RU2362225C2 - Stabilising material and method of producing thereof - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to compositions and methods of producing stabilising material, used in atomic power engineering for retaining spent nuclear fuel and solid radioactive wastes of atomic power stations. The stabilisation material contains a magnesium phosphate binder and a powdered part. The powdered part contains aluminium oxide (Al₂O₃), magnesium oxide (MgO) and a boron-containing component. The stabilising material contains the following, wt %: magnesium phosphate binder 56-66; powdered part 44-34; composition of the powdered part: Al₂O₃ 78-90; MgO 22-10; boron containing component (in terms of boron) 0-1; composition of magnesium phosphate binder: P₂O₅ 30-40; MgO 7-8.5; H₂O makes up the rest. The method of producing stabilising material involves mixing the powdered part with the binder at temperature not above 25°C, obtaining a fluid mixture with viscosity of 300-1000 cSt with subsequent solidification of the stabilising material. The mixture is used for retaining spent nuclear fuel or for other purposes during the "life-span" of the mixture in liquid state.

EFFECT: longer "life-span" in liquid state, increased mechanical strength and reduced acidity.

11 cl, 2 tbl

Description

The invention relates to compositions and methods for producing a stabilizer material used in nuclear energy for fixing spent nuclear fuel (SNF) and curing liquid and solid radioactive waste. The fixation of spent nuclear fuel with a self-curing stabilizer material allows for rigid fixation of nuclear fuel and prevents spillage of fissile material during its transportation and storage. Of the binders, phosphate cements are of greatest interest for use as a stabilizing material. Phosphate cements are readily soluble in acids, which greatly simplifies the reprocessing of spent fuel after aging.

Known phosphate cements, hardening at room temperature, which have found application as dental cements and refractory concrete, consisting of a mixing fluid (binder) and a powder part (S.L. Golynko-Wolfson, M.M.Sychev, L.G. Sudakas , LN Skobko. Chemical fundamentals of technology and the use of phosphate binders and coatings. - Chemistry Publishing House, 1968, 188 pp .; P. P. Budnikov, L. V. Khoroshavin. Refractory Concrete with Phosphate Binders. - "Metallurgy", M., 1971; V. A. Kopeikin, A. P. Petrova, I. L. Rashkovan. Materials based on metallophosphates. - Publishing house "Chemistry", M., 1976, 200 pp .; M.M.Sychev. Inorganic adhesives. - “Chemistry”, 1986, 157 pp .; US 2937101 A, 05.17.1960; SU 420585 A, 05.26.1972). The mixing liquid is a solution of phosphoric acid, aluminophosphate binder, as well as solutions of phosphates of various metals. The powder part of such cements is zinc oxide, magnesite, alumina, calcined dunite, chromite and various additives. The disadvantages of phosphate dental cements and refractory concrete are the short “life time” in the liquid state and the exceptionally high viscosity of the mixture.

The closest in technical essence to the claimed stabilizer material is zinc phosphate cement (preservative) (RU 2271588 C2, 04/12/2004), consisting of aluminophosphate binder and powder based on zinc oxide with additives of oxides of various metals. The ratio of mass fractions of powder to aluminophosphate binder is 1: 2. The disadvantages of the proposed material are low pH in the hardened state, a small "lifetime" in the liquid state and a slow increase in strength during hardening.

The aim of the invention is the creation of a stabilizer material for fixing spent nuclear fuel and solidification of liquid and solid radioactive waste, devoid of the above disadvantages. The goal is solved by the proposed stabilizer material, including a magnesium phosphate binder and a powder part, consisting of aluminum oxide (Al ₂ O ₃ ), magnesium oxide (MgO) and a boron-containing component (BC).

