IMPLEMENTATION OF A FATIGUE MONITORING SYSTEM IN MOCHOVCE UNIT 1 AND 2 FIRST RESULTS

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1 International Conference Nuclear Energy in Central Europe 2001 Hoteli Bernardin, Portorož, Slovenia, September 10-13, 2001 www: tel.: , fax: Nuclear Society of Slovenia, PORT2001, Jamova 39, SI-1000 Ljubljana, Slovenia IMPLEMENTATION OF A FATIGUE MONITORING SYSTEM IN MOCHOVCE UNIT 1 AND 2 FIRST RESULTS Wilhelm Kleinöder, Gerhard Schön Framatome ANP Freyeslebenstraße 1, Erlangen, Germany wilhelm.kleinoeder@framatome-anp.de, gerhard.schoen@framatome-anp.de Stefan Horvath, Frantisek Kalmancai Slovenské Elektrárne, a.s. Atómové elektárne Mochovce Mochovce, Slovenská republika ABSTRACT Within the modernization of the VVER440 NPP s of Mochovce Unit 1 and 2, the EUCOM Consortium (Siemens AG and Framatome S.A.) installed, amongst other diagnostic systems, the fatigue monitoring system, FAMOS. The first step was the preparation of a fatigue manual, where those systems, components and component parts were identified, which undergo fatigue more quickly than other components. For the implementation of the system a basic and a detailed design was carried out. The commissioning tests on site showed the high quality and accuracy of the system so the final acceptance was confirmed. After two years of operation FANP received the data for evaluation of the second cycle. The quick evaluation with the FAMOS Stage 2 Software showed that there are no unexpected loads. However, for FANP, it was very surprising that the temperature amplitudes and the frequency of the temperature fluctuations in the surge line are very moderate compared to German plants. This result was achieved by improvement of some operating procedures after the first cycle. The fatigue monitoring system, FAMOS, is one of the basic diagnostic systems installed in Mochovce unit 1 & 2. Although the results after two years of operation seem to be very positive, it is too early for a final assessment. The evaluation for Unit 1 & 2 must be continued within the following years of operation. 1 INTRODUCTION Mochovce is the first Russian-designed nuclear power plant in Central and Eastern Europe where a comprehensive program of measures was implemented in a European cooperation to upgrade the plant to an internationally accepted safety level in the course of plant completion. The upgrading and completion of this Slovakian nuclear power plant thus represents significant progress in the sense of the long-term policy of the European Union to reduce the safety risks of earlier generations of Russian-designed nuclear power plants in Central and Eastern Europe

2 505.2 The safety upgrade is the result of successful, close cooperation between Western institutes for nuclear safety, the International Atomic Energy Agency IAEA and the Slovakian licensing authority ÚJD (Úrad Jadrového Dozoru). This applies both to specification of the program of safety improvements and to the cooperation between companies from Western Europe, Russia, the Czech Republic and Slovakia in the implementation of these measures. The first idea for fatigue monitoring in Mochovce unit 1 and 2 was born in 1988 with the detailed design of the execution project for a so-called system Z. This was a product of ENERGOVYZKUM Brno. The system was designed for data acquisition, storage and sorting of relevant data for fatigue monitoring of a few locations at the steam generator and pressurizer, during normal operation. The system, with 23 measuring channels, was delivered to Mochovce but was not installed. In 1995 it was decided within the frame of the TSSM (Technical Specification Safety Measurement) that the Siemens fatigue monitoring system, FAMOS, would be installed instead. The reasons were, the larger number of measurement nodes, and therefore the possibility to provide better monitoring of all relevant primary system as well as secondary system components. The Czech company EGV Brno (which is one part of the former ENERGOVYZKUM Brno) was also involved in the implementation of the new Siemens/Framatome ANP system in Mochovce. Further the final supplier of the NPP, Skoda Slovakia, participated in the installation and commissioning of the new system. 2 OVERVIEW OF THE FAMOS SYSTEM 2.1 The Four Steps of FAMOS The fatigue monitoring system FAMOS is divided into a preparatory phase and three implementation stages. Stage 0: Fatigue manual Stage 1: Implementation of the measurement system Stage 2: Quick evaluation on the load level Stage 3: Fatigue analysis A separate functional unit consisting of hardware and software has been developed for each of the three implementation stages. The three functional units work autonomously of each other in time and space and can therefore be implemented independently in several steps. The data acquisition unit runs on-line in the power plant. The data measured by the special FAMOS instrumentation, at present temperatures only, are processed together with the other plant operating data obtained from the plant computer (temperatures, pressures, water levels in vessels, flow rates, valve positions etc.). Data evaluation, and later fatigue analysis, is also performed on personal computers. At first, on-line data acquisition generates a flood of information that can be meaningfully handled only by computerized analysis, as an integral part of the fatigue monitoring system. If on-line fatigue monitoring is conceived as an information source, the copious original flow of data which must be reduced in stages and produce, ultimately, a single value, a procedure such as that illustrated in Figure 1 has to be used.

