Application of Small Punch Testing Methods for Thermal Ageing Evaluation on Samples from Primary Piping of NPP

Transcription

1 Application of Small Punch Testing Methods for Thermal Ageing Evaluation on Samples from Primary Piping of NPP Jana Petzová 1, a *, Martin Březina 1, b, Miloš Baľák 1, c, Ľudovít Kupča 1, e 1 VUJE, a.s., Okružná 5, Trnava, Slovakia 2 STU MtF Trnava, Slovakia a jana.petzova@vuje.sk, b martin.brezina@vuje.sk, c milos.balak@vuje.sk, e ludovit.kupca@vuje.sk * the corresponding author Abstract. During a long-term operation of NPPs, the changes of structural material properties occur. To ensure the safe and reliable operation, it is necessary to monitor and evaluate these changes mainly on components from primary circuit of NPPs. One of the dominant ageing mechanisms of NPP components besides the radiation embrittlement and the fatigue loads is the thermal ageing. The thermal ageing is temperature, material and time dependent degradation mechanisms due to long term exposure at the operating temperature of 570 K. The reactors in Slovakia have been behind several decades in operation. While the monitoring of the radiation degradation is standardly performed using surveillance programs during all plant life operation, the monitoring and evaluation of thermal ageing of primary circuit components started only few years ago. The concept of the thermal ageing assessment was divided into two possible approaches of evaluation. The first is long-term exposition of surveillance specimens. And the second approach is a direct surface sampling of heavy components after several years of operation. The Small Punch Test (SPT) methods are used for evaluation of main material properties in this TAM project. By the SPT technique it is possible to evaluate the basic tensile properties as the ultimate tensile strength and the yield stress of the tested materials from a very small amount of obtained material. Present paper summarizes the results obtained from evaluation of original primary piping material of NPP in Bohunice and NPP in Mochovce. There will be presented SPT results obtained due to testing of material removed by surface sampling of original primary pipings. Keywords: Thermal ageing, nuclear power plant, small punch test, primary piping Introduction The long-term operation of nuclear power plants can cause changes in the properties of structural materials of the components important to safety. Besides the radiation embrittlement and the fatigue loads, it is possible to consider the thermal aging as one of the dominant ageing mechanisms [1]. Thermal ageing is temperature, material state (microstructure) and time

2 dependent degradation mechanism due to long-term exposure at the operating temperature of 570 K. The reactors in Slovakia have been in the operation for several decades and the monitoring of the thermal ageing of the components related to safety is essential for the planned prolongation of NPP`s lifetime and with respect of its reliable and safe operation. Programs for long-term monitoring of primary circuit structural material properties changes were prepared on the base this knowledge, regulatory requirements [2] and NPP s operator demands. The concept of the thermal ageing assessment was divided into two possible approaches to evaluation [3, 4]: Long-term exposure of blocks of original materials that are exposed to the same thermal load as the monitored components. Pieces of materials were inserted into the boxes which were mounted in the ring form holder placed on the main primary circuit pipe (see Fig. 1 and Fig. 2). This method of monitoring has been used in Bohunice Unit 3 and in Mochovce Unit 1 NPPs. The monitored materials were made of reactor pressure vessel (RPV), primary piping (PP), steam generator shell (SG) and surge line steels (SL). Direct sampling of components important to NPP safe operation and a detailed analysis of the removed samples. The possibility of such sampling was computationally checked and the maximum allowable cutting depth was determined [5]. In the frame of the project of thermal ageing monitoring (TAM), it was planned to perform surface sampling of primary piping, steam generator shells and reactor pressure vessels at Bohunice and Mochovce NPPs, too. Removed samples were analysed to determine the components actual state. The SPT technique for determination of mechanical properties such as tensile properties and transition temperature were applied due to a small amount of removed materials. Figure 1: Placement of the ring on the primary piping

3 a) b) Figure 2: Testing installation of the ring of boxes containing the monitored materials piping in (a) NPP Bohunice Unit 3 and (b) NPP Mochovce Unit 1 After sampling, cutting and mechanical testing the remains of the samples will be used for supplementary tests: chemical analysis, micro hardness measurement, microstructure and sub-microstructure analysis, sensitization of austenitic steel. The first surface sampling of primary piping in NPP Bohunice was performed in June 2013 and the samplings from the other components at Bohunice and Mochovce NPPs were carried out subsequently in the years The paper deals with description of sampling from primary piping in NPP Bohunice Unit 3 and NPP Mochovce Unit 1. All obtained results are necessary to compare with reference testing on the materials which are used for long-time exposure during next years. Principle of surface sampling The direct sampling of the primary circuit components and evaluation of removed samples is a quick method used to determine the current state of materials. Removing materials from operated devices using classical techniques represents an inappropriate interference with the integrity of the assessed part. These classical destructive methods require subsequent repair of the removed parts, including welding, heat treatment and a range of non-destructive testing. The possibilities of material sampling from components within the nuclear industry are described in [6].

