Patent Number: 046876057
Section: description

DETAILED DESCRIPTION OF THE INVENTION In the following description, like reference characters designate like or corresponding parts throughout the several views of the drawings. Also, in the following description, it is to be understood that such terms as "forward", "rearward", "left", "right", "upwardly", "downwardly", and the like, are words of convenience and are not to be construed as limiting terms. In General Referring now to the drawings, and particularly to FIG. 1, there is illustrated a block diagram of the basic, successive, interrelated stages of the automated nuclear fuel rod production system of the present invention, being generally designated by the numeral 10. Each of these stages will be described in detail hereafter with reference to the process steps depicted in FIGS. 2 to 5 and the equipment schematically illustrated in FIGS. 6 to 8. However, before proceeding into a detailed discussion of the various stages of the automated system 10, a brief overview of the automated system 10 will be presented. In a first stage of the automated system 10, represented by block 12 in FIG. 1 which contains the caption, Powder Formulation & Processing, a suitable radioactive gas, such as uranium hexafluoride (UF.sub.6), is converted into another radioactive substance in powder form, such as uranium dioxide (UO.sub.2), which is then blended into a suitable composition for pellet fabrication. Next, the blended powder is formed into green pellets in a second stage of the automated system 10, being represented by block 14 in FIG. 1 and designated Pellet Fabrication. After fabrication, the green pellets are sintered, sampled, ground, inspected and stored, all of which steps are included under the heading, Pellet Processing, and represented by block 16 in FIG. 1. Concurrently, as the pellets are being fabricated and processed, the other primary part of a nuclear fuel rod, the hollow tube, is being prepared for assembly with the pellets. Block 18 of FIG. 1, called Tube Preparation, represents such activity. Finally, steps carried out in assembling the prepared tubes and stored pellets together and in inspecting the assembled fuel rod are represented by block 20 of FIG. 1 and termed Fuel Rod Fabrication & Inspection. Powder Formulation & Processing Turning now to FIGS. 2, 6 and 7, there is shown the process steps and equipment involved in the first and second stages, the powder formulation and processing stage 12 and the pellet fabrication stage 14, of the automated fuel rod production system 10. The formulation and processing of a suitable radioactive compound in powder form will be described in this section, with its formation into pellets being reserved for the next section. The first, or powder formulation and processing, stage 12 of the automated system 10 begins when, as per block 22 of FIG. 2, cylinders containing raw uranium hexafluoride (UF.sub.6) gas are released from storage as needed and installed in one of several vaporization units, such as vaporizer vessels 24 illustrated in FIGS. 6 and 7. To vaporize the UF.sub.6 gas as per block 26, the cylinders are heated to a temperature of approximately 180 degrees F. in the vessels 24 by circulating hot water sprays therein. The resulting UF.sub.6 vapor is supplied under pressure from the cylinders in the vessels 24 through a pair of gas flow lines 28 to a pair of kiln units, such as rotary kilns 30. In the preferred embodiment, as depicted in FIG. 7, a pair of the vaporizer vessels 24 are connected in flow communication with each of the kilns 30 so as to provide sufficient excess capacity to ensure a continuous supply of vapor to the kilns at the same time as a depleted one of the cylinders in one of the vessels is exchanged for a fresh cylinder. The kilns 30 implement the IDR process, as per block 32 of FIG. 2, by converting the gaseous UF.sub.6 to UO.sub.2 powder, first, through reaction of the gas with superheated steam at the feed ends of the kilns and, then, through reaction of their intermediate products with a counter-current flow of steam and hydrogen at the product ends of the kilns. The UO.sub.2 is discharged by gravity flow from the product ends of the kilns 30 to a temporary storage, as per block 34 of FIG. 2, in the form of a plurality of check hopper units 36. The check hopper units 36 which together continuously receive uranium dioxide powder from the lower end of the kilns 30 are each of a size to ensure geometric control of the UO.sub.2. Powder entering the check hopper units 36 is continuously sampled by a time proportional sampler and analyzed for acceptable quality as to fluoride and moisture content as per block 38 of FIG. 2. The moisture check is made here in order to initiate the exercise of moderation control over the powder during the blending of the