EXPERIENCES ON MASSIVE QUALIFICATION TEST BLOCK COMBINING MANY INSPECTION OBJECTS

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1 EXPERIENCES ON MASSIVE QUALIFICATION TEST BLOCK COMBINING MANY INSPECTION OBJECTS Raimo Paussu, Jani Pirinen; Fortum Power and Heat Ltd, Espoo, Finland Petri Luostarinen, Fortum Power and Heat Ltd, Loviisa NPP Iikka Virkkunen, Mika Kemppainen; Trueflaw Ltd, Espoo, Finland Reijo Pirttilahti; Mekelex Ltd, Somerniemi, Finland ABSTRACT Fortum operates two VVER-440 nuclear power plants units in Loviisa, Finland. The in-service inspections are qualified according to Finnish qualification rules, closely following ENIQ recommended practices. Over the years, Fortum has gathered experience on inspection qualifications and on fabrication of own test blocks and defects. The on-going qualifications of outside UT inspections of bottom area of the pressurizer are presented in this paper. The inspection areas are longitudinal weld in bottom, inner radius areas of two surge line nozzles, nozzle welds to bottom and weld joints of inside structures on bottom. Following the ENIQ methodology, both open and blind samples are required for qualification. The outside UT inspections, scanning along the bottom surface using long sound paths will need massive and representative mock-up to fully simulate the real inspection situation. It was decided to implement all these requirements and inspection objects in the same massive mock-up. The solution was to order one massive bottom weighting over 7 tons and to weld one layer strip cladding and grind it smooth before starting the fabrication of actual inspection objects. This approach gave several advantages: representative mock-up, versatile use of different flaw types and fabrication processes, practical training for the next full size component (RPV emergency cooling nozzles). It also came with some challenges: transportation arrangements, efforts for various aspects of production processes and devices needed for the massive mock-up. The paper discusses these various aspects in practical test block design, manufacturing of inspection objects and their defects, and lessons learned from the project. INTRODUCTION Loviisa NPP unit 1 started operation in 1977 and unit 2 in Pressurizer is located in a closed space in reactor building and component is really hard to replace if needed. Due to too rigid support structure of LO1 pressurizer, the prognosis of service life of support structure analyzed by the utility specialist was two years, figure 1. Fatigue issues and long service life of pressurizer was not considered in finishing work of component structures. Figure 1 - Original support structure of LO1 pressurizer 28

2 The new support structure of LO1 pressurizer had to be redesigned and totally repaired during the first outage in The welds of inside structures of lower bottom area of LO1 unit had to be finished by grinding in outage Also the ladders had to be removed from the critical nozzle radius area of lower bottom. New support structure of LO2 pressurizer could be assembled at fabrication plant before the delivery to site, figure 2. Figure 2 - Support structure of LO2 pressurizer Need to reduce thermal loading of surge line nozzles was obvious based on utility analysis and sleeve with new design and thermal isolation was assembled during outages 1984 (LO1) and 1986 (LO2), figure 3. One fixing weld of sleeve at LO1 was leaking out off nozzle in 1988 through a plugged thread hole, used for helium test of sleeve tightness before start-up. Reasons were the high thermal stresses and slag defect of sleeve weld; ligament above the slag line was broken and boron water was able to pass inside the nozzle. Luckily, boron cake on nozzle surface exposed the leak. Crack was removed and the sleeve weld repaired during outage. Leakage control was assembled using existing thread holes into gap of corrosion sleeve. Now, the service life of Loviisa plants is prolonged to 50 years. Consequently, the whole pressurizer structures have been divided to important structure assemblies and systematically considered against that goal and faced up to consider the possible failure mechanisms and possible failures during the whole service life. Figure 3 - Old (left) and new surge line nozzle structure 29

