Method for producing uranium-gadolinium nuclear fuel

Abstract

The invention relates to the nuclear industry, and more particularly to technology for producing a ceramic nuclear fuel for fuel rods of a nuclear power plant. A method for producing uranium-gadolinium nuclear fuel comprises preparing an initial powder of uranium dioxide, mixing same with a powder of triuranium octoxide and gadolinium oxide, granulating the powder mixture, mixing the granulate with a solid lubricant and a pore-forming agent, compressing the moulding powder, sintering the pellets produced, and polishing said pellets. Sintering is carried out at an atmospheric moisture content of from 8500 to 34000 ppm at a preliminary heating stage, from 9200 to 35000 ppm at a high-temperature dwell stage, and from 5100 to 6000 ppm at a cooling stage. The high-temperature dwell time during sintering is from 5.5 to 8.0 hours. The technical result of the invention is that of reducing the hydrogen content of uranium-gadolinium fuel pellets, as well as stabilizing the density and thermal properties thereof, which contributes to the safe operation of the fuel in the reactor.

Description

[0001] Method for manufacturing uranium-gadolinium nuclear fuel FIELD OF TECHNOLOGY

[0002] The invention relates to the nuclear industry, in particular, to the technology of manufacturing ceramic nuclear fuel for fuel elements (fuel rods) of nuclear power plants.

[0003] The development of advanced nuclear fuel with an extended cycle requires the use of uranium-gadolinium pellets with an increased content of burnable absorber (8.00 wt. % and more). The use of gadolinium in fuel allows for the alignment of energy release fields in the active zone and compensation of the reactivity margin during a campaign. Such fuel must meet high requirements.

[0004] The fuel properties are mainly formed during the sintering operation, which consists of three main stages: preliminary heating, high-temperature holding and cooling. One of the main tasks of the sintering process is to achieve a given level of the required properties, which in the case of fuel with a high gadolinium oxide content (8.00 wt. % and more) are density, porosity, thermal stability (additional sintering), grain size, hydrogen content, etc. Temperature, isothermal holding time, and the oxygen potential of the sintering atmosphere have a strong effect on these properties. An increase in the dew point and, accordingly, an increase in the oxygen content in the sintering atmosphere contribute to an increase in the diffusion of uranium and gadolinium ions, which is necessary for the formation of a solid solution of UO2- GCI2O3. This in turn affects the receipt of stable positive results in "additional sintering" and a decrease in the hydrogen content in the tablets.Oxygen included in CO2 has high activity. And due to the low solubility of CO2 in the crystal lattice of UO2-GCI2O3, closed pores are filled with CO2, not hydrogen, which reduces the likelihood of hydride corrosion of fuel element cladding and, as a consequence, depressurization.

[0005] "Resinterability" means a change in the geometric dimensions and / or density of tablets during repeated thermal exposure outside the reactor. Using the results of testing tablets for "resinterability", it is possible to predict the compaction and shrinkage of tablets during operation in a reactor in a steady-state mode, and depending on the magnitude of changes in the density and dimensions of tablets (diameter) during "resintering", models of in-reactor changes in the density and dimensions of tablets under irradiation have been developed.

[0006] Practice has shown that additional compaction can proceed at different rates depending on the fuel properties and irradiation conditions. Therefore, stabilization of additional compaction ("additional sintering") and other properties allows to reduce possible risks and guarantee long-term operability of the fuel in the reactor.

[0007] LEVEL OF TECHNOLOGY

[0008] A method for producing nuclear fuel pellets is known, which includes the preparation of press powder, pressing, sintering and grinding. At the stage of preparing uranium oxide, both defective sintered pellets and grinding dust and UO2 powder with a content of U in the pellets that differs from that required by the technical documentation are used as raw materials. 235 , U 236 and other uranium isotopes, and at the stage of preparing the press powder, the resulting uranium oxide is added to the batch in an amount of up to 20 wt. %, and the required U content is achieved in the technical documentation 235 , U 236and other uranium isotopes in tablets are achieved by varying the amount of additives in the oxide-oxide batch, both separately and in mixtures with UO2 powder, depending on the U content in them 235 , And 236 and other uranium isotopes (RU 2396611, published 10.08.2010). The disadvantage of this method is the high temperature (1750 °C), the duration of sintering (~ 30 h), and the moistening of the sintering environment not at all stages of sintering (8000 - 12000 ppm), resulting in a large spread in the results for "additional sintering". At the same time, the resulting density of the tablets is quite low (< 10.15 g / cm 3), and the hydrogen content is not given at all. Also, a significant disadvantage of this invention, at present, is the limited use of gadolinium oxide to 7.0 wt. %. This is due to the fact that the development of nuclear fuel is aimed at increasing the burnout depth and the duration of the irradiation cycle between reloads. The ways to achieve these goals are carried out, including by increasing the content of the burnable absorber in the fuel.