same which occurs next. In the preferred embodiment seen in FIG. 7, a pair of the check hopper units 36 are connected in flow communication with each of the kilns 30 such that as at least one of the check hopper units of one pair is being filled from its respective kiln, at least one of the other check hopper units is dispensing its powder while the powder in another of the check hopper units is being sampled. With such arrangement, a sequence of operation can be implemented whereby powder is continuously dispensed from at least one of the check hopper units 36 while in-line sampling of powder is carried out at another unit. Powder of acceptable quality is discharged continuously from at least one of the check hopper units 36 via a pneumatic transfer line 40 to a plurality of blending units, such as the bulk blenders 42 seen in FIGS. 6 and 7. Powder found to be unacceptable with respect to fluoride and/or moisture is transferred via safe geometry transfer containers (not shown) to a powder rework station (not shown) for further treatment to reduce fluoride and/or moisture content. Reworked powder which meets specifications is then returned to the process stream at the blenders 42. Each of the bulk blenders 42, in the preferred embodiment, has a 5000-Kg capacity and is used to produce a homogeneous blend which meets product specifications, as confirmed by the performance of a chemical analysis per block 44 of FIG. 2. Each conical shaped blender 42, preferably being three in number, has a rotating internal screw to ensure thorough blending, as per block 46 of FIG. 2. The use of large blenders 42 reduces the number of individual blends which must be made and minimizes blend-to-blend variations. Further, because of the large capacity of the blenders 42, complete elimination of all powder from a batch of one concentration from the kiln and blender components is not now necessary before powder from another batch of a different concentration can be introduced into the blenders. Transport line diverter valves 48 associated with each of the blenders 42 are actuated to direct powder via a cyclone receiver 50 to one of the blenders while blended powder in another of the blenders is inspected as per block 52 of FIG. 2 and blended powder from the remaining blender is dispensed to pellet fabrication operations immediately downstream of the blenders. In such manner, blended powder is dispensed continuously for uninterrupted pellet fabrication. Also, as indicated by blocks 54, 56 of FIG. 2, powder from dirty and clean scrap processing, after an adequate moisture check as per block 58, can be transferred to the blenders 42 for blending. Pellet Fabrication Blended powder from the blenders 42 is transferred via a pneumatic transfer line 60 to pellet fabricating units 62, preferably two in number, which begins the second, or pellet fabrication, stage 14 of the automated system 10. The entire green pellet fabrication process is controlled and operated as an automated integrated system with the pelleting equipment of the units 62 being arranged vertically to permit gravity transfers of material and to minimize floor space requirements. The equipment is enclosed and is subject to controlled ventilation to prevent the spread of airborne particles. As per block 64 of FIG. 2, blended UO.sub.2 powder released from the one of the blenders 42 which happens to be dispensing at the time is fed on demand, via transfer line 60, into a powder compactor 66 of each of the pellet fabrication units 62 as seen in FIG. 6. In the compactor 66, powder is compacted by a slugging press into small wafers or slugs which flows downward to a granulator 68 at the next lower level of each fabrication unit 62. The inlet of the granulator 68 is close-coupled with the discharge of the compactor 66 so that the two devices operate simultaneously. As per block 70 of FIG. 2, the slugs are granulated in the granulator 68 to a composition resembling freeze-dried coffee. Next, as per block 72 of FIG. 2, the granules are combined, on a proportional basis, with a suitable lubricant, such as zinc stearate, and rolled to produce a press feed material that has improved flowability. (The zinc stearate serves as a die lubricant during pellet pressing which follows.) Finally, the granule and lubricant mixture are formed, as per block 74 of FIG. 2, into green pellets by a pellet press 76 at the lowest level of each of the fabrication units 62. The pellets are typically compacted into cylindrical bodies 5/8ths-inch long and 3/8ths-inch in diameter with a 60 percent theoretical density that equals 10.3 grams per cubic centimeter. Ordinarily, the pellet fabricating units 62, operating at a rate of only about one-half of their combined capacity, provide a continuous stream of green pellets which is sufficient for feeding the processing