NEW APPROACHES FOR FLOW-ACCELERATED CORROSION

Transcription

1 NEW APPROACHES FOR FLOW-ACCELERATED CORROSION M. Bakirov a, H. Cheng b, V. Levchuk a, L. Selesnev a, A. Eremyn a IAEA-CN a Centre of Materials Researches and Lifetime Management (CMSLM), Moscow, Russia b Nuclear Power Engineering Section, Division of Nuclear Power, IAEA, Vienna, Austria address of main author: testm@orc.ru NPP s main equipment and pipelines manufactured from carbon steels are subjected to erosion-corrosion (flow accelerated corrosion FAC) wear. Malfunctions in Russian NPPs operation as a result of FAC occur on average 3 times per year. In the Figure 1 there are performed statistic data on metal wall thinning for typical elements (T-joints, bends, transitions, after fitting) of the piping and equipment on WWER and RBMK NPPs. These data have been received from operational inspection sheets to the end of From the Figure 1 it is seen that only about 10% of the elements are subjected to the intensive wear (more than 20 % of wall thinning) and are potentially dangerous considering probable reaching of critical metal wall thickness which is set according to normative Codes. For the rest 90 % of the elements metal wall thinning is not significant (less than 20 %) during the whole period of operation. So, these elements are not potentially dangerous to be ruptured as a result of FAC wear. no thinning 48% of elements thinning less than 10% 25% of elements thinning m ore than 30% 4% of elements 20-30% thinning 6% of elements а) WWER NPP s elements 10-20% thinning 17% of elements no thinning 43% of elements thinning less than 10% 32% of elements thinning m ore than 30% 4% of elements 20-30% thinning 7% of elements 10-20% thinning 14% of elements b) RBMK NPP s elements FIG. 1. Statistic data on metal wall thinning for WWER and RBMK typical elements

2 Also it should be noted that a design margin being set on metal wall thinning owing to foresee corrosion process is 1,2 mm on 30 years of operation it is 0,04 mm per year. Actual rate of metals FAC wear can achieve 0,2-2,5 mm per year - that is dozens times more than the design thinning margin. So, the main issue now consists in recognizing the most dangerous zones with intensive FAC wear, in-time operational diagnostics of metal condition, FAC wear forecasting and prevention of piping and equipment malfunctions caused by metal destruction due to intensive uncontrollable FAC wear. Current situation in nuclear industry in Russia Scopes and periodicity of FAC operational inspection on NPP s are set by Typical programs of operational inspection of equipment and piping base metal and weld joints acting in nuclear industry. These programs differ for each type of the reactor assemblies. NPP s operational experience shows the following: there are places which have not been included into the program of the periodical operational inspection and where considerable rates of FAC wear exist; there is a redundant scope of the inspection for elements with not significant FAC wear; there is no elements classification reflecting risk rates of FAC damaging, now operational inspection is to be done for all typical elements with the same periodicity, i.e. elements specifics are not considered; there is an imperfection in data collection and systematization system sharp necessity of creation of the common electronic data base exists; there is an insufficient analysis of periodical inspection results and no forecast of FAC dynamics are done. So, NPPs have a very hard task to provide reliably in-time revealing of elements with extreme or non-accepted metal wall thinning. Current technology of FAC operational inspection of the equipment and piping requires carrying out point inspection of the wall thickness in accordance with PNAE G , and following disadvantages of that point method are seen: preliminary trimming of the surface is required; discreteness of the point inspection the method does not enable to obtain common picture of metal wear and to reveal reliably local zones of maximum thinning; the method does not provide reiteration and comparability of periodical inspection results; problem of dense corrosive deposits influence on inspection results have not been solved until now; influence of a human factor on inspection results. The way to solve all mentioned problems is in using of automatic wall control systems which enable to make complete scanning of the surface with computer form of results recording. 2

