Proceedings of the National Seminar & Exhibition on Non-Destructive Evaluation. NDE 2009, December 10-12, 2009

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1 210 Proceedings of the National Seminar & Exhibition on Non-Destructive Evaluation Four Decades of Operating Experience on Detection, Condition Monitoring and Health Assessment of Tarapur BWR Nuclear Surface Condensers Role of NDE, at Tarapur Atomic Power Station-1&2, NPCIL, India A. Ramu, A.Jabbar, N.K.Dhanmeher, N.K.Roy, V.S.Daniel, Ravindranath, S.Bhattacharjee and U.Ramamurty Tarapur Atomic Power Station -1&2, Nuclear Power Corporation of India Limited, PO: TAPP; Dist: Thane; Maharashtra , India akkalaramu@gmail.com; aramu@npcil.co.in NDE 2009, December 10-12, 2009 Abstract Tarapur Atomic Power Station is a twin-unit Boiling Water Reactors (BWRs) is one of the vintage reactors operating at its best efficiency meeting the needs of both Maharashtra and Gujarat States. The rated capacity of each unit was 210 MWe but it was re-rated to 160MWe in the year subsequent to the isolation of Secondary Steam Generators (SSGs). Since then both the units are in operation at its rerated capacity. The station has completed 40 years of successful, commercial and safe operation. Main condenser is a one of the vital conventional system components which has direct bearing on cycle efficiency. The tube material is Aluminum Brass with Aluminum-Bronze tube sheets. The tubes are lightly rolled, seal welded without filler material and expanded with the tube sheets. The station has observed various failure mechanisms in the condenser tube material & its associated auxiliary equipments and remedial measures were taken from time to time based on the Operating Experience (OE) feedback received from overseas BWRs. TAPS has established various NDE techniques to effectively detect these degradations mechanisms. Ageing effects is one of the concerns of older generation plants and methodologies need to be adopted to identify various degradation mechanisms. This paper describes various degradation mechanisms of surface condensers, their detection and condition monitoring & health assessment using various NDE techniques. Also mentions about the inspection methodologies followed at Tarapur to effectively detect the degradation of tubes & other auxiliaries by optimizing inspection techniques and detecting capabilities. Keywords: Surface Condenser, Aluminum Brass, Aluminum Bronze, Boiling Water Reactors, Inlet-end erosion, Fretting failure, De-zincification. Tarapur Atomic Power Station is a twin-unit Boiling Water Reactors (BWRs) and is one of the vintage reactors operating in the world at its best efficiency meeting the needs of both Maharashtra and Gujarat States. Both the units were commissioned in the year 1969 with rated capacity of each unit was 210MWe. Similar vintage reactors operating world-wide are Oyster Creek in USA and Tsuruga of Japan. These units were re-rated to 160MWe in the year subsequent to the isolation of Secondary Steam Generators (SSGs) of secondary cycle due to problems associated with tubes. Since then both the units are in operation at its rerated capacity. The station has completed 40 years of successful, commercial and safe operation. Ageing effects is one of the concerns of older generation plants and methodologies are being adopted to identify various degradation mechanisms and are eliminated either by design modification or by replacements with better resistant materials. This process is required to enhance the life of components for continued service. TAPS had also taken up corrective steps in this regard and a comprehensive study was conducted in association with the expertise from Nuclear Power Corporation of India Limited (NPCIL)-HQ and various agencies of Bhabha Atomic Research Centre (BARC). 1. Introduction The condenser of Tarapur BWRs is a Steam Surface type of Condenser and is one of the vital conventional system components which have direct bearing on cycle efficiency. The operating cycle is Regenerative Rankine Cycle, in which part of the steam is extracted from different stages of turbines (HP&LP) to regenerate the condensate thru. Feed Water Heaters (FWH) for improving cycle efficiency. Initially TAPS-BWRs worked on Dual-Cycle which is designed to operate with both Primary and as well as Secondary Cycle. 70% power output was from the Primary Cycle, i.e., from Reactor Pressure Vessel (RPV-Boiler) and balance 30% was from secondary cycle SSGs. The unit was designed to operate with load following characteristics and

