The Manufacture of Uranium Dioxide Fuel in Pellet Form

N\ THE MANUFACTURE OF URANIUM DIOXIDE FUEL IN PELLET FORM Arnold Blum Iscar Ltd , Nahariya Introduction Commercial nuclear fuel for conventional pressurized water reactors (PWR) is being produced in tonnage quantities on the basis of low enriched uranium oxide (UO ), i.e., UO in which the uranium is enriched to 5% or less in the fissionable isotope U-235. The bulk of the fuel proces- sed is of an enrichment between 2-3% U-235, While the fuel is generally drawn from the enrichment plants in the form of UFR, there are a variety of processes by which it is converted to uranium oxide. The method of conversion has a significant bearing on the properties of the uranium oxide powder which serves as the stating material for pellet fabrication This problem is being dealt with by another, and will not ba treated here, A fossil-fuel furnace is principally designed to generate heat, but in the process of doing so, also produces fay-products in the form of solid residues and gaseous effluents. A pressurized water reactor is ana- logous in its mode of operation. One of the solid residues of the fuel is the metal plutonium, which can be re-cycled and used in subsequent fuel cores. The fabrication of plutonium containing fuel, irrespective of its use, falls outside the scope of this paper. The fabrication of low enriched uranium oxide pellets is in essence a process for generating ceramic bodies, but the fabrication processes are complicated by problems of nuclear safety, security, health & safety, high intrinsic value, enrichment control, scrap recovery, accountability, effluent control and environmental protection, and the high sensitivity of the fuel to even minor contaminants, which are of no consequence in - 2 - conventional ceramics. The problems peculiar to the production of nuclear fuel result in a high overhead operation, with a fixed cost unheard of in most other industrial operations. The basic process, and some of the problems alluded to abova, will be dealt with in the following parts of this paper. The Basic Process 1. Powder Since the mechanism by which UO^ powder is converted to fuel pellets is based on the application of pressure and heat, the required magnitude of these, depends in large measure on the intrinsic surface energy of the powder. On the National Archives in Washington, D.C. is found the inscription: "The Past is Prologue". The truth of this state- ment can nowhere's be demonstrated as effectively as in the fabrication of UO, pellets. The impact of powder properties on the economics of fuel fabrication is overwhelming, in regard to the reguired compacting pressures, sintering temperatures and sinterinq times, and the resulting density and integrity of fuel pellets which have a direct bearing on product yield. At the time of my last involvement in the fabrication of pellets, about one year ago, there was still raging a controversy among fuel designers as to the most desirable structure of sintered fuel.. Since in the fission process, fission gases are generated, these must be acco- modaterl inside the fuel pellets if these are not to disintegrate into dust under the influence of internal gas pressure. On one side of the argument were lined up the advocates of moderate density pellets (92 - 93% of theoretical) characterized by a microstructure with a high density matrix interspersed by well rounded, uniformly distributed pores. - 3 - Such a structure, because of the spherical nature of the porosity, would not be expected to shrink significantly under the influence of the hiqh temperatures prevailing inside the fuel under reactor operating condi- tions . On tho other side of the controversy were the advocates of high density pellets (96% of theoretical and higher) who argued that on high burn- up, fuel shrinkage was bound to occur even if the pores were well rounded in medium density structures, and such shrinkage would tend toward fuel clad failure by tube buckling. Some such failures had just been discovered in operating reactors, and these reactors shut-down by order of the U.S. A.E.C. The outcome of this controversy will h»ve a decided impact on the types of powders required. High density pellets require powders with a specific surface of 3 rn /gram and up and Fisher Sub sieve Sizes in the range of 0.8 - 1.1 microns. Medium density pellets require lower specific surfaces, about 2 2.5 - 2.8 m /gram and Fisher Sub Sieve Sizes of 1.2 - 1.5 microns. There are, of course, other means of affecting the density of fuel pellets, such as the magnitude and kind of the lubricant addition and the mariner in which it is applied; the magnitude of U,0o additions, used as the preferred method of recycling sintered scrap; the compacting pressure; and the sintering temperature. But the predominant factor in achieving a desired sintered