RU2244351C2 - Finely dispersed solid coolant and its production process - Google Patents

Abstract

FIELD: nuclear engineering.

SUBSTANCE: finely dispersed solid coolant meant for use in nuclear reactors is produced in the form of spherical particles. Each spherical particle of finely dispersed solid coolant has nucleus of amorphous nongraphitic carbon material whose diameter amounts to 0.3 - 0.6 of coolant particle diameter. Proposed coolant production process includes compaction of particles by pyrolysis of hydrocarbons in fluidized bed. Source material for coolant particle nuclei is usually one of resins such as universal cation-exchange resin KU-23. Resin particles are heated to 1000 - 1200 °C and treated at this temperature with hydrocarbon-hydrogen mixture.

EFFECT: enhanced stability and reliability of heat transfer.

7 cl, 2 tbl, 5 ex

Description

The invention relates to the field of operation of nuclear reactors, in particular to emergency situations associated with overheating of the core, loss of coolant, etc.

Known solid heat carrier of graphite-containing material, providing increased heat removal in the active zone of a nuclear reactor (Bulkin Yu.M. et al. A.S. No. 1600554, MKI G 21 C 15/24, published on 05/10/1988).

The disadvantage of this coolant is that heat removal is carried out only due to the radiant heat exchange of the core with a cylindrical wall of the coolant.

Known finely dispersed coolant of a nuclear reactor made in the form of particles of graphite, silicon carbide, metal (aluminum, zirconium, etc.) (Heinlein FA Improvements in relatinq to atomic power plant. UK patent No. 875872, MKI G 21, NKI 39 (4), Declared September 19, 1958, published August 23, 1961). During rotation of the hopper, a solid finely dispersed heat carrier in contact with a stationary core and a secondary heat exchanger mixes, which improves heat removal from the core and increases cooling efficiency.

The described technical solution has the following disadvantages:

- intensive abrasion of solid coolant particles during rotation of the hopper can lead to a change in the operating characteristics of the reactor;

- high voltages in the buildings of the active zone and the secondary heat exchanger in contact with the stirred solid coolant;

- coolant particles having high hardness and sharp edges, for example, made of silicon carbide, contribute to intensive abrasive damage to the core shells, the secondary heat exchanger and the hopper.

These shortcomings do not allow to achieve sufficient reliability and safety of a nuclear reactor.

The closest analogue (prototype) to the proposed solid finely dispersed heat carrier is a finely dispersed solid heat carrier of graphite-containing material containing particles of various sizes and shapes (Riqq S., Greenlees FM / Nuclear reactor. UK patent No. 1,309.883, MKI G 21 D 5/00 , NKI G 6 C 30X363. Declared February 18, 1972, published March 14, 1973).

During the operation of the reactor, an increase in the finer and smallest fraction of the coolant occurs due to the abrasion of the coolant particles. Because of this, the filling of the coolant has a different density, which does not allow to ensure the stability of the speed of movement of the coolant through the active zone and, therefore, the stability and reliability of heat removal. In addition, with increasing mass of the smallest fraction, the coolant becomes prone to sticking, which contributes to disruption of the reactor due to a decrease in the rate of movement of the coolant (up to a stop) through the active zone. The closest analogue (prototype) of the method for producing a solid finely dispersed coolant is a method of compacting carbon-graphite particles by pyrolysis of hydrocarbons in a fluidized bed (RF patent No. 2166806 dated 09.02.2000, MKI G 21 C 15/24). The disadvantage of this method is the low radiation resistance of the particles due to swelling of the core under the action of radiation.

The objectives of the proposed invention is to increase the resistance of heat-transfer particles to abrasion, increase the stability of the rate of movement of the heat-transfer agent through the core and, therefore, the stability and reliability of heat removal, align the neutron flux field during normal operation of the reactor and increase the nuclear safety of the reactor in emergency situations by intensifying cooling processes core and increase radiation resistance of particles.

The solution of these problems in the proposed solid finely dispersed coolant is achieved by the fact that, compared with a solid finely dispersed coolant for nuclear reactors in the form of spherical particles based on carbon-graphite material, spherical particles contain nuclei of an amorphous non-graphite carbon material with a diameter of 0.3-0.6 of the diameter of the coolant particles. Additionally, the core particles contain a metal selected from the group consisting of gadolinium, europium, samarium, and the metal content is selected in the range of 0.1-30.0 wt.%. Additionally, the coolant particles contain a surface layer of a composition of silicon carbide and carbon with a thickness of 0.01-0.03 particle diameter.

In addition, in the known method for producing a solid finely dispersed coolant for nuclear reactors in the form of spherical particles based on carbon-graphite material, including compaction of particles by pyrolysis of hydrocarbons in a fluidized bed, ion-exchange resins are selected as the source material for the particle nuclei, for example, universal KU-23 cation exchange resin, resin particles heated to 1000-1200 ° C and at this temperature is treated with a mixture of hydrocarbon and hydrogen, and then at a temperature of 1300-1400 ° C is treated with a mixture of hydrocarbon and argon.

