Method for producing nuclear reactor fuel microelements

FIELD: nuclear power engineering; nuclear reactor fuel microelements covered with four-layer shielding coating.

SUBSTANCE: proposed method involves sequential fluid-bed deposition of coating layers onto fuel microspheres. First low-density pyrocarbon layer is deposited by pyrolysis of acetylene and argon mixture of 50 volume percent concentration at temperature of 1450 °C. 85 - 95 % of second layer is deposited from high-density pyrocarbon by pyrolysis of acetylene and argon mixture of 40.0 - 43,0 volume percent concentration, and of propylene and argon mixture of 30.0 - 27.0 volume percent concentration at temperature of 1300 °C; 5 - 15 % of coating is deposited by pyrolysis of propylene and argon mixture of 5.0 - 10.0 volume percent concentration doped with 0.5 - 1. 5 volume percent of methyl trichlorosilane. Third layer of silicon carbide is deposited by pyrolysis of methyl trichlorosilane and argon mixture of 2.5 - 3.0 volume percent concentration in hydrogen-argon mixture at temperature of 1500 °C. Upon deposition this layer is treated with hydrogen at temperature of 1750 -1800 °C for 20 - 30 minutes. 90 - 95 % of fourth layer is deposited by pyrolysis of acetylene and argon mixture of 40.0 - 43.0 volume percent concentration and of argon and propylene mixture of 30.0 - 27.0 volume percent concentration at temperature of 1300 °C. Upon deposition of 90 - 95 % of fourth-layer pyrocarbon coating thickness 5 - 10 % of coating is deposited by pyrolysis of propylene and hydrogen mixture of 3.0 - 5.0 volume percent concentration.

EFFECT: enhanced service life of fuel microelements due to reduced damage probability during their manufacture and in service.

1 cl, 6 dwg, 1 tbl

 

The invention relates to the field of nuclear energy, in particular to microwell high temperature gas cooled reactors (HTGR) high safety of operation.

One of the ways to improve the safety of operation of nuclear reactors is the use of fuel elements (cartridges), in which the fuel is localized in the microvolume of spherical particles of fissionable material, each of which has a protective coating of non-fissile material with characteristics that are optimized in accordance with the type of reactor and its operation conditions [Kotelnikov RB, Bashlykov S.N., Chestnut A.I., Menshikov I.E. high Temperature nuclear fuel. Ed. 2-E. M.: Atomizdat, 1978, 432 S.].

Regardless of the type of design of the fuel rods of the nuclear fuel in all types of HTGR is in the form of microspherical particles with layers of protective coating so-called microtalon-MT.

In modern MT HTGR number of protective layers on the fuel microsphere (TM), as a rule, does not exceed four (figure 1):

- first - low-density (buffer) layer of pyrocarbon (Rus);

- the second internal high-density isotropic eng;

third - silicon-carbide (SiC);

- fourth outer high-density isotropic eng.

The main task of the buffer Rus is providing free volume of the gaseous products of Deleni the (GPA) and rauhouse under irradiation TM, reducing thermal stresses due to differences in coefficient of linear thermal expansion of the material TM and subsequent coatings, as well as protection internal Rus and SiC layer from damage by fission fragments.

Internal high-density isotropic Rus is a barrier for GPA and most solid products division (TPD). His task is to protect the SiC layer from the effects of the fission fragments and protection of TM against the penetration of chlorine in the deposition of a layer of SiC.

The SiC layer is the main diffusion barrier for TPD, and also ensures the durability of the coating in General. Experimental studies show that the integrity of the SiC layer provides retention of almost all (maybe with the exception of the silver isotopes) of fission fragments at the required level.

Outer high-density isotropic Rus is an additional diffusion barrier and protects the SiC layer from mechanical damage during subsequent processing steps with MT.

In the irradiation process, the MT is the number of physical and chemical transformations, which can cause the destruction of the SiC layer and the coating as a whole, which will lead to increased leakage of fission products. Under normal operating conditions most of the HTGR (temperature exposure ˜900-1300°, fluence fast neutrons ˜4·1021n/cm2 and deep burnup fuel, the main factors determining the destruction of the coating MT are:

I. the Development of mechanical stresses in power coating layers under irradiation.

II. Chemical interaction of fission products with coating material (oxidation Rus, the interaction of SiC, for example, with palladium and so on).

III. The damaging effects of irradiation on materials surfaces.

IV. The presence of initial defects in the coating after fabrication MT, as well as residual stresses in the layers of the coating and on the border between the layers, causing the formation of additional defects in the fuel elements and in the early stages of irradiation, especially in conditions of thermal Cycling, temperature gradient and neutron flux.

The known method according to which the first buffer layer Rus precipitated on TM fluidized bed at a temperature of pyrolysis 1500°from acetylene (C2H2), and the second high-density isotropic Rus from acetylene-propylene mixture at 1100-1300° (U.S. Pat. U.S. No. 3554783, IPC 7 G21C 3/00).

