In the context of long-term operation or lifetime extension most regulatory bodies demand from utilities and operators of nuclear power plants to monitor and evaluate the fatigue of system, structures and components systematically. As does the Swiss Federal Nuclear Safety Inspectorate ENSI. The nuclear power plant Goesgen started its commercial operation in 1979 and will go into long-term operation in 2019. The increased demand for monitoring and evaluating fatigue due to the pending long-term operation led the Goesgen nuclear power plant to expand the scope of their surveillance and therefore to install AREVA’s fatigue monitoring system FAMOSi in the 2014 outage. The system consists of 39 measurement sections positioned at the primary circuit and the feed-water nozzles of the steam generators. The locations were chosen due to their sensitivity for fatigue. The installed FAMOSi system consists of a total of 173 thermocouples which were mounted in order to get the necessary input data for load evaluation. The advantage of FAMOSi is the possibility to obtain real data of transients near places with highest fatigue usage factors. Examples of steam generator feed-in during heating-up and cooling-down will be given. In addition, spray events before and after the installation of closed loop controlled spray valves will be compared. The measurements and the results of the load evaluation are not only of interest for internal use e.g. in regard to optimization of operation modes (e.g. load-following), but must also be reported to ENSI annually. In addition, by evaluation of stresses and determination of usage factors combined with an optimization of operation modes an early exchange of components can be avoided.
The fatigue assessment of power plant components based on local fatigue monitoring approaches is an essential part of the integrity concept and modern lifetime management. An integral approach like the AREVA Fatigue Concept (AFC) basically consists of two essential modules: realistic determination of occurring operational thermal loads by means of a high end fatigue monitoring system and related highly qualified fatigue assessment methods and tools. The fatigue monitoring system delivers continuously realistic load data at the fatigue relevant locations. Consequently, realistic operational load sequences are available as input data for all ensuing fatigue analyses. This way, realistic load data are available and qualified fatigue usage factors can be determined. The mode of operation of the fatigue monitoring system will be explained in the framework of a live demonstration by means of the FAMOSi (i = integrated) demonstration wall. The workflow starts with the continuous online measurement of outer wall temperatures transients on a pipe. Visualization is implemented within the FAMOSi viewer software. In a second step, inner wall temperatures are directly calculated. In a third step, the resulting linearly elastic stress history will be calculated as the basis for subsequent code conforming fatigue assessment. Subsequently, the related advanced fatigue assessment methods of the three staged AFC-approach are addressed.
For plant lifetime extension programs, AREVA provides a customized solution for thermal load monitoring and fatigue calculation. Requirements like environmental assisted fatigue effects or flexible plant operation (load following) have crucial impacts for a detailed fatigue assessment. With a modularized monitoring concept and automated simplified fatigue estimation technique, plant components can be monitored very cost efficiently with validated methodologies, providing flexible, fast and accurate results. The layout of the modularized monitoring system is simplified and different options can be added on demand. With the automated fatigue assessment, thermal transients can be classified and a cumulative usage factor can be estimated on-line. With the help of this method, the impact of thermal transients to the component fatigue can be evaluated just in time. That can be a piece of important information to get for a better knowledge about the plant behavior during flexible operating (load following) modes.
Within the AREVA Fatigue Concept (AFC) a new method for fast fatigue evaluation for highly loaded nuclear power plant components was developed. This method uses FAMOS (FAtigue MOnitoring System) measured data from the outside surface of a pipe and can evaluate a fatigue level for the component for the thermal event plug flow. The measuring location of FAMOS is chosen close to a fatigue relevant component and the points of interest are at the inner surface of the component. The calculated inner wall temperature time history will be transferred to the inner surface of the component. The thermal load cycles are well known after that step and the stress time history and also the strain rates would be calculated with the Green’s function approach. Unit transients will be used to calculate principle stresses and shear stresses in all fatigue relevant locations within the monitored component. Pressure cycles will also be evaluated with the Green’s function approach. After the calculation of the equivalent stresses the mechanical load cycles can be classified by the use of the rainflow algorithm. Comparisons (Miner’s rule) with the fatigue curve results in fatigue levels for all relevant locations within a component. In the current approach, the conservatism will be reduced with this method, and an enveloping fatigue level can still be calculated. In another words, for highly loaded components, using the current methodology can provide a more realistic stress calculation and enveloping fatigue level calculation. Depending on the number of load cycles, the new and more stringent requirements can be complied.
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