[0009] A method of the company "ASEA-Atom" for the production of urangadolinium fuel is known, in which the sintering of tablets is carried out in a tunnel-type furnace under conditions of controlled partial pressure of oxygen. Sintering is carried out in an atmosphere of a mixture of CO2 + H2 or CO + CO2 / H2 (the partial pressure of oxygen at 1973 K was 0.25x10' 6 Pa). This produces tablets with a density of 10.16 g / cm3. 3 (DE 3609663, published 10 / 30 / 1986).

[0010] The disadvantage of this method is that it produces low density (< 10.20 g / cm3 ) uranium-gadolinium pellets. The use of such low-density fuel contributes to the problem of shell hydrogenation due to residual moisture.

[0011] The closest to the proposed method is a method for producing uranium-gadolinium nuclear fuel, which includes preparing the initial uranium dioxide powder, mixing it with uranium oxide powder and gadolinium oxide, granulating the powder mixture, mixing the granulate with a solid pressing lubricant, pressing and sintering to obtain tablets, and grinding the resulting tablets. 1.2 wt. % of a blowing agent in the form of azodicarbonamide is gradually added to the powder mixture, with part of the blowing agent added at the granulation stage, and the remaining part of the blowing agent added at the stage of mixing the granulate with a solid pressing lubricant, which is aluminum stearate (RU 2750780, published 02.07.2021).

[0012] The disadvantage of the prototype method is the additional operation of introducing a pore-forming agent to stabilize the “additional sintering”, as well as maintaining the dew point not at all stages of the sintering operation, which can lead to an increased hydrogen content in the tablets and the formation of small cracks due to a lack of oxygen during sintering.

[0013] DISCLOSURE OF INVENTION

[0014] The objective of the present invention is to ensure optimal performance and stable characteristics of uranium-gadolinium fuel pellets (density, “sinterability”, grain size, hydrogen content).

[0015] The technical result of the invention consists in reducing the hydrogen content in uranium-gadolinium fuel pellets, as well as in stabilizing the density and “additional sintering”, which contributes to the safe operation of the fuel in the reactor.

[0016] The technical result is achieved in that the method for producing uranium-gadolinium nuclear fuel includes preparing the initial uranium dioxide powder, mixing it with uranium oxide powder and gadolinium oxide, granulating the powder mixture, mixing the granulate with a solid lubricant and a blowing agent, pressing the press powder, sintering the resulting tablets and grinding the tablets. Sintering is carried out at an atmospheric humidity of 8,500 to 34,000 ppm at the preheating stage, 9,200 to 35,000 ppm at the high-temperature holding stage, and 5,100 to 6,000 ppm at the cooling stage.

[0017] The high-temperature soak during sintering is from 5.5 to 8.0 hours. This method helps to reduce the hydrogen content in uranium-gadolinium fuel pellets and ensures stable operational properties of ceramic nuclear fuel.

[0018] The invention is illustrated by:

[0019] Fig. 1 shows the characteristic porosity (a) (x100), microstructure (b) (x400) of uranium-gadolinium tablets sintered with additional oxygen supply at all stages of sintering, including cooling.

[0020] IMPLEMENTATION OF THE INVENTION

[0021] The implementation of the described method is demonstrated using several variants of manufacturing uranium-gadolinium tablets.

[0022] Examples of implementation of the proposed method:

[0023] Option 1:

[0024] 15% uranium oxide, 8.0% gadolinium oxide and alloying additives were added to the initial UO2 powder, and the mixture was mechanically processed in a vibration mill. 0.2% solid lubricant and 0.1% blowing agent were added to the resulting mixture, and the mixture was mixed in a three-axis mixer. The press powder was pressed on a rotary press and sintered in an argon-hydrogen environment (6.0% U) at a temperature of 1680 °C. The level of humidification of the sintering atmosphere at the preheating stage is ~ 8500 ppm, at high-temperature holding ~ 9200 ppm, at the cooling stage ~ 5100 ppm. Maintaining the required dew point is achieved by adding carbon dioxide. The high-temperature holding time was ~ 5.5 h. The grinding operation was carried out on a centerless grinding machine (“dry” method).

