REPAIR OF NE KRSKO STEAM GENERATOR TUBES BY SLEEVING METHOD

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1 HR REPAIR OF NE KRSKO STEAM GENERATOR TUBES BY SLEEVING METHOD Prepared by: Kresimir Gudek, Drago Pete Krsko Nuclear Power Plant, Vrbina 12, 8270 Krsko, Slovenia ABSTRACT In order to decrease the percentage of plugged tubes, i.e. to avoid exceeding of plugging margin in the future, NE KrSko introduced in 1993 Sleeving as the repair method of degraded steam generator (SG) tubes. TIG welded sleeves developed by ABB CE to span defects in roll transition zone or just above the tubesheet (RTZ sleeve) and around tube support plates (TSP sleeve) have been installed during 1993 and In this document both type of TIG welded sleeves are presented including design, installation and examination. Plant specific qualification process and experience from both sleeving campaigns are presented too. At the end there is described new developed ABB Reactor weldless type of sleeve (PLUSS sleeve) which is selected by NE Kr ko as a preferable sleeving option that could replace plugging. 149

2 1.0 INTRODUCTION The operation of Pressurized Water Reactor (P WR) steam generators results in some instances with localized corrosive attack on the inside (primary side) or outside (secondary side) of the steam generator tubing. Such corrosive attack interfere to tube wall integrity in two ways - like reduction of tube wall or cracking. Based on applicable codes and regulations, degradation up to 40% of wall thickness (plugging or repair criteria) does not require any corrective action because SG tubing has been designed with considerable margin between the actual wall thickness and the wall thickness required to meet structural requirements but exceeding this value tube has to be repaired or plugged. Shortly after start up (since 1985), NE KrSko was faced with degradation processes in SG tubing inspectioned by Eddy Current (ECT). Up to year 1993 NE Kr ko used to perform only plugging on tubes exceeding plugging criteria because steam generators have been designed with plugging margin of 18% (percentage of tubes which could be plugged without decreasing of power). Approaching to the plugging margin NE Kr ko maintenance staff started to think about sleeving like relatively new repair method instead of plugging to keep plant operability on 100% of power. At NE Krsko steam generators the most of tube defects are placed at the top of the tube sheet area (TTS) and at the tube support plates (TSP) intersections, both on the "hot leg" side. Some service companies developed sleeves for the tube repairs in these locations. Fig. 1 shows a NPP Kr ko SG's structure (Westinghouse D-4 model, with tubing 3/4" OD x 0.043", ALLOY 600 MA). From technical point of view, sleeve is part of tube with outside diameter very close to the inner tube diameter inserted into the parent tube with a minimal gap. Sleeve spans defect between two joints - welded or mechanically expanded or in combination of these two ways. Both joints have to assure leak-tightness to prevent reactor coolant leakage from primary to secondary side in steam generator. At that time (1993) industry was offering several types of sleeves which could be divided into the two main groups based on technology: welded and mechanically expanded sleeve. There are two types of welded sleeves and they are related to the welding processes used for joint: TIG and LASER. Also, for weldless types there are several technics used: explosion, hydraulic and rolling expansion. Evaluating techniques and sleeve types, industry and plants experience and references NPP KrSko has chosen for the first campaign TIG welded sleeve designed by ABB CE. 150

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4 2.0 DESIGN OF TIG WELDED SLEEVES ABB CE has developed and designed two different welded sleeves: RTZ (roll transition zone) sleeve for spaning defects in top of the tube sheet and TSP (tube support plate) sleeve for defects placed at tube support plate intersections. Alloy 690 was selected for sleeve material due to increased corrosion resistance and perfect welding properties. 2.1 RTZ Sleeve RTZ sleeve design is shown of Fig. 2 bellow. Hydraulically expanded region at upper part of sleeve assure contact with the tube necessary for welding using TIG process. Lower sleeve joint is achieved using hard rolling combined with friction and sealing by carbide layer called microlock. Tube Figure RTZ Welded Sleeve

5 2.2 TSP Sleeve Fig. 3 shows TSP sleeve with expended regions above and below tube support plate. Leak tight joint is achieved by welds..tube,weld Tube Support 3.0 INSTALLATION Figure 3 TSP Welded Sleeve 3.1 RTZ Sleeve Process starts with cleaning of tubes in region where sleeve will be installed. Cleaning is performed by steel brushes and stream of air. Verification of cleanliness is done by visual examination using endoscope. After sleeve insertion in to the position follows expansion of upper joint with very preciselly controlled hydraulic equipment. TIG welding process without filler material is used to weld upper joint. In case of rejected weld it is possible to make rewelding. Upper weld is examined by NDE. Finally, lower joint is done by mechanical rolling same as at installation of mechanical removable ABB plug. Post weld heat treatment is option. Basically, if sleeve is installed for longer period heat treatment is necessary but for shorter time of operation sleeve is fully safe without this step which consumes significant time on critical path of an outage. 153

