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10 years of experience with Westinghouse fuel at NPP Temelin


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10 years of experience with Westinghouse fuel at NPP Temelin

  1. 1. 10 years of experience with Westinghouse fuel at NPP Temelin Daniel Ernst & Luka Milisdorfer VVER 2010 Prague, November 1-3, 2010
  2. 2. 2VVER 2010 History Contract with Westinghouse was signed in 1993 First fuel assembly (FA) VVANTAGE-6 - was loaded into the reactor of Unit 1 in July, 2000 . ..and first criticality was achieved 3 months later 8 cycles at Unit 1 (Cycle 5 was divided on Cycle 5 and 5A) and 7 cycles at Unit 2 These 15 cycles represent 960 FAs 490 FAs (U1) + 470 FAs (U2) Average burnup of unloaded F/As was 37.800 MWd/tU and maximum average burnup of F/A was app. 44.500 MWd/tU 67 leaking FAs, 27 FAs were repaired and reused
  3. 3. 3VVER 2010 Content History Core Design, Safety Evaluation and Core Monitoring Fuel Problems Post Irradiation Inspection Program Conclusion
  4. 4. 4VVER 2010 Core Design and Safety Evaluation Transfer of computational software and methodology (NRC licensed). Computational aids were modified by Westinghouse for usage with hexagonal core (ANC-H; PHOENIX-H, etc.) Set of core design trainings for EZ personnel several of them as on-job-trainings important know-how transfer allows CEZ to perform its own core design-related calculations Close WEC- EZ joint cooperation in loading pattern search the preliminary patterns (with brief safety evaluation) were found by EZ and then fully evaluated by Westinghouse and delivered as a final core design. Over time the participation of the CEZ personnel in core design process becomes more and more significant
  5. 5. Core Design and Safety Evaluation Complicated core design: Real cycle lengths significantly departs from the scheduled ones Core Design constraints caused by IRI and grid-to-rod fretting Common core design limitations and efforts: safety aspects, reactor vessel fluency minimalization; economical aspects. In despite of the above-mentioned limits the safe, operation- friendly and reasonable cost-effective loading patterns were found Low leakage cores reactor vessel fluency minimalization Cores which helps to mitigate the IRI issue. Cores with sufficient and comfortable margins to operational limits (peaking factors) Cores with minimalized sixth asymmetry forcing function, etc. 5VVER 2010
  6. 6. Core Design and Safety Evaluation During IRI and grid-to-rod fretting problems detailed core design guideline was created to mitigate/eliminate the above-mentioned issues. Since the irreparable leakage of the fuel, which were several times determined during outages the Contingency Plans was always created before each fuel reload helps with quick redesign The core redesign were often performed in challenging time range days/few weeks 6VVER 2010
  7. 7. Core Design and Safety Evaluation The flexible fuel assemblies design was the main presumptions for successful core design: Fuel enrichment according to core designer requests (restricted only by range 0.7 5.0 % w/o) Axial and primarily radial profiled fuel assemblies Fine tuning of burnable absorbers using integrated fuel burnable absorbers (IFBA) rods from 0 to 84 IFBA rods per assembly The loading pattern proposals safety evaluation: Using the bounding principle Performed for reasonable cycle burn-up window - mostly ± 20 EFPD Safety margins sufficient for safe and comfortable operation 7VVER 2010
  8. 8. Core Monitoring The Westinghouse core monitoring system BEACON based on Rhodium self-powered incore detectors was modified for use for hexagonal core BEACON TSM (Technical Specification Monitoring) used at Temelin For the first cycles the version BEACON 2.0.1 were used the core monitoring model development was fully performed by Westinghouse The modified De-coupled version BEACON 6 was fully applied in 2008. The preceding system tuning and test opartion was performed in close WEC-CEZ cooperation 8VVER 2010
  9. 9. Core Monitoring The BEACON Model Development Training was performed in 2009 The training was performed as a on-job-training (U2C7) The BEACON models were than develop by CEZ personnel and verified by Westinghouse. In 2010 the training for determining self powered detectors cross-section constants for using with fuel containing gadolinium It allows CEZ to develop the core monitoring system model for the cores with new type of fuel supplied by JSC TVEL 9VVER 2010
  10. 10. 10VVER 2010 Fuel Problems Problems with RCCAs (Rod Control Cluster Assembly) during operation Incomplete Rod Insertion (IRI) Fuel Assembly (FA) / Fuel Rod (FR) Bow Excessive FA Growth Leaking FRs
