By Brian Schimmoller, Contributing Editor
The nuclear power industry has added the equivalent of five 1,000 MW reactors over the past 30 years through power uprates at existing plants. While most have proceeded uneventfully, a few “extended” power uprates (EPUs) at boiling water reactors have encountered structural stability issues with the steam dryer. As a result, the U.S. Nuclear Regulatory Commission (NRC) is exercising increased scrutiny when licensees apply for EPUs.
The steam dryer is a non-safety-related component that removes moisture from the steam before it passes to the steam turbine. Drier steam results in increased power production and reduced moisture carryover minimizes the opportunities for component corrosion. EPUs increase the amount of steam flowing from the reactor to the steam turbine, resulting in higher flow velocities and potentially leading to flow-induced vibration of components in the flow path. “In some cases, the vibration can result in stresses high enough to initiate high-cycle fatigue cracking,” said Kevin Walsh, senior vice president, nuclear services, for GE Hitachi Nuclear Energy.
This is what happened at Exelon’s Quad Cities nuclear power station in Illinois following a 17.8 percent power uprate. Quad Cities Unit 2 had to shut down twice–in June 2002 and June 2003–to diagnose and repair the steam dryer. In October 2003, Quad Cities Unit 1 experienced similar problems and had to shut down for repairs. The cracking progressed to the point where pieces of the steam dryer hood separated from the steam dryer and entered the steam line system.
Exelon’s Dresden plant also discovered steam dryer cracking in 2003. And the steam dryer at Entergy’s Vermont Yankee plant has aroused considerable attention due to crack indications discovered during a detailed inspection associated with an EPU approved in 2006.
An extended power uprate, however, does not necessarily equate to high-cycle fatigue; each EPU represents a unique set of circumstances with a unique set of plant effects. At Vermont Yankee, for example, the cracks have been traced to intergranular stress corrosion cracking, not metal fatigue from additional steam flow, according to John Dreyfuss, director of nuclear safety assurance with Entergy Nuclear.
Opposition surrounding the Vermont Yankee uprate highlights the elevated concern around uprates in general and steam dryer issues in particular. While the steam dryer does not perform a safety function, it must maintain its structural integrity during normal operation and transient events. The Quad Cities experience, in which a loose dryer part lodged in a safety component, prompted the NRC to require detailed structural analysis prior to approving an EPU.
“The analysis must show that the predicted stresses under EPU conditions will be below the fatigue initiation threshold and that there is sufficient margin in the analysis predictions to bound the analytical uncertainty,” said Walsh.
The industry’s steam dryer repair experience indicates that there is little margin available to accommodate the increased EPU loads and to bound the analytical uncertainty. Demonstrating that existing steam dryers are structurally sound for EPU operation, therefore, will be difficult. All plants that have performed EPU uprates since Quad Cities have either modified or replaced the steam dryer to provide this additional structural capacity and margin, according to Walsh.
NRC promotes a conservative and comprehensive approach for resolving steam dryer concerns, but is open to more than one methodology to demonstrate steam dryer structural integrity. GE Hitachi Nuclear Energy has developed a refined model to calculate loads and stresses on steam dryer components and to make accurate margin predictions at EPU conditions.
The NRC guidance demands a detailed data collection and evaluation effort, according to Walsh. The first step is to obtain plant-specific operating data, which typically requires instrumentation installation during a plant shutdown. Data is then collected and analyzed for about six months, after which the plant operator decides how to proceed: no action required, modify the existing steam dryer, replace the steam dryer or reduce the steam load to the dryer. Steam dryer replacement represents the longest and costliest option, requiring two outages to implement: the first for instrument installation and the second for dryer installation. Replacement provides the greatest certainty in operation and regulatory approval, however, and can be considered an investment in long-term operation during the license renewal process.
Based on industry experience and vendor input, Vermont Yankee implemented pre-emptive strengthening modifications to the steam dryer prior to operation at uprated power levels, according to Entergy’s Dreyfuss. Also as a condition of its EPU license, Vermont Yankee will perform detailed visual examination for three successive outages and operators will closely monitor plant parameters such as moisture carryover into the steam lines, reactor power and level to identify unexplained changes that could be indicative of dryer performance problems.
With capital costs for new capacity at all-time highs, the ability to “mine” existing assets for more capacity is an increasingly attractive option. While the industry has already done much mining, a substantial amount of untapped resource remains. The NRC web site lists 17 approved EPUs to date. Four applications are under review and 17 more are expected. As such, steam dryer issues will remain a focus for some time.