A method of making fuel pellets from uranium-molybdenum powder

Abstract

The invention belongs to the nuclear industry and can be used for the production of fuel pellets from uranium-molybdenum metal powder enriched with uranium-235 to 7%, which can be used as fuel elements for nuclear reactors. The sintering of the pellets is carried out in an inert argon atmosphere at a temperature in the range of 1100°C to 1155°C, as the initial powder, a particle size of 160 μm is used, with a molybdenum content of 9.0 to 10.5 wt.%. The powder is preheated at a temperature of 500°C for 10-20 hours (in an argon medium), and then cold-pressed into pellets in a mold at a pressure of up to 950 MPa. According to another method, when preparing uranium-molybdenum pellets using a binder (plasticizer), the pellets are heated in argon from 300°C to 450°C for a duration of 2-4 hours before sintering, in order to remove the binder. The invention allows to increase the uranium content in the fuel, to reduce the heat accumulated in the core of the nuclear reactor, and to reduce the energy release in the event of a violation of the normal operating conditions of the nuclear reactor, which will improve its safety and emergency stability.

Description

Technical Field

[0001] This invention pertains to the nuclear industry and can be used to produce fuel pellets from uranium-molybdenum metal powder. The metal powder is enriched with uranium-235 to 7%, and can be used as fuel elements in nuclear reactors. Background Technology

[0002] The known standard techniques for producing fuel pellets from uranium dioxide (enriched to 5% with uranium-235) ceramic nuclear fuel include: the production and preparation of uranium dioxide powder (mixed with binder), pressing the pellets in a mold, sintering the pellets in a gaseous medium, dry or wet grinding of the pellets, drying and controlling the pellets to meet the requirements of technical specifications and drawings, packaging qualified products, and delivering the products for assembly into nuclear reactor fuel elements (“Research, Development, Production and Operation of Fuel Elements for Power Reactors. Edited by F. G. Shetnikov – Moscow: Energoatomizdat Publishing House, 1995, Vol. 1, pp. 93–106”).

[0003] A method for producing a uranium-molybdenum alloy containing 9 wt.% molybdenum (U) is known [Eiss A.L. Kalish HS3rd Nucl. Eng. and Sci. Conference, USA, Paper 178, 1958]. In this method, uranium and molybdenum powders are mixed for 2 hours without the addition of a binder. After mixing, the uranium and molybdenum powders are pressed into blocks approximately 4.5 cm long with a square cross-section of 1.9 cm × 1.9 cm, as well as cylindrical blocks. Sintering is performed in a vacuum at 1150 °C for 4 hours. Metallographic analysis of the cold-pressed and sintered samples shows that the alloying elements react completely with each other. The density of the resulting core is 17.18 g / cm³.

[0004] The disadvantage of this method is that the molybdenum is not evenly distributed throughout the volume in the sample obtained by this method, and high pressure is required during the cold pressing of the mixed uranium-molybdenum powder.

[0005] In addition, a method for preparing ceramic fuel pellets for nuclear reactor fuel elements is known, the method comprising: preparing uranium dioxide powder mixed with a binder, two-stage pressing of the pellets and sintering thereof (see: Russian Federation Patent No. 2421834, IPC G21C 3 / 02, Invention Notification No. 17, dated June 20, 2011).

[0006] The disadvantages of this method are: the two-stage pressing of uranium dioxide pellets and the use of plasticizers in the preparation of the pressed powder.

[0007] The closest method is a method for preparing ceramic fuel pellets for nuclear reactor fuel elements (Russian Federation Patent No.: 2360308, IPC G21C 3 / 62, Invention Notification No. 18, dated June 27, 2009). This method includes: preparing uranium dioxide powder enriched to 1.6-5% uranium-235, mixing it in stages with a dry binder (metal-free) and uranium oxide powder, pressing the pellets in a mold, thermally debinding the binder, sintering the pellets in a gaseous reducing medium, wet grinding the pellets with a diamond grinding wheel, drying the pellets, and removing the waste.

