RU2360307C2 - Method of fuel pellet production for nuclear fuel elements - Google Patents

Abstract

FIELD: chemistry; power engineering. ^ SUBSTANCE: invention relates to nuclear industry, particularly to production of fuel pellets from uranium dioxide (VO2), produced by ADU method for nuclear fuel elements. Essence of the invention in the following: ammonium polyuranate (ADU-process) is precipitated in a stage, before mixing powder of uranium dioxide is put through screen, mesh of which is not more than 630 micron with simultaneous reducing and sifting through the same oversize screen, mixing is carried out together with dry organic binding agent 1,2-distearylethylenediamine - DISED of formula C38H76O2N2 and powder of uranium oxide concentrate having total specific surface area not more than 1.2 m2/g and particle size 40 micron, pressing is carried out in compact density equalled 5.4-5.6 g/cm3. Ammonium polyuranate is precipitated with ammonium diuranate solution, the concentration of which is 50-80 g/l, pH=6.4-7.0, feed speed - 200-400 l/h, powder of uranium oxide concentrate is derived by oxidising sintered pellets preliminary reduced so that the particle size is not more than 5 mm in furnace during 3-4 hours at 360-380C and air consumption equalled 100 l/h per 1 kg of uranium dioxide. ^ EFFECT: precipitation of powder on the condition providing stable properties of powder, decrease of particle size, content increase of back uranium oxide concentrate admixture, increase of compacting pressure with reducing negative consequences for each applied procedure. ^ 2 cl, 2 tbl, 2 ex

Description

The invention relates to the nuclear industry, in particular to the manufacture of pelletized fuel from uranium dioxide (UO ₂ ) manufactured by the ADU method for fuel elements of nuclear reactors.

Known methods for the manufacture of pelletized nuclear fuel by direct compression of uranium dioxide with a dry binder, for example, according to RF patents No. 2158971 dated 1999.04.21, No. 2210821, MKI G21C 21/00, 08.20.03, No. 2110122, MKI G21C 21/10, 10.08. 03, in which the powder after its receipt and before the process of pressing the tablets is not subjected to any processing except mixing with a binder and additives of reverse uranium oxides. This scheme is called direct dry pressing.

The obtained sintered tablets have to be subjected to mechanical processing and grinding in order to obtain a higher class of manufacturing accuracy in diameter than can be done by powder metallurgy methods. This operation causes the appearance of unacceptable chips on the surface of some tablets. Chips are formed both during grinding and after small mechanical impacts due to the development of microcracks.

As experience in the manufacture of tablets shows, literature data on the properties of ceramics (see Obert L. Fragile Rock Destruction. Destruction. Volume 7. M. Mir. 1976, report of OAO NZHK No. 39-42-01 / 730 dated September 14 .05), the probability of formation of cracks and chips under the action of mechanical loads is proportional to the number of structural heterogeneity elements inside ceramic products - pores of irregular shape, accumulation of small pores, and the formation of large grains. The type, form, and amount of these formations are affected by the properties of the initial powder of uranium dioxide and its particles, the type of binder, the presence of modifying additives, the uniformity of the charge, and the pressing conditions. The likelihood of formation of inhomogeneities that weaken the strength of the tablets is especially high when using dry pressing.

Thus, the characteristics of all operations in the cycle “obtaining powder - preparing powder for pressing - pressing and sintering of tablets” affect the level of marriage.

Powders made by the ADU method have an initial high internal heterogeneity - the powder particles vary in morphology, size, density and strength. However, this plays a positive role in ensuring the technological properties of the dioxide — fluidity, compressibility, and sintering. Each group of particles with internally homogeneous properties, but different between lobes, provides one necessary quality, almost without violating the other. For example, more durable particles allow the pressure of the pressing to be transferred from the punch to the inside of the pressing, while soft particles, deformed, fill the space between them. Large agglomerates give the powder fluidity, while small ones reduce the porosity of the compact. Dense particles increase the bulk density of the powder, loose increase sintering. There is no ideal powder with the maximum values of all technological properties, we can only talk about their optimal ratios, when, for example, you can immediately direct the powder to pressing without any modifications.