The known method, whereby high-density isotropic pyrocarbon precipitated in a fluidized layer by pyrolysis of propane, propylene (C3H6or butadiene at a temperature of 1250-1300° (U.S. Pat. France No. 1593145, IPC 7 G21C 3/00).

The known method according to which a layer of silicon carbide precipitated in the fluidized bed when the temperature 1650± 25°and the concentration of methyltrichlorosilane (CH3SiCl3in the hydrogen of 2.5% vol. (Voice E.H., Scott V.C. The formation and structure of silicon carbide pyrolytically deposited in afluidized bed of microspheres, In.: Special Ceramics 5, Eds P. Popper at al. The British Ceramic Research Assoc., 1972, p.1-32).

Known deposition method is a four-layer coating in a fluidized bed TM, according to which the first buffer eng produced by pyrolysis With2H2at a temperature of 1250°With the second high-density isotropic eng produced by pyrolysis of C2H2-C3H6mixture at a temperature of 1300°With the third SiC layer produced by pyrolysis of CH3SiCl3at a temperature of 1500°and the fourth high-density isotropic eng produced by pyrolysis With2H2-C3H6mixture at a temperature of 1300° (Huschka H., Vugen P. Coated fuel particles: requirements and status of fabrication ethnology. - Nuclear Technology, V.35, September, 1977, p.238-245).

The disadvantage of these methods is that when the four-layer deposition coatings on TM using the conditions specified for individual layers is possible uncontrolled damage to the fragile silicon carbide single MT in the party of particles. The likelihood of damage to the SiC layer in a separate MT increases significantly in the presence of residual stresses in the coating material, as well as growth defects, especially on the outer surface of the silicon carbide layer.

N is the most closest analog prototype of the proposed technical solution is the way, whereby fluidized bed TM at 1450°and concentration With2H250% vol. precipitated the first buffer layer Rus, at 1300°from a mixture of C2H2(40-43%) and C3H6(30-27 vol.%) precipitated the second high-density isotropic layer Rus, at a temperature of 1500°With a mixture of CH3SiCl3with hydrogen precipitated the third layer of silicon carbide, the fourth high-density isotropic layer Rus precipitated on the mode of the second layer (U.S. Pat. Germany No. 2626446, IPC 7 SS 11/02).

The disadvantage of this method, like the previous, is the emergence in the process of deposition of residual stresses in the four-layer coating, which are the cause of damage to the SiC layer (figure 2) and the fourth Rus (3) separate MT in a large array of particles at the production stage. The influence of residual stresses effect at early stages of irradiation, resulting in damage to the SiC layer in the form of bundles (figure 4) and the detachment of local areas of the second Rus layer from the SiC (figure 5), limiting the life of MT.

The destruction of at least one of the three power coating leads to an increase of the output PD of the MT and thereby limit the life of the latter.

The authors proposed technical solutions faced the challenge of increasing resource use MICROTEL nuclear reactor by reducing damage pokr is involved at the stage of coating deposition TM, as well as during their operation.

The problem is solved in that at the stage of obtaining microtalon nuclear reactor with four-layer protective coating, comprising the sequential deposition on fuel microspheres protective layer coating in a fluidized layer, after deposition of 85-95% of the thickness of the pyrocarbon coating of the second layer 5-15% cover precipitated by pyrolysis of propylene concentration in the mixture with argon 5,0-10,0% vol. with the addition of 0.5-1.5 vol.% methyltrichlorosilane, after deposition of the third silicon carbide layer hold his treatment in hydrogen at a temperature of 1750-1800°C for 20-30 min, and after deposition of 90-95% of the thickness of the pyrocarbon coating of the fourth layer, 5-10% coverage precipitated by pyrolysis of propylene concentration in the mixture with hydrogen 3,0-5,0%vol.

The causal link between the essential features and the technical solution is as follows.

The sequential deposition of each layer four-layer coating on TM in the fluidized layer, the following processes occur:

- at the stage of deposition of SiC between him and second Rus arise mechanical stresses caused by the difference in coefficient of linear thermal expansion of the contacting pairs. Moreover, for individual MT from a large array of particles these stresses can be significant. Under conditions of thermal Cycling of particles in key is present layer this will lead to an increase in the probability of damage to the SiC layer.

In the deposition of 5-15% coverage of the second pyrocarbon layer by pyrolysis of 5.0-10.0% vol.% propylene in mixture with argon with the addition of 0.5-1.5 vol.% methyltrichlorosilane formed the boundary layer of pyrocarbon-doped silicon, the coefficient of linear thermal expansion which is close to the value of the coefficient of linear thermal expansion of SiC coating.

- upon receipt of SiC layer in it there are residual stresses, relaxation whose deposition is virtually impossible. In terms of the stress state increases the likelihood of damage in thermal Cycling conditions in a fluidized bed, and in the early stages of irradiation MT.

