THERMAL STRATIFICATION MONITORING OF ANGRA 2 STEAM GENERATOR MAIN FEEDWATER NOZZLES

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1 2009 International Nuclear Atlantic Conference - INAC 2009 Rio de Janeiro,RJ, Brazil, September27 to October 2, 2009 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: THERMAL STRATIFICATION MONITORING OF ANGRA 2 STEAM GENERATOR MAIN FEEDWATER NOZZLES Miguel Cisternas 1 1. Eletrobrás Termonuclear S.A. - ELETRONUCLEAR Departamento GAN T Rua da Candelária, 65 6º andar Rio de Janeiro, RJ mcister@eletronuclear.gov.br ABSTRACT In former times several utilities reported a large temperature excursion in the Steam Generator Main Feedwater nozzles during heat up and cool down of the nuclear plants. In Angra 2 this behavior was also observed and, besides that, thermal stratification during the hot standby mode of operation has also been measured. This behavior was detected thru a temperature monitoring program, in which a set of thermocouples were installed on the external surface of the pipe with the aim to monitor thermal transient. Because the stratification loading has not been defined in the design phase of the system, it is necessary to check the effect of this phenomenon on the fatigue analysis of the piping. This stratification leads to undesired temperature gradients which can conduct to strain and consequently stress in the nozzle material. This behavior can be part of the cause of accelerated fatigue of the component. The objective of this paper is to present the thermal stratification monitoring program implemented in Angra 2 and determined the usage factor of the components due to the phenomenon. 1. INTRODUCTION The starting point for Thermal Stratification is the U.S. Nuclear Regulatory Commission (USNRC) Bulletins and Bulletin is about Thermal Stresses in Piping Connected to Reactor Coolant Systems (auxiliary lines) and Bulletin has references to Pressurizer Surge Line Thermal Stratification. In Angra 2 NPP, Surge Line was analyzed considering the phenomenon of thermal stratification. Operational experience shows that other piping can operate with stratified pipes, who in certain conditions can lead the piping to be submitted to fatigue. Taking all these facts into consideration, ELETRONUCLEAR developed a thermal stratification monitoring system. 2. THERMAL STRATIFICATION The minimum prerequisite for the initiation of the phenomenon of thermal stratification are the inadequate lay out of the piping (horizontal pipe) and low velocities of the flow (stagnation or circulation of fluids in layers in the pipe). Another kind of this phenomenon can start when a hot straight pipe is suddenly filled with cold water. Besides a thermal shock, there will be stresses in the pipe as a result of thermal stratification. After a period of time, the pipe wall and the cold water in contact with it have the same temperature, whereas the remainder of the pipe, which is not in contact with the water, remains hot. This behavior cause additional deformation as well as bending moments. In other words, the temperature difference between the hot water (top) and cold water (bottom) in a piping is known as thermal stratification and is shown next in Fig. 1.

2 Figure 1. Thermal stratification. The effect of thermal stratification on the piping can be separated in two parts [1]: Local Stratification, which causes stresses similar to thermal gradients stresses for Class 1. This thermal condition can produce significant thermal stresses in the pipe wall. Thermal stresses increase, consequently fatigue life is reduced; Global Stable Stratification, which causes bending. This thermal condition can lead to piping displacements exceeding those considered in original design Thermocouples Configuration Two typical configurations are used in Angra 2 NPP and are shown next in Fig. 2: Figure 2. Typical thermocouples configuration.

3 The thermocouples are installed on the external surface of the pipe, in several sections of the piping, and have the function of determine the operational transients. Fig. 3 shows full configuration of thermocouples at Pressurizer Surge Line of Angra 2 NPP. Figure 3. Full configuration of thermocouples Data Acquisition Unit FieldChart Novus The FieldChart software allows displaying the Field Logger's data (Fig. 4) in digital and graphic format, in batches or in real time, and in real time trend and historic trend views. The Microsoft Windows-compatible software allows to zoom, superimpose and link graphs, and print or export all data. Figure 4. Field Logger.