3 The pre- and in-service inspections are planned and implemented according to deterministic component program and rules of ASME Section XI. Inspection interval is 10 years and critical objects are inspected with 4-6 years intervals. Weld crown of dissimilar weld of surge line nozzle is removed from the start of in-service inspections to optimize the inspection possibilities. The role of reliable ISI inspections of pressurizer has now more significance to verify and guarantee the integrity of critical areas considering the prolonged service life. That is the reason for this massive test block implemented. The qualifications of NDE inspections will be implemented following ENIQ recommendations and Finnish qualification instructions accepted by the Finnish Regulator (STUK). DESIGN AND FABRICATION OF TEST BLOCKS The utility had purchased some main components and piping blocks originated from the nuclear project stopped in Poland, and the idea to start test block fabrication using the available material came from Loviisa NPP. Later on, RPV material segments were purchased from Greifswald NPP. The first test block fabrication trials to produce flaws started late 1995 using private welder, experienced NDE and welding inspector and quality and welding engineer of technical support organization of the utility. This action has continued up today with about 90 fabricated test blocks (1)-(7). The team has a small development budget available for welding and crack trials, EDM trials and validations of thermal fatigue and for special devices needed. The fabrication of test blocks is realized based on qualification needs of Loviisa NPP and their budget on case by case offer-order basis. Tight cooperation with Finnish companies Mekelex (EDM machining) and Trueflaw Ltd (thermal fatigue cracks) has started already early in their starting history and the cooperation still continues and develops. MASSIVE TEST BLOCK OF PRESSURIZER BOTTOM The most critical inspection object is the inner radius of surge line nozzle, corrosion sleeve and its fixing weld in addition to dissimilar weld of nozzle. Also fixing welds of thermal sleeve holders are located near the critical inner radius area, figure 3, are important. Longitudinal weld (ESW) of bottom is located between the surge line nozzles. Ring welds of support cylinder of heaters are also important inspection objects. All these welds are UT inspected from outside surface with long sound paths. The simulation of the real inspection situation requires the same thickness as the bottom has. The solution was to order bottom from Germany with the same dimensions and shape as the pressurizer bottom, figure 4. Figure 4 - Half-finished bottom The frame structure around the bottom and the legs were designed at Loviisa NPP to simulate the inspection circumstances under the bottom, figure 5. 30

4 Figure 5 - Frame and legs of test block Frame, legs and SAW cladding welding on bottom and inner radius area using strip electrode and the same cladding thickness were ordered by Loviisa NPP as sub-delivery from Finland. Due to fabrication and more flexible inspection reasons, it was decided to fabricate separate test blocks (open and blind) for the dissimilar weld of surge line nozzle and pipe-to-surge line nozzle weld, figure 6. CS surge line nozzle Dissimilar weld SS reducer piece SS surge line pipe Figure 6 - Open and blind test blocks of dissimilar and pipe weld of surge line nozzles PREPARATIONS NEEDED FOR MASSIVE TEST BLOCK OF PRESSURIZER BOTTOM The massive test block sets many requirements to be considered before getting the test block into workshop. Transport company for heavy components was available at Loviisa city. Heavy duty truck could easily lift and move the test block beside the vehicle. Moving of test block inside the workshop was settled by welding two U-profiles together and by purchasing roller set to roll along guides and on the floor. Workspace behind the doors had to be framed with curtains from ceiling to floor to limit grinding dust. Ventilation of grinding and welding space was settled using wind unit for getting dust and flue gases out of the working space. The same guides and rollers were used also in other working places at Trueflaw and Mekelex where test block was transported. 31