  11. 11. 11VVER 2010 Fuel Problems - IRI Description Some RCCAs (on both Units) do not drop fully to the bottom position (IRI Incomplete Rod Insertion) - root cause was changes in FA geometry Worst case - Unit 1, June 2 test showed two RCCAs stopped above the level of the hydraulic dashpot, in other words they failed to meet the Limit Condition unscheduled outage for refueling Since the problem became known, the function of RCCAs dropping has been repeatedly tested at intervals specified by SUJB The interval for tests was 30 days (in 2005) and is 150 days (in 2010) Based on previous tests fuel vendor: provides prediction of number of IRI for upcoming test we were/are better prepared for given situation performs a safety assessment
  12. 12. 12VVER 2010 Fuel Problems IRI (cont.) Corrective actions BCO (Bases for Continuous Operation) Non fuel modification was done in 2005 - control rod drive shaft connecting bars were modified (drilled holes, increased weight the similar way as on the other VVER1000 units) FA design modifications (2006-2007) tube-in-tube dashpot design and top nozzle modifications (Phase 0) fuel rod loading equipment alignments a new structural materials of FR cladding and other FA components (Phase 1X) A methodology for core design with bowed and twisted FAs has been developed
  13. 13. 13VVER 2010 Fuel Problems IRI (cont.) Odstran ni IRI na ETE modifikaci paliva Westinghouse 0 5 10 15 20 25 30 35 40 45 50 55 1.8.2004 9.11.2004 17.2.2005 28.5.2005 5.9.2005 14.12.2005 24.3.2006 2.7.2006 10.10.2006 18.1.2007 28.4.2007 6.8.2007 14.11.2007 22.2.2008 1.6.2008 9.9.2008 18.12.2008 28.3.2009 6.7.2009 14.10.2009 22.1.2010 PoetIRI U2EOC2 U1EOC3 U1EOC4 U1EOC5 U1EOC5AU2EOC3 U2EOC4 U1EOC6 U1EOC7 U2EOC5 U2EOC6
  14. 14. 14VVER 2010 Fuel Problems FA/FR Bow Description The mechanical deformations of FR and FA, i.e. the deviations from their ideal geometric shapes and the elongation of fuel assemblies (radiation growth), come as a natural phenomenon accompanying the production of energy inside the cores of nuclear reactors FAs prone to geometry changes due mostly to the radiation growth of FRs and FA skeleton. The changes translate themselves into FA bow and twist The first cores in the Temelin NPP have been designed to tolerate the flow area reductions by as many as 51%. All subsequent core designs were calculated with the conservative assumption that the FRs could touch each other FA bow has affected refueling of core, reshuffling sequence of FAs was changed, the special dummy assemblies have been used to load bowed and twisted FAs However, FA bow and twist is not a safety problem, it is an usual phenomenon in all reactors in the world
  15. 15. 15VVER 2010 Fuel Problems FA/FR Bow (cont.)
  16. 16. 16VVER 2010 Fuel Problems FA/FR Bow (cont.)
  17. 17. 17VVER 2010 Fuel Problems Excessive FA Growth Comparison of three different designs of VVANTAGE-6 fuel 0 2 4 6 8 10 12 14 16 0.0 2.0 4.0 6.0 8.0 10.0 Fast Neutron Fluence, (x 1E21 NVT) FuelAssemblyGrowth,mm T1 Growth Updated Best Estimate Curve Updated Upper Limit Curve T2 Growth Phase 1X Growth T1 design: - original design - loaded to Unit 1, first core only - Zircaloy-4 T2 design - loaded to Unit 2, first core and all reloads for both Units till 2006 - Zircaloy-4 Phase 0 - loaded to both Units in 2006 - Zircaloy-4 Phase 1X design - all reloads from 2007 till now - ZIRLO
  18. 18. 18VVER 2010 Fuel Problems Excessive FA Growth (cont.) F/A Growth vs. WEC database Zr-4 Fuel Assembly Growth Data Base 0.000 0.050 0.100 0.150 0.200 0.250 0.300 0.350 0.400 0.450 0.500 0.00 2.00 4.00 6.00 8.00 10.00 12.00 Fast Fluence (1e21 nvt) FuelAssemblyGrowth(%) Zr-4 Fuel Assembly Growth Data Base Temelin 1 EOC 1 and 2 Data Temelin 2 EOC 2 Data Temelin 1 EOC 3 Data Temelin 1 EOC 4 Data Temelin 2 EOC 3 Data Temelin 1 EC 5A Data - ZIRLO
  19. 19. 19VVER 2010 Fuel Problems Leaking FRs Description The nuclear reactor installed at the Temelin NPP contains 163 fuel assemblies and each assembly holds 312 fuel rods. All nuclear power plants are designed on assumption that a certain number of the rods develop leaks The number of fuel rods supposed to become leaky is determined with reliance on safety analyses and detailed in Technical Specifications in the form of Safety Limits & Conditions - at the Temelin NPP the fuel rods are checked for tightness through monitoring the specific activity in the primary circuit the overall specific activity limit of 3,7x109 Bq/l Summary of maximum activities found in the primary circuit coolant on both Units (Bq/l): 2003 2004 2005 2006 2007 2008 2009 2010 Unit 1 5,9x10 6 2,4x10 6 3,6x10 6 6,3x10 6 7,9x10 6 7,1x10 6 6,3x10 6 4,3x10 6 Unit 2 2,7x10 6 7,0x10 6 3,3x10 6 6,4x10 6 8,5x10 6 5,8x10 6 6,3x10 6 6,6x10 6