Additionally, processing at 1000-1200 ° C is carried out with a mixture of methane and hydrogen in a volume ratio of (0.65-0.80) :( 0.35-0.20), and at 1300-1400 ° C, processing is carried out with a mixture of methane and argon in volume ratio (0.20-0.25) :( 0.80-0.75). Additionally, the resin particles are impregnated with an aqueous solution of a metal selected from the group consisting of gadolinium, europium, and samarium before heating. Additionally, the heat carrier particles are treated at 1350-1550 ° C with a mixture of methyltrichlorosilane, methane and hydrogen in a volume ratio of (0.02-0.05) :( 0.01-0.03) :( 0.97-0.92).

The proposed solid heat carrier and the method for its preparation are justified as follows. The presence of amorphous nuclei in a solid coolant particle leads to increased particle strength during reactor irradiation when pyrocarbon and graphite are compacted (Sawa, K., Minato, K., Investing of irradiation Performance of High Burnup HTGR fuel. - J. of Nucl. Science and Technol. Vol. 36, No. 9, 1999, p. 781-791). Compressive stresses in coolant particles (Miller GK, David AP, Dominic J. et al. Consideration of the effects on fuel particle behavior from shrinkage cracks in the inner pyrocarbon layer. - J. of Nucl. Materials, 295 (2001), p. 205-212) are compensated by the possibility of deformation of the amorphous core. All temperature treatments in accordance with the claims are carried out at a temperature not exceeding 1550 ° C, i.e. lower than carbon graphitization temperature (Virgiliev Yu.S., Kurolenkin E.I. Change in the structure of graphite under neutron irradiation. - Issues of Atomic Science and Technology. Ser .: Physics of radiation damage and radiation materials science, 1991, issue 3 (57), p.20-26).

A decrease in the diameter of the core of amorphous carbon material below a value of 0.3 of the particle diameter does not compensate for compressive stresses, and an increase above 0.6 of the particle diameter significantly reduces the particle strength.

The addition to the nuclei of metal particles selected from the group consisting of gadolinium, europium, and samarium is due to the possibility of equalizing the temperature field of the nuclear reactor during its operation, when the content of the neutron-absorbing metal in the nuclei of the particles is 0.1-10.0 wt.%, And preventing emergency heating, when the metal content can be 10-30 wt.%. At present, gadolinium additives are used directly in nuclear fuel to equalize the temperature field (White Paper of Nuclear Power Engineering. Monograph. Edited by E.O. Adamov M., NIKIET, 1998), which leads to a significant complication of the process of nuclear fuel regeneration caused by the need to separate gadolinium from uranium to a level of 10 ^-5 wt.%.

An additional layer of the composition of silicon carbide and carbon on the surface of the particles of the coolant reduces the abrasion of the particles by 2-3 times (see examples of implementation). When applying an additional layer of the composition of silicon carbide and carbon, a decrease in the layer thickness below 0.01 of the particle diameter does not reduce abrasion, and an increase in the layer thickness above 0.03 of the particle diameter causes an increase in stresses in the transition layer of the pyrocarbon-composite and thereby abrasion resistance.

A decrease in the concentration of methyl trichloroilane and methane below 0.02 vol. shares and 0.01 vol. fractions, respectively, leads to a sharp decrease in the growth rate of the coating, an increase in the corresponding concentrations above 0.05 vol. shares and 0.03 vol. fraction causes a decrease in the strength of the outer coating and the abrasion of the particles. Similarly, when the deposition temperature of the composition is lower than 1350 ° C, the growth rate of the coating decreases, and above 1550 ° C, the porosity increases, with a corresponding decrease in abrasion resistance.

During the operation of a nuclear reactor, the use of a coolant with a neutron-absorbing metal content of 0.1-10.0 wt.% Allows you to get a uniform temperature profile in the core from the center to the periphery when organizing the flow of coolant with different contents of the absorber to different zones along the radius.

In an emergency, when the temperature rise of the reactor is due to an increase in the neutron flux, the proposed coolant reduces the temperature of the reactor by absorbing neutrons and thereby reducing the rate of fission of uranium-235 with a corresponding decrease in the rate of heat release. In such a situation, it is advisable to use a solid coolant with a neutron-absorbing metal content of 10-30 wt.%.

Table 1 shows the neutron capture cross sections of elements (and their isotopes) in the composition of the proposed coolant (Tables of physical quantities. Ed. Kikoin I.K. M .: Atomizdat, 1976, p. 907).