5 Practical difficulties and challenges Parallel grinding work and fabrication of other test blocks behind the curtains was not possible due to disturbing grinding noise. The works had to be organized by turns. Totally the grinding work took about four months before the flaw fabrication could be started on bottom. INSPECTION OBJECTS OF TEST BLOCK Inspection difficulties and challenges The inner radius area was challenge for manual scanning with traditional probes (in times before phase array probes). Scanning with various angles of incidence against radial flaws using a guide fixed around the nozzle should confirm to coverage of the whole volume. A simple radial notch was used as reference reflector earlier. Scanning of welds of inside structures is easier to cover from different directions by scanning from bottom surface. Weld locations are permanently marked on outer surface of bottom to simplify the further ISI inspections. The future inspection technique is open and new vendor will be selected based on suitable technical and economical offer. Technique and personnel will be qualified using this and other suitable test blocks. Selected inspection objects The critical nozzle inner radius areas and corrosion sleeve welds, nozzle welds to bottom, holder welds of sleeves around nozzle holes and cylinder weld of support cylinder of heaters are the selected, welded objects, figure 7. Figure 7 - Inspection objects of test block The L-weld of bottom (ESW) is missing, but the location of weld is defined between nozzles and defects are fabricated in the imagined weld borders. Due to symmetric structure of test block, it was easily divided into open and blind sections to fulfill the ENIQ qualification recommendations. FLAW TYPES SELECTED FOR INSPECTION OBJECTS OF TEST BLOCK Test block design and selection of flaw types to be fabricated are performed by the utility based on the input data for qualification of pressurizer inspections. Detection targets and critical flaw sizes have also guided the selection of flaw fabrication methods. 32

6 FLAW TYPES SELECTED FOR INSPECTION OBJECTS OF TEST BLOCK Test block design and selection of flaw types to be fabricated are performed by the utility based on the input data for qualification of pressurizer inspections. Detection targets and critical flaw sizes have also guided the selection of flaw fabrication methods. From the utility point of view, when deep surface or sub-surface cracks are needed to be fabricated into finished component surface, welding of solidification cracks is the easiest, cheapest and often representative enough as fatigue cracks, compared to other type of flaw options, to be used in ferrite and austenitic structures. Shallow solidification cracks are hard to produce as surface breaking. Inner radius area of nozzles The deep cracks are produced by welding solidification cracks and shallow cracks with thermal fatigue by Trueflaw Ltd. Also narrow EDM notches with shape of crack front will be used. The crack tip of solidification crack can be curved in dendrite direction or be straight. It cannot be noticed while welding. Crack can also have interruptions in thickness and length direction especially in ferrite material. The crack follows closely the welding movement. Crack opening is typically larger compared to tight fatigue cracks. The ferrite / austenite border of crack has to be kept on the same level as inner radius nearby the radial crack, figure 8. Many times it is hard to get the crack to follow up to surface breaking. Middle part of surface pass did not crack in some radial inner radius due to cooling stresses in nozzles. However those cracks could be easily broken open with few thermal fatigue cycles by Trueflaw. SS Crack composition SS CS CS nozzle Figure 8 - Solidification crack and crack borders Sleeve weld inside the nozzle hole, sleeve holder welds on bottom cladding surface, support cylinder welds of heaters and longitudinal ESW weld of bottom (imagined weld line) Slag simulation is used inside the weld volume (sleeve weld) based on findings in pressurizer and slag combined with s-crack under cylinder and holder welds are used to simulate fabrication defect and possible cracking initiated from fabrication flaw. Solidification cracks are used at the edges and beside the full penetration fillet welds of holders to simulate fatigue cracks, figure 7. Planar deep flaw simulations using aid pieces, welded through into opening volume, are used as flaws parallel to weld and located on vertical weld surface as sub-surface and surface simulating cracks and LOF. Deep openings with vertical surface along weld groove line are grinded and aid pieces are welded to be part of weld volume, figure 9. Solidification cracks were used to simulate transversal cracks of ESW weld. Figure 9 - Welding of planar sub-surface flaw simulation 33

