Patent Number: 046876057
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to production of nuclear fuel rods for use in nuclear reactors and, more particularly, is concerned with a unique automated system for the production of nuclear fuel rods starting from the conversion of a radioactive gas to powder, through the fabrication of the powder into pellets, to completion of the assembly of the fuel rods. 2. Description of the Prior Art Conventional nuclear reactors include fuel elements, generally called fuel rods. The fuel rods contain fissile material and are grouped together in arrays which are organized to produce a neutron flux in the reactor core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat. A coolant such as water is pumped upwardly through the arrays of fuel rods in the reactor core in order to extract some of the heat for the production of useful work. Typically, a fuel rod is composed of an elongated hollow metallic tube which contains the nuclear fuel material in the form of a stack of cylindrical fuel pellets. The tube is closed at its opposite ends by upper and lower end plugs which are rigidly attached to the tube ends by girth welds so as to hermetically seal the tube. During operation of the reactor core, the fuel rods are subjected to high temperatures and pressures within the core which cause elongation of the tube and pellets due to thermal growth and vibration of the fuel rods due to coolant flow. Thus, the pellets are fabricated to very exacting dimensions so as to produce a controlled diametrical clearance between the pellets and the inside of the tube to accommodate pellet growth due to thermal expansion and fuel swelling. Additionally, in view that pellets are brittle and will easily chip upon impact, a coil spring is ordinarily disposed within the tube between the upper end plug and the top of the pellet stack to restrain damaging impacts between the pellets due to rod vibration. Fuel rod manufacturing conventionally involves beginning with the radioactive material in powder form and then blending it to the desired chemical composition. The properly blended powder is then made into pellets by first forming it into slugs, then granulating the slug and mixing a lubricant with the granulates, and lastly pressing the lubricated granulates into green pellets. The green pellets are fed into a sintering furnace where high temperatures sinter the pellets in a hydrogen atmosphere to achieve the required density and microstructure. After exiting the furnace, the sintered pellets are fed to a wet grinding process for grinding them to precise dimensions. Before insertion into the fuel rod tube, the finished pellets are visually inspected. After the pellets are placed in the tubes, the completed fuel rods are subjected to several different inspections. Due to the fact that fissile material is involved, fuel rod manufacturing up to the present time has been carried out in conformity with the regulatory requirement of geometric control of the radioactive material being converted into fuel pellets. Geometric control relates to a safeguard which eliminates the possibility of a chain reaction occurring by limiting the quantity of radioactive material assembled together to an amount significantly less than the critical mass needed for fission. This safeguard was implemented by the performance of a high degree of manual handling of radioactive materials and fuel rod components during the various manufacturing stages of what has been termed a batch mode of operation. For instance, a batch of radioactive material of a given enrichment had to be processed completely through a given stage of the manufacturing process and the equipment emptied of all residual material of that enrichment before material from a different batch having a different enrichment could be processed. Thus, with respect to each batch, typically, in the initial stage each worker carried an individual container filled with a small quantity of radioactive material from the batch in powder form to the blender. Once the material was properly blended, the worker then manually transferred the blended powder to the pelleting stage where this quantity of material was fabricated into pellets. The green pellets were then loaded manually by the worker into a sintering boat which was taken to the infeed end of a sintering furnace and then unloaded manually. After being conveyed through the furnace, the sintered pellets were manually picked up and fed to the wet grinding station. Then, the ground pellets were manually placed on inspection trays. After visual inspection, the pellets were manually inserted into tubes which in the meantime had been manually handled through various stages involving the inspection and cleaning of the tubes and attachment of end plugs thereto. Another regulatory requirement which was implemented most effectively by the performance of a high degree of manual handling of radioactive materials and fuel rod components during the various stages of the manufacturing operation was the need for traceability of the radioactive material from its initial powder form to its final form as pellets in a completed fuel rod. Without much difficulty, a worker who began with a certain quantity of radioactive material from a known batch and transported it through successive stages of the manufacturing operation could identify which completed fuel rods contained material from the particular batch. While the high degree of manual involvement in fuel rod manufacturing up to the present time has assisted the nuclear industry in meeting the regulatory requirements of geometric control and traceability and thus has served the industry well over the past several decades, such involvement has tended to constrain improvement in manufacturing productivity and product quality. Consequently, a need has evolved for a different approach to fuel rod manufacture which promises increased manufacturing efficiency and productivity and improved product quality and reliability while at the same time meets all regulatory requirements. SUMMARY OF THE INVENTION The present invention provides a unique automated system for the production of nuclear fuel rods which is designed to satisfy the aforementioned needs. Starting from the conversion of a radioactive gas to powder, through the fabrication of the powder into pellets, to completion of the assembly of the fuel rods, the present invention provides interrelated fuel rod production stages which fully integrate and transform what are otherwise conventional manufacturing process steps per se, some of which were used heretofore in conjunction with a batch mode operation, into a dedicated, continuous or paced, flow mode of operation. The automated system of the present invention is designed to have a production capacity of 400 MTU (metric tons uranium) of fuel rods per year