The stabilizer material has a composition, wt.%:

magnesium phosphate binder 56-66; powder part 44-34;

composition of the powder part:

Al2O ₃ 78-90; MgO 22-10; BC (in terms of B) 0-1;

composition of magnesium phosphate binder:

P ₂ O ₅ 30-40; MgO 7-8.5; H ₂ O rest

An increase in the content of a magnesium phosphate binder in a mixture of more than 66% significantly reduces the mechanical strength of the stabilizer material. An increase in the content of the powder part in the mixture of more than 44% leads to a significant decrease in the lifetime of the mixture in the liquid state. In accordance with the requirements for the stabilizer material, its liquid life should be at least one hour. During this time, it is necessary to fill the liquid with SNF stabilizer and prepare equipment for subsequent operations.

An increase in the Al ₂ O ₃ content in the powder part of more than 90% leads to a significant increase in the viscosity of the mixture, which complicates its use for fixing SNF. An increase in the MgO content in the powder part of more than 22% significantly reduces the lifetime of the mixture in the liquid state.

The concentration of P ₂ O ₅ in the binder within 30-40% corresponds to the region of maximum solubility of MgO in phosphoric acid. A decrease or increase in the concentration of P ₂ O ₅ compared with the limiting values leads to a decrease in the concentration of MgO in the binder and, consequently, to a decrease in the mechanical strength of the stabilizer material. The binder has a pH of 0.8-1.5 and a viscosity of 70-150 cSt.

Alumina with a specific surface area of 50-150 m ² / g and a granule size of less than 0.1 mm is used as alumina. Alumina not only serves as a filler, but also due to the chemisorption of phosphates on the surface of primary particles, it contributes to the creation of a supersaturation system in the cementing phase, which is hydrated disodium magnesium phosphate MgHPO ₄ · 3H ₂ O.

As magnesium oxide, fused magnesite with a granule size of less than 0.1 mm is used. Caustic magnesite powder, dunite and other magnesium-containing raw materials can also be used, which are preliminarily annealed at temperatures above 1500 ° C to reduce the reactivity of magnesium oxide.

A method of obtaining a stabilizer material involves mixing the powder part with a binder to form a fluid suspension with a viscosity of 300-400 cSt. With increasing time, the viscosity of the mixture increases to 1000 cSt and it loses fluidity and sets. The time from the beginning of mixing to the moment of setting corresponds to the lifetime of the mixture in the liquid state (setting time). During this time, a liquid stabilizer material with a viscosity of 300-1000 cSt should be submitted for SNF fixation. Further, in the stabilizer material, which has lost fluidity, hardening processes occur, characterized by an increase in mechanical strength.

The hardening process of the stabilizer material is carried out at a temperature of 15-100 ° C and relative humidity of 70-100%. An increase in temperature of more than 100 ° C leads to significant mechanical stresses in the material due to different temperature coefficients of expansion of the liquid and solid phases of the mixture. Lowering the temperature below 15 ° C is associated with significant costs for cooling the mixture. At a relative humidity of less than 70%, the uniformity of the surface and inner layers of the material is significantly violated.

When the powder part is mixed with a binder, a chemical interaction of magnesium oxide with phosphoric acid, which is present in the binder, occurs, accompanied by concentrated heat. The rate of a chemical reaction, and therefore the lifetime of a mixture in a liquid state, is very dependent on temperature. In order to avoid heating the mixture, it is necessary to organize heat removal so that the temperature of the stabilizer material in the liquid state does not exceed 25 ° C. As a powder part, a mechanical mixture of alumina, magnesium oxide and boron-containing component powders is used. Separate mixing of the powdered components is possible. First, alumina is mixed with a binder, and then a mechanical mixture of magnesium oxide and boron-containing component is added. To reduce the heating of the mixture, the binder is cooled to a temperature of 5-18 ° C before the introduction of the powder part.

