Pellet fabrication

Abstract: A method of fabricating pellets which comprises subjecting dry powder or a mixture of powders to attrition and thereafter conditioning the powder or mixture such that it is capable of free flow and pressing the free-flowing powder or mixture into green pellets having a required density.

Description: The present invention concerns the fabrication of pellets, in particular nuclear fuel pellets comprising uranium dioxide or a mixture of uranium and plutonium dioxides. The fabrication of pellets basically involves the pressing of powder into green pellets which are then sintered to a required density. The powder feed to the pelleting presses must have good pour qualities such that it will flow freely and evenly into the press dies. To achieve good pour qualities it is accepted practice to granulate the powder using either a binder or binderless technique. The binder technique involves the addition of a proprietary organic binding agent to the powder. The powder is slurried with solvent and binder, the slurry is spray-dried to dispel solvent and granules are discharged and classified on vibratory screens. Alternatively, powder mixing with binder and solvent can be achieved in a dough-mixer, which can also be used to dispel solvent during the mixing process. In the binderless technique, powder is pre-compacted into briquettes which are then fractured, sieved and conditioned to produce granules for subsequent pelleting operations. The initial step in both the binder and binderless techniques can involve attrition of the powder feed, conveniently by ball milling. This step serves to break down the powder and in the case where the feed is a mixture of different powders to achieve homogenisation. As a result of further work and study of the binderless technique it was observed that a free-flowing feed supply for the die presses can be provided instead of the usual. A method known as the "AUC" route for the production of a pourable UO₂ or UO₂/PuO₂ mixed powder which is a free-flowing feed supply and may be directly pressed is described in the Journal of Nuclear Materials, Vol. 106, No. 1/3, April 1982, pages 15 to 33. The method described in that reference only works with a specific powder having specific ideal powder characteristics, ie "a pourable powder with adequate sintering activity," which result from the use of specific process conditions. It is admitted in the reference that a milling stage may be necessary for various reasons before further processing of the powder, e.g. to provide an increase in the sintering activity of the UO₂ powder, a change in the powder characteristics which cause an undesirable pellet structure and homogeneous admixture of additives containing Pu, Gd etc. Where milling is required the reference teaches that granulation either as wet granulation or pre-compaction and dry granulation is required after milling. EP-A- 0036214 corresponding to US-A 4676935 describes a method for the treatment of mixed UO₂ and PuO₂ powders for the production of nuclear fuel pellets and involves the step of milling the powders. However, in that case the method is aimed at increasing pellet solubility in nitric acid and requires milling of powders to 2 micrometers which will give extreme difficulty if applied to plutonium oxide due to dispersion of contamination and design difficulty in providing adequate containment. Also, after milling the step of granulation is applied so that the production of a free-flowing material does not follow after milling without granulation. According to the present invention a method of producing nuclear fuel pellets comprises producing a free-flowing powder by a process which includes a step of milling a powder comprising uranium oxide or a mixture of uranium oxide and plutonium oxide, and pressing the free-flowing powder into green pellets having a required density characterised in that the milling step is carried out without any pre-compaction and granulation of the product of that step and that the product of the milling step is treated by conditioning by rolling to provide free flow of the powder, the conditioning step being carried out after the milling step. The invention omits the pre-compaction and granulation stages after milling from the existing binderless route for pellet production and for convenience can be termed the "short binderless route". We have observed that a free-flowing feed supply for the die presses can be provided from the product of a milling step where that is necessary with omission of the previously used pre-compaction and granulation steps and the omission of such steps allows saving in plant and equipment. In the method according to the present invention a lubricant such as zinc stearate can be introduced into the mill, preferably before the introduction of the powder, in order to prevent compaction within the mill. The invention will be described further by reference to the following examples utilising uranium dioxide powder and a mixture of uranium dioxide and plutonium dioxide powders. The characteristics of uranium dioxide and 600°C calcined plutonium dioxide employed as starting materials are:- Tests were performed on UO₂ powder alone and on a mixture of 96% UO₂ and 4% PuO₂ powders, both with and without the precompaction and granulation stages. Attrition by ball milling can be performed in a rolling polypropylene mill pot using hardened steel balls at a ball/charge weight ratio of approximately 6/1. The balls ranged in diameter from 6 to 19mm. Preferably the mill was run for a short time interval of about one minute with approximately 0.1% zinc stearate powder before adding the charge. Where used, precompaction was achieved in a 25mm die fitted to a 10 ton hydraulic press. The precompaction pressure was adjusted to result in a precompact density of about 4.5 g/cc. The precompacts were broken and granulated through a stainless steel mesh. Conditioning can be achieved by rolling either powder or granules in a cylinder. Conveniently, after conditioning, approximately 0.2% zinc stearate can be added to serve as a lubricant for the subsequent pelletising operation. Green pellets were pressed hydraulically in a 9.9mm diameter die. Geometric green densities were measured on the pellets pressed at various pressures in the range 2.36 to 6.30 Te/cm². Green pellets can be sintered in a 4.6% H₂-Ar atmosphere using a batch furnace operating at a maximum temperature of 1650°C. The heating rate was 200°C/hour and the normal sintering time at maximum temperature was 6 hours. UO₂ powder from the milling stage was divided into two batches, batch 1 being processed by the normal binderless route to include precompaction and granulation and batch 2 being processed by the short binderless route. A comparison of the densities of the green and sintered pellets is given in the following table. Similar comparisons were made with batches 3 to 6 of a mixture of 96% UO₂, 4% PuO₂ powders. Batches 3 and 4 were subjected to the full binderless route and batches 5 to 6 to the short binderless route with the results as given in the following table. With 100% UO₂, the results show that somewhat higher green and sintered density pellets are produced by the short binderless route. With 96% UO₂ - 4% PuO₂ the results show that no significant difference exists between the densities of green and sintered pellets obtained by the two binderless routes and demonstrate the feasibility of the short binderless route for the fabrication of oxide nuclear fuel pellets. As an alternative to a conventional rolling ball mill it is possible to employ an attritor mill in which the balls are driven within an upright cylinder or barrel having a rotatable paddle. Thus an attritor mill having a charge of 18Kg hardened stainless steel balls was run at speeds of 120 to 240 rpm (revolutions per minute) with milling times of between 10 and 30 minutes. Results indicate that 10 minutes Attritor milling at 180 rpm yields a powder with very similar tap-density and specific surface area to that achieved by six hours ball milling.