[0008] The disadvantages of this method are that it requires a lot of energy during the pressing process and the resulting fuel has a low uranium content. Summary of the Invention

[0009] The purpose of this invention is to develop a method for producing fuel pellets from a uranium-molybdenum alloy enriched to 7% uranium-235. This alloy is used in fuel elements of nuclear reactors, thereby improving the safety of nuclear reactor operating conditions and their operational efficiency.

[0010] According to the first and second embodiments, the technical result of the present invention is to produce uranium-molybdenum pellets enriched with uranium-235 up to 7% for nuclear reactor fuel elements, thereby increasing the uranium content, reducing the heat accumulated in the nuclear reactor core, and reducing energy release in the event of violation of normal nuclear reactor operating conditions, which will improve its safety and emergency stability.

[0011] The method for preparing fuel pellets from uranium-molybdenum powder for use in nuclear reactors, based on the technical results of the first scheme, includes: powder preparation, pressing the pellets into a mold, sintering them in a gaseous medium, grinding, drying, and rejecting waste. Furthermore, the sintering of the pellets is carried out in an inert environment, and uranium-molybdenum powder enriched to 7% with uranium-235 and having a molybdenum content of 9.0 to 10.5 wt.% is used as the initial powder.

[0012] The particle size of the uranium-molybdenum powder does not exceed 160 micrometers. Before pressing the pellet into the mold, the uranium-molybdenum powder is heated at 500°C in an argon atmosphere for 10-20 hours. The pellet is pressed into the mold under a pressure of up to 950 MPa. The pellet is sintered in argon at 1100°C-1155°C for 4-12 hours.

[0013] The method for preparing fuel pellets from uranium-molybdenum powder for use in nuclear reactor fuel elements, based on the technical results of the second scheme, includes: powder preparation, staged mixing with a binder, pressing the pellets in a mold, thermal removal of the binder, sintering the pellets in a gaseous medium, grinding, drying, and rejecting waste. The sintering of the pellets is carried out in an inert medium. The initial powder used is uranium-molybdenum powder enriched to 7% uranium-235 and containing 9.0–10.5 wt.% molybdenum. The particle size of the uranium-molybdenum powder does not exceed 160 micrometers. The pellets are pressed into the mold under pressure up to 950 MPa. The pellets are heated in an argon medium at 300°C to 450°C for 2–4 hours to thermally remove the binder.

[0014] The core was sintered in an argon atmosphere at 1100℃–1155℃ for 4–12 hours.

[0015] This method of manufacturing fuel pellets for nuclear reactor fuel elements using uranium-molybdenum metal powder enables the production of uranium-molybdenum pellets enriched to 7% uranium-235, with a molybdenum content of 9.0-10.5 wt.%, and a pellet density of not less than 15.7 g / cm³ (exceeding 90% of the theoretical density). Compared to uranium dioxide fuel and low-enriched uranium-235, nuclear reactors using uranium-molybdenum pellets are more efficient due to the increased thermal conductivity of uranium-molybdenum fuel, as the density of uranium-molybdenum pellets is 1.5 times greater than that of uranium dioxide pellets. Because the density of uranium-molybdenum alloys (molybdenum content of 9.0-10.5 wt.%) is higher than that of uranium dioxide, the higher uranium mass charge in the nuclear reactor allows for increased fuel assembly cycle time without the need to increase fuel enrichment. Uranium dioxide has a density of 9.7 g / cm³ based on uranium, while uranium-molybdenum alloy (molybdenum content 9.0-10.5 wt%) has a density of approximately 15.75 g / cm³ based on uranium. Therefore, with the same amount of uranium-molybdenum fuel in the reactor, the amount of fissile components increases to approximately 60%.

