16x16 NEXT GENERATION FUEL POOL SIDE EXAMINATION AFTER END OF FIRST CYCLE

Transcription

1 16x16 NEXT GENERATION FUEL POOL SIDE EXAMINATION AFTER END OF FIRST CYCLE Marcio Adriano C. Silva 1, Rosvita Gold Matthes 1 1 Indústrias Nucleares do Brasil (INB) Rodovia Presidente Dutra, Km Resende, RJ, Brazil marcio.adriano@inb.gov.br rosvita@inb.gov.br ABSTRACT Indústrias Nucleares do Brasil (INB), KOREA Nuclear Fuel Company, Limited (KNFC) and Westinghouse Electric Company LLC (Westinghouse) have worked together in order to create a product (Nuclear Fuel Assembly) to fulfill the severe conditions required to high burnup operation. In order to operate with more economic load pattern and to increase the profits, a new fuel assemblies design (16NGF - Next Generation Fuel) was created in order to allow the utilities uses more reactive cores. A higher discharge burnup means better uranium utilization, reducing the uranium requirements and the amount of fuel assemblies disposed during the utility life. After design phase be finalized, the next important activity was the fabrication of some Lead Tests Assemblies (LTA) and introduce than in a core to be irradiated. It has been done in KNFC factory and four Fuel Assemblies were load in Kori-2 nuclear power plant (Kori- South Korea) in February of After 13 month under operation these four LTA were evaluated following a rigorous procedure known as Pool Side Examination. Describe the methods and techniques applied during this evaluation as well as the results with regards the 16NGF behavior under operation are the main goals of this paper. 1. INTRODUCTION Industrias Nucleares do Brasil (INB), KOREA Nuclear Fuel Company, Ltd. (KNFC), and Westinghouse Electric Company (Westinghouse) have jointly designed an advanced 16x16 Westinghouse type PWR fuel assembly. This advanced 16x16 Westinghouse type PWR fuel assembly is an integral part of the utilities fuel management strategy. This same fuel design has also been developed for future use in Krsko Unit 1 (in Slovenia). The major reason for initiating this joint development program was to update the current 16x16 fuel assembly, which is also called 16STD. Nuclear design activities in the areas of fuel cycle cost and fuel management were performed in parallel to the fuel assembly design efforts. As the change in reactivity due to the change in the fuel rod diameter influences directly the amount of Uranium and related services, the evaluation was performed aiming at having a fuel rod diameter that achieves the cycle requirements with the optimized Uranium cost. The optimum fuel rod diameter and supporting mechanical design features were based on these fuel cycle cost and fuel management evaluations. After design phase be finalized, the next important activity was the fabrication of some Lead Tests Assemblies (LTA) and introduce than in a core to be irradiated. It has been done in KNFC factory and four Fuel Assemblies were load in Kori-2 nuclear power plant (Kori-