3 505.3 Functional Unit Data Processing Results Stage 1 Data Acquisition Relevant Data Stage 2 Data Evaluation Load Level (T, p) Stage 3 Fatigue Analysis Strain Level ( σ, ε ) Graphic Display of Load and System Data Quick Look Evaluation Temperatures and Pressure Acting on the Individual Item Fatigue U sage Factor Mode of Operation Load Case Identification Frequency of Temperature and Pressure Fluctuations Temperature and Stress Profiles at Predefined Locations Rainflow Evaluation of Stress Ranges Figure 1: Data reduction Stage 1 collects all desired data on-line. Stage 2 is performed at intervals, when significant quantities of data have accumulated in Stage 1. The data is correlated by component and analyzed at the load level (temperatures, pressures). As the final step, in Stage 3, the stress history and ultimately the cumulative usage factor is calculated for the component using the measured load profiles. This approach makes it possible to do only what is necessary, when it is necessary. The acquired database remains intact in long-term storage. Significant information can be gained even at the load level in Stage 2 and may prompt the plant operator to consider improving his plant operating practices and possibly even enable him to reduce the fatigue rate. Thus a beneficial influence can be brought to bear on the plant's fatigue status even without that status having been explicitly identified. 2.2 Stage 0: Fatigue manual The preparatory phase, known as Stage 0, consists of drawing up a fatigue manual for the power plant in question. The fatigue manual identifies those components which, on the basis of the design calculations but also in the light of the local loadings known from operating experience, are expected to undergo fatigue more quickly than other components. The selection may be restricted to the components of the primary coolant pressure-retaining boundary. But in the interests of overall plant availability, it is now common practice to include some secondary-side systems, too. The fatigue-enhancing transients (example Figure 2) are identified separately for each component, and the parameters to be examined for fatigue monitoring are defined. These include parameters that are used to analyze the plant status. The instrumentation already in place for monitoring operation of the plant is used as far as possible, but in order to gain a realistic picture of local loads, additional thermocouples have to be installed. The final step in the preparatory phase is the compilation of an instrumentation map. Thus, the fatigue manual gives information on the where, why and how for the local FAMOS instrumentation.