2 NDE 2009, December 10-12, Fig. 1 : Dual Cycle of TAPS-1&2 BWRs without changing the reactivity mechanism pattern the power can be varied with SSGs steam out put to HP turbine at 5 th Stage. The purpose of Condenser is to condense the exhaust steam from the turbine for re-use in the closed cycle to maximize turbine efficiency by maintaining proper vacuum. As the pressure of condenser is lowered (vacuum is increased), the enthalpy in the turbine also will increase. This results in (a) increased turbine output (b) Reduced Steam flow and (c) Increased Plant efficiency. Therefore it is necessary to maintain the condition of surface condenser vis-à-vis its vacuum as high as possible. The surface condenser is once-through condenser at Tarapur and is cooled by sea water drawn from Arabian Sea through intake canal. In case of TAPS-BWRs low impurity concentrations, especially chlorides & sulfates during operations is prime importance. Ingress of chloride content in the primary system affects the performance of Austenitic Stainless Steel components, Stress Corrosion Cracking (SCC) and hence it is essential to keep the condenser leak tight from ingress of sea water. Non-availability of surface condenser results in energy loss thus economic penalty; therefore it is important to detect various degradation mechanisms that affects condition of surface condensers using different NDE techniques and should be mitigated appropriately. In view of this, utmost importance has been given to assess the condition with online performance monitoring techniques. TAPS has also experienced various degradation mechanisms and developed various inspection methodologies with operating experience feed back (OE) from overseas reactors and in-house experience as well. The following paragraphs indicate various degradation mechanisms detected and mitigated with developed/enhanced inspection methodologies from time to time at TAPS. 2. Description-Surface Condensers The Surface Condenser is a rectangular-shaped, welded steel plate unit of the single-pass, vertically divided type. The general technical specification of Tarapur condensers is given in Table 1. The tube material is Aluminum Brass with Aluminum-Bronze tube sheets. The tubes are welded in both tube sheets. The inlet end of tube is flared for smooth entrance flow characteristics. The tube bundle is sloped from outlet end to inlet end of the condenser to facilitate drainage of cooling water when the units are not in operation. The tubes are lightly rolled, seal welded without filler material and expanded with the tube sheets. Table 1 : General Specifications of TAPS-1&2 Surface Condensers Description Surface Area Specifications 171,050 Square feet No. of tubes 16,334 Number of 1"OD 18 BWG tubes Number of 1"OD 16 BWG tubes (Impingement Areas of Tube Bundle) 300 Tube Material: Main Tube Bundle Tube Material: Impingement Areas Tube sheet Material Aluminum Brass Aluminum Brass Aluminum Bronze Effective tube Length 40-0" Actual tube lengths required for tube replacements /32" Number of Tube Support Plates 11

3 212 Ramu et al. : Proceedings of the National Seminar & Exhibition on Non-Destructive Evaluation The degradations mechanisms depend upon various factors including material of construction, operating conditions and other external factors. Table 2 gives various components of surface condenser with Material Of Construction (MOC). The degradation mechanism of each component is discussed in detail in the subsequent paragraphs and operating experience of past four decades has also been included Inlet-end Erosion [2,3] This type of degradation occurs as a result of turbulence created by cooling water entry from a larger area water box to much smaller area of tube ID. Due to this eddies forms at tube inlet end typically within H 1"-2" distance from tube entry. The appearance of this degradation starts with pitting and aggravates with operating time hence causes sea water Table 2 : Material Specifications of TAPS-1&2 Surface Condensers Component Description Condenser Shell Material Specifications Tube Sheet Aluminum Bronze (P No. 35) ASTM-B-171 (UNS # 61400) Tubes Tube Support Plates Water Boxes Condenser shell-to- Water box XJ Turbine Exhaust Casing-to-Shell XJ Tube Support plates to shell supports Impingement Baffles Hot well Tray Penetration Nozzles Aluminum Brass (P No.32) ASTM-B 111 (UNS # 68700) Belt type Dog- Bone shaped Rubber (Neoprene) Structural Steel Alloy Steel Fig. 2 : PVC/Nylon Tube inserts at Inlet-end of tube sheet 3. Degradation Mechanisms- an Overview The units are in operation since 1969 and have experienced various degradation mechanisms associated with tubes, water boxes & its associated piping on cooling water side and structures/internals on steam side. Initial tube failure rate was very high which might be attributed to fabrication related defects but subsequently the failure rate has come done and stabilized for both the units. However, the total number of tubes failure rate is more in unit no.1 as compared to unit no.2. Therefore, comprehensive study was carried out and condition assessment of all the associated components has been