RU2498430C2 - Silica-alumina filter for high-temperature chemical adsorption of caesium isotope vapours - Google Patents

Abstract

FIELD: power engineering.

SUBSTANCE: invention relates to the field of processing of gaseous radioactive wastes, namely, to high-temperature chemical adsorption with a silica-alumina filter, vapours of radioactive caesium isotopes, forming during thermal treatment of caesium-containing radioactive materials. Chemical adsorption of caesium vapours is carried out on a silica-alumina filter with a disordered structure, specific surface of up to 101 m²/g, open porosity of up to 84 vol.% and content of amorphous phase of up to 95 wt %. The filter is made of porous light-weight chamotte of grade ShL-0.4, both source one and previously thermally treated at 1350-1500°C for 3 hours. The filter is made in a cylindrical mould that is convex at the ends with concentric grooves on them.

EFFECT: invention makes it possible to increase efficiency of a filter when catching caesium vapours.

3 cl

Description

The invention relates to the field of neutralization of gaseous radioactive waste (RAW) generated during the regeneration of irradiated nuclear fuel (SNF), during the solidification of liquid high-level waste (HLW) and in the high-temperature synthesis of glass and ceramic cores of ionizing radiation sources (III).

Heat treatment of spent nuclear fuel, vitrification of liquid HLW, as well as high-temperature synthesis of glass and ceramic cores of cesium-containing III are accompanied by the volatility of its vapors.

Given the strict standards for permissible emissions of radioactive cesium isotopes into the environment, the efficient capture of its vapors in the gas cleaning system is an important technological and environmental task.

Two methods of trapping cesium vapor are fundamentally possible, differing both in the nature (nature) of the process and the place of its implementation in the technological scheme:

1. “Wet” capture - low-temperature condensation of cesium vapors in a gas purification system (scrubber type apparatus: bubblers, packed and spraying venturi columns, centrifugal washers, etc.). This method has the following disadvantages:

- the formation in the gas treatment system of secondary liquid radioactive waste;

- radioactive contamination of equipment and communications, leading to additional exposure of personnel;

- significant energy costs to obtain high cleaning ratios.

2. “Dry” capture - high-temperature chemisorption of cesium vapor by aluminosilicate filters located in the heat treatment zone of cesium-containing radioactive materials. The amorphous aluminosilicate phase of these filters at high temperatures (not lower than 700 ° C) has a high reactivity with respect to cesium vapors, which allows it to be fixed in stable aluminosilicate crystalline phases.

The volatilization of gaseous and volatile radioactive isotopes, in particular cesium, is accompanied by the processes of heat treatment of spent nuclear fuel [Beznosyuk VI, Galkin B.Ya. and others // Radiochemistry, 2007, vol. 49, No. 4, p. 344-338].

For the curing of liquid HLW, the two-stage vitrification processes, including the calcination of liquid HLW and the vitrification of calcine, are most widely used. To capture particles of calcine, aerosols and vapors of volatile radioactive isotopes, in particular cesium, in the vitrification of liquid HLW, a gas cleaning system is used, which includes scrubbers, condensers, droplet eliminators (HEME and MERA filters), aerosol filters (metal-ceramic, metal-fabric, fiberglass), adsorbers and other elements [Design and Operation of Off-Gas Cleaning Systems at High Level Liquid Waste Conditioning Facilities, Technical Reports Series No. 291, IAEA, Vienna, 1988, p.31-48]. In the head elements of the gas cleaning system (scrubbers, condensers), cesium vapors are trapped due to low-temperature condensation ("wet" capture).

The industrial-scale two-stage vitrification process for liquid HLW AVM AVM [Design and Operation of Off-Gas Cleaning Systems at High Level Liquid Waste Conditioning Facilities, Technical Reports Series No. 291, IAEA, Vienna, 1988, p. 48-52] includes their calcination rotary tube furnace and vitrification of calcine in an induction heating melter. The vapor-gas mixture (ASG) generated in calcine and melter is sent to a scrubber, in which aerosol particles of calcine are dissolved in a stream of boiling nitric acid. The resulting solution is returned to the calciner for processing after mixing with the original liquid HLW. From the scrubber, the PGS is sent to a condenser (to remove water vapor and partially nitric acid), an absorption column (to recombine nitric acid), a wash column, coarse and fine filters and is released into the atmosphere. This set of equipment provides a high degree of ASG from aerosols and volatile radioactive isotopes. In particular, the total coefficient of CGS removal from cesium is 1.2 · 10 ⁹ . In this case, up to 1.4% of the cesium from the vitrification received is captured in the condenser. This condensate belongs to secondary liquid HLW, which need vitrification. In addition, the constant recycling of cesium scrubber (up to 10% of the vitrified) into the initial liquid HLW leads to its constant concentration (accumulation) in them.