4 505.4 Figure 2: Example of measured temperature and pressure loading 2.3 Stage 1: Implementation of the measurement system In implementation of Stage 1, the data acquisition system is installed. As of this time, it delivers on-line load and system data. This data is not evaluated on-line but goes into longterm storage. Evaluation is performed at a later point in time, in Stage 2 at the load evaluation level and in Stage 3 at the stress and fatigue analysis level. The hardware installed consists of the measurement electronics, the data acquisition PC and software and the thermocouples, which are arranged in measurement sections of either two or seven thermocouples. 2.4 Stage 2: Quick evaluation on the load level Since the visual analysis of measured data profiles is relatively time-consuming, a tool has been provided to permit quick simultaneous evaluation of the fatigue-related parameters in the period of observation for all components. The temperature and pressure profiles associated with the components are examined for fluctuations by means of a rainflow algorithm [1]. An example of quick evaluation output is given later in Figure 4. The result consists essentially of a list that identifies, for each component, the frequencies of occurrence of certain defined classes of temperature and pressure fluctuations. The counts from sub-periods are accumulated. Such lists very quickly reveal periods in which individual components have experienced no or only moderate fatigue-related loads. 2.5 Stage 3: Fatigue analysis Stage 3, the fatigue analysis proper, is necessary only if Stage 2 has identified significant loadings. The fatigue analysis converts the pressure and temperature profiles measured at the component into stress profiles. The procedures used to calculate the stresses are matched to the capacity of the PC. The stress fractions resulting from slug flow, thermal stratification, internal pressure and any section forces and moments possibly present, are superimposed to give the total stress. The time histories of the stress intensities are evaluated by means of the rainflow algorithm in accordance with the applicable codes and standards. The fatigue fractions resulting from the stress cycles recognized are taken from the code design fatigue curves and accumulated by Miner's [2] linear method. The result of the

5 505.5 calculation is the partial usage factor U in the period of observation t for various locations at the component. If the cumulative usage factor U 0 at the time of commencement of monitoring is known, the momentary fatigue status U can be obtained as the sum of U 0 and all the partial usage factors U i hitherto calculated, i.e. U = U 0 + Σ U i. Then the fatigue rate α = U/ t can be applied to extrapolate the cumulative usage factor at any time in the future. The FAMOS fatigue analysis software can be configured for various component geometries (nozzles, vessels, tube sheets, pump and valve bodies). The effort involved consists of pre-calculation of stress responses to standard transients. These are stored on the PC as baseline parameters in the configuration data file. The computational part of the system allows calculation and comparison of individual usage factors for specific transients taking into account the measured loading history. The initial cumulative usage factor U 0 can be determined by summation of the partial usage factors from the transients that have occurred in the past. It is also possible to input simulated temperature and pressure histories to investigate how a change in operating mode, other transient time histories, or an alternative design would affect the fatigue performance of the component. Thus the system can be of advantage as early as in the component design stage. 3 IMPLEMENTATION OF THE FAMOS SYSTEM IN MOCHOVCE 3.1 Preparation of a Fatigue Manual As the first step in implementation of the fatigue monitoring system, FAMOS, a fatigue manual was prepared for the Mochovce NPP s. For carrying out this work, a lot of original documents like system drawings and operational procedures, most of them in the Czech language, needed to be investigated. So the basic input for the manual was provided by the Czech company EGV, Brno. Additional experience was provided by Framatome ANP as they had previous knowledge of a VVER type reactor in Kola (Russia). The fatigue manual contains proposals for the locations of the additional FAMOS instrumentation and also for the operational signals to be linked to FAMOS. These locations, which undergo fatigue at a faster rate than others, were determined by: evaluation of system data evaluation of system layout experience from system component manufacturers operational experience from other VVER experience from Framatome ANP-reactors with FAMOS instrumentation. From the investigated spectrum of systems and components in this report the following systems were selected for FAMOS additional instrumentation: Piping DN 250 of low pressure system, 4th loop, cold leg Piping DN 100 of high pressure emergency feeding system Surge Line Pressurizer Steam Generator, 1st and 4th loop, feed-water piping. In the first step, 16 measurement sections with 87 thermocouples were suggested, from which 5 measurement sections have 2 thermocouples and 11 measurement sections have 7 thermocouples. This configuration was the basis for the selection of the hardware during the basic and detailed design. An example of the instrumentation plan for the Surge Line is given in Figure 3.

