Reactor Pressure Vessel Thermal Cycling Evaluation
I. PURPOSE AND SCOPE
This project will provide an operating transient monitoring system for assessing the fatigue effects of thermal cycling of reactor coolant system carbon steel components. Also, fatigue in stainless steel components not previously reviewed will be assessed under this project. Technical Specification Table 5.7.1-1 describes the number of thermal cycles (e.g., scrams from 100 percent power, loss of feedwater heaters, improper start of a cold recirculation loop, and reductions in power from various power levels) allowed on the reactor pressure vessel (RPV) during its initial 40 year lifetime. This project also includes identifying procedure revisions and Technical Specification and/or Final Safety Analysis Report (FSAR) changes necessary for converting from thermal cycle counting to a more accurate fatigue monitoring program. Actual operating data will be used to determine these fatigue effects and their impacts on remaining component life. A primary benefit will be reduced inspection frequency per NUREG 313, Rev. 2. Additionally, this monitoring system may be used to monitor when the RPV temperature drops below 525F for evaluation of radiation induced effects.
This project incorporates previous efforts to assess the RPV and associated components having the greatest potential for thermal cycle fatigue. Specific project tasks include:
1. Review the efforts made to date relative to the RPV thermal cycling/fatigue issue and determine their applicability to Technical Specifications.
2. Develop a program to simplify/enhance fatigue tracking within the requirements of the applicable Technical Specifications, including required procedure revisions and Technical Specification/FSAR changes.
3. Calculate the actual fatigue effects on the Unit 1 feedwater nozzles accumulated during the time frame between the completion of the RPV Thermal Cycling Study (by General Electric) and the implementation of ENP-44 (Feedwater Nozzle Thermal Data Acquisition and Usage Calculation Procedure). Calculate actual fatigue effects experienced on the Unit 2 feedwater nozzles from initial plant start-up to the implementation of ENP-44.
4. Establish a manual fatigue monitoring program (currently scheduled to be in place by the end of 1992) to provide an interim monitoring capability until the automatic system is in place.
5. A plant modification is planned which will provide automatic data retrieval and analysis by interfacing the data through the plant process computer to an analysis software program (Fatigue Pro). This automatic method will eliminate manual data collection by Operations and Technical Support (strip chart data retrieval and recording). Implementation of this modification is planned in conjunction with the feedwater sparger replacement modifications (PID G0029A).
The project will result in a determination of the need, expected costs, and schedule for monitoring RPV thermal cycling. This information could be used to support continued plant operations for the full reactor vessel design lifetime and for any potential license renewal period.
II. EVALUATION
Schedule Index: 10 - Evaluation of thermal cycling for reactor vessel components such as the inlet feedwater nozzles could have an impact on the availability of the feedwater system, impacting nuclear safety (0.2 x 32). Unit availability could be reduced if the fatigue effects exceed design allowances; however, the study and surveillance programs may help prevent a premature shutdown of the unit and may aid any potential license renewal efforts (0.2 x 12). The plant is somewhat enhanced by improved analysis and surveillance capabilities for assessing plant operations relative to the plant Technical Specifications (0.2 x 8).
Economic Aspects: Allocating resources to this study now could provide valuable information to justify continued plant operation in the future and for possible license renewal at the end of design life.
Related Standards: The applicable Work Management Policies are material condition and operating parameters.
Other Considerations: The scope of this study would not require any outage or other project related work for its completion. However, the project on removal and analysis of reactor in-vessel irradiation specimens (G0159A), which involves the changes in material properties of the RPV, may have implications for the thermal cycling analysis.
III. CONCLUSION
This study will ensure that the plant can complete its initial operating life of 40 years, and it will gather information that could be useful for possible license renewal. The study started in 1988 and will be continued as scheduled so that the thermal cycling fatigue effects on the reactor pressure vessel and associated components can be determined. It is also important to assess more accurately the extent that the related Technical Specifications impact current and future plant operations.
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