2 South Korea) in February of After 13 month under operation these four LTA were evaluated following a rigorous procedure known as Pool Side Examination. Describe the methods and techniques applied during this evaluation as well as the results with regards the 16NGF behavior under operation are the main goals of this paper. 2. ADVANCED PWR DESIGN FOR WESTINGHOUSE 16X16 TYPE PLANTS INB, KNFC, and Westinghouse have jointly designed an advanced 16x16 Westinghouse type Pressurized Water Reactor (PWR) fuel assembly. This advanced 16x16 Westinghouse type PWR fuel assembly, which will be implemented in Angra Unit 1 (in Brazil) and Kori Unit 2 (in Korea) is an integral part of the utilities fuel management strategy. Additionally, the owners of the Krsko nuclear power plant in Slovenia have been involved in the development program and have expressed interest in deploying the advanced fuel bundle in the future. The advanced fuel assembly design for the Angra-1, Kori-2 and Krsko 16x16 Westinghouse Type Plants in Brazil, Korea and Slovenia, respectively, had the following program objectives: Batch Average burnup greater than 55 MWd/kgU. More than 20% increase in DNB Margin over existing 16x16 Westinghouse Fuel Assembly. Through-grid dynamic buckling strength and dynamic stiffness sufficient to meet LOCA/Seismic-related design criteria. Demonstration of compatibility between the 16NGF Fuel Assembly and the existing Westinghouse 16x16 fuel assemblies in use in both the Republic of Korea, Brazil and Slovenia. The major focus for this joint development program was to update the current 16x16 fuel assembly, which is also called 16x16 Standard (16STD), being utilized at the Angra-1, Kori-2, and Krsko sites. The main reason for the design change from 16STD to the proposed next generation fuel assembly was to update the 16x16 design to current Westinghouse technology. As such, the advanced 16x16 fuel assembly was designed for peak rod average burnups of up to 75 MWd/kgU and will use an optimized fuel rod diameter (i.e. 9.1mm OD ZIRLO fuel rods), neutronic efficient components (i.e. ZIRLO Mid grids), ZIRLO Intermediate Flow Mixer (IFM) grids to improve Departure from Nuclear Boiling (DNB) margin, optimized ZIRLO guide thimble tubes (i.e. tube-in-tube dashpot) from an IRI standpoint, debris filter bottom nozzle and Protective Grid (P-Grid) to enhance debris filtering and capturing efficiency, and updated mechanical design features that allow for top-down fuel rod reconstitution (i.e. removable top nozzle). The main changes were made to the fuel rod diameter, middle structural grids, intermediate flow mixing grids and the guide thimble/instrumentation tubes. The features of the 16NGF assembly designs include: Fuel Rod Diameter Optimization; Annular axial blankets; ZIRLO TM Enhanced structural middle grids; ZIRLO TM Enhanced Intermediate Flow Mixing (IFM) grids; Removable top

3 nozzle; Debris Filter bottom nozzle; Reduced rod bow Inconel top grid; High burnup Inconel bottom grid; Inconel protective bottom grid and Tube-in-tube Thicker Guide thimble tubes High Burnup Capability A higher discharge burnup means better uranium utilization, reducing the uranium requirements and the amount of fuel assemblies disposed during the utility life. The peak rod burnup up to 75 GWd/MTU and region batch burnup up to 55 GWd/MTU were the targets for the 16NGF project. In this new design, many features were implemented to reach this goal. An important feature is the use of ZIRLO TM cladding tubes for the fuel. ZIRLO TM features a balanced combination of good corrosion performance, superior mechanical properties, and dimensional stability. The excellent performance and robustness of ZIRLO TM has been proven over 15 years of experience. More than 2.5 million rods have been irradiated in 48 commercial power reactors, up to over 70 MWd/kgU. Hence, ZIRLO TM is the advanced cladding of choice for high duty operation, including uprating, extended burnup, longer cycles, and modern coolant chemistries. All the mechanical components were designed taking in account the higher fluence that will be achieved. In addition, the fuel rods have to accommodate more gaseous fission products that are released to the plenum region Low Parasitic Neutron Absorption using ZIRLO TM Mid and IFM Grids The 16NGF design uses ZIRLO TM instead of Inconel in the grids in the active length of the fuel (Mid Grids and IFM Grids). This was done in order to take advantage of the lower neutron absorption in the ZIRLO TM material, leading to a reduction in the fuel cycle cost Improved Thermal Margin In support of obtaining the maximum thermal margin with the 16NGF fuel assembly design, the following information shows the reasons for choosing the 16NGF Mid and IFM grid vane patterns, the 16NGF Mid and IFM grid vane shapes, and the number of IFM s to be added. Consideration was given for the Mid / IFM grid vane patterns to minimize both fuel assembly and fuel rod vibration. Also consideration was given for the Mid / IFM grid vane features for improved flow blockage area and resultant improved DNB margin. One of the goals of the 16NGF program was to increase DNB performance by 20% over 16STD (10% in power). Adding IFM s improves DNB performance by approximately 20% due to the increase in turbulence and mixing. Conversely, reducing the rod diameter penalizes DNB performance since there is less heat transfer surface area available for the same power. For example, a reduction from to inches decreases DNB performance by about 8%. Therefore, the net DNB benefit in this example would be 20-8 = 12%. In order to meet the program objective of 20% DNB margin benefit, DNB effects of mid-grid design optimization were also included to increase DNB performance. Therefore, the effect of the grid design on DNB margin should also be taken into account. This effect is called grid benefit.