done. In this process, some of the tubes were removed from each condenser (healthy tube & leaky tube) and sent for failure analysis and condition assessment. The shell side of surface condenser being radioactive, the analysis was carried out by Post Irradiation Examination Division (PIED) at BARC Mumbai. Degradation mechanisms have been identified in Surface condenser & its associated components at TAPS are (a) Inlet-end erosion (b) Erosion-Corrosion (c) Mechanical damage (d) fretting failure (e) Under-Deposit Corrosion and (e) General Corrosion. These degradations could be detected and mitigated in a phased manner based on in-house engineering analysis & Operating Experience (OE) feed back received from time-to-time from overseas BWRs. Fig. 3 : Tube Inserts details in use at TAPS-1&2sheet in-leakages. This mechanism has been overcome by introducing Plastic tube inserts, made of PVC material. The design & construction of tube inserts is such that it allows smooth entry of cooling water into tubes thus the turbulent area falls within the tube insert. This mechanism is detected by visual examination using Optical Fibroscopes and wall reduction can be determined by Eddy Current Testing (ECT) method. One has to be very careful in selection of tube insert material. In appropriate selection/inadequate design of tube insert might cause increased turbulences behind the insert end Erosion-Corrosion [2,3] Erosion-Corrosion is a result of disruption of oxide film typically formed on the metal surface [1]. This disruption is caused by turbulences producing local shear forces

4 NDE 2009, December 10-12, Fig. 4 : One of the failed tubes shows disruption of oxide film Fig. 6 : Tube ID surface showing axial grooves formed due to movement of foreign objects. Fig. 5 : Tube ID surface showing Erosion and fine pits at the disrupted oxide film location Fig. 7 : Tube ID surface showing axially grooved region containing perforation. continuously breakdown or prevent the formation of protective oxide film on the tube surface. Erosion-Corrosion is often occurs in case of partial blockage of tube, may be caused due to debris and/or foreign material. This mechanism has been eliminated to some extent by cleaning of the tube ID surfaces at regular intervals by isolating each half of condenser water boxes. However, complete mitigation is not feasible as the cooling water is from Sea-intake canal and the water silt content depends upon the seasonal variations. Removal of turbulence causing foreign material from the tubes and preventing re-entry back into the tubes should the option to eliminate erosion-corrosion degradation mechanism. The condition of protective film would be difficult to estimate by NDE methods, however, sample tube failure analysis would indicate the performance of remedial measures adopted, as detailed above. Visual Exanimation after thorough tube cleaning with water jet followed by pneumatic pressure jet is being followed at present. This would ensure the cleanliness of each tube thus absence of foreign material inside the tubes can be verified & documented. Visual Examination is one of the effective NDE methods with which most of the problems can be identified during preliminary inspections. VT has been useful because, the rubber ball shooting, generally done at the time of tube cleaning might stuck within the tube along the length, which would otherwise block the flow as well as causes tube erosion-corrosion. Based on the experience the stuck foreign objects may find its way to outlet side depending upon the tide level/pump start/stops. Fig.6 & 7 shows such phenomenon observed in some of the analyzed tubes of TAPS Pitting [2,3] Pitting is a localized corrosion that occurs preferentially under the deposits at tube ID surface. These deposits occur as a result of break down of protective film. This degradation mechanism has been observed in one of the remove tube analyzed for residual life assessment. Pitting has been noticed underneath of deposits. Tubes fouling in the form of mud deposition, foreign objects are some of the major contributors to pitting corrosion of Aluminum Brass tube ID surfaces. Cleaning with high velocity water jets may help to keep the condenser tubes clean and fouling can be well controlled. This degradation mechanism has been well understood and addressed at TAPS by scheduling tubes cleaning at regular intervals of unit operation. Tube cleaning is done after reducing the power up to 60MWe, and isolating each half of condenser water box one by one. It is also ensured that debris is removed completely from the tube sheets at inlet and tube inserts are in place as per the design intent. Visual inspection is essential for ensuring satisfactory removal of debris from the tube sheet. The failure analysis of one of tube indicated this aspect predominantly and hence cleaning methodology has been changed since then and through visual examination of all the tubes is being followed. The methodology has improved the tube leak detection in hydrostatic testing in four ways: (1) Proper tube cleaning with high pressure water jet followed by pneumatic ensured positive removal of mud/debris from each tube (2) Cleaning also removes the fine saw dust which is used for on-lining plugging of very minute seepage leaks from tubes (3) it allows the leakage path opened fro leak