However, to improve the quality of tablets, it may be necessary, for example, to increase the sintering ability of the powder, and to provide fluidity and bulk density, which are usually reduced in this case, to provide additional means. Therefore, in order to simultaneously achieve such conflicting goals as high quality tablets and low manufacturing costs, you should look for the optimum throughout the entire production cycle of powders and tablets. There is a known method for producing uranium dioxide powder using ADU technology in two stages (see Technological Instruction of OAO NZKhK No. 25000.00779), in which deposition is carried out in a cascade of reactors: in the first one, a lower pH value is maintained, in the last one a higher one. In the second stage of polyuranate precipitation, the structure and chemical composition of the precipitate formed in the first stage changes. Under conditions of higher pH values in the second stage, there is a reduced solubility of the salt and increased supersaturation of the solution relative to the first stage. Therefore, in the second stage of precipitation, crystallites of smaller size are formed. Small crystallites form loose agglomerates or settle in the form of a "coat" on salt particles obtained in the first stage. At the same time, crystallites formed in the first and second stages can be redissolved due to the possession of excess energy. This effect helps to reduce the overall saturation of the solution. There is a combination of two opposite effects. The powder thus obtained consists of particles with various properties. Their ratio in the finished product, and hence its technological properties, are determined by the deposition modes in the first and second stages - pH, temperature, solution feed rate. Modes of obtaining powder and its average properties are shown in table 1 and 2:

Table 1 Uranium Dioxide Production Modes Parameter pH The concentration of the solution, g / l Reextract rate, l / h Two-stage Stage 1 - 6.0-7.0 Stage 1 - 55-65 Stage 1 - 350 precipitation Stage 2 - 7.8-8.2 Stage 2 - <5 Stage 2 - 350 Single stage precipitation 6.4-7.0 55-65 230 l / h

table 2 Parameters of uranium dioxide after receiving and after grinding fractions of more than 630 microns Parameter Bulk density with shaking, g / cm ³ Full specific surface, m ² / g The proportion of powder particles> 630 μm% The average particle diameter, microns Two-stage deposition Before chopping 2.9 Before chopping 3.9 Before grinding 48% Before grinding 580 (up to 1250) After grinding 3.1 After grinding 0 After grinding 200 (up to 630) Single stage precipitation Before chopping 2.4 Before grinding 4.1 Before chopping 10 Before chopping 300 After grinding 2.6 After grinding 4.2 After grinding 0 After grinding 200

However, with two-stage deposition, this is difficult to do because of the complexity of finding the optimum among many factors and their ambiguous influence on the final result. Maintaining the found modes stable is difficult. The powder has a wide fractional composition from microns to 1500 microns, large particles of more than 1250 microns are crushed on an annular grinding and scattering device. To reduce particle size by grinding, it would be necessary to grind almost the entire volume of powder. Its return to the original powder would lead to a violation of the free flow of the party. Tablets made from such a powder using a dry binder, zinc stearate or DISEDA have matrix discontinuities in the form of discontinuities, caverns, triangular pores between large particles. Reactive oxide-oxide additives accumulate between particles, forming joint porosity. In order for the tablets to have an open porosity of not more than 0.5%, it is necessary to limit its content to a few percent. Rejection by chips of sintered tablets made from this powder is 10-18%.

There is also known a method with grinding uranium dioxide in ball mills (see RF patent No. 2165651 from 2000.02.18). The crushed powder does not contain large agglomerates, which create inhomogeneities in the form of pore clusters in sintered tablets, which makes it possible to add reverse nitrous oxide up to 10–20%, which favorably affects the tablet structure — uniformity and continuity. But in such cases, pre-compaction of the powder, granulation, rolling, such as, for example, according to RF patent No. 2001109717 of 2001.04.12 or US patent 4637900 of 01.20.87, which significantly increases the cost of the process, leads to additional return and irrecoverable losses, are used.