[0025] Option 2:

[0026] To the initial UO2 powder were added 15% uranium oxide, 8.0% gadolinium oxide and alloying additives, mechanically processed in a vibratory mill. To the resulting mixture were added 0.2% solid lubricant and 0.1% blowing agent, mixed in a three-axis mixer. The press powder was pressed on a rotary press and sintered at a temperature of 1680 °C for 8 hours in a reducing atmosphere of hydrogen with nitrogen (= 25% N2). The gases were saturated with water vapor, and CO2 was supplied at the cooling stage. The humidification level of the sintering atmosphere at the preheating stage was ~ 34,000 ppm, at the high-temperature holding ~ 35,000 ppm, at the cooling stage ~ 6000 ppm. The grinding operation was carried out on a centerless grinding machine ("dry" method).

[0027] For comparison, the properties of uranium-gadolinium tablets are given, which are sintered in reducing environments (including with the addition of N2 or Ar) with humidification at all stages except cooling (options 3, 4).

[0028] Option 3 (for comparison):

[0029] 15% uranium oxide, 8.0% gadolinium oxide and alloying additives were added to the initial UO2 powder and mechanically processed in a vibratory mill. 0.2% solid lubricant and 0.1% blowing agent were added to the resulting mixture and mixed in a three-axis mixer. The press powder was pressed on a rotary press and sintered at a temperature of 1680 °C for 5.5 hours in an argon-hydrogen environment (6.0% H2). The level of humidification of the sintering atmosphere at the preheating stage was ~ 8650 ppm, at the high-temperature holding ~ 8600 ppm, CO2 was not supplied at the cooling stage. Maintaining the required dew point is achieved by adding carbon dioxide. The grinding operation was carried out on a centerless grinding machine ("dry" method).

[0030] Option 4 (for comparison):

[0031] 15% uranium oxide, 8.0% gadolinium oxide and alloying additives were added to the initial UO2 powder, and the mixture was mechanically processed in a vibration mill. 0.2% solid lubricant and 0.1% blowing agent were added to the resulting mixture, and the mixture was mixed in a three-axis mixer. The press powder was pressed on a rotary press and sintered at a temperature of 1680 °C for 8 hours in a reducing atmosphere of hydrogen with nitrogen (~ 25% N2). The gases are saturated with water vapor. The level of humidification of the sintering atmosphere at the preheating stage was - 21,000 ppm, at the high-temperature holding ~ 25,000 ppm, humidification was not performed at the cooling stage. The grinding operation was carried out on a centerless grinding machine ("dry" method).

[0032] The quality characteristics of uranium-gadolinium tablets for each manufacturing option are given in Table 1.

[0033] Table 1.

[0034] *TP – theoretical density.

[0035] In Table 1, variant 1 should be compared with variant 3 (sintering atmosphere of Ar / 6 % H2), and variant 2 with variant 4 (sintering atmosphere of H2 / 25 % N2). The presented results show that the level of humidification at all sintering stages plays an important role in stabilizing the properties of the ceramic fuel. With a high level of humidification at all sintering stages, including with additional introduction of oxygen in the form of CO2 during cooling (variants 1, 2), the density and "post-sintering" values ​​are stabilized, and the hydrogen content decreases compared to variants 3, 4, where CO2 was not supplied at the cooling stage. Thus, the proposed method for producing urangadolinium fuel allows reducing the hydrogen content in sintered pellets, and also contributes to stabilizing the density and "post-sintering" of the fuel pellets.The practical use of the proposed technology for manufacturing uranium-gadolinium fuel has made it possible to increase the reliability of fuel elements with fuel containing a burnable absorber in the form of gadolinium oxide additive in an amount of > 8.0 wt. %, and has also made it possible to reduce the level of fuel damage.

Claims

9 Invention formula 1. A method for producing uranium-gadolinium nuclear fuel, which includes preparing a starting uranium dioxide powder, mixing it with uranium oxide powder and gadolinium oxide, granulating the powder mixture, mixing the granulate with a solid lubricant and a blowing agent, pressing the press powder, sintering the resulting tablets and grinding the tablets, characterized in that sintering is carried out at an atmospheric humidity of 8,500 to 34,000 ppm at the preheating stage, 9,200 to 35,000 ppm at the high-temperature holding stage, and 5,100 to 6,000 ppm at the cooling stage.

2. The method according to paragraph 1, characterized in that the high-temperature holding time during sintering is from 5.5 to 8.0 hours.

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