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Transactions of the Korean Nuclear Society Autumn Meeting
PyeongChang, Korea, October 25-26, 2007
Long Term Asset Management (LTAM) Strategy for Feedwater Heater at Kori 3 & 4 Nuclear Power Plants Se Youl Won and Young Sheop Park Nuclear Engineering & Technology Institute, Korea Hydro & Nuclear Power Co. Ltd., 25-1 Yuseong-Gu, Daejeon, KOREA, 305-343, [email protected] The purpose of the feedwater heating system is to increase plant thermal efficiency by preheating the As the electric power industry becomes more condensate/feedwater prior to its entering the steam competitive, Long-Term Asset Management (LTAM) generator. The system is comprised of various strategy of systems, structures, and components (SSCs) components and controls, which provide the following becomes more important to keep nuclear power plants economically viable throughout their remaining licensed - Maintain proper water levels in the feedwater heater operating terms, whether 40 or 60 years. This paper and drain for maximum efficiency of the system provides an optimized LTAM plan for the feedwater - Supply heating steam to the feedwater heaters heater at Kori 3&4 nuclear power plants. - Provide a flow path for the return of the condensed extraction steam drains to the feedwater and condens- - Allow collection from all feedwater heater drains Based on Life Cycle Management(LCM) - Prevent the entrapment of non-condensable gases sourcebook, LCM implementation guide conducted by The feedwater heaters contain a number of sub- EPRI, and Exelon's LTAM strategy, LTAM plans are components including tubes, tubesheets, baffles, shields implemented for providing early identification and plates (or impingement plates), tube tie and spacers, reduction of risks and vulnerabilities to power tube supports, the drain nozzles, and the steam inlet and production from long-term aging degradation and outlet nozzles. The scope of this paper includes only the obsolescence that may not be addressed by existing passive mechanical components and sub-components of nuclear plant feedwater heaters such as shell, tubes, tube The Long Term Asset Management (LTAM) process support and impingement plate: is intended to provide an effective long-term planning tool for minimizing unplanned capacity loss and 2.3 Operating and Maintenance Experience investments consistent with plant safety and an The environments and operating conditions in which identified plant operating strategy. Such an operating feedwater heaters operate can cause significant strategy might include license renewal or retaining the problems such as fatigue, erosion, corrosion, cracking, option for license renewal. LTAM plan addresses such and vibration. If not properly monitored, inspected, and management, preventive maintained, the feedwater heaters will incur damage over time, causing a negative impact to plant efficiency. replacement or redesign of an SSC important to safety and plant operation. Therefore, LTAM planning is a variable process to systematically identify and examine the important SSCs, optimize their contribution to plant performance, reliability, safety, and value, and prepare long-term maintenance management plans through the technical and economic evaluation. 2.1 Basis for Selection of Feedwater Heater for LTAM Planning Basis for selection of feedwater heater are that feedwater heaters are important to power production and plant efficiency, and are subject to significant operating stress and degradation. Also, they have a history improvements in feedwater heater operation directly affect the plant thermal cycle. Figure 1. Basic outline of plant feedwater heater arrangement 2.2 Feedwater Heater Function and Scopes Transactions of the Korean Nuclear Society Autumn Meeting
PyeongChang, Korea, October 25-26, 2007
From EPRI 1003470, 29 incident event items pertaining Table 2. PM template for the feedwater heater at Kori 3 & 4 to the feedwater heater are presented. Of these events: Main Class. Heat Exchanger - Seventeen events (59%) involved inspection and disc Functional Importance Determination (FID) overy of shell thinning (near the nozzle inlet area) - Nine events (31%) involved tube leakage Feedwater Heater High Low High Low High Low High Low - Two events (7%) dealt with manways and gaskets Service Condition - One event (3%) dealt with partition plate repairs CHS CLS CHM CLM MHS MLS MHM MLM Condition Monitoring Task The operating history for the feedwater heater was Performance Monitoring 1M assembled from the plant's scram and trip reports, and operating and maintenance experience report for the Operator 's Walk year 2003-2006 at Kori 3 & 4. The event causing Time Directed Task plant's scram and trip is not reported. But, there are two Internal Inspection F ailure Finding Task event reports addressed by operating and maintenance experience. Plant-specific events by operating and Applied to HP Heater to CHS and Applied to LP Heater to CHM based on FID maintenance experience are presented as in Table. 1. steam erosion, FAC, improper impingement plate Table 1. Plant events database information on feedwater installation plate installation. heaters at Kori 3 & 4 The maintenance and monitoring condition of FW Heater generally retain good shell thickness condition at High level alarm is occurred at LP Heater 1A for normal operation. Through derated Kori 3 & 4. But, currently, tube plugging rate for LP 2004-09-16 maintenance, one LP Heater string is isolated. Then, five damaged tubes are found at LP Heater 3A. After ECT inspection, 28 tubes are plugged.