equipment located downstream. Thus, if one of the fabricating units 62 happens to be temporarily out of commission, the other one can take up the slack and, by operating at or near its capacity, supply the total requirement of green pellets for the next pellet processing stage 16 of the automated system 10. Pellet Processing At the beginning of the third, or pellet processing, stage 16 of the automated system 10, empty boats 78 are advanced in a procession thereof toward, while boats 80 loaded with green pellets are moved away from, the discharge of the pellet press 76 by a conveyor 82. Only an end of the conveyor 82, as depicted in FIGS. 6 and 7, is associated with the pellet fabrication units 62. Most of the conveyor 82 is seen in FIG. 8 wherein it is arranged to deliver boats to and remove boats from respective infeed and discharge ends 84,86 of a plurality of sintering furnaces 88 as well as other processing equipment to be discussed later. In addition to conveying the boats, the conveyor 82 provides in process storage of both empty and full boats. At the discharge of the fabrication units 62, the green pellets are gently loaded in an orderly array within the molybdenum sintering boats 78,80 and then moved by the conveyor 82 to a branch 90 thereof where a shuttle car 92 delivers individual loaded boats 80 to the infeed ends of the furnaces 88. A boat 80 loaded with green pellets at the infeed end 84 of one sintering furnace 88 is conveyed through the furnace by a walking beam device employed by each furnace and emerges as boat 94 loaded with sintered pellets. In each furnace 88, the pellets are sintered, as per block 96 of FIG. 3, to a specified 95 percent theoretical density in a hydrogen atmosphere at approximately 1750 degrees C. to achieve the required density and microstructure. The boat handling and furnace operations are mechanized in their entirety and monitored and controlled as an integrated system from a single control station. Multiple sintering furnaces 88, such as three in number, are used to allow excess capacity so that a continuous stream of sintered pellets can be provided to the remainder of the processing equipment even when one of the furnaces is temporarily out of commission. After the boats 94 loaded with sintered pellets exit the discharge ends of the furnaces 88, they are automatically transported by the conveyor 82 to a sampling station 97 where representative ones of the sintered pellets in each boat are randomly sampled and their density inspected, as per block 98 of FIG. 3. Low density pellets are routed by a branch (not shown) of the conveyor 82 for resintering in the furnaces 88. High density (overdense) pellets are routed to clean scrap recovery, as per block 56 of FIG. 2. Other measurements and checks are performed of the pellet samples, as per blocks 100 and 102 of FIG. 3, some of which take several days before the pellets are finally approved. Therefore, the unapproved sintered pellets are advanced through the next step in the pellet processing stage 16 and thereafter stored where they will await approval before assembly into a fuel rod. The boats 94 of unapproved sintered pellets are conveyed to one of a pair of unloading units 104, as seen in FIG. 8, where the boats are unloaded and the pellets oriented in single file are fed to one of a pair of grinding units 106. The pellets are centerless ground in a dry grinding operation, as per block 108 of FIG. 3, using a diamond grinding wheel to achieve acceptable surface finish and proper diameter. The material removed during grinding is collected by a dust collection system and the recovered swarf is collected, as per block 110 of FIG. 3, and returned to clean scrap recovery, as per block 56 of FIG. 2. The ground pellets are then fed in single file to one of a pair of inspection stations 112 where on-line diameter and surface quality inspection is carried out, as per block 114 of FIG. 3, by suitable devices paced to the operation of the grinding units 106. Also, additional tests are performed on the pellets, as per blocks 116,118 of FIG. 3. Unacceptable pellets are sorted and sent to either dirty scrap processing, as per block 54 of FIG. 2, or clean scrap processing, as per block 56 of FIG. 2, depending on the particular contaminant and/or defect associated with the pellet. Acceptable pellets are loaded, row by row, onto clean pellet trays which are then routed by a tray transfer device 120 into an auto storage and retrieval system 122, as seen in FIG. 8, to an identified storage position, as per block 124 of FIG. 3. The pellets stay in the system 122 pending receipt of quality control approval of their earlier sampling and, after approval is received, until required for fuel rod tube loading. The storage area of the system 122 is designed to hold a 3-4 day requirement of pellets. Such excess