3 Main strategy of a new design-experimental approach application for perfection of FAC wear inspection on NPP s Currently concern Rosenergoatom is developing 4-years Program of FAC works intended to improve normative documents and to optimize scopes and periodicity of FAC operational inspection of the secondary circuit equipment and piping of WWER The program aims implementation of the complex of activities used to prevent ruptures and to improve erosioncorrosion resistance of NPP equipment and piping. Goal of the initiated work lies in elaboration of scientific theoretical and of the methodological basis of physical chemical and mathematics models, definition of technical solutions and method of diagnostics, forecast and control of the erosion-corrosion processes. Also, it is very important to increase erosion-corrosion resistance of the metal, increase of reliability and safe operation of the power equipment of the second circuit of NPP with WWER by use of elements FAC monitoring. As a polygon for FAC works implementation there were used 4 one-type WWER-1000 units of Balakovo NPP. These units perfectly suit for FAC problems resolving due to the following factors: all units are of the modern design, they are under operation for the long period (units were set in operation step-by-step with a period between sets of about 2 years). As a consequence, these units already have piping and equipment elements, being damaged by FAC mechanism; secondary circuit piping have single-type layouts, identical materials, thermal-hydrodynamic parameters of the flow, water-chemistry parameters, etc. there is a possibility in the frames of this work to collect all the necessary data regarding the really operating NPP elements. In 2005 specialists of the Center of material science and lifetime in cooperation with NPP staff had executed on Balakovo NPP, Unit 3 the main algorithm of a design-experimental approach for the purpose of FAC problem solving, also inspection of NPP elements by use of modern equipment was carried out. In the frames of this work there was carried out training and certification of NPP staff for application of new equipment and methods. Training was held in a new Scientific Certificate Center by use of real samples with FAC wear cut on NPP. On a first stage of the work there was carried out an enlarged collection and analysis of initial data for the consequent design-experimental substantiation of the pipelines operational reliability. It was analysed about 1500 elements of the secondary circuit piping and for the inspection were chosen 30 zones after the throttling, regulating and transition elements. Chosen elements were subjected to intensive FAC wear during operation and are the most critical from the point of potential rupture as a result of critical metal wall thinning. Accepted design-experimental approach includes implementation of the following main stages: Elements classification concerning risk rates of FAC damaging. Elaboration and application on NPPs of new methods and equipment of the operational inspection. 3

4 Elaboration of a design-experimental procedure for FAC rates definition. Verifying stress calculation for determination of the minimum acceptable wall thinning levels. Actions intended for increase of FAC resistance of NPP equipment. Elements classification concerning risk rates of FAC damaging First of all it is necessary to do an enlarged collection of initial data on NPP for all 4 units in the following scope: analysis of piping layouts; material data, piping data sheets, certificates, physical-mechanical properties; water chemistry data for the whole period of NPP operation; thermal-hydro-dynamic parameters of the working fluid, phase composition; collection of the operational inspection results (wall thickness, defectability, cases of damages occurrence), scope, periodicity and types of the control; working modes - continuous operation or periodic switch-on; revision of input data - geometry, chemical composition, mechanical properties; determination of the areas with restricted access for the inspection; choosing the definite number of inspection zones, which overlap the total scope of the areas to be inspected from the position of possibility to distribute the inspection results in determined areas on the other elements; on a base of the determination of the most probable damage mechanisms (erosioncorrosion, pitting, stress corrosion cracking, drop-impact erosion, cavitation erosion), choosing the most effective methods and tools for inspection. All input data are collected into a common electronic data base which will be used for a comprehensive analysis of NPP secondary circuit elements. In parallel specification of the necessary and sufficient for analysis dada is to be done and all not available on NPP data are to be collected in-site and incorporated into the common data base. To define the most critical zones having maximum FAC wear it is necessary to elaborate procedure of the comprehensive analysis of the secondary circuit elements considering an experience of NPP operation and mutual discussion between NPP staff, general designer of the equipment and other nuclear industry institutes. Implementation of the comprehensive analysis algorithm enables to classify all elements on groups depending on rates of FAC risk wear and to segregate the first group of the most dangerous zones including zones which have not been included into operational inspection program on NPP. On a base of received data it is possible to elaborate and approve Work program for execution of expert operational inspection on the pilot NPP units applying new modern methods and equipment. 4

5 Elaboration and application on NPPs of new methods and equipment of the operational inspection. Center of material science and lifetime in cooperation with Special scientific engineering company (Ukraine) had elaborated and manufactured an improved Electro-Magnetic-Acoustic (EMA) tester for wall thinning measurements which enables to make continuous scanning of the measured surface. General view of the device is performed on a Figure 2. FIG. 2. General view of EMA-tester. This devise compared to the traditional US-testers has the following advantages: enables to perform continuous scanning of the surface and to obtain 3D-shape of FAC wear; excludes gaps of local wall thinning; inspection process without preliminary surface preparing; the device has PC-form of inspection results records; lubrication or contact fluid is not necessary. On Figures 3, 4 it is performed an example of 3D-shape of wall thinning obtained during the expert operational inspection on Balakovo NPP, Unit 3 in