fuel pellet structure lies in the quality of the starting powder. Attempts were made in correlating the particle size distribution with the sintering behavior of powders, using 3 different types of instruments. None of the results were successful. - 4 - Another important quality attribute of UO powders is that of the 0/U ratio. Since UO is not the hiohest oxide of uranium, it can readily oxidize in the'finely'divided state. A certain amount of re-oxidation is almost inevitable as UO powder exits from the reduction step and is auto- natically conveyed through mechanical treatments to the weighing and blending station. Dry ice is used to provide a CO blanket over exposed UO and also to extinguish fires which sometimes start spontaneously in exposed containers. U/0 ratios greater than 2.15 are not acceptable for pellet powder. Great care is exercised in maintaining the chemical and isotopic purity of UO- powders. Since mechanical conveyances are used in the preparation of the powder, there is the danger of contamination by abrasion of the contacting surfaces, principally by iron group metals. If is for this reason that 400 series stainless steel is preferred as material of cons- truction of contacting surfaces. This series of stainless, being magnetic, can be removed by magnetic separation. Neutron absorbers, such as cadmium, used as plating for steel faste- ners, and silver, used as a constituent of braze joints, are carefully excluded from powder and pellet facilities. So are boron, and the rare earths. The powders are analyzed for a lona list of elements, each speci- fied as to the permissible maximum concentration allowed. Isotopic analysis by raass-spectograph is required for each powder and pellet lot. In a plant processing a diversity of enrichments, a thorough "isotopic" cleanup between different enrichments is a major effort. Ins- pection and release by Quality Control is required before the facility may be used for a new enrichment. - 5 - 2. Press Feod Preparation I all automatic compactino presses the reproducibility of green density, weight and heiaht of a compact depends on reproducibly volume- tric die filling. In order to assure this, the powder must be rendered flowable in a reproducible manner. This requires the addition of organic lubricants which tend to hold tooether aggregates of primary particles to form spheres or granules which flow into the pressing die. If the spheres are imperfect, or fines are present in the press feed, they tend to cause improper die filling by "bridging", and the weight of material delivered to the dies fluctuates within excessive limits. Conventional press-feed preparation uses liaht compaction of the raw powder into low density compacts which are pushed through a screen and so broken up into granules. These granules are blended with a small percen- tage of an organic binder and rounded off into balss during the blending operation, so as to make them free-flowing. Some processors screen the blended material in order to remove fines and large balls, which impede flow. More advanced means of accomplishiRo this parpose are by the use of a "spray-drier". In such equipment a slurry composed of raw powder, an organic vehicle and the binder is pumped t.o a drying tower, where it is sprayed upward from a nozzle and the volatile vehicle is evaporated by a counter-current flow of hot, inert gas, causing the droplets of slurry to congeal into small balls or spheres which are then removed from the tower. Such press feed is more uniform, softer and freer of contamination, since it has not been exposed to abrasion as in the case of mechanical granulation. - 6 - However, nuclear safety aspects of the spray-drying techniaue have to be carefully engineered, due to the exposure of the UO-to fair-sized volumes of hydrogenous vehicles. Yet other press-feed results directly from the powder making process, in which the primary aggregates are generated in spherical form. Such aggregates are free flowing without an intermediate processing step. Lu- bricant must then be added to the dip walls during the pressino operation, since the close fitting punches cannot operate without lubrication. 3. Pellet Pressing Si.nce a 3000 kg. lot of powdr;r is converted into several hundred thousand pellets depending on pellet size and density, the press must be capable of high productivity and the tooling must be highly abrasion resis- tant to survive a production run. Cemented tungsten carbide tooling is the- refore standard for UO' pellet production. To minimize pellet-cladding interaction, as fuel clad tubes shrink onto the fuel stack, it has been found advisable to chamfer fuel pellets. Furthermore as fuel center line temperatures tend to be far higher than periphereal temperatures, there is a non-uniform axial expansion of fuel pellets in an operating reactor.

Load more