5 214 Ramu et al. : Proceedings of the National Seminar & Exhibition on Non-Destructive Evaluation The Surface Condenser in discussion is in service for the past 4-decades hence ageing effects of material should also be taken into consideration while assessing the condition of tubes for continued service. In view of this, Eddy Current Testing of the tubes has been taken up in a phased manner for condition assessment. So far no degradation of the examined tubes was observed due to vibration induced failure mechanism Uniform Corrosion [2,3] Fig. 8 : Deposits at tube ID at 20X magnification Aluminum Brass tubes are susceptible to uniform corrosion. The tube removed for failure analysis and condition assessment was thickness measured by ultrasonic testing and no considerable reduction in thickness noticed. Hence, this type of degradation mechanism has not been experienced by TAPS, however localized pitting results in perforations initiated from tube ID were observed at many locations. Fig. 9 : Internal shallow Pittings observed under the deposits 20X magnification detection during surface condenser hydrostatic testing and (4) Positive identification & subsequent leaky tubes plugging would facilitate trouble-free unit s operation as well as eliminates chloride intrusion in the primary system, which a prime contributing factor for SCC. Fig. 12 : Perforations observed within the erosion area of tube Inner Surface-initiated from ID Fretting/Vibration Induced failure mechanism [2,3] Aluminum Brass tubes are susceptible to vibration induced damage. This may occur when tube are excited by the exhaust steam flow from LP turbine to vibrations. This phenomenon can only be seen on Outer surface of the tube below the tube Support Plates (TSPs). The removed tubes which were analyzed did not show this type of degradation mechanism. At TAPS surface condenser shell side inspection is possible during refueling outages (once in 18 months) or any other planned outages. Fig. 13 : Perforation as seen from OD surface Fig. 10 : Plugged tube is being removed for failure analysis Fig. 11 : External surface of tube at Tube Support Plate-no reduction in Thk Mechanical damage due to external factors [2,3] Surface Condenser tubes are supported by 11 TSPs and these in turn take support of condenser shell with various internal structurals. Shell side of condenser has many components such as baffles, internal piping/partition plates etc., which are provided to meet the functional requirements of the system. Total 71 penetrations exist in each condenser t facilitate various systems requirements. One of such critical systems is Primary Feed Pumps (BFPs) minimum recirculation

6 NDE 2009, December 10-12, Inspection & testing plays a key role during shut down in detection & identification of leaky tubes. 4.1 ON-Line detection (Vacuum) Method [2] Fig. 14 : Tube punctured due to BFP minimum recirculation line end plate. Fig. 15 : Tube damage due to external structural member lines laid below the tube bundle with supports from hot well tray. The failure of supports leads to tube puncture due to mechanical damage. This problem was reviewed & analyzed by in-house expertise and design modifications have been implemented to overcome such failure mechanism. A detailed inspection was essential to ascertain the cause of failure and its effect on adjacent tubes. All the tubes can not be assessed for extent of damage due to external impingement attack, Eddy Current Testing was adopted to identify & confirm the healthiness of affecte5d tubes. 4. Non-Destructive Testing (NDE) Methods for Detection, Monitoring & Condition Assessment [2] The performance of surface condenser is monitored while in operation in terms of heat duty, various performance parameters like LMTD, ÄT, ÄP, vacuum and also the condensate water quality in terms of conductivity etc., any sea water in-leakages will rise the conductivity which is a concern, as most of the primary system piping is made of Austenitic Stainless Steel which is susceptible to Stress Corrosion Cracking (SCC). Performance of the surface Condenser depends on may influencing parameters out of which tube fouling affects not only the heat transfer rate but also contributes to tube failure rate. Therefore, it is necessary to monitor the above performance parameters and schedule tubes cleaning & inspection. Inspection