4 A special quasi non-destructive sampling equipment SSamTM-2 (Rolls Royce) is used for the surface sampling. This device is designed as a universal system for the sampling of different devices without any significant affecting of the component surface and without necessity to repair or modify the component. The SSamTM-2 sampling system provides a remotely operated material sampling capability. The machine s unique hemispherical cutter is able to remove a sample without mechanical distortion or thermal degradation of the material. The sampling procedure produces a depression in the base material approximately 0.8 mm deeper than the sample thickness. A schematic depiction of the surface sample removal is shown in Fig. 3. Figure 3: Schematic depiction of the surface sample removal Figure 4: Surface sampling from primary piping on NPP Bohunice Unit 3

5 Figure 5: Removed sample from primary piping and detail of piping surface after sampling Surface sampling provides a small amount of material only, the critical point of material evaluation is just cutting and preparation of specimens. The real surface sampling from primary piping on NPP Bohunice Unit 3 is illustrated in the Fig. 4. Within TAM project, there were carried out four surface samplings from each component, specifically there were removed two samples from base material and two samples from weld metal of chosen safety-related component. The Fig. 5 illustrates removed sample from primary piping and detail of piping surface after sampling. Precise cutting of samples The samples from surface sampling represent a very valuable experimental material and their proper subdivision is a very important activity. The precise cutting of samples to obtain semifinished specimens are carried out by a computer controlled EDM (Electrical Discharge Machine) with a very thin molybdenum wire ø0.1 mm [7]. The cutting process using EDM provide high cutting precision and low cutting width, so as to obtain the saving of experimental material. An example of sample partitioning is presented in the Fig. 6. Figure 6: Removed sample after EDM cutting SPT method

6 The principle of SPT testing procedure is a penetration of a disk shape specimen by a hemispheric rod and recording of loads and deflections during the test. The SPT testing procedure in accordance with [8] is more detail described in [9]. Disk shaped specimen has 8 mm in diameter and 0.5 mm in thickness. Using relatively simple system with recorders of the load and deflection values, we can obtain the following data of basic mechanical properties: The yield stress and ultimate tensile strength, which are well correlated with the parameters Py and Pmax, respectively. Ductile-brittle transition temperature (DBTT) measured by the Charpy test can be estimated from the results of temperature dependence of small punch energy determined from the area under the load deflection curve. The testing is performed on a standard tension test machine, equipped with a data recorder for the registration of load-deflection curves. For the realization of SP tests there are used special testing holders depending on the testing temperature. For low-temperature tests, a special thermal insulated chamber is used. The specimen is cooled using liquid nitrogen vapors or directly by a liquid nitrogen. The temperature of the specimen is measured by a calibrated thermocouple. For testing at elevated temperatures a special testing furnace is used. SPT procedure in TAM project is applied as the new method for estimation of base mechanical properties of the evaluated materials. Specifically, small punch tests are used for determination of: tensile strength from reference blocks of materials and small removed surface samples and the temperature dependence of SPT energy only from reference blocks of materials. Material The primary piping of NPP is standardly made from the austenitic stainless steel grade 08Ch18N12T. The chemical composition of the primary piping materials and real analysis of samples are shown in Table 1 and Table 2. The microstructure of the base material is composed of polyedric austenite grains with a relatively large amount of annealing twins, with a minimal amount of δ-ferrite, non-metallic inclusions and secondary phases in the form of carbides and carbonitrides (Figure 7a). The microstructure of weld metal is consisted of austenitic dendrites, small amount of δ-ferrite and the secondary phase inclusions (Figure 7b).