Example 1. For the preparation of the stabilizer material, alumina was used ground alumina grade G-00 with a granule size of less than 0.1 mm, manufactured by OJSC SU AL, a branch of UA3-SU AL, Kamensk-Uralsky, Sverdlovsk area. Alumina had a specific surface area of 90 ± 5 m / g, determined by low temperature nitrogen adsorption. Fused periclase, a dusty fraction of the PPP-85 brand with a granule size of less than 0.1 mm, manufactured by Ogneupory OJSC, Bogdanovich, Sverdlovsk Region, was used as magnesium oxide. Magnesium phosphate binder was prepared using a powder of magnesite caustic grade PMK-87, GOST 1216-87, manufacturer of OJSC “Combine Magnesite”, Satka, Chelyabinsk region and thermal phosphoric acid of 70% concentration, grade B, GOST 10678-76. A binder was obtained by dissolving at 80-100 ° C magnesite powder in an orthophosphoric acid solution of 45.7-55% concentration. In the binder, the magnesium concentration was determined by the trilonometric method, and phosphate ions with preliminary precipitation in the form of ammonium phosphoromolybdate in nitric acid (G. Charlot. Methods of analytical chemistry. Quantitative analysis of inorganic compounds. T.2, Publishing House "Chemistry", M., 1969 g). The optimal composition of the stabilizer material, wt.%:

magnesium phosphate binder 61; powder part 39;

composition of the powder part:

ground alumina; brand G-00 84.8; fused periclase PPP-85 14.5; boron oxide (in terms of boron) 0.8;

composition of magnesium phosphate binder:

P ₂ O ₃ 33.1; MgO 8; H ₂ O rest

The volumes of the mass being simultaneously sealed were small: for 25 ml of a magnesium phosphate binder having a density of 1.46 g / cm ³ , the calculated amount of the powder part was added as a mechanical mixture. Mixing was carried out in a water thermostat, having previously cooled the binder to 16 ° C, with continuous stirring. Due to the course of the chemical reaction, the temperature of the mixture increased and it was maintained within 19-20 ° C during the entire mixing time. After 30 minutes, the liquid mixture was poured into cylindrical polyvinyl chloride tubes with a diameter of 1 cm and a height of 1.2 cm, sealed on one side with adhesive tape and kept to harden. Then, the second end part of the tube was sealed with adhesive tape and the samples were placed for aging at room temperature in a hermetically sealed vessel. At certain time intervals, the hardened stabilizer material in the form of cylinders was removed and the mechanical compressive strength (P _m ) was determined in accordance with GOST 24409-80 using an RPG-10 hydraulic press with an ISTS-1 digital force meter.

In the remaining part of the liquid sample, the presence and setting time (T _cx ) were controlled. During the setting time was taken from the beginning of mixing to the moment of setting. The presence of setting was recorded at the moment when the sample interface, placed in a vertical position, did not change its shape for 30 seconds.

To determine the viscosity of the composition in the liquid state, the volume of the sealed sample was increased four times. The viscosity of the mixture was determined using a viscometer B3-246, GOST 9075-75.

The acidity of the stabilizer material in the hardened state was evaluated by the pH value of the aqueous extract. For this, the samples were ground, a weighed sample of the ground sample in the amount of 10 g was poured into 30 ml of demineralized water, kept for one day with stirring, and the pH value was determined.

When a powder part is mixed with a binder of the above composition, a homogeneous flowing suspension is formed having a viscosity of 350 cSt. With increasing exposure time, the viscosity of the mixture increases and, finally, it loses the fluidity of setting. The "life time" of the stabilizer material in the liquid state was 1.6 hours. The mechanical compressive strength of the hardened stabilizer material after one day exposure at room temperature turned out to be 110 ± 15 kg / cm ² , and the pH value of the aqueous extract was 5.5, which is close to a neutral environment.

Examples 2-8. Table 1 shows the effect of temperature on the "life time" of the stabilizer material in the liquid state (T _cx ), which has the composition and preparation method, as in Example 1. Mixing was carried out in a water thermostat at different temperatures of cooling water.

Table 1
The effect of temperature on the "lifetime" of the stabilizer material in the liquid state Example Number Temperature ° C T _c , h 2 fourteen 2.7 3 19 1,6 four 23 1,2 5 27 0.8 6 33 0.5 7 39 0.4 8 49 0.1

At a mixture temperature of 14 ° С, the “lifetime” is 2.7 hours. With an increase in temperature, the “lifetime” is significantly reduced and at 49 ° С it is 0.1 hours. To provide T _cx about one hour the temperature of the mixture in the liquid state should not exceed 25 ° C.