Table 1
The neutron capture cross section of the proposed metals Element Isotope σ, barn Gadolinium Natural
155
157 44620
56200
242000 Europium Natural
151
152 4400
7800
5500 Samarium Natural
149
151 5830
40800
12400

The data in Table 1 show that the neutron capture cross sections of the proposed additives in the coolant significantly exceed the capture cross sections for such common absorbers as boron (σ = 750 bar) and cadmium (σ = 1400 bar).

The authors experimentally found that when the content of absorbing metals is less than 0.01 wt.%, The heat removal rate does not change, and when the content is above 30 wt.%, The fragility of the heat-transfer particles and, accordingly, their abrasion increases.

Heat treatment (pyrolysis) of resin particles with a mixture of hydrocarbon and hydrogen at a temperature of less than 1000 ° C is ineffective, since the processes of particle compaction with pyrocarbon proceed at low speeds. At a pyrolysis temperature of more than 1200 ° С, the rate of pyrocarbon deposition on the particle surface dominates the compaction processes.

When heat treated with a hydrocarbon-argon mixture in the temperature range 1300-1400 ° C, the maximum deposition rate of high-density (1.75-1.90 g / cm ³ ) pyrocarbon is realized. At temperatures above 1400 ° C, the density of pyrocarbon decreases sharply (up to 1.50-1.60 g / cm ³ ).

During the heat treatment of particles in the first stage, when the volumetric content of methane in the mixture with hydrogen is less than 0.65 vol. fraction, the rate of compaction of the pore space decreases, and when the methane content is more than 0.80 vol. undesirable processes of homogeneous nucleation and growth of soot particles are intensified significantly.

At the second stage of heat treatment, the methane content in the mixture with argon is less than 0.2 vol. fractions of a low deposition rate of pyrocarbon, and with a methane content of more than 0.25 vol. the share sharply increases the intensity of soot formation.

The proposed solid finely divided coolant and method for its preparation are illustrated by examples of implementation.

Example 1

Using a sieve analysis, fractions of resin particles 0.7-1.0 mm in diameter were isolated from a batch. In shape, the particles were separated using an inclined (at an angle of 10 °) vibrating plate.

300 g of ion exchange resin (universal cation exchanger KU-23 TU10P-388-69) were impregnated with an oxalic acid gadolinium solution (1 l of a solution with a gadolinium concentration of 50 g / l). The cation exchange resin was dried at a temperature of 150 ° C in air. In a fluidized bed apparatus (particle fluidization was carried out with argon) at a temperature of 200-600 ° C, gadolinium cation exchange resin was carbonized for 1.0 hour, and the final heat treatment forming the porous structure of the carbon skeleton and gadolinium carbide was carried out for 1.5 hours at a temperature 1100 ° C. After completion of the heat treatment, the gadolinium content in the carbon particles was about 80 wt.%, The particle diameter was 0.40-0.48 mm, and the shape coefficient (D _max / D _min ) <1.1.

The pore space of the particles was densified in a fluidized bed apparatus at a temperature of 1100 ° C with a volume ratio of CH ₄ : H ₂ = 0.70-0.30 (total gas mixture consumption of 1200 l / h). The time of the compaction stage is 1.0 hour. The pyrocarbon coating was deposited (the second stage of processing) was carried out at a temperature of 1350 ° C with a volume ratio of CH ₄ : Ar = 0.22-0.78 (total gas mixture consumption of 1500 l / h). The time of the deposition stage of the pyrocarbon layer is 1.5 hours. Particles were obtained with a diameter of 0.8-0.9 mm and a density of 1.9-2.0 g / cm ³ and a gadolinium content of 20 wt.%, With an amorphous core diameter of 0.40-0.48 mm (ratio of core diameter to diameter particles amounted to 0.45-0.50). The sphericity of the particles after compaction and deposition of the pyrocarbon coating did not change compared to the initial ones.

Example 2

As the substrate used particles obtained in example 1.

Sealing of the pore space was carried out analogously to example 1.

The pyrocarbon layer was deposited (the second stage of processing) was carried out at 1450 ° C and a volume ratio of CH ₄ : Ar = 0.35-0.65. In this case, a coating density of 1.55 g / cm ³ with a gadolinium content of 26 wt.% And a particle density of about 1.70 g / cm ^{3 were} obtained. Due to the low density of the outer coating, the particles are subject to abrasion, which manifests itself in a significant dust removal during their transport in the core.

Example 3

As the substrate used particles obtained in examples 1 and 2.

Sealing the pore space (the first stage of processing) was carried out at a temperature of 1250 ° C and a volume ratio of CH ₄ : H ₂ = 0.85-0.15. At the same time, the compaction process slowed down significantly, and the process of pyrocarbon layer buildup dominated. After the implementation of the second stage of processing, analogously to example 1, particles with a density of 1.60 g / cm ³ with a gadolinium content of 28 wt.% Were obtained. This is due, first of all, to the low efficiency of the compaction stage (overestimated in comparison with the nominal values of the pyrolysis temperature and the concentration of CH ₄ ).