4 The design of the 16NGF mid-grid is employing the latest Westinghouse DNBimprovement technology and lessons learned from the previous development efforts. Based on experience the optimized grid benefit ranged from 8 to 10%. In the case of the 16NGF design, this 8-10% grid benefit should offset the 8% rod diameter penalty for reducing the fuel rod diameter. Therefore, the preliminary best estimate of the DNB performance improvement for 16NGF is approximately 20-22%. These estimates are based on Westinghouse experience with currently available DNB test data NGF LTA AND FULL REAGION IMPLEMENTATION Before a full region implementation, the 16NGF development program was planed to implement four LTA at Kori-2 cycle 20 on January 2005 and at Angra-1 cycle 16 on May ELETRONUCLEAR intends to implement the first region of 16NGF at Angra-1 in the Cycle 19, beginning of 2012, soon after replacement of the Steam Generator taking full advantage of the new ZIRLO TM fuel core and uprating the power. For both utilities (Angra-1 and Kori-2), the four LTA will be burned for 3 cycles and examined (PSE - Pool Side Examination) after each cycle, and then one of the four is planned to be loaded at fourth cycle for higher burnup. As can be understood, there are different 16NGF implementation strategic and schedule for KNFC and INB companies. INB and ELETRONUCLEAR intend to take advantage of the new fuel after replacing Angra-1 Steam Generator what is scheduled to be conclude in Considering KNFC schedule, the first 16NGF nuclear fuel has been burned in Kori 2 plant since January This first cycle ended in February 2006 ( 19.5 MWd/KgU) when the four 16NGF LTA assembly were submitted to a detailed examination in a sided controlled pool (Pool Side Examination). Taking advantage of INB (Brazil) and KNFC (South Korea) jointly development for the 16NGF nuclear fuel, INB has witnessed the Pool Side Examination activities and had access to the results. The purpose of this Pool Side Examination was to verify the performance and integrity of the irradiated 16NGF LTA as will be presented and discussed in the following items Visual inspection All four 16NGF irradiated LTA inspected had no indication of the Grid or Fuel Rod fretting wear as well as any other unexpected material or geometric alteration Fuel assembly bowing and twist Two 16NGF irradiated LTA were measured regarding bowing and twist. Basically this measurement was performed by means of comparison with a reference line positioned parallel to the Fuel Assembly to be measured (Figure 1). Fuel Assembly bowing and twist measured results were obtained when Videotape Recording (VTR) pixels were counted, compared with a known reference and then processed by image analysis.

5 Figure 1. 16NGF Fuel Assembly bowing and twist Measurement. After all measurement, no unexpected results were found out either to bowing or to twist evaluation Fuel rod bowing (channel closure) Two 16NGF irradiated LTA were measured regarding fuel rod bowing (channel closure). This measurement was performed by means of Videotape Recording (VTR) when the pixels were counted, compared with a known reference and then processed by image analysis. After all measurement, no unexpected results were found out to channel closure Fuel assembly grid width All grids were measured for an irradiated 16NGF LTA considering its all four faces. In order to perform these measurements, it was used a accurate caliper instrument attached to a LVDT sensor (Linear Variable Differential Transformer sensor) which was specifically developed for this kind of task. During the measurement procedure, the caliper was put in contact with the fuel assembly grids and many points and position were measured. The results were then compared with a standard (no irradiated) grid and the grid lateral growth were obtained. After all measurement, no unexpected results were found out to fuel assembly grid growth Fuel rod diameter alteration A total of forty fuel rod diameter were measured for an irradiated 16NGF LTA considering its all four faces. In order to perform these measurements, it was used another accurate caliper instrument attached to a LVDT sensor (Linear Variable Differential Transformer sensor) which also was specifically developed for this kind of task. During the measurement procedure, the caliper was put in contact with the fuel rod and many points and position were measured (Figure 2).