6 505.6 Figure 3: Instrumentation plan for the Surge Line The fatigue manual also contains additional locations which could possibly be instrumented with FAMOS measurement sections in the future. 3.2 Implementation of the Measurement Hardware The measurement electronics were designed for the 14 measurement sections selected within the fatigue manual. But as additional locations are provided for in the fatigue manual, the cabinet was prepared for possible future extension. The whole measurement chain consists of: The measurement sections with the thermocouples The terminal boxes with the thermal compensation The dataloggers with the programmable plugs and The data acquisition PC and software The fatigue monitoring system, FAMOS, also needs operating signals for verification and for validation purposes. So it was necessary to establish a serial line (RS 232) connection to the plant operating computer. The connection was realized with an additional data link computer as other diagnostic systems such as LPMS (Loose Parts Monitoring System) also needed additional signals from the plant computer. It is important to note that for the measurement sections a new design was suggested and realized by EGV. Instead of two separated mounting and fixing strips, one integral part was realized. This allows an easier assembly and disassembly procedure under radiation conditions.

7 Quick Evaluation with FAMOS Stage 2 First Results The diagnostic department of Mochovce has a trained staff for all diagnostic systems, so the local specialists provide the evaluation of the measured results for FAMOS. Although they have an MS-DOS evaluation utility for the FAMOS Stage 2, they carry out considerable additional analyses with the measured data. Framatome ANP is still in contact with the Mochovce staff and also received some data from the second operating cycle of Unit 1. Our own evaluation with the FAMOS Stage 2 software package achieved interesting results compared to our experience to date with German NPP s. Figure 4 shows the thermal stratification and the temperature shock behaviour of the Surge Line in Mochovce Unit 1. The thermal stratification reaches a maximum value of only 120 K (part no. 6 and 9). Experience with German plants shows values of up to 180 K. Temperature shock reaches values of up to 200 C (without start-up and shut-down) in German plants MOCHOVCE 1FAMOS STUFE 2 SRE012 V 2.4/1 SCHNELLAUSWERTUNG STAND : 11/04/01 10:15:55 ZEITRAUM : SIEHE LISTE DER AKKUMULIERTEN ZEITRÄUME SCHWANKUNGSBREITE MIT TOLERANZ ± 20 GRAD C BZW. BAR BAU- MATRIX EIN- TEIL NUMMER HEIT K Thermal stratification 10 C Thermal shock (MS5) 6 11 K Thermal stratification 12 C Thermal shock (MS6) K Thermal stratification 14 C Thermal shock (MS7) 8 15 K Thermal stratification 16 C Thermal shock (MS8) 9 17 K Thermal stratification 18 C Thermal shock (MS9) K Thermal stratification 20 C Thermal shock(ms10) Figure 4: Temperature Fluctuations at Surge Line The discussion with the EMO staff showed that they had changed some of the operating procedures after the first operating cycle of Unit 1. This was directly a result of the measurement values obtained within the first operating cycle. The future operating cycles have to show whether the obtained improvement can be confirmed. It is also planned to extend the measurement sections at the feed-water pipeline of the Steam Generators to all six loops. Up to now only loop 1 and 4 is instrumented. The new instrumentation shall show whether and how the behaviour of the six loops differs. 4 CONCLUSIONS The implementation of diagnostic systems like the fatigue monitoring system FAMOS in the NPP s Mochovce Unit 1 and 2 showed good results after the first few cycles of operation. Without carrying out detailed fatigue analyses it was possible to improve operating procedures in such a way, that the fatigue load of the primary piping system was decreased. Over the years this will lead to a very low fatigue usage factor of the components concerned. Fatigue analyses can be carried out on the basis of real operating loads and the collected data will also be the basis for the lifetime management of the power plants.

8 505.8 REFERENCES [1] M. Matsuishi, T. Endo Fatigue of Metals Subjected to Varying Stresses Kyushu District Meeting of the Japanese Society of Mechanical Engineers, March 1968 also T. Endo et al. Damage Evaluation of Metals for Random or Varying Loading Proceedings of the 1974 Symposium on Mechanical Behavior of Materials, Vol.1, The Society of Material Science, Japan, 1974, pp [2] M. A. Miner Cumulative Damage in Fatigue ASME Journal of Applied Mechanics, 1945, pp. A159-A164