6 Wall thickness, mm Axial direction, mm Circumferential direction, hours mm FIG. 3. 3D-shape of FAC wear surface Circumferential direction, hours, Axial direction, mm mm FIG. 4. Flat topogram of FAC wear surface 6

7 Application of the modern equipment of operational inspection on NPP with continuous scanning enables to exclude a local wall thinning, to improve reliability of inspection results, to provide record of inspection data in automatic mode and facilitates incorporation of inspection results to the common data base. Proposed form of inspection data record allows to compare results of the periodical operational inspection, to asses dynamics of FAC wear progress during different operational periods. 3D-shape of FAC wear surface can be used as initial data for verifying stress analysis defining minimum allowable wall thickness levels. In parallel with wall thickness measurements there was carried out an inspection of the actual stressed state for 30 elements. Stressed state was measured since residual stresses are one of causes of intensive FAC wear processes. Stressed state was measured by use of magnetic structure tester. Example of stressed state inspection results are performed on a Figure 5. Obtained stresses in the element metal allow to precise FAC zones with maximum wear. Circumferential direction, hours Axial direction, mm Stress, MPa FIG. 5. Results of stressed state measurements Elaboration of a design-experimental procedure for FAC rates definition For elaboration of the Procedure for FAC rates definition two approaches should be used: 3Dmodelling of flow hydrodynamics and neuron network planning. For determination of the thermo-hydrodynamic parameters of the flow including definition of flow rates and flow energies are used standard finite element programs for thermal-hydro-gasdynamic calculations (for example, ANSYS/CFX-5.x, FLUID). Results of hydrodynamic modeling are also used for optimization of inspection zones. On Figures 6, 7 it is shown an example of 3D hydrodynamic modeling of feed water piping elements on WWER-1000 NPP. 7

8 FIG. 6. 3D-layout of the feed water piping Flow velocity field (element а) Turbulent energy pulsations field (element а) Flow velocity field (element b) FIG. 7. Hydrodynamic flow modeling 8

9 Received results enable to make correction of zones for wall thinning inspection. IAEA-CN For the purpose of forecast of FAC process dynamics the most perspective way is using a technology of neuron networks: input data for the analysis are hydrodynamic modeling results, electronic data base and results of expert operational inspection of wall thinning in the worst places. Principle of a neuron network work is shown on Figure 8. Input 1 Synapses Input 2 Output Input 3 Neuron kernel Hidden neurons FIG. 8. Principle of a neuron network work By use of neuron networks technology it is easy to investigate a dependence of forecasted value from independent variables. For example, taking parameters of the working medium (ph, T, composition), metal, flow as initial data and information from data base enables to use neuron networks for making forecast of metal wall thinning in a definite zone and FAC process progress. One more advantage of neuron networks lies in the fact that building of neuronstructure model happens during a process of network self-training without human participation. Design-experimental approach for FAC processes modeling gives a base for creation of a soft for FAC rates calculation (FAC Codes) in a single-phase and double-phase flows. To verify an certify FAC Codes it is used existing electronic data base as well as experimental tests on a FAC test-bench in accordance with specially elaborated program of experiments with variation of different parameters effecting on FAC rates. Implementation of the above mentioned works enables to elaborate and certify a procedure of design-experimental substantiation of zones, scopes and periodicity of the operational inspection of NPP piping and equipment. Verifying stress calculation for determination of the minimum acceptable wall thinning levels Existing in Russia normative documents for the stress calculation of the minimum acceptable wall thinning of the elements with FAC wear consider only uniform wear shapes and do not consider the real shape of FAC wear. 9

