluation of thermal performance
Expert consulting on water chemistry practices
Development and application of chemical and mechanical cleaning techniques
Economic assessment of strategic options

Evaluation of Steam Generator Thermal Performance
Over the past ten years, an increasing number of steam generators have begun to experience degradation in thermal performance due to an assortment of causes, including corrosion deposit fouling and instrument error among others. This degradation can result in costly reductions in the electrical generating capacity of the plant.

DEI has developed an in-depth approach to analyzing decreased steam generator thermal performance. This program includes fouling factor calculations based on plant instrument data, detailed uncertainty analysis, and evaluation of the thermal properties of corrosion deposit layers using measured physical and chemical properties measured in our laboratory. The ultimate goal is an understanding of the root cause(s) of reduced thermal performance levels at each individual plant. As of 2003, more than 25 plants have been analyzed using this methodology.

Primary and Secondary Side Water Chemistry
DEI helps utilities resolve complex primary and secondary side water-chemistry problems. Several nuclear utilities have utilized our expertise in assessing the benefits and disadvantages of various water chemistry options, such as use of on-line boric acid and high hydrazine in the secondary water loop. Because of our comprehensive knowledge of steam-generator degradation processes, we are able to quantify the potential benefits of the options while also identifying the potential risks associated with water chemistry changes.

DEI has also helped the industry prepare primary and secondary side water chemistry guidelines through the Electric Power Research Institute (EPRI).

Assessment of Strategic Options
Drawing on DEI's expertise in a variety of steam generator areas, we have developed a program for analyzing the critical technical inputs that impact steam generator reliability and efficiency—and combining the results with an economic evaluation that systematically assesses different strategic options. The technical inputs include detailed thermal performance evaluations, projection of future heat-transfer fouling rates, and statistical prediction of tube degradation. The results are used to perform quantitative evaluation of numerous candidate strategies, including chemical and mechanical cleaning processes, changes in operating temperature targets, power uprates, and replacement of the steam generators with more advanced designs. This type of analysis has been used to assist roughly a dozen different utilities in developing long-term plans for their steam generators.