methodologies are selected such that positive identification of leaky areas Tube leaks are detected by off-line as well as on-line, in case of on-line one of the condenser water box will be isolated after reducing the power to lower value and tube leak checks are done with either (a) manometer (b) Cellophane paper (18µm/27-28gm/m 2 ) and/or (c) soaping (soap suds) methods. During this check the tube side in under full vacuum and its detection sensitivity better. However, due to field constraints (water box area temperature, 100% RH and poor ventilation) it would be difficult to achieve the desired results. Moreover it would not be possible check all the tubes for positive tube leak identification, as 8167 tubes are to be checked after temporarily plugging the outlet end of each tube under test. It is a time consuming job and hence most vulnerable areas such as (a) tubes adjacent to the earlier failure zones and (b) tube in front of impingement baffles are covered & checked specifically. The high humidity and high radiation levels present in the vicinity due to general contamination restrict on-load tube plugging operations to the minimum. Fine saw dust addition at inlet to Circulating Water pumps can tolerated for tiding over building up of conductivity due to minor leaks in condenser tubes. This will enable for proper scheduling of tube leak checks off-line. Saw dust is added with a CAUTION to prevent entry of large wooden pieces etc., which might lodge in tubes, causes erosion damage to tubes. 4.2 OFF-Line Methods [2] Surface condenser & its associated components condition assessment is done using various NDE methods. The inspection methods used are (a) Visual Testing (VT-1) (Direct & Remote visual techniques) (b) Liquid penetrant Testing of Weld joints (c) Ultrasonic Testing (UT) for thickness gauging (d) Eddy Current Testing (ECT) and (e) Hydrostatic Testing (VT-2). Specific Codes are applicable to detect & application of these NDE methods for surface condensers are not fully available. These inspection methods/techniques have been established by TAPS from four-decades of Operating/ Feedback experience (OE). Tube leak checking procedures have been established and enhanced based on OE. After tubes cleaning, shell side is filled with demineralized water up to 30-0" static head, ensures sufficient hydrostatic pressure for positive leak identification. a) Visual Testing (VT-1& VT-3): This method is very effective in detecting various degradations associated with the surface condenser components. In Visual Examination, both direct & remote visual examination techniques are utilized to assess the component s integrity. The components covered in VT are (a) shell side internals (b) water box internals surfaces (lined with neoprene rubber) (c) waste plates/sacrificial anodes. The condition of inlet tube ends are assessed by remote visual examination method, using Fibroscopes. This is to monitor tube inlet conditions, vulnerable to inlet-end erosion

7 216 Ramu et al. : Proceedings of the National Seminar & Exhibition on Non-Destructive Evaluation c) Ultrasonic Thickness Gauging (UTG) Surface Condenser shell, water boxes and inlet/outlet piping is made of carbon steel material. Health assessment of these components is felt necessary for continued service. Thickness gauging of water boxes was done, assessed, evaluated & documented. d) Eddy Current Testing (ECT) Fig. 16 : Arrangement of sacrificial anode plates in each water box Tarapur Condensers have been in operation for the past 40years and hence assessment of tubes for continued service is essential. Therefore, tubes are subjected to periodic ECT for estimating the extent of degradation. Results are analyzed by the expertise and compared with the metallurgical studies conducted on failed tube samples. Tube plugging criteria has been established on these parameters. e) Hydrostatic Testing (VT-2)-Condenser tubes Prior to conducting this test, tube sheets dryness is achieved either by air flushing and/or by heating the tube sheets by high energy flood lights. Tubes integrity is assessed by filling the condenser shell by demineralized water step-by-step so that sufficient time is allowed to seepage water to come out of the leak path. Qualified inspection personnel in VT are employed to perform this activity. The leaky tubes identified in VT are plugged as per the established procedures. f) System Leakage tests(vt-2) System leakage test is necessary to ensure the integrity of surface condenser pressure boundary which is opened to facilitate the above inspection & testing. This also ensures that performance of components after the maintenance works performed during the shut down. Fig. 17 : Sacrificial anode plates lay out in each water box as explained above. This degradation mechanism is can also be detected by ECT & confirmed by