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  • EPRI | Nuclear Sector Roadmaps August 2016

    IN USE: STEAM GENERATOR DEGRADATION PREDICTION AND MITIGATION

    ISSUE STATEMENT

    Steam generator life-cycle management must balance the implementation of measures to mitigate or repair existing degradation and the application of techniques to reduce the likelihood of future degradation against the associated costs. Recent operating experience demonstrates that current knowledge of relevant degradation mechanisms and existing mitigation options may be inadequate to accurately project and maximize steam generator life: Fatigue failure and primary-to-secondary leaks have

    occurred in three steam generator tubes caused by signifi-cant deposit accumulation on the tube support plates.

    Denting of steam generator tubes at the top of the tubesheet has occurred in at least ten plants, has occurred in as many as 1,500 tubes in a single plant, and has caused stress corrosion cracking and tube repair in at least three plants.

    Stress corrosion cracks are being detected in Alloy 600TT and Alloy 800NG steam generators.

    In plants that have implemented power uprates, deposit accumulation has limited thermal performance.

    This roadmap addresses steam generators with 600MA, 600TT, 690TT, and 800NG tubing.

    DRIVERS

    Operational DriversUnacceptable levels of deposit accumulation can result in stress corrosion cracking, water level oscillations, less than optimal thermal performance, and/or fatigue failure of steam generator tubes. At the plant level, this can lead to an unplanned outage, an unplanned tube plugging campaign, a reduction in power output, reduction in the maximum allowed power level, or primary-to-secondary leakage. These activities can significantly increase worker radiation expo-sure and plant operating costs.

    Asset ManagementAsset management decisions associated with power uprates, license renewal, and life extension should consider the oper-ating life of a steam generator and the possibility of a first and second steam generator replacement. The models used today to assess steam generator management options and mitigation options will influence whether a plant chooses to uprate power or extend the license.

    Regulatory DriversRegulators are concerned that deposit accumulation in steam generators may reach levels that adversely affect steam generator performance prior to implementation of deposit removal activities by utilities. For example, the U.S. Nuclear Regulatory Commission raised this issue in NRC Informa-tion Notice 2007-37. NRC expectations for reviewing infor-mation related to Alloy 600TT cracking were communi-cated in Information Notice 2013-11.

    RESULTS IMPLEMENTAT ION

    Upon completion of this work, it is expected that utility pro-gram managers and steam generator vendors will have robust models and an adequate knowledge base to develop plant-specific steam generator life management strategies that effectively balance the risk of deposit accumulation, stress corrosion cracking, and thermal performance against other costs and benefits and inform long-term reliability decisions. Specific implementation plans include: Developing the basis for a plant-specific strategy for man-

    aging steam generator deposit accumulation using the new report PWR Steam Generator Deposit Removal Strate-gies Sourcebook.

    Predicting thermal performance consequences of deposit formation on the tubes using the updated PWR Steam Generator Deposit Characterization Sourcebook and new steam generator thermal-hydraulic software code.

    Assessing the risk of lead-induced stress corrosion crack-ing and implementing inhibitors using information in a new report, Lead-induced Stress Corrosion Cracking: Mech-anism and Inhibitors.

  • Materials Degradation and Aging August 2016

    PROJECT PLAN

    The project plan is shown in the flowchart. The primary focus is on strategic planning to maximize steam generator life and optimize steam generator performance. The work is divided into four areas:

    Deposit ManagementSecondary side water chemistry in conjunction with deposit removal techniques control the deposit accumulation on steam generator tubes and tube support plates.

    EPRI has updated the Dispersant Application Sourcebook to include lessons learned and recommendations from plants using on-line dispersant application. Dispersants suspend corrosion products in the steam generator bulk water and repel corrosion products away from the tubes/deposits, thereby preventing corrosion products from adhering to the tubes/deposits. Ongoing research and field trials are explor-ing the possibility of using dispersants during outages (steam generator wet lay up) to remove deposits from steam genera-tor tubes and during start up (long-path re-circulation) to prevent corrosion products from reaching steam generators. For additional information on dispersant application, see the roadmap titled Dispersant Application for Fouling Mitigation.