7 Nozzle-to-bottom welds Nozzle-to-bottom weld is needed for simulation of the real scanning situation of inner radius area from outside surface. Scanning guide or manipulator are fixed around the nozzle or pipe fixed to nozzle. Nozzle weld is scanned only from the nozzle side and full size test block was not earlier available. Surface fatigue cracks are produced into weld root surface using welded bending arm, figure 10. Figure 10 - Welding of aid piece for producing fatigue crack on weld groove Crack surfaces are fitted together and finished to required size and shape into groove surface before welding the joint. In addition, planar flaw simulations using aid pieces are used to produce tight crack like reflectors into weld grooves before welding the joint together. Narrow transversal EDM notches with shape of crack front will be machined into weld root of nozzle weld. Removable pipe will be fixed to nozzle confirming the fixing of inspection manipulator. SUMMARY AND LESSONS LEARNED The positive attitude of the utility and need to improve and qualify inspection techniques, know-how of components, fabrication and welding, and motivated inspector and welder are the key elements to execute test blocks for qualification. SAW welding of cladding layer on inner surface of bottom was challenge for the welding company. Poor connection of parallel, adjacent strip welded passes and many welding defects in cladding caused the long grinding job before the bonding of cladding could be inspected. Repair and grinding of cladding took about four months of production time and caused extra costs. Perhaps the weakest point was that the team had no chance to inspect the flaws because the vendor for pressurizer inspections was not yet considered and selected. This uncertain situation and total absence of information about sizes, response and quality of flaw for qualification is not wise and normal. Inspection services should always be guaranteed for test block fabrication as natural, important fabrication phase. Trueflaw has found a working solution to produce thermal fatigue cracks using long heating arms. The basic concept for EDM machining is ready and planned by Mekelex. Assembly of EDM system for this bottom job is now going on. Next step is to implement practical trials with sparkling device and find the proper parameters for the new device. The last flaws will be fabricated during summer The blind sector of bottom shall be covered somehow to avoid seeing the flaws. FUTURE CHALLENGES General storage solution of test pieces (open and blind) is not yet organized and resolved. Premises for this kind of heavy test blocks are missing (handling, inspection, storage). The next, heavy duty fabrication objects are the inner radius sections of emergency cooling nozzles (TH) of RPV available for inspection trials and qualification at Loviisa NPP. 34

8 Figure 11 - TH nozzle inner radius areas of RPV Arrangements will be organized during this summer. Closed working place inside the vessel sets safety aspects to be taken care of before fabrication actions. Future continuation of fabrication team depends on qualification needs of Loviisa NPP. REFERENCES 1) Paussu, R., Pitkänen, J., Elsing B., Särkiniemi, P., Jeskanen, H., "Ultrasonic inspection of reactor pressure vessel from outside surface, qualification and assessment of inspection at Loviisa NPP", 15th World Conference on NDE. Rome, IT, Oct ) Paussu, R., Kemppainen, M., Pitkänen, J., "Practical Examples of Application of Different Qualification Defects", 8th ECNDT, Barcelona, June ) Kemppainen, M., Virkkunen, I., Pitkänen, J., Paussu, R., Hänninen, H., "Comparison of realistic artificial cracks and in-service cracks", NDT & Ultrasonics, Vol.8, no.3, March 2003, pp ) Kauppinen, P., Paussu, R., Sarkimo, M., Pitkänen, J., "Qualification of ultrasonic inspection of austenitic piping by practical trials with test blocks containing defects", 8th Int. Conference on Material issues in Design, Manufacturing and Operation of Nuclear Power Plants Equipment", St.Petersburg, June, ) Paussu, R., Virkkunen, I., Kemppainen, M. Utility aspect of applicability of different flaw types for qualification test pieces, Proceedings of the 6 th International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurised Components, 2007 Oct 8 10, Budapest, Hungary, pp ) Virkkunen, I., Kemppainen, M., Ostermeyer, H., Paussu, R., Dunhill, T Grown cracks for NDT development and qualification, InSight, May ) Virkkunen, I., Kemppainen, M., Paussu, R., Pirinen, J., Luostarinen P., Proposed improvements for use of different qualification defect types - three generations of defects. 35