10 Real shape of FAC wear can be obtained by use of modern equipment realizing the method of continuous scanning which allow to receive general picture of FAC wear as well as real shapes of local zones of wall thinning. A new stress calculation is intended to receive allowable geometrical shapes of wall thinning as well as wear depth. Relationships between the real shape of the defect and its depth enable to define a current condition of the element from the point of its further reliable operation and to forecast its residual lifetime. On a Figure 9 it is performed an example of lifetime curves building for definition of the residual lifetime of the element considering the real wear shapes: zone I supercritical condition, zone II noncritical condition. Zone I Boundary lifetime curve Zone II FIG. 9. Definition of the residual lifetime of the element with FAC wear Actions intended for increase of FAC resistance of NPP equipment. As the actions intended for increase of erosion-corrosion resistance of NPP equipment the following actions can be outlined: elaboration of recommendations for choosing of the chemical composition of the materials used for manufacturing of NPP piping and equipment: definition of the minimum allowable contents of alloying elements in carbon steels; applying on NPPs a technology of universal protective coating placing (thickness up to 0,02 mm) on the internal surface of the metal for the purpose of increase the wear resistance and lifetime of elements operation; passivation of piping and equipment inner surface by means of protective films creation. 10

11 Clarification of the main stages of FAC Program In a Table 1 the main stages of proposed FAC Program are presented. TABL. 1. The main stages of FAC Program Items of FAC Program implementation 1 Selection of pilot NPP units for implementation of the new design-experimental approach for FAC problem resolving. 2 Collection, systematization and analysis of the input data from the pilot NPP concering data as regards to equipment and piping FAC wear progress. 2.1 Analysis of piping layouts, features of the construction of different elements and equipment. 2.2 Collection of certificates, data sheets and design data for the different elements of equipment and piping: geometry, steel grades, mechanical properties, chemical composition, etc. 2.3 Collection of water chemistry data for the whole history of NPP operation. 2.4 Collection of data on thermo-hydro-dynamic parameters of the working fluid, its phase composition. 2.5 Collection and analysis of the operational inspection results (wall thickness measurements, metal flaws, cases of damages occurrence), data on scope, periodicity and types of the operational inspection. 2.6 Collection of working modes data - continuous operation or periodic switchon. 2.7 Improvement of the electronic data base and creation of primary atlases of erosion-corrosion wear for different types of NPP elements. 3 Specification of the necessary and sufficient scope of data for analysis, in-site collection of all data not available on NPP and its incorporation into the common data base. 4 Elaboration and implementation of an algorithm for comprehensive analysis of the secondary circuit elements to define zones which are most susceptible to FAC wear. 4.1 Forming of a work group including NPP specialists, representatives of the general designer of the equipment and specialists from other nuclear industry institutes. 4.2 Taking into account the extensive experience of NPPs operation, collective definition of the most critical zones with maximum FAC wear rates. 4.3 Primary classification of all elements on groups (about 3 groups) depending 11

12 Items of FAC Program implementation on rates of FAC risk wear. 4.4 Analysis of the selected elements: definition of the worst elements (first group no more than 20% of a total elements number). 4.5 Application of standard finite element programs for thermal-hydro-gasdynamic calculations to make correction of zones for wall thinning inspection. 5 Development and certification of a new equipment and procedure for metal wall thickness measurements to enable obtaining of 3D wear shapes applying automatic electro-magnetic acoustic scanners for continuous wall thickness inspection providing. 5.1 Technical realization of the wall thickness inspection method which allows to control large plots without preliminary metal surface preparing and without use of sophisticated automatic scanners. 5.2 Automatic inspection data recording and 3-D wear shapes reconstruction. 5.3 Elaboration of a standard and uniform view of inspection results performing. 5.4 Taking into consideration an effect of solid corrosive deposits in the inner surface of the metal on inspection results, improving reliability of inspection results. 5.5 Creation of a bank of test-samples with real FAC wear cut on NPPs, elaboration of certification procedures. 5.6 Elaboration of a soft for making comparison of periodical inspection data. 6 Development and certification of a new equipment and procedure for metal residual stresses measurements in elements subjected to FAC wear for the purpose of obtaining real stressed state of NPP elements and using this data as one of input parameters for FAC wear rated determination. 7 Elaboration of a procedure for in-site measurement of a metal chemical composition determination of alloy elements mass content effecting on FAC rated decrease chrome, cuprum, molybdenum. Recommendations for definite equipment application in-site. 8 Elaboration of a Work Program for execution of expert operational inspection on the pilot NPP units of the elements from 3 ranked groups applying new modern methods and equipment. 8.1 Inspection in the worst places being selected. 8.2 Inspection in the new places which have not been included into the Operational Inspection Program. 8.3 Selective inspection in the places which are less dangerous for FAC wear (groups II and III). 8.4 NPP staff training and certification for using new equipment and procedures 12