    Crystal habit modifiers can be used to control or tailor deposit properties. Results from preliminary experiments look promising. If specific amines or their degradation prod-ucts are found to significantly affect the crystal habit of cor-rosion product particles, this would allow utilities to engi-neer heat transfer enhancing deposit layers on steam generator tubes and to facilitate removal of the deposit from the tube. The EPRI Steam Generator Management Program and the EPRI Chemistry Program plan to explore the appli-cation of this technology to nuclear power plants.

    Techniques to mitigate denting of the tubes at the top of the tubesheet will be developed based on an assessment of the underlying mechanism. The operational effects of deposit accumulation at tube support plates will be assessed using a new transient analysis method. An online monitoring tech-nique that uses existing plant measurements to estimate the accumulation of deposits on the tube support plates is under development.

    The PWR Steam Generator Deposit Control and Removal Strategies Sourcebook will evaluate the characteristics of deposit samples removed from steam generator tubes, his-torical deposit removal activities, and secondary chemistry that can be used to develop recommendations for an overall deposit control and removal strategy.

    PredictionTwo empirical models are under development to estimate deposit accumulation. One will predict deposit accumula-tion on the tubes in the tubesheet region and relate this to stress corrosion cracking and the second will predict deposit accumulation on the top tube support plate. Generic predic-tions (Pressurized Water Reactor Generic Tube Degradation Predictions) will be developed to estimate the future extent of tube degradation in steam generators based on existing degradation. The results of these two models and the generic predictions are used as input to the overall steam generator life strategy.

    The new steam generator thermal-hydraulics software code (Triton) and deposit formation module will be used to pre-dict the location of deposit accumulation in the steam gen-erator. The new steam generator thermal-hydraulics software code (Triton) will also overcome deficiencies in the current code (ATHOS/SGAP) by removing geometry constraints to enable evaluation of modern and future steam generator designs.

    Thermal PerformanceA plants thermal margin decreases when it implements a power uprate, makes other operational changes, or when deposits accumulate on steam generator tubes. A chapter on tube fouling in the updated Characterization of Pressurized Water Reactor Steam Generator Deposits report will describe how thermal performance may be factored into a steam generator life management strategy. In addition, utili-ties and vendors will be able to predict thermal performance using the new steam generator thermal hydraulics code (Triton).

    Lead-Induced Stress Corrosion Cracking InhibitorsAdvanced alloys such as 800NG and 690TT are signifi-cantly more resistant to stress corrosion cracking than Alloy 600MA. However, stress corrosion cracking occurs in 800NG and 690TT in laboratory test environments con-taining lead. Autoclave testing will be used to develop a fun-damental understanding of the lead-induced stress corrosion cracking mechanism. Subsequently, mitigation options will be developed.

  • EPRI | Nuclear Sector Roadmaps August 2016

    RISKS

    Indications of Alloy 600TT steam generator tubing deg-radation are becoming more frequent. It is unknown whether these indications are anomalies or the start of a long-term trend due to thermally activated mechanisms like those which have affected Alloy 600MA tubing. Research into Alloy 600TT degradation mechanisms should be performed in time to meet industry needs.

    Crystal habit modifiers and film forming amines hold promise to change the characteristics of corrosion product deposition to facilitate removal and to enhance heat trans-fer. These technologies are in an early stage of develop-ment and the expected benefits may not be achieved.

    RECORD OF REVISION

    This record of revision will provide a high level summary of the major changes in the document and identify the Road-map Owner.

    revision description of change0 Original Issue: August 2011

    Roadmap Owner: Heather Feldman1 Revision Issued: December 2011

    Roadmap Owner: Heather Feldman

    Changes: Updated flowchart, added (Deposit) to title

    2 Revision Issued: August 2012 Roadmap Owner: Heather Feldman

    Changes: Updated flowchart to include milestones. Changed title from Steam Generator Life (Deposit) Management to Steam Generator Degradation Prediction and Mitigation.

    3 Revision Issued: December 2012 Roadmap Owner: Heather Feldman

    Changes: Updated flowchart.4 Revision Issued: August 2013

    Roadmap Owner: Rya

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