#4B of Kori 3 unit and LP #4C of Kori 4 unit is 8.1% During a maintenance outage, HP Heater 6A shell was disassembled and inspected and 8.4%, respectively and is approaching to 10 % 2005-09-04 inside internal instrument. Due to un-installing impingement plate, six tubes and tube support plate are damaged by high-temperature and pressure steam.
plugging rate which is a plugging limit. Therefore, Long * Note : PD-Planned Derate and FD-Forced Dereate & Middle Facility Investment plan should be scheduled 2.4 Determination of Current Maintenance Activities for monitoring continuous condition and replacing the FW Heaters. For preventing FW Heater failure, the Most of the preventive activities are performed proper re-tubing and re-bundling plan with erosion during a refueling outage. These are applied for the resistant shell and tube material is required and the determination of appropriate Preventive Management whole replacement plan of FW Heater is considered (PM) frequencies, taking into account the past operating referring to an expected FW Heater lifetime that is experience, feedback from the craft, safety and non- approximately 20 years less than overall plant lifetime. safety service, cyclic duty and environment as well as In the near future, LTAM strategy alternative will be the refueling cycle. At the end of last year, KHNP made driven to a large extent by the plant operating strategies PM Template to maintain efficiently the integrity of that are being followed or evaluated, and by the current components and systems based on recommendation of reliability of the FW Heaters. Then, through the EPRI and Exelon. This PM Template is classified into economic evaluation using the net present value (NPV), three categories which include critical, duty cycle, and the optimum LTAM strategy will be identified. service condition of Kori 3&4 Functional Important Determination (FID). Table 2 shows the template of the current preventive maintenance activities. For performance monitoring, it is conducted to monitor The approach to improving the long-term asset decreasing performance occurred by corrosion, erosion, management and long-term condition of FW Heaters are tube crack, scale or deposition impurity and to analyze summarized as follow based on the economic evaluation trending of Terminal Temperature Difference, TTD. For in the near future. NDE inspection, it is implemented to investigate the - Plan replacement by considering tube plugging limit, internal and external erosion, tube scale, tube crack or a measurable effect on thermal performance, and tube tube defection, inlet nozzle condition of shell, shell wall experiencing a failure mechanism. thinning using ECT, PT, UT, and VT. Lastly, operator's - Require the use of FAC and erosion resistant material round include activities like connection, external of shell and tube. leakage sense, operating parameters, level, and drain - Develop monitoring the condition of the heater such flow rate surveillance. Also, tube damage and internal as NDE inspection using PM Template. leakage are inspected for prohibiting decreasing of heat transfer area caused by dust, corrosion, and damage. 2.5 LTAM Strategy [1] EPRI, Feedwater Heater Maintenance Guide, 1003470, Major failure mechanisms of FW Heater are tube [2] EPRI, Life Cycle Management Sourcebook – Volume 10 : Feedwaters, 1009073, December 2003. erosion, shell thinning, and impingement plate damage [3] EPRI, Life Cycle Management Planning Sourcebook – by tube-and-tube collision, fretting, support plate fault Overview Report, 1003058, December 2001.

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