13 Items of FAC Program implementation for operational inspection. 9 Elaboration of a design-experimental approach and creation of codes for making a forecast of FAC wear progress in different NPP elements by using the electronic data base with NPP input data, the results of the expert operational inspection. 10 Development and manufacturing of an experimental test-bench for carrying out the real etalon erosion-corrosion tests. Implementation of different experiments and experimental results using for verification of the design-experimental codes for FAC rates calculation. 11 FAC calculative codes modification and verification. Attestation of FAC codes intended for determination of FAC wear rated in arbitrary elements Attestation of the code for calculation of FAC wear rates in a single-phased flow Attestation of the code for calculation of FAC wear rates in a double-phased flow. 12 Performing a set of calculations for different NPP elements for the purpose of definition of FAC wear rated by using the certified FAC codes. Making a secondary classification of all elements on groups (3 groups) depending on rates of FAC risk wear. 13 Elaboration and attestation of a design-experimental FAC Procedure for substantiation of zones, scopes and periodicity of FAC wear operational inspection of NPP equipment and piping. 14 Elaboration and attestation of normative documents for the stress calculation of the minimum acceptable wall thinning of the elements with FAC wear considering the real shape of FAC wear. 15 Investigation of different steel grades FAC resistance depending on a mass content of different alloy elements in a metal composition using a test-bench intended for investigation of FAC process under high-speed drop-impact water action. 16 Recommendations for increase of erosion-corrosion resistance of NPP equipment Elaboration of recommendations for choosing of the chemical composition of the materials used for manufacturing of NPP piping and equipment: definition of the minimum allowable contents of alloying elements in carbon steels Applying on NPP a technology of universal protective coating placing (thickness up to 0,02 mm) on the internal surface of the metal for the purpose of increase the wear resistance and lifetime of elements operation Passivation of piping and equipment inner surface by means of protective films creation. 17 Elaboration of procedures for collection and systematization of operational inspection and design data (wall thickness measurements, ultra sonic inspection, water chemistry, design parameters, metal chemical composition, residual stresses). 13

14 Items of FAC Program implementation 18 Modification of Regulative Documents acting in NPP industry, correction of Typical Programs of Operational Inspection optimization of zones, scopes and periodicity of operational inspection on a base of: design-experimental FAC Procedure for substantiation of zones, scopes and periodicity of operational inspection; analysis of the results of regular operational inspection on NPPs; calculative results of FAC rates definition for arbitrary NPP elements by use of improved FAC Codes. 19 Elaboration of normative base for diagnostics, monitoring and residual lifetime assessment of NPP equipment and piping. 20 Making recommendations to the General Designer for making a new improved design of NPP components. Conclusion Described design-experimental approach for FAC problem solving will enable to execute the following works: elaboration and certification of the procedure of design-experimental substantiation of zones, scopes and periodicity of the NPP elements operational inspection; elaboration and certification a new Regulatory Document of stress calculation for definition of the minimum acceptable wall thickness levels considering real wear shape, FAC rates and inaccuracy of devices for wall thickness measurements; improving the current Regulatory Documents and correcting of the Typical programs of operational inspection optimization of zones, scopes and periodicity of the inspection; elaboration of recommendations for operational lifetime prolongation of the WWER second circuit s elements by means of increasing of erosion-corrosion resistance of the new and operating energetic equipment and of the equipment, exceeding the design lifetime; improving of safe and uninterrupted work of the power unit due to in-time prediction of the most damaged elements behavior from point of view of erosion-corrosion effects and determination of lifetime for the equipment, operating in the conditions of increased thermalmechanical and hydro-dynamical loads, which cause intensive erosion-corrosion wear of the metal; creation of system for monitoring of erosion-corrosion conditions of the second circuit s elements of WWER power units; optimization and effectiveness increasing of diagnostic and preventive regulations, inspection methods of erosion-corrosion state definition and prevention of emergency situations and unplanned shutdowns occurrence as a result of damaging of NPPs working elements metal; creation of the normative base for data collection, diagnostic and monitoring of the second circuit s equipment and pipelines of WWER power unit. giving the recommendations to the main designer for making a new improved design of NPP components. 14