WWER Type Fuel Manufacture in China

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1 WWER Type Fuel Manufacture in China Yang Xiaodong P.O. Box 273, CJNF, YiBin City, Sichuan, China, [Fax: (+86) ] Abstract: At CJNF, a plan was established for implementation of technical introduction of WWER FAs manufacture. And it was confirmed by Russian experts, under which the technical introduction was implemented. On the basis of the technical documentation provided by Russian Party, CJNF established the technical specifications, drawings and the documents associated to processes and quality management applicable for CJNF to standardize the manufacture of WWER FAs. Depending on the practical process capacity, CJNF, as far as possible, adopted directly the technical requirements provided by Russian Party, focused on resolving the technical difficulties in the manufacture and ensured the success in introducing the technology of WWER FAs manufacture. Key words: Fuel assembly; fuel rods; top nozzle; pellets; grids 1. INTRODUCTION China Tianwan NPP is the design of WWER-1000/428 type reactor. In accordance with the Contract signed by and between both Chinese and Russian Parties on December 29, 1997, Russian Party implemented the technology transfer to Chinese Party of manufacturing WWER-1000 fuel assemblies (FAs) in order that the WWER-1000 FAs which were manufactured in China could be refueled in cycle 5 reload of Tianwan NPP. China Jianzhong Nuclear Fuel Co., Ltd. (hereinafter referred to as CJNF ) is the licensee of the technology of manufacturing WWER-1000 FAs transferred by Russia Party. The schedule for implementing the introduction of technology of manufacturing WWER-1000 FAs was established by CJNF according to the Contract and the construction situations of CJNF production line, which was reviewed and confirmed by Russian Party. With the strict implementation schedule, the introduced technology was accomplished under the accurate schedule. With the assimilation of technical specifications, drawings and associated process documents provided by Russian Party, CJNF established its own applicable technical specifications, drawings and associated process documents, and adopts the processes and the technical requirements proposed by Russian experts as direct as possible depending on the actual workmanship of CJNF. In order to ensure the success in introduction of manufacture technology, CJNF highlighted resolving the technical difficulties in manufacture of WWER FAs. 2. TECHNICAL DIFFICULTIES AND SOLUTIONS IN MANUFACTURE OF WWER-1000 FAs 2.1 Improvement of WWER FAs In comparison with the FA for V-320 type reactor, the improved FA are characterized and of different as follows: 1) The specific burnup and the utilization rate of nuclear fuel are raised by the reduction of 235 U enrichment for equilibrium reload to 3.9% from 4.4% and the increase of 1 / 5

2 average discharge burnup to 43 GWD/tU. 2) For the springs of FA top nozzle, a newer and firmer material nickel based alloy has been utilized to improve the hold-down stability. (Previously, the springs of FA top nozzle were manufactured of stainless steel with poorer elastic performance and smaller compression stroke - 27mm of ultimate stroke, and the new springs with bigger compression stroke - 41mm of ultimate stroke.) 3) In FA top nozzles, the single springs are used to hold the bushings; the single compression of springs holding guide thimbles compensates the non-uniformity of the bushings due to thermal expansion and irradiation growth. 4) The outer diameter of guide thimbles are increased as 13 ± 0.05mm and the rigidity is enhanced. 5) Top nozzle is mechanically connected with fuel bundle with detachability. 6) The diameter of guide thimble tip holes is optimized to mm. There are 4 holes in a tip, of which 2 holes can ensure the water flowrate. Therefore, even if 2 holes were blocked by foreign materials, the other 2 holes would still function. 7) The structure of fuel rod (FR) end plug is modified to facilitate pushing of FRs during assembling the FR bundle. 2.2 Design Characteristics of WWER FAs WWER FA is a unique hexahedral FA for pressurized water reactor. With comparison to the square FAs in western countries, it is characterized as follows: 1) Fuel pellet is designed with central hollow. The density of such pellet is bigger than those of other design, as (the nominal value of other design is 95%T.D). Fig.1 WWER fuel assembly schematic figure 2) With different enrichments in a FA. A bundle consists of 311 FRs with different enrichments (at least 2 enrichments) which are inserted into a skeleton. The low enrichment FRs are arrayed inside the circumferential edge of FA; the low leakage array mode is also adopted in FAs. 3) Unique top and bottom nozzles design. Top nozzle assembly is a complicated one, which consists of 23 types (116 pieces) of small components and parts assembled by welding; the design concept is completely different from the others, for instance, the top nozzles for AFA3G and Performance+ are manufactured mainly by welding and machining. The tow design concepts are applied, that is as far as possible made of the cast and as far as possible composed of smaller parts. 4) Unique spacer grids design. The in-reactor operation experience shows the springs in the cells of such spacer grids have no significant loose and enable support 4 cycles of in-reactor operation for over 5 years. The design concept is fully different from that of the double metal spacer grids. 2 / 5