Since the use of powder grinding is required in case of adverse fluctuations in the properties of the powder, a method of selective technology is known, for example, according to the application of OAO NPKhK for RF patent No. 2005117546 with priority dated June 7, 2005, in which tests and diagnostics of the technological properties of the powder launched into production , and then adjust the technological scheme for the manufacture of tablets depending on the test results. This method allows to increase the yield, but is limited in application, does not solve the issue of stabilization of production, does not always allow you to adjust the properties of the obtained powder, requires a set of equipment that will not always be involved. Therefore, a method is needed that gives maximum stability of the properties of uranium dioxide, even if at the same time maximum processability is not achieved.

It is known (see W. Timmermans, A. Van Heck-Hennen. J. of Nuc. Mat. 71 (1978), Sintered characterization of U02 powder) that an increase in compression pressure reduces the size of the voids in a compressed tablet, but increases wear of the press tool, the likelihood of the formation of transverse cracks and the appearance of re-pressing effects, which increase the rejection of sintered tablets, reduce direct yield.

The closest in technical essence and the achieved effect is the method (see RF patent No. 2158030 - prototype), in which reverse oxide-oxide is added to the uranium dioxide powder obtained by the two-stage deposition scheme by the ADU process, with a particle size of not more than 1250 μm obtained by burning the marriage of tablets at a temperature of 500-600 ° C and a particle size of up to 100 microns, and a dry binder - zinc stearate. Next, the powder is fed by gravity to a rotary press, where tablets are pressed to a density of 4.9-5.1 g / cm ³ , which are then heated in a furnace at a temperature that provides thermal removal of the binder, sintering of the tablets in a gaseous reducing medium. Sintered tablets are subjected to wet grinding with a diamond wheel, dried and discarded those that have deviations in appearance in the amount of 10-18% of the batch. The disadvantage of this method is the inability to achieve high quality fuel pellets due to the heterogeneity of the pressing, and hence the heterogeneity of its sintering, and, as a result, vulnerability to chips, pores and other defects that reduce yield.

The technical task of this invention is the development of such a technological scheme in which the maximum set of tools is used that increases the technological properties of the powder and the quality of sintered tablets, but in such a set and in such "soft" modes, which do not require a radical restructuring of the line, complicating and increasing the cost of a new of the method, that is, the use of the principle of "maximum benefit at minimum cost", namely the deposition of the powder in a mode that gives stable properties of the powder, reducing particle size, increasing the content anija reverse additives mixed oxide, increased compaction pressure while taking measures to reduce the negative effects of each used operations.

The problem is solved by using all known methods for increasing the strength and uniformity of tablets at all stages of the manufacture of powders and tablets, but to the extent and form that does not require increased costs of materials, equipment and time. Moreover, due to all the effects achieved, albeit partially, not only the total result, but also the integral result is obtained. Each technological improvement used compensated for the shortcomings of other improvements, namely, in the method for producing pelletized fuel for fuel elements of a nuclear reactor, including the production of uranium dioxide powder by the ADU process, mixing it with a binder and uranium oxide powder obtained by oxidizing sintered tablets, pressing thermal binder removal, sintering of tablets in a gaseous reducing medium, grinding of tablets with a diamond wheel, according to the invention the precipitation operation of the ammonium polyuranate of the ADU process is carried out in one step, before the mixing operation, uranium dioxide powder is passed through a sieve with a mesh size of not more than 630 μm with simultaneous grinding and sifting of the plus fraction through the same sieve, the mixing operation is carried out with a binder organic dry binder 1 , 2-distearylethylenediamine - DISED with the formula C ₃₈ N ₇₆ O ₂ N ₂ and with uranium oxide oxide powder having a total specific surface of not more than 1.2 m ² / g and a particle size of less than 40 microns, the pressing operation is carried out in the density of the compacts equal to 5.4-5.6 g / cm ³ .

The problem is also solved by the fact that in the method the precipitation operation of ammonium polyuranate is carried out at a concentration of ammonium diuranate solution equal to 50-80 g / l, pH 6.4-7.0, flow rates 200-400 l / h, nitrous oxide powder Uranium is obtained by oxidation in a furnace of pre-crushed to a particle size of not more than 5 mm sintered tablets for 3-4 hours at a furnace temperature of 360-380 ° C and an air flow rate of 100 l / h per kilogram of uranium dioxide.