Example 4

300 g of an ion exchange resin with a particle diameter of 0.7-1.0 mm were impregnated with an oxalic acid gadolinium solution (1.0 l of a solution with a gadolinium concentration of 0.25 g / l). Drying, heat treatment, densification of the surface of the layer and coating was carried out analogously to example 1. Particles were obtained with a diameter of 0.8-0.9 mm, a density of 1.9-2.0 g / cm ³ with a gadolinium content of 0.1 wt.%, The ratio the diameters of the nuclei to the diameter of the particles was 0.45-0.60.

Example 5

300 g of an ion exchange resin with a particle diameter of 0.7-1.0 mm were impregnated with a gadolinium solution, densified by heat treatment and a surface layer of pyrocarbon was applied as in Example 1. Additionally, the particles were treated at 1450 ° C with a mixture of methyltrichlorosilane, methane and hydrogen in a volume ratio of 0.03: 0.01: 0.96, respectively, for 20 minutes. The coating thickness of the silicon carbide - pyrocarbon composition was 18 μm, i.e. 0.02 particle diameter.

Table 2 provides the characteristics of the abrasion of particles and the results of measuring the abrasion obtained by moving batches of particles weighing 200 g (examples 1-5) in a closed loop of 2 m long, 1 cm in diameter for 600 hours. The coolant particles were moved along the contour with an average linear speed of 0.4 m / s. The characteristic of the abrasion of the particles is the value of the decrease in particle mass after testing (Δm).

table 2
Characteristics of coolant particles and abrasion test results No. D _core , mm D _particles , mm C _Gd , wt.% Δ _SiC-C , μm Δm, mg ρ _frequent , g / cm ³ 1 0.40 0.8-0.9 twenty - 350 1.9-2.0 2 0.40 0.75-0.85 26 - 2 · 10 ⁴ 1.7 3 0.40-0.48 0.9-1.1 28 - 4 · 10 ³ 1,6 4 0.40-0.48 0.8-0.9 0.1 - 400 1.85-1.95 5 0.40-0.48 0.8-0.9 twenty eighteen 150 1.9-2.0

From the data of table 2 it follows that the implementation of the process of obtaining particles of a coolant in examples 1, 4, 5 according to the parameters specified in the claims, allows to obtain a solid finely dispersed coolant with high values of density and strength of particles and low abrasion.

The proposed coolant can be used to operate reactors with a content of neutron-absorbing metals at a level of 1.0-10.0 wt.% In an emergency associated with an increase in temperature and criticality, the proposed coolant with a content of neutron-absorbing metals at a level of 10-30 wt.% Is forcibly sent into the coolant circuit, sharply reducing criticality and, accordingly, core temperature.

Claims (7)

1. A solid finely dispersed coolant for nuclear reactors in the form of spherical particles, characterized in that the spherical particle of a solid finely divided coolant contains a core of amorphous non-graphitized carbon material, which has a diameter of 0.3-0.6 of the diameter of the coolant particle. 2. The solid finely divided coolant according to claim 1, characterized in that the cores contain a metal selected from the group consisting of gadolinium, europium, samarium, and the metal content is selected in the range of 0.10-30.0 wt.%. 3. The solid finely divided coolant according to claim 1, characterized in that the particles contain a surface layer of a composition of silicon carbide and carbon with a thickness of 0.01-0.03 particle diameter. 4. A method of obtaining a solid finely dispersed coolant, comprising compaction of particles by pyrolysis of hydrocarbons in a fluidized bed, characterized in that ion-exchange resins are selected as the starting material for the core of the coolant particles, for example, universal KU-23 cation exchange resin, the resin particles are heated to 1000-1200 ° C and this temperature is treated with a mixture of hydrocarbon and hydrogen, and then at a temperature of 1300-1400 ° C is treated with a mixture of hydrocarbon and argon. 5. The method according to claim 4, characterized in that the treatment at 1000-1200 ° C is carried out with a mixture of methane and hydrogen in a volume ratio of (0.65-0.80) :( 0.35-0.20), and at 1300 -1400 ° C treatment is carried out with a mixture of methane and argon in a volume ratio (0.20-0.25) :( 0.80-0.75). 6. The method according to claim 4, characterized in that the resin particles are impregnated before heating with an aqueous solution of a metal selected from the group consisting of gadolinium, europium, samarium. 7. The method according to claim 4, characterized in that the coolant particles are further treated at a temperature of 1350-1550 ° C with a mixture of methyl trichlorosilane, methane and hydrogen in a volume ratio (0.02-0.05) :( 0.01-0.03 ) :( 0.97-0.92), respectively.