Treatment in hydrogen SiC layer, when it is the outer coating on the particle at a temperature of 1750-1800°C for 20-30 min promotes relaxation of residual stresses therein due to thermal creep.

- Rus outer layer composed MT performing a protective function for the protection of SiC from mechanical damage, experiences tensile stress. The tear resistance of external Rus layer, in conjunction with other characteristics of the material, significantly depends on the quality of its surface, which is the absence of Macrovision, macropores of diameter greater than 1000 Å etc.

In conditions, when 5-10% of the outer PN the layer precipitated by pyrolysis of propylene concentration in the mixture with hydrogen 3,0-5,0%vol., forms a smooth, without noticeable protrusions of the surface with a predominant concentration of closed pores with a diameter of 60-200 Å.

An example implementation of technical solutions

Fuel microspheres of uranium dioxide with a diameter of 500 μm are precipitated in a fluidized bed at a temperature of pyrolysis 1450°With the first buffer Rus layer from C2H2-Ar at concentrations2H250% vol. and the total gas flow 1500 l/h

After deposition of the desired thickness of the buffer Rus stop filing With2H2and the particles are supported in a state of fluidization due to the inert carrier gas argon.

Due to the reduction applied to the heater electric power, reduce the temperature of the fluidized bed to 1300°and feed in a reaction zone a mixture With2H2(40%vol.) and C3H6(30 vol.%) with argon at a total gas flow of 1500 l/h precipitated 85-95% of the thickness of the second Rus layer. After that, at constant temperature, stop the supply of the reaction gases (C2H2and C3H6), and the remaining 5-15% of the thickness of the doped silicon of the second layer precipitated by pyrolysis of propylene in mixture with argon 5,0% vol. with the addition of 0.5 vol.% methyltrichlorosilane.

The third layer of silicon carbide precipitated at a temperature of 1500°from a mixture of methyltrichlorosilane with hydrogen at a concentration of CH3SiCl32,0 is B.% and the flow of hydrogen on fluidization 1600 l/h After deposition of the desired thickness of the SiC layer stop the flow of CH3SiCl3in the reaction zone. The particles are in a state of fluidization in the reaction zone due to the consumption of hydrogen in the amount of 1600 l/h After that due to the increase applied to the heater electric power particles are heated to a temperature of 1800°aged at this temperature for 20-30 minutes

After reducing the temperature of the fluidized bed to 1300°from a mixture With2H2(40%vol.) and C3H6(30 vol.%) with argon precipitated 90-95% of the thickness of the fourth Rus layer. The total gas flow is 1500 l/h stop flow in the reaction zone With2H2and C3H6and the total flow is compensated by the increase of hydrogen supply. The remaining 5-10% of the thickness of the fourth layer at a constant temperature of 1300°precipitated by the pyrolysis of propylene concentration in the mixture with hydrogen to 3.0 vol.%.

Comparison of deposition conditions four-layer protective coating and operational characteristics of MT obtained by a known method, with MT on the proposed technical solution is given in the table.

As follows from the table, the proposed method of obtaining microtalon nuclear reactor (examples 2, 3, 4) in comparison with the known method (example 1) provides increased ESRC operation due to higher radiation resistance of the second Rus layer, less damage SiC and external Rus layers. With exorbitant parameters (examples 5, 6) the operating characteristics of the MT decline sharply.

The method of producing microtalon nuclear reactor with four-layer protective coating, comprising the sequential deposition on fuel microspheres layers of coating in a fluidized bed, in which the first layer of low-density pyrocarbon precipitated by the pyrolysis of acetylene concentration in the mixture with argon 50% vol. at a temperature of 1450°, 85-95% of the second layer of high-density pyrocarbon precipitated by pyrolysis of a mixture of acetylene concentration in the mixture with argon 40,0-43,0% vol. and propylene concentration in the mixture with argon 30,0-27,0% vol. at a temperature of 1300°With, a third layer of silicon carbide precipitated by pyrolysis of methyltrichlorosilane with a concentration in the mixture of the hydrogen-argon 2,5-3,0% vol. at a temperature of 1500°C and 90-95% fourth layer of high-density pyrocarbon precipitated by pyrolysis of a mixture of acetylene concentration in the mixture with argon 40,0-43,0% vol. and propylene concentration in the mixture with argon 30,0-27,0% vol. at a temperature of 1300°C, characterized in that after deposition of 85-95% of the thickness of the pyrocarbon coating Deut the layer 5-15% cover precipitated by pyrolysis of propylene concentration in the mixture with argon 5,0-10,0% vol. with the addition of 0.5-1.5 vol.% methyltrichlorosilane, after deposition of the third silicon carbide layer hold his treatment in hydrogen at a temperature of 1750-1800°C for 20-30 min, and after deposition of 90-95% of the thickness of the pyrocarbon coatings fourth layer of 5-10% coverage precipitated by pyrolysis of propylene concentration in the mixture with hydrogen 3,0-5,0%vol.



 

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