6 The results were then compared with a standard (no irradiated) fuel rod and the fuel rod diameter alteration were obtained. After all measurement, no unexpected results were found out to fuel rod diameter alteration. Figure 2. Fuel rod diameter measurement Fuel rod oxide thickness All fuel rod oxide thickness for last two fuel rod row were measured for an irradiated 16NGF LTA considering its all four faces. In order to perform these measurements, it was used an accurate ECT (Eddy Current) sensor instrument which also was specifically developed for this kind of task. During the measurement procedure, the ECT equipment was positioned between two fuel rod rows and the oxide thickness were measured continually for all fuel rod in that particular row (Figure 3). The results were then compared with some standard thickness film so that the oxide thickness for each fuel rod were obtained. After all measurement, no unexpected result were found out to fuel rod oxide thickness. Figure 3. Fuel rod oxide thickness measurement by ECT.

7 3.7. Fuel rod growth Two irradiated 16NGF LTA were submitted to fuel rod growth measurements considering its all four faces. In order to determine the fuel rod growth, the shoulder gab between the fuel rod top and the bottom nozzle surface were measured and compared with the standard dimension. The difference were evaluated and converted to the real fuel rod growth. Similarly, the shoulder gap measurement was performed by means of Videotape Recording (VTR) when the pixels were counted, compared with a known reference and then processed by image analysis. After all measurement, no unexpected results were found out to fuel rod growth Fuel assembly growth Two irradiated 16NGF LTA were submitted to fuel assembly growth measurements considering its all four faces. In order to determine the fuel assembly growth, all grid axial position were measured after a zero point had been set up under the fuel assembly bottom nozzle. The measured axial grid position as well as the overall assembly length were compared with standard dimension and then the differences were evaluated and converted to the real fuel assembly growth. After all measurement, the average fuel assembly growth result were considered a little greater than expected value. Two possible reasons were considered to explain this unexpected result: Measurement uncertainty mainly due to thermal gradient along of the spent fuel pool. Fuel assembly guide thimble fluence (thermal and neutron) less than expected what has allowed a more significant fuel assembly growth (specifically to kori-2 plant which the distance between core plates is the biggest compared with Angra-1 and Krsko plants). For the next 16NGF Pool Side Examination scheduled to the end of cycle two, especial attention will be dedicated to evaluate the 16NGF growth progress. 4. CONCLUSIONS After 16NGF LTA Pool Side Examination measurement results had been carefully analyzed, it was concluded that their performance under normal operation during the first cycle was within of the expected. As a consequence, the four 16NGF LTA fuel assemblies have been returned to operation in the Kori-2 plat in order to be burned for the second cycle.

8 REFERENCES 1. E. F. Pulver, E. Faria, L. Sadde, S. K. You, S. J. King, R. Esteves, K. L. Jeon, Core Management Improvements at Angra Unit 1 and Kori Unit 2 using an Advanced 16x16 Westinghouse Type PWR Fuel Assembly, American Nuclear Society (ANS) Advances In Nuclear Fuel Management III Conference (ANFM 2003), Hilton Head Island, South Carolina, USA, October 5-8, Michael L. Boone, S. J. King, E. F. Pulver, Kyeong-Lak Jeon, and R. Esteves, Development of an Advanced 16x16 Westinghouse Type Fuel Assembly for Slovenia, 5 th International Conference on Nuclear Option in Countries With Small And Medium Eletricity Grids, Dubrovnic Croatia, May 16-20, S. J. King, K. L. Jeon, R. P. Knott, Y. H. Kim, K. T. Kim, J. L. Norrell and R. Esteves, Inrtroduction of Advanced 16x16 and 17x17 Fuel Assemblies into Korean PWRs, 14 th Pacific Basin Nuclear Conference, Honolulu, Hawaii, USA, March 21-25, Esteves. R.G, King S.J. and Kyeong-Lak Jeon, Desenvolvimento de um Elemento Combustível Avançado Tipo Westinghouse PWR 16x16, Para Angra-1, Chamado 16NGF (Next Generation Fuel), X Congresso Brasileiro de Energia, Rio de Janeiro, 26 a 28 de Outubro de Rosvita G. M., Marcio A. C. Silva, GEACO.N/NT-41- Avaliação de Performance por Pool Side Examination do 16NGF após primeiro ciclo de irradiação como LTA em Kori2, INB Internal Technical Report, 20 de Abril de 2006.