3 5) Some characteristics of FR design. A FR consists of such components and parts: top end plug, cladding tube, bottom end plug, UO 2 pellets and locator (spring). The cladding tube and the top and bottom end plugs are made of Zr-1%Nb alloy, i.e., E100 alloy. In order to protect the cladding tube from extruding by the coolant during operation, the FR is pressurized with 2.0±0.25MPa of helium. The locator fixes the fuel stack in the specified location during transportation or process operation. The end plugs are engaged with the grippers of FR withdrawal and insertion equipment. The bottom end plug is cut into two sections in the tail, in such manner when assembling the FR bundle, the tail is inserted into the hole of lower grid and then the two sections are expanded to engage with the lower grid. The force withdrawing a FR from the grids is 50 kgf min. Such structure guarantees the force enough to hold FRs avoiding the vibration fretting. In cold state, the height of fuel stack is 3530mm and the temperature of cladding external surface is less than 352. The other design features are different from the western design. 2.3 Difficulties in WWER FA Manufacture WWER FA manufacture consists of powder production, pelletizing, FR and FA assembling. The difficulties in the manufacture are: 1) for UO 2 pellet, the pressing of green pellets with central hollow and the control of pellet density;. 2) for top nozzle, more new processes are increased due to the complexity of structure and the difficulty of machining. 3) for spacer gird, it is a honeycomb structure and has 331 cells formed by 311 bushings, of which 311 cells are used to support FRs and the other 20 cells are used to support the guide thimbles of control rods, the central tube and instrumentation tube. The cells are stamped into a protruding triangle with 0.25mm thin-wall tubes; the cells are tightly arrayed each other and fixed with the rims. The cell to cell and the cell to rim is connected by spot welding; the convexity of a cell and its elasticity guarantee the good contact with a FR free of any clearance. 15 levels of spacer grids form the holding of FRs with approximate 45 kgf, ensuring no vibration friction. 4) for resistance butt welding and ultrasonic examination, the bottom end plug of FR is welded by electron beam welding machine, and the weld is examined by ultrasonic plus X-ray. It is required that 0.2mm defect can be found by X-ray examination; the top end plug of FR is welded by resistance butt welding machine and at the same time MPa helium is pressurized inside the FR. The weld shall have no protruding and it is necessary to perform ultrasonic examination for the weld width more than 1.5mm. FRs are welded by electron beam welding machine in CJNF for many years, and CJNF has accumulated richer experience in this aspect. However, CJNF had no such resistance butt welding and ultrasonic examination before. 5) for FR bundle, the coated and lubricated FRs are automatically pushed into skeleton at the speed of 1 rod per 20 seconds. 2.4 Measures Guaranteeing Product Quality In order to overcome effectively various difficulties in the technology introduction, CJNF mainly got a start in the following aspects: implementing introduction and assimilation of foreign countries mature technologies and experiences; improving the internal quality management and control; organizing to hold special meetings to investigate 3 / 5

4 and resolve the difficulties in WWER FAs manufacture; adequately utilizing the current technical capacity to develop the new processes meeting the product quality and also accommodating our current machining level. (1)Utilization of foreign country s mature experiences Manufacture of WWER FAs is a brand new subject of CJNF, but the basis of accomplishing the project is to take adequate use of the foreign country s mature experiences. Firstly, CJNF translated the transferred documentation into Chinese, and then organized the concerned department to hold special meetings to discuss the differences of manufacture processes and inspections so as to establish a strong basis for understanding and mastering the processes of WWER FAs manufacture. In addition, according to the technology transfer contract between Chinese and Russian Parties, the technology transfer is divided into the following stages: assessment for construction of production line, training and couch session in Russia, evaluation for technical documentation, preliminary qualification and final qualification. For example, WWER qualification was completed in three stages from April 20th 2009 to April 1st 2010 (this refers to the time when Russian experts started and ended working in CJNF): preliminary qualification for processes, semi-final products after production line putting into running; preliminary qualification for individual items of production line, processes, semi-final products at the stage of FA dummies fabrication; final qualification for production line, processes, items during fabrication of the first four WWER-1000 FAs. During implementation of qualifications, In these stages, the qualification was completed through CJNF s schedule and thorough organization as well as the close cooperation of Russian experts. The corresponding memorandum was signed by both Chinese and Russian experts; In these stages, Russian Party provided the qualification program by which CJNF established the individual preliminary qualification programs. After these programs were reviewed and approved by Russian experts, the qualification was completed, including pellets, parts and components, FR bundle, FA, welding, inspection, and so on. The qualification results were confirmed by Russian experts; In these stages, Russian experts and the representatives from customer witnessed the qualifications. (2)Improving internal quality management and control Based on understanding and assimilating the updated technical documents and drawings provided by Russian Party, CJNF established a series of standards, norms and technical specifications, such as List of Process and Technical Documents Applicable for Manufacture of WWER FAs, Plan for Implementation of Tools and Fixtures for WWER FA manufacture, etc. Depending upon the practical manufacture level, CJNF directly adopted some technical specifications provided by Russian; and according to the self documentation system and the recommendations from Russian experts, CJNF established 22 technical specifications and a huge number of inspection documents (including routine cards, inspection sheets, traceability cards, etc.) and manufacturing process documents (including process cards and sequence cards). Total quality education to the staffs has been developed to analyze deeply various reasons for quality incidents. In the activities of product quality assurance, the staffs are 4 / 5

5 required to: enhance the diathesis, keep calm while an incident occurs, make careful operation, maintain high sense of responsibility, and implement strict management and supervision. (3) Considering WWER FA Manufacture as a focus CJNF regularly held the company-level and technical adaptation meetings in which the main leaders took part and the introduction of WWER FA manufacture technology was coordinated as an most important work to ensure the parallel implementation of the technical introduction, manufacture and adaptation. For the issues of machining tools and fixtures, CJNF held special meetings to check and review the drawings of tools and fixtures to find out the resolutions. The project manager of CJNF went deep into workshops and organized technical discussion meetings in which Russian experts took part to establish the test plans and the detailed production schedule and resolve the process problems so that the production qualification was guaranteed. 3. CONCLUSION With over 20 year history of nuclear fuel manufacture, CJNF has accumulated higher capacity of manufacturing fuel assemblies. However, for, CJNF has some difficulties of manufacturing WWER FAs due to their design features. CJNF systematically organized and overcame many technical difficulties and completed the transfer of technical documentation, the review of transferred documentation, the qualification, etc. By now, CJNF has succeeded in manufacturing 108 FAs, indicating the technical introduction is successful. 5 / 5