capacity ensures that continuous assembly of pellets with tubes can be accommodated while awaiting up to 2 days to receive results of laboratory tests on the pellet samples. Furthermore, tray movement into, from, and within the storage and retrieval system 122 is directed and controlled in such manner that the system has the capability to trace the location of individual trays or of groups of trays as required to maintain traceability. Tube Preparation The remaining two stages of the automated fuel rod production system 10, the tube preparation stage 18 and the fuel rod fabrication and inspection stage 20 which are arranged in tandem, are carried out concurrently with the first three stages described above. By so doing, tubes will be prepared and ready for insertion of nuclear fuel pellets by the time processing of the pellets has been completed whereby a continuous (paced) assembly line type production of fuel rods can be achieved. The steps involved in the preparation of fuel rod tubes will be described in this section, while the assembly and inspection of the fuel rods will be reserved for the final section. Referring now to FIGS. 4 and 8, the tube preparation stage 18 begins when, as per block 126, tubes are taken from storage and delivered to a tube indexer system 128, seen in FIG. 8. The tube indexer system 128 is a synchronous transporter which transfers tubes through the various preparation and inspection operations of this stage. In the system 128, multiple indexing units are used with transition and feed devices separating the units. The transition and feed devices provide a pause in the system which increases system availability. Initially, the serial number of each tube is read using an automatic image recognition device (not shown) which verifies the correct label and enters all information for the tube into the traceability system. Then the tube is indexed by the system 128 to a checker station 130 seen in FIG. 8 where, as per block 132 of FIG. 4, the tube is checked to see if it is clear internally. If the tube is not clear an operator is alerted and the tube is not indexed. From the station 130, the tube then goes to a cleaner station 134 where, as per block 136 of FIG. 4, a tube cleaner engages the tube end (normally the lower end), grips it and with a rotating action wipes the end with a cleaning material. The cleaning media is discarded to a collection can, the head of the cleaner retracts and prepares for the next cycle. The lower end of the tube is now prepared for receiving an end plug at a next, tube plugger station 138 of FIG. 8. After being moved to station 138, the tube is gripped by a clamp and, as per block 140 of FIG. 4, a plug is pressed into the tube end. Then, the plugged tube is advanced to a weld station 142, inserted into a weld chamber and, as per block 144 of FIG. 4, a girth weld is made on the tube-to-plug joint. When completed, the tube is transferred to a transition device (not shown). Between the weld station 142 and the downstream inspection operations coming up next, the tubes are surged to form a break between the two indexing transporters of the indexer system 128. Then, from the transition device, the tube is advanced to a weld physicals check station 146 seen in FIG. 8 where, as per block 148 of FIG. 4, the weld beam on the tube is checked for diameter and surface discoloration. After weld physicals check, each tube is transferred to a weighing station 150 where the tube is weighed as per block 152 of FIG. 4. Finally, the tube weld is ultrasonically inspected at station 154 of FIG. 8, as per block 156 of FIG. 4. If the weld is accepted the tube is transferred downstream to a tube transporter (not shown) where it is transferred axially in preparation for fuel pellet loading operations. Tubes with rejected bottom end welds are removed from the process stream to a repair station (not shown), as per block 158 of FIG. 4, where the tube end plug is removed and the tube is again recycled through the tube preparation operations as described above. Fuel Rod Fabrication & Inspection In the fifth and final, or fuel rod fabrication and inspection, stage 20 of the automated system 10, the fuel rod tube prepared during the previous stage 18 and pellets stored in the storage and retrieval system 122 are brought together. The UO.sub.2 fuel pellets are loaded into the tube, then a spring is inserted and an upper end plug is applied and welded to the tube, after which the tube is internally pressurized and sealed. These are the basic assembly steps. They are followed by a multiple of inspection operations, although a few checks are interspersed between the assembly steps. In a tube indexing system 160, a synchronous transporter transfers tubes through the various fabrication and inspection operations. Groups of operations are separated by