The proposed solution is new and has an inventive step, since the following measures have been used to increase the continuity and uniformity of the tablets:

- reduced maximum particle size of the resulting uranium dioxide;

- increased content of working oxide-oxide;

- increased pressing pressure, which are implemented as follows.

The reduction in particle size of the powder was achieved in two successive ways. The first is a change in the deposition mode of polyuranates so that the agglomerates of uranium dioxide are smaller. This was done using the single-stage deposition mode. The second is the reduction in the size of the sieve at the screening and grinding plant of the resulting uranium dioxide powder. The sieve was selected with the smallest mesh size from those with which the crushed portion did not yet violate the fluidity of the powder. These methods can reduce the average particle size by half.

However, it remains at a high level - hundreds of microns, while it is desirable to have it at the level of tens of microns. Although the effect of segregation of dioxide particles and added uranium oxide is weakened, it still remains. To increase the degree of mixing of working oxide-oxide with uranium dioxide and, as a result, increasing its content in the charge, it is necessary to obtain it as finely as possible by oxidizing the sintered tablets at the lowest possible, from a practical point of view, temperature at which the oxidation process is already quite fast, but elastic the properties of the dioxide matrix are still low. Then, fracture stresses due to a decrease in the density of oxidizing dioxide in sintered tablets occur at small volumes of oxidation and the size of the resulting fragments is small - microns to tens of microns. The degree of homogeneity of the mixture can be increased by additional processing in a hammer mill, in which not so much grinding of large particles occurs, as the introduction of small nitrous oxide in the pores of the remaining large agglomerates.

To reduce the pore-forming ability of nitrous oxide and to reduce the size of most residual pores below the diffusion length of ions during sintering, it was necessary to increase the density of the compacts and the pressure of their pressing, but in such a way as to avoid the phenomenon of repressing - repressive cracks. It is determined during testing of uranium dioxide for sintering by pressing tablets at different pressing pressures. The limiting value of the pressing pressure P _{gr is} determined at which the delamination of the end is observed. Working pressure is determined by the formula P = 0.8P _gr .

To minimize the mechanical effect on the tablet, the thickness of the layer to be removed during grinding, the diameter of the press tool is selected in accordance with the selected pressing pressure so that the minimum diameter of the tablets after sintering corresponds to the nominal diameter of the finished tablets.

To exclude increased wear of the press tool with increasing pressing pressure, it was necessary to choose a dry binder that more effectively reduces internal friction during pressing. Studies have shown (report on the research work of JSC NZHK No. 39-42-01 / 290 of 04/21/03) that DIESED is distearylethylenediamine, an organic substance with the formula C ₃₈ H ₇₆ O ₂ N ₂ , in this respect more effective than stearate zinc used in the prototype. Moreover, it does not form solid deposits in the sintering furnace. In addition, the plasticizing effect on the mixture and the pressed tablet is exerted by small nitrous oxide when the content in the mixture is 5% or more.

Single-stage deposition, in addition to reducing the particle size of the powder, additionally saves consumables (ammonia), stabilizes the properties of uranium dioxide, which further reduces grinding tolerances and reduces the volume of control operations.

The use of a thinner working oxide of uranium oxide in an increased amount (more than 5%) gives the plastic properties of the mixture and pressed tablets, further increasing their uniformity and reducing their chipping.

Increasing the pressing pressure also strengthens the pressed tablet, reduces the loss of product on pressing, reduces shrinkage during sintering, which allows more precise control of the grinding allowance. To do this, select the diameter of the press tool accordingly.

The use of DISEDA as a dry binder and lubricant that reduces the pressing force

The combined effect of using the proposed method allows to obtain an increase in tablet yield by 5-10%.

The method is implemented as follows.