Example 9. The composition of phosphate cement, wt.%:

binder 67 powder part 33

composition of the powder part:

ground alumina G-OO 89 fused periclase PPP-85 eleven

The composition of the magnesium phosphate binder and the preparation procedure, as in example 1. The setting time of such a composition was 3.5 hours. However, the strength of daily cement was at the limit of sensitivity of the determination method and amounted to 7 ± 2 kg / cm ² , which is clearly not enough to use it as a stabilizing material.

Examples 10-14. Table 2 shows the main properties of the stabilizer material in the liquid and solid state (setting time (T _c ), mechanical strength after one day exposure (P _m ) and the pH of the water exposure) depending on the MgO content in the powder part. The composition of the binder and the preparation procedure, as in example 1. From the table it follows that by changing the composition of the stabilizer material, it is possible to change its properties over a wide range.

table 2
Stabilizer material properties Example Number The content of the powder part,% The MgO content in the powder part,% T _c , h P _m , kg / cm ² pH 10 38,0 eleven 2,3 45 4.8 eleven 38.6 13 1.8 80 5,0 12 39.2 fifteen 1,6 110 5.5 13 39.7 16.7 1.3 130 6.3 fourteen 40,2 18,4 1.06 135 7.2

Thus, the proposed composition and method for producing a stabilizer material can significantly increase the "lifetime" in the liquid state, the mechanical strength in the solid state and reduce acidity.

Claims (11)

1. The stabilizer material for fixing spent nuclear fuel and curing liquid and solid radioactive waste from nuclear power plants, including a magnesium phosphate binder and a powder part, including aluminum oxide (Al ₂ O ₃ ), magnesium oxide (MgO) and boron-containing component (BC), characterized in that the stabilizer material has a composition, wt.%:
magnesium phosphate binder 56-66 powder part 44-34
composition of the powder part:
Al ₂ O ₃ 78-90 MgO 22-10 BC (in terms of boron (B)) 0-1
composition of magnesium phosphate binder:
P ₂ O ₅ 30-40 MgO 7-8.5 H ₂ O rest 2. The stabilizer material according to claim 1, characterized in that alumina with a specific surface of 50-150 m ² / g and a granule size of less than 0.1 mm is used as alumina. 3. The stabilizer material according to claim 1, characterized in that fused periclase powder with a granule size of less than 0.1 mm is used as magnesium oxide. 4. The stabilizer material according to claim 1, characterized in that the magnesium-containing raw materials used are magnesium-containing raw materials annealed at temperatures above 1500 ° C. 5. The stabilizer material according to claim 1, characterized in that boron-containing compounds with a granule size of less than 0.1 mm are used as a boron-containing component. 6. The stabilizer material according to claim 1, characterized in that the magnesium phosphate binder has a pH value of 0.8-1.5 and a viscosity of 70-150 cSt. 7. A method of obtaining a stabilizer material according to any one of claims 1 to 6 by mixing the powder part with a binder followed by curing the mixture, characterized in that the mixing process is carried out at a temperature not exceeding 25 ° C to obtain a fluid mixture with a viscosity of 300-1000 cSt which is used to fix spent nuclear fuel or to solidify radioactive waste during the “lifetime” of the mixture in a liquid state. 8. The method according to claim 7, characterized in that a mechanical mixture of aluminum oxide, magnesium oxide and boron-containing component is introduced into the binder. 9. The method according to claim 7, characterized in that the alumina is first introduced into the binder, and then the magnesium oxide and boron-containing component. 10. The method according to claim 7, characterized in that the binder is cooled to a temperature of 5-18 ° C before mixing with the powder part. 11. The method according to claim 7, characterized in that the hardening of the stabilizer material is carried out at a temperature of 15-100 ° C and a relative humidity of 70-100%.