6 RTZ sleeve can be installed in about 90% of tubes. Tubes on periphery is difficult to sleeve because of channel head geometry and tools capability. Installation process which is very complicated requires nearly 100 specialists and supporting workers (jumpers) during 24 hours work, and 20 tons of remote controlled equipment as manipulators, lighting, air-conditions, computers, cameras, different tools etc. Steam generators have to be drained and dried on primary and secondary side. Both atmospheres have to be strictly controlled because success of welding depend on inside humidity conditions. 3.2 TSP sleeve TSP sleeving process differs from RTZ sleeving process because both joints (upper and lower) are welded. Because of this, NDE and heat treatment take longer time and production rate of TSP sleeve installation is approximately 20% less than RTZ sleeves installation. Today, development of tools enables TSP welded sleeving up to the 5-th tube support plate and sleeves can be installed in 95% tubes. 4.0 EXAMINATION The critical moment after installation is NDE. Three methods are used for sleeve welds examination. Visual testing is performed by remote controlled endoscope. Experience learned us that this method provides the most reliable informations about weld quality in conjunction with other NDE methods. Data recorded on video tape can be very useful to make easier ECT, especially to distinguish surface indications, like roughness, from defects to be plugged. During both sleeving campaigns NE Kr ko performed pressure test (water standing test) because this very simple method is capable to detect leakers missed by NDE (case in both campaigns). To check sleeve to tube weld fusion and completeness ultrasonic testing (UT) is applied using immersion technique. Final decision about weld acceptance is based on ECT which establishes baseline data. Today, for this method are developed very sensitive probes which are capable to detect very small defects. 5.0 QUALIFICATION Sleeve to tube assembly and functional integrity has to be maintained in normal conditions during operation as well as in accidental conditions. ASME B & PV code defines comprehensive and extensive testing program for material and design to proove sleeving like safe repair method. Laboratory testing program is consists of tests like cleaning, expansion, welding, different leakage tests performed on full-scale loop test setup, than fatigue, collapse, corrosion, axial load test, NDE etc. 154

7 6.0 EXPERIENCE FROM OUTAGES '93 AND '96 To perform successful sleeving campaign plant staff for steam generators maintenance have to be perfectly informed about sleeving processes through specific training program prepared by contractors to get necessary knowledge and experience. This job has to be well prepared from plant side. That means that for each tube which is sleeving candidate some measures and checking have to be performed. The most important restriction is sludge height. Welding performance in sludge area is very difficult and usually results with rejected joints. Therefore, eddy current sludge measurement will realise tubes with minimum 12 mm sludge level below positions of future weld joints. In addition to all restrictions of technique it is not recommended to sleeve first two rows because in next inspections it will be very difficult to examine those tubes with ECT. Calculation showed that sleeving campaign is the most cost benefit for number of over 200 tubes. It is not good decision to install more than one sleeve in one tube. Expenses per saved tube are much higher and in next outages, parent tube between two sleeves will be very difficult to inspect. In both campaigns NE KrSko decided to return also some plugged tubes into operation. These tubes have been deplugged on the"hot leg" side and examined on full length. Tubes with only one indication were added on sleeving list and directed for deplugging on "cold leg" side. In tubes with more indications plugs have been installed on "hot leg" side back. Deplugging tubes just on one side decreases cost and time. Taking into considerations all mentioned restrictions and experience sleeving campaign can be optimized. 7.0 ABB WELDLESS SLEEVE (PLUSS SLEEVE) Last two years NPP Krsko has been in very close contact with ABB Reactor during developing of new sleeve type without welding. An idea is to make joint between tube and sleeve performing just expansions. Concept of new pressure boundary created between sleeve and tube is based on different yield strengths of materials. Sleeve material (ALLOY 800) has lower yield strenght. The elasticity of the tube is much higher, but its real spring back effect is blocked by the plastically expanded sleeve resulting in a contact pressure between tube and sleeve. This contact pressure provides all codes and regulations requirements at ambient temperature as well as at load parameters and accidental conditions. With 6 expansion zones below and 6 above TSP, or 6 expansions above and hard rolling below TTS, joints are assured. Such configuration is capable to fulfill all requirements. Fig. 6 shows PLUSS Sleeve design. 155

8 N Stem Figure 4 Pluss Sleeve 8.0 SUMMARY Good experiences with installed welded sleeves since 1993 is confirmed by fact that up to last inspection no one indication is found in sleeve region. Next target is to keep plant operating on full power till year 2000 when steam generators will be replaced. Krslco plant is just passing through PLUSS sleeving licencing process waiting for positive decision from URSJV*. With new sleeve plant will get possibilities to save tubes degraded also up to 10-th TSP as well as drastically reduce cost and time for outage. This sleeve is perfect solution for short tenri repair, because this method does need welding, heat treatment, extensive NDE and it is close to expenses to mechanical plugging. While RTZ PLUSS sleeves are already in operation in Belgium and Korean plants, NE Krslco will be first one which will install TSP PLUSS sleeves. * Slovenian Nuclear Authorities References: ABB CE ABB Reactor ABB Reactor "Reports on Testing and Qualification of the Sleeving Process" "SG Tube Repair for W Designed Plants with 3/4" Alloy 600 Tubes Using PLUSS Sleeves, GBRA B "Scope of Products, Steam Generator Services" 156

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