[0016] It is known that [VV Kalashnikov, VVTitov, G. Ya. Sergeev, A.G. Samoilov. "Uranium-Molybdenum Alloys in Reactor Construction," Journal of Atomic Energy, Vol. 5, No. 4, October 1958, p. 422] that when the γ phase in U-Mo synthesis is released at temperatures of 350–5500°C, the γ phase transforms into a eutectoid mixture of α-uranium and the intermetallic compound U₂Mo (γ' phase). However, this process is slow. When the initial powder is held in a vacuum at 500°C for 10 to 20 hours, partial phase separation occurs, forming the γ phase, as well as a eutectoid mixture of the α and γ' phases. Using a powder that is a mixture of eutectoid phases, a robust "pristine" pellet is obtained at a lower pressure (below 950 MPa). Subsequent sintering of such pellets (pistons) at temperatures above 1100°C completely re-transforms the eutectoid phase into the γ state.

[0017] Optimal results can be obtained by holding initial uranium-molybdenum metal powder with a molybdenum content of 9.0 to 10.5 wt.% in an inert gas medium at 500 °C for 10 to 20 hours. This is because the initial metal powder in the form of uranium γ phase undergoes partial phase decomposition, decomposing into γ phase and α and γ' phases of the eutectoid mixture. Therefore, robust “original” pellets with pressures up to 950 MPa can be obtained.

[0018] Reducing the molybdenum content in the initial uranium-molybdenum powder to below 9.0 wt.% increases the density of the sintered uranium-molybdenum pellets (reaching below 19.05 g / cm3 when the molybdenum content is zero), while increasing the molybdenum content in the initial uranium-molybdenum powder by more than 10.5 wt.% leads to a decrease in pellet density, which is prohibited by the requirements of fuel pellet preparation technology for nuclear reactors.

[0019] Raising the sintering temperature above 1155℃ causes the uranium-molybdenum core to melt, while temperatures below 1100℃ make it impossible to prepare cores without internal pores. Meanwhile, the core density remains very low, only 13–14 g / cm³.

[0020] Optimal results were obtained for uranium-molybdenum pellets with a molybdenum content of 9.0 to 10.5 wt.% at sintering temperatures of 1100°C to 1155°C (in an inert atmosphere) for 4 to 12 hours.

[0021] The developed method enables the extraction of the γ phase from uranium-molybdenum powder used in nuclear reactor fuel elements, where molybdenum is the primary alloying element that helps maintain the uranium γ phase across the entire operating temperature range of the fuel element. Molybdenum not only alters the phase transformation kinetics to achieve a randomly oriented, fine-grained structure but also stabilizes the uranium γ phase, thereby improving the efficiency of the fuel element.

[0022] Because uranium-molybdenum fuel has a higher thermal conductivity than uranium dioxide fuel, it can reduce the heat accumulated in the reactor core, thereby reducing energy release under conditions that violate the normal operation of the nuclear reactor, and improving its safety and resilience to emergencies. Detailed Implementation

[0023] The following describes examples of recommended methods for producing uranium-molybdenum fuel pellets.

[0024] Example 1 (according to the first embodiment). As the initial powder, uranium-molybdenum powder with a molybdenum content of 9.0 wt.% in the alloy was used, prepared from a similar alloy ingot enriched to 7% uranium-235 by centrifugal spraying. The centrifugally sprayed uranium-molybdenum ingot with a molybdenum content of 9.0 wt.% in the alloy allowed for a uniform molybdenum content in the initial powder. The powder was sieved through a sieve with an aperture size of 160 μm. The sieved uranium-molybdenum powder with a molybdenum content of 9.0 wt.% was heated at 500°C for 20 hours in a vertically loaded vacuum resistance furnace (CSBE type) (in an argon medium). The prepared powder was pressed into a cylindrical mold at a pressure of 750 MPa without the addition of a binder (plasticizer). The core blocks were sintered in an argon atmosphere (with a water content not exceeding 80 ppm) at a temperature of (1125 ± 10 / - 5) °C, and held isothermally for 4 hours in a vertical vacuum resistance furnace (SBE type) (or XERION XVAC-2200). Heating was carried out in an argon flow of 1 L / min at a heating rate not exceeding 5 °C / min, and held isothermally, followed by cooling in a static argon atmosphere at a cooling rate of (15–20) °C / min. Afterwards, the core blocks were ground, dried, and discarded to meet technical requirements.