Claims: A method of producing nuclear fuel pellets by a binderless technique which comprises producing a free-flowing powder by a process which includes a step of milling a powder comprising uranium oxide or a mixture of uranium oxide and plutonium oxide, and pressing the free-flowing powder into green pellets having a required density characterised in that the milling step is carried out without any pre-compaction and granulation of the product of that step and that the product of the milling step is treated by conditioning by rolling to provide free flow of the powder, the conditioning step being carried out after the milling step. A method as in Claim 1 and wherein the milling step is carried out in an attritor mill in which a paddle is employed to assist the action of balls. A method as in Claim 2 and wherein the milling step is carried out in an attritor mill wherein balls are driven within an upright cylinder or barrel having a rotatable paddle. A method as in Claim 2 and wherein the paddle is rotated at a speed of between 120 revolutions per minute and 240 revolutions per minute. A method as in Claim 3 and wherein the milling time is between 10 and 30 minutes. A method as in any one of Claims 1 to 4 and wherein the powder comprises uranium oxide and plutonium oxide which are added together by mixing. A method as in any one of Claims 1 to 5 and wherein the milling step includes the use of a lubricant. A method as in Claim 6 and wherein the lubricant is zinc stearate. A method as in any one of Claims 1 to 7 and wherein a lubricant is added to the powder after the conditioning step. A method as in Claim 8 and wherein the lubricant added after conditioning is zinc stearate.

Inventors: HOLBURT GEOFFREY ALAN, LAWRENCE ANTHONY EDWARD, MARSHALL ALAN, HOLBURT, GEOFFREY ALAN, LAWRENCE, ANTHONY EDWARD, MARSHALL, ALAN
Applicants: BRITISH NUCLEAR FUELS PLC
Application year: 1988
Main classification: G21C3
IPC codes: G21C21, C04B35, G21C3