4 2.3 Monitoring Locations in Main Feedwater Line The Main Feedwater nozzle (MFN) is connected horizontally to the Steam Generator as shown next in Fig. 5: Figure 5. Steam Generator Main Feedwater Nozzle Locations Temperature measurements locations close to Main Feedwater Nozzles: Main Feedwater Line (four loops): 4 sections with 11 thermocouples 2.4 Hot Standby Mode of operation of Angra 2 NPP There are several modes of operation of Angra 2 NPP. In order to clarify the subject of this paper, two modes of operation are shown in Fig. 6: Figure 6. Modes of Operation of Angra 2 NPP

5 Measured performed during hot standby mode of operation, captured thermal stratification that was not specified in the existing thermodynamic loading transients, for the Steam Generator Main Feedwater Nozzles of Angra 2. These temperatures are shown in Fig Thermal stratification during hot stand-by mode of operation at MFN loop 1 Fig.7 shows Steam Generator Main Feedwater nozzle at loop 1: Transient ºC ºC Transient ºC ºC Thermal Stratification T= 81ºC Figure 7. Thermal Stratification at loop 1 close to Main Feedwater nozzle Thermal stratification during hot stand-by mode of operation at MFN loop 2 Fig.8 shows Steam Generator Main Feedwater nozzle at loop 2: Figure 8. Thermal Stratification at loop 2 close to Main Feedwater nozzle

6 2.4.3 Thermal stratification during hot stand-by mode of operation at MFN loop 3 Fig.9 shows Steam Generator Main Feedwater nozzle at loop 3: Figure 9. Thermal Stratification at loop 3 close to Main Feedwater nozzle Thermal stratification during hot stand-by mode of operation at MFN loop 4 Fig.10 shows Steam Generator Main Feedwater nozzle at loop 4: Figure 10. Thermal Stratification at loop 4 close to Main Feedwater nozzle

7 2.5 Summary of Results of Thermal Stratification during Heat up and Cool down Table 1 show a comparison of thermal stratification of Main Feedwater Line between measured values and design values during heat up and cool down of the plant. Table 1. Results of thermal stratification during heat up and cool down 2.6 Summary of Results of Thermal Stratification during Hot Standby mode Table 2 show maximum measured values of thermal stratification of Main Feedwater Line during hot standby mode of operation of the plant [2]. Table 2. Results of thermal stratification during hot standby 2.7 Effect of thermal stratification Besides thermal shock (transients), the effect of thermal stratification are considered by determination of piping bending moments due to stratification and by calculating additional local stresses due to non-linear temperature difference across the pipe diameter. At the present work the local stresses due to stratification are considered as a term to be adding to Eq. 11 of ASME NB [3]. Eq. 11 is defined as stress intensity range resulting from primary plus secondary stresses and peak stresses. The term to be adding to Eq. 11 is shown next:

8 K 3.E.α. T 3 (1) Where, K 3 E α T 3 stress indices (K 3 = 1,0 for straight pipe) modulus of elasticity at room temperature linear coefficient of thermal expansion at room temperature absolute value of the range of the maximum difference between the linear topto-bottom temperature 2.8 Summary of Fatigue Analysis of Main Feedwater Line Table 3 presented a summary of fatigue analysis of Main Feedwater Line at section close to the Steam Generator nozzle. Cumulative usage factor (CUF) considering original events for 40 years life time including Thermal Stratification occurring while the plant is starting up or cooling down is shown. Another evaluation presented the CUF at the present day considering the occurred events during 9 years operation (without cyclic Thermal Stratification). Finally an evaluation considering only the cyclic Thermal Stratification occurred and not specified in the existing thermodynamic loading transients is exposed. Table 3. Summary of Fatigue Analysis of Main Feedwater Line 3. CONCLUSIONS Thermal Stratification of the Main Feedwater Line occurs while the plant is starting up or cooling down, or when the plant is in hot standby mode of operation. Besides global thermal stratification (unexpected movement), Angra 2 discovered cyclic thermal stratification (pipe cracks), acting simultaneously at Main Feedwater Line. This kind of thermal stratification was not specified in the existing thermodynamic loading transients for the Main Feedwater Line.

9 In order to reduce the effects of the thermal stratification at the Main Feedwater Line, which can result in a large fatigue and a short load life, Angra 2 is now improving the hot standby mode of operation. REFERENCES 1. User s Guide, PIPESTRESS version 3.6.0, January M. Cisternas, Thermal Stratification Monitoring in Angra 2 NPP, Nuclear Plant Fatigue Applications Workshop, Charlotte, NC, January 21-23, Structural Integrity Associates, Inc, Approach for Evaluation of Piping Affected by Stratification and Thermal Cycling, AFD