  20. 20. 20VVER 2010 Fuel Problems Leaking FRs (cont.) FRI faktor FRI ETE 2005 - 2010 0 20 40 60 80 100 120 140 160 leden05 bezen05 kvten05 ervenec05 zai05 listopad05 leden06 bezen06 kvten06 ervenec06 zai06 listopad06 leden07 bezen07 kvten07 ervenec07 zai07 listopad07 leden08 bezen08 kvten08 ervenec08 zai08 listopad08 leden09 bezen09 kvten09 ervenec09 zai09 listopad09 leden10 bezen10 kvten10 as [m s] FRI [Bq/g] HVB 1 HVB 2
  21. 21. 21VVER 2010 Fuel Problems Leaking FRs (cont.) Sipping systems are used to search the leakers: On-line Sipping - the system applies a gas method of continuous changes in the Xe 133 concentration Off-line sipping - the system used to conduct qualitative measurements of a FA found damaged in an attempt to reveal the scope of damage done Fuel Inspection & Repair Equipment a system used to identify and repair leakers: mobile equipment (for both Units) designed and manufactured by Westinghouse during outage is FRIE assembled on the floor of the Reactor Hall and than is transported into the fresh fuel cask pit next to spent fuel pool the refueling machine moves fuel assemblies in and out of the FRIE the fuel assembly rests on a turntable which allows it to be rotated 360 degrees during the inspection process is designed to provide complete fuel inspection for entire FA the fuel assembly UT inspection system is used to locate leaking fuel rods the failed fuel rod storage basket (FFRSB) provides storage for the failed fuel rods leakers are replaced with dummy rods.
  22. 22. 22VVER 2010 Fuel Problems Leaking FRs (cont.) Unit 1 End of Cycle # of Leakers # of Repaired Leakers # of Leaked F/Rs 2 1 0 0 3 5 1 1 4 6 1 1 5 6 6 8 5A 4 2 3 6 7 7 10 7 3 1 1 8 0 0 0 Unit 2 End of Cycle # of Leakers # of Repaired Leakers # of Leaked F/Rs 2 3 2 5 3 10 2 4 4 5 2 2 5 7 2 3 6 5 0 0 7 5 1 2 An overview of leaks disclosed in fuel at the Temelin NPP
  23. 23. 23VVER 2010 Fuel Problems Leaking FRs (cont.) Root cause of leaking FRs is grid-to-rod fretting that means vibration of FRs in spacer grids Fuel Rod Fuel Rod with fretting marks
  24. 24. 24VVER 2010 Fuel Problems Leaking FRs (cont.)
  25. 25. 25VVER 2010 Fuel Problems Leaking FRs (cont.) FR separation event at Unit 2 Fuel reconstitution was being performed on assembly BE24 One leaking fuel rod was to be removed and replaced by a stainless steel rod After withdrawing about 150 cm of the fuel rod, the rod jerked suddenly and fractured Visual examination of the fracture location indicated the separation was caused by secondary hydriding (there had been no indications of significant hydriding from the visuals) Extensive through-wall grid-to-rod fretting was found Visual examinations of the fractured ends on both rod segments showed intact fuel pellets with no empty area, indicating that no fuel pellets were lost during the fracture event.
  26. 26. 26VVER 2010 Fuel Problems Leaking FRs (cont.)
  27. 27. 27VVER 2010 Post Irradiation Inspection Program (PIIP) Post Irradiation Inspection Program (PIIP) substituted the Lead Test Assembly program for new fuel design program was structured to meet requirements of the Czech Regulatory Body (SUJB) additional proof of material compatibility, analytical methods support and verification, overall thermomechanical performance demonstration 8 selected FAs (these 8 FAs were pre-characterized ) The inspection program included: FA measurements Visual Examination FA Bow, Twist and Overall FA Length Measurement Peripheral FR Corrosion Measurements Single FR measurements Visual Examination Corrosion Measurements Profilometry Measurements RCCA inspections Measurements of Total Wear of the Tube Cross Section
  28. 28. 28VVER 2010 Post Irradiation Inspection Program (PIIP) cont. PIIP results Positive: visual examinations revealed little or no corrosion on the cladding surfaces confirmation of compatibility of Zircaloy alloy with the VVER water chemistry there was no wear of any RCCA rodlet (criterion is wear less than10%)
  29. 29. 29VVER 2010 Post Irradiation Inspection Program (PIIP) cont. PIIP results Negative: FA twist, bow and length measurements showed an unexpected behavior FR visuals of leaking FRs indicated GRF
  30. 30. 30VVER 2010 Conclusion During first decade of the Temelin NPP operation, formidable mission of implementing and development program of a new fuel system has been accomplished. Indeed, it raised more or less serious problems which all were solved together with the fuel vendor and we can say they are under the control. Today, we are standing at the beginning of new era, waiting what will be brought by the future.