which represents a significant departure from the existing batch system capable of producing approximately 200 MTU per year. Major factors contributing to productivity and quality improvements in the automated system are as follows. First, the use of the Integrated Dry Route (IDR) method, instead of the Ammonium Di Urinate (ADU) method, for the conversion of UF.sub.6 gas to UO.sub.2 powder. This process, licensed from British Nuclear Fuels Limited, consistently produces a more fabricable powder of higher sinterability than is achieved using the ADU conversion process. Second, the utilization of bulk blending methods contributes to improved product consistency. These methods used in association with pneumatic conveying significantly reduce material handling requirements, personnel exposure and airborne contamination levels. Third, integration into the automated system of product inspection operations utilizing advanced technology and of automation of materials handling and process control reduces operator dependency and improves product quality. Fourth, line manning and operation of the total system (UF.sub.6 through completed fuel rods) is on a team basis. This is expected to improve morale and group productivity, and reduce manpower requirements. Fifth, increased automation of the production line results in fewer personnel actually working on the line. Improved containment and processing methods, and use of advanced ventilation systems will minimize operator exposures and resultant lost time due to work restrictions. Finally, implementation of a fully integrated Management Information System (MIS) employing automated data imput provides a central information network for management visibility, and production planning and control. The MIS will ensure material accountability, quality control and product traceability so as to satisfy both customer and regulatory requirements. Accordingly, the present invention sets forth an automated system of nuclear fuel rod production composed of a plurality of interrelated stages. First, a radioactive powder formulation and processing stage comprises the combination of: (a) a plurality of kiln units for receiving a radioactive material in the form of a gas and converting the radioactive material to the form of a powder; (b) a plurality of check hopper units being connected in flow communication with each of the kiln units for receiving the powder from the kiln units, for holding the powder for sampling and inspection and for dispensing the powder therefrom such that as at least one of the check hopper units is being filled from its respective one kiln unit, at least another of the check hopper units is dispensing its powder whereby powder can be continuously dispensed from at least one of the check hopper units; (c) a plurality of blending units connected in flow communication with the check hopper units for receiving the powder from the check hopper units and for blending the powder into a radioactive composition suitable for fabrication into a nuclear reactor fuel, the plurality of blending units being fewer in number than the check hopper units; and (d) valve means for causing the filling of one of the plurality of blending units at a time with powder from the check hopper units such that as one of the blending units is being filled, blended powder is being dispensed from another of the blending units whereby blended powder can be dispensed continuously from the blending units for subsequent fabrication into a form suitable for use as nuclear fuel. Also, the plurality of blending units includes yet another blending unit containing blended powder which can be analyzed as the one of the blending units is being filled with powder and as blended powder is being dispensed from the other of the blending units. Second, the automated system of nuclear fuel rod production includes a radioactive pellet fabricating stage comprising a plurality of pellet fabricating units for receiving blended powder of a radioactive composition suitable to be used as nuclear fuel. The pellet fabricating units are operable to press the powder into slugs, then granulate the slugs, next mix the granulated powder with lubricant and finally form the mixture into a succession of green pellets of the radioactive composition. The fabricating units are capable of providing a continuous stream of the green pellets even when one of the fabricating units is temporarily out of commission. Third, the automated system includes a pellet processing stage comprising the combination of: (a) a plurality of sintering furnace units, each being adapted to receive green pellets at an infeed end thereof, to sinter the pellets as they are moved through the furnace, and to discharge the pellets at the completion of sintering, the sintering furnaces being greater in number than the green pellets fabricating units so as to be capable of providing a continuous stream of sintered pellets even when one of the furnaces is temporarily out of commission; and (b) means for receiving the sintered pellets in a successive manner after discharged from the sintering furnaces for periodically sampling random ones of the sintered pellets. More particularly, the pellet processing stage further comprises: (c) a multiplicity of sintering boats; (d) means for conveying the sintering boats and for providing surge storage thereof; and (e) means for loading green pellets from the pellet fabrication units into the sintering boats on the conveying means. The conveying means is operably arranged with the plurality of sintering furnace units such that each of the loaded boats is directed to the infeed end of one of the furnaces, is moved through the one furnace for sintering the pellets loaded therein and is directed from a discharge end of the one furnace at the completion of sintering. Still further, the pellet processing stage includes: (f) a plurality of pellet grinding units for grinding the sintered pellets to precise predetermined dimensions; (g) means for unloading sintered pellets from the boat on the conveying means in single file into the pellet grinding units; (h) a plurality of inspection units for inspecting the ground pellets; (i) a pellet storage and retrieval unit for receiving the inspected pellets and storing the same; and (j) means for conveying the pellets in single file from the grinding units through the inspection unit to the storage and retrieval unit. Finally, the automated system for fuel rod production includes tube preparation and fuel rod fabrication and inspection stages comprising: (a) means for preparing fuel rod tubes for receiving the stored pellets; PA1 (b) means for assembling the tubes and pellets into completed fuel rods; and PA1 (c) means for inspecting the completed fuel rods. These and other advantages and attainments of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention.