RU2569928C2 - Method of producing pelleted fuel for fuel elements - Google Patents

Abstract

FIELD: atomic physics.

SUBSTANCE: invention relates to the nuclear industry and production of pelleted fuel. A method of producing fuel includes mixing UO₂ powder, enriched with uranium U-235 to 5 wt %, with U₃O₈ powder in an amount of less than 15 wt %; premixing the mixture of uranium oxides with dry binder - zinc stearate to 0.3 wt % and a blowing agent - azodicarbonamide to 1 wt %; compacting, vibration milling and vibration granulation of the prepared mixture; final mixing - homogenisation with the dry binder - zinc stearate, such that the total amount of zinc stearate is not greater than 0.5 wt % taking into account the previously added zinc stearate. The method is universal and is suitable for producing fuel pellets on a "dry" scheme using any UO₃ powder obtained via an ADU process.

EFFECT: high strength of compacted products, achieving low thermal stability values, high strength of sintered pellets.

3 cl, 2 tbl

Description

The invention relates to the nuclear industry and may find application in enterprises for the manufacture of pelletized fuel from uranium dioxide.

A known method for the manufacture of pelletized fuel for fuel elements, including the preparation of a press powder of uranium dioxide UO ₂ enriched in uranium 235 to 2-5%, mixing with a dry binder - zinc stearate [Zn (C ₁₇ H ₃₅ COC) ₂ ] and oxide powder uranium U ₃ O _8, compressing the lubricated in a matrix, the thermal removal of the binder, sintering the pellets in a reducing gas medium, wet grinding diamond wheel of tablets, drying and culling defective tablets (development, manufacture and operation of fuel elements en rgeticheskih reactors. / Ed. FG Reshetnikov. M .: Energoatomisdat 1995 Book 1, pp. 66-68, 93-94, 94-95, 96-100, 101-102).

In this method, the preparation of uranium dioxide UO ₂ through ammonium polyurinate (ADU process) is associated with a compound of ammonium diurinate. Long experience with this technology indicates that obtaining stable quality UO ₂ dioxide powders is difficult (see Book 1, p. 66-68).

The disadvantage of this method is that the preparation of a press powder from UO ₂ powder obtained by the ADU process often does not provide satisfactory tablet quality.

Closest to the claimed invention is a technical solution described in patent RU No. 2158030 dated 11/18/1998, IPC G21C 3/62. G21C 21/10.

According to this patent in the production of molding powder as the raw uranium dioxide powder of UO ₂ is used UO ₂ powder obtained by wet conversion from the reduction of ammonium diuranate (ADU) process, which was used without sealing operations (briquette pressing, crushing, grinding, sieving ), mixture of uranium dioxide UO ₂ with dry binder - zinc stearate is carried out in stages wherein the first stage is mixed 10.5 parts of uranium dioxide UO ₂ powder, with the entire amount of zinc stearate is required per game, in watt swarm step the resulting mixture is mixed with 5-10 parts of uranium dioxide UO ₂ powder, in a third step the resulting mixture is mixed with the rest of uranium dioxide UO ₂ powder and a uranium oxide U ₃ O ₈ obtained after thermal oxidation air marriage tablets of uranium dioxide UO ₂ (obtained by the ADU process) to uranium oxide U ₃ O ₈ , zinc stearate, previously introduced into the press powder, is used as a lubricant during tablet pressing.

The disadvantage of this prototype is that the manufacture of fuel pellets from press powder is difficult, since when molding the compacts, a significant amount of scrap occurs in the form of chips on the side and end surfaces of the compacts and in the form of a detachment of the end surface of the compacts. Rejection in the form of a detachment of the end surface of the compacts is characteristic when using an untreated uranium dioxide powder UO ₂ (obtained by the ADU process) due to the fact that the primary agglomerates of the powders UO ₂ obtained by the ADU process have a dendrite morphological structure, and therefore molding moldings from them, it is necessary to apply greater molding pressure to break agglomerates.

The elimination of these drawbacks in the claimed method is proposed through the use of preliminary compaction of a mixture of powders of uranium oxides with a dry binder and pore former, then vibration grinding and vibration granulation, which results in an increase in the strength of compacts, a decrease in thermal stability, and an increase in the strength of sintered tablets.

The aim of the invention is to increase the strength of the compacts due to preliminary machining of the mixture of the initial powders of uranium oxides with a dry binder and a blowing agent and due to the two-stage input of a dry binder. In addition, this results in lower values of thermal stability, increased strength of sintered tablets.

The essence of the invention lies in the fact that the powder of uranium dioxide UO ₂ obtained by the method of wet transformation with recovery from ammonium diuranate (ADU process) is mixed in an inert gas medium with uranium oxide U ₃ O ₈ obtained from marriage of tablets after thermal oxidation by air, without compaction operations, they are mixed with a dry binder — zinc stearate and a pore former — azodicarbonamide (chemical formula C ₂ H ₄ N ₄ O ₂ ), the mixture of powders is compacted, then the vibro-grinding and granulation of the powder mixture are carried out, then final mixing with zinc stearate is carried out, moldings are molded into a mold, sintering in a reducing medium is carried out, then wet grinding is carried out with a set of diamond wheels, then tablets are dried and rejected that do not meet appearance requirements.

The objective of the present invention is to increase the strength of the compacts, increase the strength of sintered tablets, achieve lower values of thermal stability of the fuel.

A method of manufacturing a tablet fuel for fuel elements includes the preparation of a press powder enriched in uranium 235 to 5% of the mass. The process is carried out with the aim of obtaining a press powder having the necessary technological properties: fluidity, compressibility and formability to ensure the formation of compacts. Flowability is the ability of a mold powder to flow and fill a mold.

Compressibility - the ability of a press powder to form compacts of a given shape, size and density as a result of molding. Formability - the ability of compacts to maintain a given shape.

Uranium dioxide powder UO ₂ , enriched with 235 to 5 wt% uranium, obtained by the method of wet conversion with reduction from ammonium diuranate (ADU process), is mixed in a drum mixer in an inert gas with uranium oxide powder U ₃ O ₈ in an amount of up to 15 % wt., enriched with uranium 235 to 5% wt., obtained from the marriage of tablets after thermal oxidation by air.

The prepared mixture of uranium oxides is discharged from the drum mixer into containers with a biased inner cone, which provides improved mixing of powder materials during rotation of the container in a two-plane mixer. A dry binder is added to containers with a mixture of uranium oxides in them - zinc stearate in an amount up to 0.3% of the mass. and azodicarbonamide in an amount of from 0.01 to 1.0 wt%, used to reduce the density of sintered tablets to the required values, and powder materials are mixed in a container fixed in a two-plane mixer.

The averaged homogenized mixture of powders from a container with a displaced inner cone is loaded into equipment of a continuous line for machining and granulating powders, in which, in the presence of an inert gas - nitrogen (to prevent oxidation of uranium oxide powders), the processes of compaction, grinding and granulation of powders are carried out sequentially. Compaction of the powder mixture is carried out in order to destroy the primary agglomerates of powders having a dendrite structure in order to increase the bulk density of the powder. Compaction of the powder mixture is carried out in the compaction chamber (roller press) by passing the powder through two sequentially installed roller pairs, where the powder is compacted. To ensure constant pressure between the rollers, a hydraulic clamp device is provided in the design of the block of the working stand.

The grinding of the powder mixture is carried out with the aim of the final destruction of the primary agglomerates to ensure the optimal microstructure of sintered fuel pellets. The grinding of the powder mixture is carried out in two successive grinding chambers located in the vertical plane of the vibrating mill. Inside both grinding chambers there are grinding bodies in the form of rods. The grinding bodies oscillate in the grinding chamber body, while rotating under the action of a rotating unbalance shaft. Due to the numerous collisions and friction of the grinding media, the mixture of powders located in the spaces between them is crushed.

Granulation is carried out to roll up the crushed particles and improve the fluidity of the powder mixture. Granulation is carried out in two successive chambers located in the vertical plane of the vibratory granulator. Oscillatory-rotational movements of the vibratory granulator and the ground mixture inside the chambers are achieved by rotating the unbalance shaft.

After passing through a continuous line of mechanical processing and granulation of the powders, the mixture of powders is discharged into containers with a displaced inner cone and the calculated amount of dry binder - zinc stearate is added to them, so that the total amount of zinc stearate, taking into account the previously introduced, does not exceed 0.5% of the mass.

Containers with a displaced inner cone, providing improved mixing of powder materials with the mixture of powders inside, are installed in a two-plane mixer and the mixture is finally mixed and homogenized for at least 2 hours. After the final mixing and homogenization, the container with the press powder is sent to the molding molding operation.

The moldings are formed with a binder in the form of zinc stearate introduced into the powder in the mold by double-sided molding.

The binder is thermally removed and the tablet fuel is sintered in a gaseous reducing medium.

Wet grinding of tableted fuel with a diamond wheel is carried out, drying and rejection of defective tabletted fuel.

As a result of the use of the invention, there is an increase in the strength of the compacts, an increase in the strength of the sintered tablets, and a decrease in the thermal stability of the fuel.

Examples of implementation

Example 1

The powder of uranium dioxide UO ₂ obtained by the method of wet transformation with recovery from ammonium diuranate (ADU process) was mixed in a drum mixer in an inert gas medium with uranium oxide U ₃ O ₈ in an amount of 5% by weight.

The resulting mixture of uranium oxide powders was loaded into a container with a biased inner cone. A dry binder — zinc stearate in an amount of 0.3% by weight — was added to a container with a displaced inner cone. and azodicarbonamide in an amount of 0.01% of the mass. and mixed powder materials in a container fixed in a two-plane mixer for 15 minutes.

The averaged homogenized mixture of powders from a container with a displaced inner cone was loaded into the equipment of a continuous line of mechanical processing and granulation of powders, in which, in the presence of an inert gas - nitrogen, the operations of compaction, grinding and granulation of powders were carried out sequentially. The compaction of the powder mixture was carried out in a compaction chamber (roller press). The powder mixture was ground in two successive grinding chambers located in the vertical plane of the vibratory mill. Inside both grinding chambers there are grinding bodies in the form of rods. Granulation was carried out in two successive chambers located in the vertical plane of the vibratory granulator.

A mechanically processed mixture of powders was unloaded into a container with a displaced inner cone and a dry binder, zinc stearate, was added in an amount of 0.2% by weight. The final mixing and homogenization of the mixture was carried out for 4 hours in a container with a displaced inner cone installed in a two-plane mixer (see table 1).

Example 2

The powder of uranium dioxide UO ₂ obtained by the method of wet conversion with recovery from ammonium diuranate (ADU process) was mixed in a drum mixer in an inert gas medium with uranium oxide U ₂ O ₈ in the amount of 7% of the mass.

The resulting mixture of uranium oxide powders was loaded into a container with a biased inner cone. A dry binder — zinc stearate in an amount of 0.2% by weight — was added to a container with a displaced inner cone. and azodicarbonamide in an amount of 1% of the mass. and mixed powder materials in a container fixed in a two-plane mixer for 15 minutes.

The averaged homogenized mixture of powders from a container with a displaced inner cone was loaded into the equipment of a continuous line of mechanical processing and granulation of powders, in which, in the presence of an inert gas - nitrogen, the operations of compaction, grinding and granulation of powders were performed sequentially. The compaction of the powder mixture is carried out in a compaction chamber (roll press). The powder mixture was ground in two successive grinding chambers located in the vertical plane of the vibratory mill. Inside both grinding chambers there are grinding bodies in the form of rods. Granulation was carried out in two successive chambers located in the vertical plane of the vibratory granulator.

A mechanically processed mixture of powders was unloaded into a container with a displaced inner cone and a dry binder, zinc stearate, was added in an amount of 0.2% by weight. The final mixing and homogenization of the mixture was carried out for 4 hours in a container with a displaced inner cone installed in a two-plane mixer (see table 1).

Example 3

The powder of uranium dioxide UO ₂ obtained by the method of wet conversion with recovery from ammonium diuranate (ADU process) was mixed in a drum mixer in an inert gas medium with uranium oxide U ₃ O _{8 in an} amount of 15% of the mass.

The resulting mixture of uranium oxide powders was loaded into a container with a biased inner cone. A dry binder — zinc stearate in an amount of 0.1% by weight — was added to the container with a displaced inner cone. and azodicarbamide in an amount of 0.5% of the mass. and mixed powder materials in a container fixed in a two-plane mixer for 15 minutes.

The averaged homogenized mixture of powders from a container with a displaced inner cone was loaded into the equipment of a continuous line of mechanical processing and granulation of powders, in which, in the presence of an inert gas - nitrogen, the operations of compaction, grinding and granulation of powders were performed sequentially. The compaction of the powder mixture is carried out in a compaction chamber (roll press). The powder mixture was ground in two successive grinding chambers located in the vertical plane of the vibratory mill. Inside both grinding chambers there are grinding bodies in the form of rods. Granulation was carried out in two successive chambers located in the vertical plane of the vibratory granulator.

A mechanically processed mixture of powders was unloaded into a container with a displaced inner cone and a dry binder, zinc stearate, was added in an amount of 0.3% by weight. The final mixing and homogenization of the mixture was carried out for 4 hours in a container with a displaced inner cone installed in a two-plane mixer (see table 1).

From the prepared press powders (examples 1-3), the tablets were molded, sintered and ground according to the modes of manufacturing standard products using technological equipment for the manufacture of tablets.

For tablets made from press powders (Examples 1-3), quality characteristics are determined. The results are shown in table 2.

Claims (3)

1. A method of manufacturing a pelletized fuel for fuel elements, including the preparation of a press powder of uranium dioxide UO ₂ enriched with uranium 235 to 5%, mixing with a powder of uranium oxide U ₃ O ₈ , with a dry binder - zinc stearate and a pore former - azodicarbonamide, molding moldings from the charge in the matrix, thermal removal of the binder, sintering of tablets in a gaseous reducing medium, wet grinding of tablets with a diamond wheel, drying and rejection of defective tablets, characterized in that in the first stage of preparation the powder presses mix the uranium oxides UO ₂ with U ₃ O ₈ ; in the second stage, the mixture of oxides obtained in the first stage is mixed with a dry binder — zinc stearate in an amount of up to 0.3% of the mass. and with azodicarbonamide in an amount of from 0.01 to 1.0 wt%, then the mixture obtained in the second stage is subjected to preliminary compaction, vibration grinding and vibration granulation, then the final mixing and homogenization of the mixture of the obtained granulate with zinc stearate is carried out so that the total amount of zinc stearate taking into account the previously introduced does not exceed 0.5% of the mass. 2. The method according to p. 1, characterized in that the mixing of mixtures of uranium oxides (UO ₂ and U ₃ O ₈ ) with zinc stearate and azodicarbonamide is carried out in containers with a biased inner cone using a two-plane mixer. 3. The method according to p. 1, characterized in that the preliminary compaction, vibration grinding and vibration granulation of the mixture of powders in the equipment of a continuous line for the mechanical processing and granulation of powders.