7 Table 1: Chemical composition of primary piping base material of NPP Bohunice Unit 3 and Mochovce Unit1 material BM - 08Ch18N12T* PP EBO3 BM PP EMO1 BM Table 2: Chemical composition of primary piping weld metal of NPP Bohunice Unit 3 and Mochovce Unit1 material WM - EA 400/10T* PP EBO3 WM PP EMO1 WM [hm.%] C Si Mn Cr Ni Ti S P Co Cu Max Max. Max *C-0.6 Max. Max. Max. Max , ±0.01 ±0.02 ±0.07 ±0.10 ± ±0.003 ± ,07 0,37 1,48 17,19 11,67 0,57 0,0075 0,013 0,024 0,017 ±0,01 ±0,01 ±0,02 ±0,07 ±0,10 ±0,10 ±0,0008 ±0,001 ±0,003 ±0,003 [hm.%] C Si Mn Cr Ni S P Co Mo V Max. Max Max. Max. Max ±0.01 ±0.02 ±0.02 ±0.07 ±0.10 ± ±0.002 ±0.003 ±0.03 ± < ±0.005 ±0.02 ±0.02 ±0.14 ±0.10 ±0.002 ±0.003 ±0.03 ±0.01 a) base material b) weld metal Figure 8: The typical microstructure of primary piping materials

8 SPT results Initial state of material properties was evaluated for the all reference materials which were used for long-term exposure in the TAM project. An optimal cutting plan was prepared for each experimental material block and the samples were divided into pieces by EDM machine. Sets of SPT samples were prepared by the VUJE standard procedure and tensile properties were determined. The testing procedure and parameters were in accordance with [7] and were presented in [10]. Summary of the tensile properties obtained on materials of primary pipings (PP) using the SP testing is listed in Table 3. Some of reference materials have never been used and others were already operated for some time, under condition of NPP primary circuit. Table 3: SPT results of tensile properties on the reference materials in TAM project SPT tensile results NPP Component Material Condition Rm [MPa] Re [MPa] Reference testing unused Ch18N12T material after operation of primary piping Weld metal after operation Reference testing material from surge line 08Ch18N10T unused after operation Sampling material from PP EBO3 Sampling material from PP EMO1 08Ch18N12T after operation Weld metal Ch18N12T after operation Weld metal Discussion and summary Based on the above presented results, it is possible to state that: It is necessary to monitor the thermal ageing of components important to safety. The thermal ageing monitoring program was prepared and implemented on the demand of NPPs operator and the Nuclear Regulatory Authority of the Slovak Republic. The important part of this program is the surface sampling. The presented results confirm the real possibilities of surface sampling of real components.

9 Sampling of the main primary circulation pipeline at Bohunice Unit 3 was performed during the regular outage in 2013 and sampling of the main primary circulation pipeline at Mochovce Unit 1 was performed during the regular outage in Because there is the lack of experimental materials, the great advantage is using of SPT methods for evaluation of the basic mechanical properties. The complete analysis of initial properties of the first part of TAM project will be used in future for the assessment of material changes due to influence of the long-term NPP operation. Acknowledgements This contribution is the result of the project implementation Long-term operation of nuclear power plant type VVER 440 regarding the environment, ITMS supported by the Research & Development Operational Programme funded by the ERDF.

10 References [1] IAEA: INTERNATIONAL ATOMIC ENERGY AGENCY, Ageing Management for Nuclear Power Plants, Safety Guide No. NS-G-2.12, IAEA, Vienna, 2009 [2] NUCLEAR REGULATORY AUTHORITY OF THE SLOVAK REPUBLIC, Safety Guide BNS I.9.2/2001 Ageing management of nuclear power plants requirements [3] Petzová, J. et al.: Project of thermal ageing monitoring of NPP primary circuit materials in NPP Bohunice and Mochovce - project design, VUJE research report No. 0360/02/2010 (in Slovak) [4] Baľák, M. et al.: Project of thermal ageing monitoring of NPP primary circuit materials - realization project, VUJE research report No. 0360/17/2010 (in Slovak) [5] Hermanský, P. et al.: Computational verification of the places for material sampling of primary main coolant piping, steam generator shell and reactor pressure vessel. Technical report VUJE No. 0360/03/2010 (in Slovak) [6] Roberts P. R. et al.: Material sampling from components within the nuclear industry and the extraction of useful materials information. Conference proceedings of 1st International conference SSTT, September 2010, Ostrava, Czech Republic [7] Baľák, M. et al.: Mass-production of SPT specimen, Conference proceedings of 1 st International conference SSTT, September 2010, Ostrava, Czech Republic [8] CEN Workshop Agreement Small punch test method for metallic materials CWA 15627:2007, December 2007 [9] Kupča Ľ. et al.: Actual status of SPT testing application in Slovak NPPs. Small Punch Test Workshop Testing and Processing of Data, Faculty of Engineering, The University of Nottingham, Nottingham, UK, June 2011 [10] Petzová, J. et al.: Application of Small Punch Testing Methods for Material Property Monitoring of Primary Piping of Nuclear Power Plant, Conference proceedings of 3 rd International conference SSTT, September 2014, Seggauberg near Gratz, Austria