[0025] Example 2 (according to the first embodiment). As the initial powder, uranium-molybdenum powder with a molybdenum content of 10.5 wt.% in the alloy was used, prepared from a similar alloy ingot enriched from uranium-235 to 7% by centrifugal spraying. The centrifugally sprayed uranium-molybdenum ingot with a molybdenum content of 10.5 wt.% allowed for a uniform molybdenum content in the initial powder. The powder was sieved through a sieve with a mesh size of 160 μm. The sieved uranium-molybdenum powder with a molybdenum content of 10.25 wt.% was heated at 500°C for 10 hours in a vertically loaded vacuum resistance furnace (CSBE type) under argon atmosphere. The prepared powder was pressed into a cylindrical mold at a pressure of 950 MPa without the addition of a binder (plasticizer). The core blocks were sintered in an argon atmosphere (with a water content not exceeding 80 ppm) at (1125 ± 10 / - 5) °C, and held isothermally for 12 hours in a vertical vacuum resistance furnace (SBE type) (or XERION XVAC-2200). Heating was carried out in an argon flow of 1 L / min at a heating rate not exceeding 5 °C / min, and held isothermally, followed by cooling in a static argon atmosphere at a cooling rate of (15–20) °C / min. Afterwards, the core blocks were ground, dried, and defective blocks were removed to meet technical requirements.

[0026] Example 3 (according to the second embodiment). As the initial powder, uranium-molybdenum powder with a molybdenum content of 9.0 wt.% in the alloy was used. This uranium-molybdenum powder was prepared by centrifugal spraying from a similar alloy ingot enriched to 7% uranium-235, and the powder was sieved through a sieve with a mesh size of 160 μm. The centrifugally sprayed uranium-molybdenum ingot with a molybdenum content of 9.0 wt.% in the alloy allowed for a uniform molybdenum content in the initial powder. As a plasticizer (binder), an 8% polyvinyl alcohol aqueous solution with 1% glycerol (3% by weight of the uranium-molybdenum alloy) was used. Mixing was performed in three stages. In the first stage, the entire amount of binder and 10 wt.% of the uranium-molybdenum alloy powder were mixed to a homogeneous mixture. In the second stage, the prepared mixture was mixed with 40 wt.% of the uranium-molybdenum alloy powder to a homogeneous mixture. In the third stage, the remaining amount of uranium-molybdenum alloy powder was added to the mixture prepared in the second stage and mixed to a homogeneous mixture. The powder was mixed in a Turbula-type mixer for 20-30 minutes. The prepared powder was pressed into a cylindrical mold under a pressure of 850 MPa. Prior to sintering, the core blocks were heated in an argon atmosphere at 300°C to 450°C for 4 hours to remove the binder. The core blocks were then sintered in an argon medium (moisture content not exceeding 80 ppm) at (1125 ± 10 / - 5)°C, and held isothermally for 4 hours in a vertical vacuum resistance furnace (CSBE type). Heating was carried out in an argon flow of 1 L / min at a heating rate not exceeding 5°C / min, followed by isothermal holding, and then cooling in a static argon atmosphere at a cooling rate of (15–20)°C / min. The core blocks were then ground, dried, and discarded to meet technical requirements.