VT. Condition assessment of waste plates is also required as it protects the tube sheet during unit s operation. Integrity assessment of 42 anode plates located in each condenser water boxes ensures protection of tube sheets & tubes. Physical verification of sacrificial anode plates is carried out & documented to preclude possibility of leaving anode plate inside the water box inadvertently, which damages the tube sheet & water box internals while unit is in operation. Rubber lining VT & holiday testing ensures the integrity and protection of water boxes against corrosion. b) Liquid Penetrant Testing (LPT): By design all the tubes are flared, light rolled, seal welded and expanded at both at inlet & outlet end. During tube leak checking if the seepage noticed at the seal weld in VT; need to be confirmed by LPT also to assess the extent of degradation and establishing the leak repair procedure. 5. Mitigation of Degradation Mechanisms-Short Term & Long term and Future Action Plans to Address Ageing Related Issues The above indicated degradation mechanisms have been detected using various inspections & testing methodologies established from time to time. The mitigative measures established and implemented effectively to limit the tube failure rate considerably. Remedial measures were taken to mitigate condenser tube failures by (a) installation of Plastic inserts at tube inlet end in 1971 (b) tubes cleaning by rubber plugs during each refueling outage (c) debris removal and tube cleaning by water-jet during short shut downs (d) Desilting of intake bays and canal to limit the silt carry over along with the cooling water flow and (e) regular dosing of cooling water by Chlorine. As both the condensers are in service for 40-years, hence a comprehensive health assessment has been performed with in-house inspection/engineering expertise. As a long-term measure, replacement of tubes with new one has been considered and a thorough review regarding selection of material was done. The material selection should eliminate the above-mentioned degradation mechanisms. Therefore,

8 NDE 2009, December 10-12, replacement with corrosion resistant material either (a) Austenitic stainless steel or (b) Titanium for sea water cooling applications has been studied. Titanium is considered to be the compatible tube material for sea water applications considering the extended operating life of the units. Titanium material has less heat transfer coefficient and hence lower wall thickness tubes are to be selected. Reduced wall thickness results in overall reduction of total tube weight and is significant effect on stresses on the turbine exhaust XJ & excessive strain on condenser supports [4]. This requires less spacing between tube support plates to avoid tube vibration related problems. High yield strength of titanium material requires compatible tube sheet material. Tube sheet holes will require grooving to facilitate tube rolling. In view of the practical difficulties, retubing with the same material has been done as it proved to be compatible with the system for 4-decades & other design parameters and successfully implemented in unit no.1, in the year The performance of condenser after re-tubing has improved as complete heat transfer area is available to take up the heat load. 6. Conclusion Tarapur nuclear surface condensers have given satisfactory service of more than three decades. Due to ageing and influence of various degradation mechanisms, replacement of tubes has been considered to be the only option available to improve the condenser performance. The operating experience feedback obtained during this period has been utilized to improve & enhance various inspection methodologies. The procedures established have been implemented and followed scrupulously to mitigate the various degradation mechanisms effectively. Acknowledgements The author is thankful to TAPS-1&2/NPCIL management in giving this opportunity to publish this technical paper in NDE The timely support provided by Post Irradiation Examination Division (PIED)/BARC in failure analysis is appreciable Also we are also thankful to Technical Committee of NDE in accepting the above technical paper in National seminar & Exhibition on Non-Destructive Evaluation-NDE References 1. Ingersoll-Rand, Instructions for Maintenance and operation of surface condenser and auxiliary equipments, 11 Broadway, New York, Bechtel Corp. Job No.4267; Bechtel Corp. Spec. No.S-4267-M3 (1963)[TAPS/MANUAL/655]. 2. K.Nanjundesawaran, Know Your Condenser,TAPS/REPORT/ 615 Rev.01 (15 th February 1984). 3. Albert Bursik & Hans-Gunter Seipp, Condenser tube failures in water-cooled condensers with copper-based alloys PPChem, (9). 4. EPRI, NP-2371, Condenser Retubing Criteria Manual, Stone & Webster Engineering Corporation, Boston, Final Report (May 1982) [TAPS/ REPORT/ 3165].