a transition device of the system 160. Rods are surged at various intervals in the fabrication and inspection stage to form a break between indexing transporters. First, prepared fuel tubes are fed from the axial conveyor to a surge conveyor of the fuel rod fabrication indexer system 160. A plurality of tubes, such as 25 tubes, are accumulated and transferred onto the pellet loading table 162 in FIG. 8 where, as per block 164 of FIG. 5, a vibratory loader is actuated and approved pellets are moved into the tubes. The pellets are transferred on trays to the loading table automatically from the storage and retrieval system 122. The pellets are then swept off of the trays onto the loading table for vibratory feeding into the tubes. Following the loading operation, tubes, now referred to as rods, are transferred to a transition section and then in order are transferred to the rod indexing transporter of the indexer system 160. At this time, each rod number is scanned and stored in a file for traceability. The first station following pellet loading in FIG. 8 is the plenum gage station 166 where, as per block 168 of FIG. 5, the rod plenum is measured and then pellets added or removed depending on the plenum measurement. After plenum adjustment, the rod is weighed at station 170, as per block 172, and then the top rod end is cleaned at station 174, as per block 176 of FIG. 5, in the same manner as the bottom end cleaning operation was carried out. Thereafter, a plenum spring is inserted into each of the rods at station 178, in accordance with block 180 of FIG. 5, followed by pressing a plug into the upper end of the rod and compressing the spring therein at plugging station 182 of FIG. 8, as represented by block 184 in FIG. 5. Fabrication of the fuel rod is completed by girth welding the plug and rod joint at girth weld station 186, as per block 188 of FIG. 5, and then pressurizing the rod with helium at station 190 with a seal weld being made on the plug end, as per block 192 of FIG. 5. Prior to the girth welding operation, the rods were surged to break the synchronous operations into a second group. Once again after fabrication of the fuel rod is finished and before inspection begins, the rods are surged to break the synchronous operations into a third group. Then the rods are fed to an indexing transporter of the indexer system 160 where at a first station 194 in FIG. 8, as per block 196 of FIG. 5, a weld physical check is made. The weld bead is basically checked for diameter and discoloration in the weld area. Following next in sequence, as seen in FIG. 8, the rod is inspected ultrasonically and by x-ray florescence at respective stations 198 and 200, as per respective blocks 202 and 204, followed by checks for straightness and length at a station 206, as per block 208 of FIG. 5, and an inspection of the tube surface for scratches and marks at a station 210, as per block 212 of FIG. 5. After surface inspection, fuel rods are transferred downstream to a transition conveyor of the indexer system 160 where they are in turn fed to a gamma scanner station 214. As per block 216 of FIG. 5, the rods are scanned automatically for presence of internal components, pellet stack continuity, enrichment verification and plenum length. Results are entered into the traceability system. After a helium leak test at a station 218, as per block 220 of FIG. 5, the acceptable rods are sent to storage, as per block 222. Rods rejected from any of the inspection operations are identified and transferred to a processing area, as per block 224 of FIG. 5, where corrective action can be taken, after which the rod is returned to the rod fabrication operations or to a fuel rod salvage location, as per block 226 of FIG. 5. From the foregoing description, it will be understood that the automated system 10 is capable of achieving a high rate of production in a dedicated, continuous (paced) flow mode of operation by providing excess capacity at critical stages of the system. It integrates process operations, quality control inspection, improved materials flow, and accountability which results in a reduction of manufacturing cycle time. Also, it incorporates improved features for the containment of special nuclear materials, with enhanced ventilation to minimize the amount of airborne material and achieve reduced occupational exposure levels, not only during routine operations, but also to facilitate containment during maintenance. It is thought that the present invention and many of its attendant advantages will be understood from the foregoing description and it will be apparent that various changes may be made in the form, construction and arrangement thereof without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the form hereinbefore described being merely a preferred or exemplary embodiment thereof.