[0027] Example 4 (according to the second embodiment). As the initial powder, uranium-molybdenum powder with a molybdenum content of 10.5 wt.% in the alloy was used. This uranium-molybdenum powder was prepared by centrifugal spraying from a similar alloy ingot enriched from uranium-235 to 7%, and the powder was sieved through a sieve with a mesh size of 160 μm. The centrifugally sprayed uranium-molybdenum ingot with a molybdenum content of 10.5 wt.% in the alloy allowed for a uniform molybdenum content in the initial powder. As a plasticizer (binder), an 8% polyvinyl alcohol aqueous solution with 1% glycerol (3% by weight of the uranium-molybdenum alloy) was used. Mixing was performed in three stages. In the first stage, the entire amount of binder and 10 wt.% of the uranium-molybdenum alloy powder were mixed to a homogeneous mixture. In the second stage, the prepared mixture was mixed with 40 wt.% of the uranium-molybdenum alloy powder to a homogeneous mixture. In the third stage, the remaining amount of uranium-molybdenum alloy powder was added to the mixture prepared in the second stage and mixed to a homogeneous mixture. The powder was mixed in a Turbula-type mixer for 20-30 minutes. The prepared powder was pressed into a cylindrical mold at a pressure of 950 MPa. Prior to sintering, the core was heated in an argon atmosphere at 300°C to 450°C for 2 hours to remove the binder. The core was sintered in an argon medium (moisture content not exceeding 80 ppm) at (1125 ± 10 / - 5)°C, and held isothermally for 12 hours in a vertical vacuum resistance furnace (CSBE type). Heating was carried out in an argon flow of 1 L / min at a heating rate not exceeding 5°C / min, followed by isothermal holding, and then cooling in a static argon atmosphere at a cooling rate of (15–20)°C / min. The core was then ground, dried, and discarded to meet technical requirements.

[0028] Industrial applicability

[0029] Figure 1 Displaying uranium-molybdenum core blocks after sintering and machining.

[0030] Therefore, compared with previously known methods, the proposed method enriches uranium-235 to 7% and molybdenum content to 9.0 to 10.5 wt.% from uranium-molybdenum powder to produce fuel pellets for nuclear reactor fuel elements with higher operating performance.

Claims

1. A method for making fuel pellets from uranium-molybdenum powder for use in a nuclear reactor fuel element, the method comprising: Preparation of the powder, pressing of the compact in a die, sintering in a gaseous medium, grinding, drying, rejection of rejects, characterized in that the uranium-molybdenum powder is heated in an argon medium at a temperature of 500°C for 10-20 hours before pressing of the compact in the die, the compact is sintered in an inert medium, as the initial powder, a uranium-molybdenum powder is used, enriched with uranium-235 to 7%, the molybdenum content being 9.0-10.5 wt.%.

2. The method of claim 1, wherein, The uranium-molybdenum powder has a particle size not greater than 160 microns.

3. The method of claim 1, wherein, The compact is pressed in the die at a pressure of up to 950 MPa.

4. The method of claim 1, wherein, The sintering of the compact is carried out in an argon medium at a temperature in the range of greater than 1100°C and less than or equal to 1155°C for 4-12 hours.

5. A method for making fuel pellets from uranium-molybdenum powder for use in a nuclear reactor fuel element, comprising: Preparation of the powder, mixing with the binder in stages, pressing of the compact in a die, thermal removal of the binder, sintering of the compact in a gaseous medium, grinding, drying, rejection, characterized in that the sintering of the compact is carried out in an inert atmosphere, as the initial powder, a uranium-molybdenum powder is used, enriched with uranium-235 to 7%, the molybdenum content being 9.0-10.5 wt.%, wherein an 8% aqueous solution of polyvinyl alcohol containing 1% glycerol is used as the binder, the amount of which is 3% of the weight of the uranium-molybdenum alloy.

6. The method of claim 5, wherein, The uranium-molybdenum powder has a particle size not greater than 160 microns.

7. The method of claim 5, wherein, The pressing of the compact is carried out in a die at a pressure of up to 950 MPa.

8. The method of claim 5, wherein, The thermal removal of the binder is carried out by heating the compact in an argon medium at a temperature of 300°C to 450°C for 2-4 hours.

9. The method of claim 5, wherein, The sintering of the compact is carried out in an argon medium at a temperature in the range of greater than 1100°C and less than or equal to 1155°C for 4-12 hours. The sintering of the compact is carried out in an argon medium at a temperature in the range of greater than 1100°C and less than or equal to 1155°C for 4-12 hours.

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