Example 1. The deposition of ammonium polyuranates was carried out according to a one-stage scheme at a temperature of uranyl nitrate of 65 ° C and a concentration of 57 g / l, pH 6.5, the feed rate of the reextract 250 l / h. Then the polyuranate powder was calcined at temperatures in the zones of the furnace 615-650 ° C and reduced in a hydrogen atmosphere at a temperature in the zones of the furnace 615-670 ° C. The resulting uranium dioxide powder was flowable, with a total specific surface area of 4.19 m ² / g and a bulk density of 2.24 g / cm ³ . The powder contained a fraction <71 μm - 42%, a fraction> 630 μm - 8%.

After reduction and stabilization, the uranium dioxide powder was passed through an annular grinding and scattering device with a 630 micron sieve installed and the entire plus fraction was crushed and sieved through this sieve, returning to the general powder flow. After grinding, the powder flowed without delay along the technological path.

Recycled uranium oxide was prepared by oxidizing pre-crushed to a size of pieces of less than 5 mm sintered tablets in an oven with a fixed layer of filling with a thickness of 10-20 mm at a temperature of 360-380 ° C with air supply to the furnace with a flow rate of at least 100 l / h for every kilogram of dioxide. The obtained nitrous oxide had a grain size of 1.6 μm, the total specific surface area was 1.1 m ² / g, and the uranium content was 84.8%. Particles, all smaller than 100 microns in size, had a developed structure, were easily destroyed into smaller particles, providing plastic properties to the whole oxide-oxide powder and mixtures with uranium dioxide. Uranium oxide was added to the dioxide in an amount of 8% and mixed in bicone at a speed of 2 rpm for 40 minutes. Then, 0.2-0.3% of DISEDA was added.

Then the mixture was gravity fed to the rotary press, where the tablets were pressed. The pressing process was carried out to a density of 5.4 g / cm ³ with a pressing pressure of 2 t / cm and a diameter of the pressing matrix of 9.58 mm. Sintering was carried out in a hydrogen atmosphere with a holding time of 4 hours at a temperature of 1740 ° C. The density of the sintered tablets was 10.62 g / cm ³ , open porosity <0.3%. Sintered tablets were ground on a centerless grinding machine at a speed of 80 tablets per minute. The yield was 92.1%.

Example 2. Made tablets according to the modes, as in example 1, with the following differences.

After mixing the powders of dioxide and uranium oxide, the mixture was ground in a hammer mill at a speed of rotation of the hammer 3 rpm As a result of intensive mixing of oxides and partial grinding of a large fraction of the powder, it is possible to increase the content of revolutions up to 15% without loss of tablet quality in terms of density and open porosity. The yield was 94.3%.

Claims (2)

1. A method of manufacturing pelletized fuel for fuel elements of a nuclear reactor, including obtaining a powder of uranium dioxide by the ADU process, mixing it with a binder and powder of uranium oxide obtained by oxidation of sintered tablets, pressing, thermal removal of the binder, sintering of tablets in a gaseous reduction medium, grinding the tablets with a diamond wheel, characterized in that the deposition operation of ammonium polyuranate of the ADU process is carried out in one step, previously before the mixing operation roshok uranium dioxide is passed through a sieve with mesh size less than 630 microns with simultaneous grinding and screening using the same sieve fraction above zero, the mixing operation is carried out with a binder bonded organic dry 1,2-disteariletilendiamin - DISED with the formula C ₃₈ H ₇₆ O ₂ N ₂ and with a powder of uranium oxide-uranium having a total specific surface area of not more than 1.2 m ² / g and a particle size of less than 40 μm, the pressing operation is carried out in a density of compacts equal to 5.4-5.6 g / cm ³ . 2. The method according to claim 1, characterized in that the precipitation operation of ammonium polyuranate is carried out at a concentration of ammonium diuranate solution equal to 50-80 g / l, pH 6.4-7.0, a flow rate of 200-400 l / h, powder uranium oxide-oxide is obtained by oxidation in a furnace of pre-crushed to a particle size of not more than 5 mm sintered tablets for 3-4 hours at a furnace temperature of 360-380 ° C and an air flow rate of 100 l / h per kilogram of uranium dioxide.