Currently, the most rational way of handling liquid HLW is considered to be extraction fractionation, which leads to re-extracts of cesium, strontium, transplutonium and rare earth elements [Romanovsky V.N. The selection of long-lived radionuclides from highly active waste, Ecological chemistry, 10, issue 1, 2001, S. 42-49]. The problem of volatility of cesium vapors is especially relevant for the vitrification of a separate fraction of liquid HLW - a re-extract of cesium, as well as for high-temperature synthesis of glass and ceramic cores of cesium-containing III, in which the concentration of radioactive cesium isotopes is very high.

Closest to the claimed one is “dry” capture of cesium vapors generated during the oxidation (high-temperature oxidation) of spent nuclear fuel, with a filter made of hollow aluminosilicate microspheres that are part of the fly ash generated by burning coal in thermal power plants [Shin JM, Park JJ, Song K.-Ch. Cesium Trapping Characteristics on Fly Ash Filter According to Different Carrier Gas / Proc. of intern. Conf. "Global'07", Boise, Idaho, USA, Sep.9-13, 2007, p.610-614]. This aluminosilicate filter, having a corpuscular, relatively ordered structure, specific surface area of 9.1 m ² / g and open porosity of 26 vol.%, Provides high-temperature chemisorption of cesium vapor, which allows it to be fixed in stable aluminosilicate crystalline phases CsAISiO ₄ and CsAlSi ₂ O _{6 6} (half-cell).

The process of separation of hollow aluminosilicate microspheres, the concentration of which in fly ash does not exceed 1.2 wt.% [Kizilstein L.Ya., Dubov IV, Shpitsgluz AL, Parada SG Components of ashes and slag of a thermal power plant. - M .: Energoatom publ., 1995. - 176 p.], Includes: flotation, magnetic, particle size and gravity separation.

The manufacturing process of aluminosilicate filters of a cylindrical or any other form from hollow microspheres includes the following stages: forming blocks from a plastic mixture (hollow microspheres, silicate binder, wetting agent), drying and firing.

The disadvantages of the aluminosilicate filter from hollow microspheres are the low specific surface area and open porosity, as well as the high cost due to the complexity and multi-stage process of isolating a fraction of hollow microspheres of the required composition and quality from flying ashes, which are waste products from the energy industry.

An object of the present invention is to increase the efficiency of an aluminosilicate filter in capturing cesium vapors that are formed during the heat treatment of cesium-containing radioactive materials. Another objective of the present invention is to simplify and reduce the cost of manufacturing an aluminosilicate filter.

To solve the tasks, it is proposed to use a filter made of porous lightweight chamotte ШЛ-0.4 [GOST 5040-96], the original or heat-treated at 1350-1500 ° C for 3 hours, as an aluminosilicate filter for trapping cesium vapor.

Lightweight refractory brick ШЛ-0.4 (chamotte) is produced in large volumes by the method of pricing from plastic refractory clay by Russian enterprises (Snegirevsky refractories, JSC, Borovichi Refractories Plant OJSC, etc.).

Fireclay is used for heat-insulating lining of units of various types in ferrous and non-ferrous metallurgy, mechanical engineering, petrochemicals. Due to its low density and thermal conductivity, sufficient mechanical strength, chamotte is a structural material that can reduce the weight of the lining, the overall dimensions of the furnaces and heat loss.

The main characteristics of fireclay in relation to the solution of tasks:

- structure - spongy, disordered;

- specific surface area - 101.0 m ² / g;

- open porosity - 84.0 vol.%;

- the content of aluminosilicate amorphous phase is 80 wt.%

- ultimate compressive strength - 1.1 MPa.

After saturation with pairs of radioactive isotopes of cesium, filters need periodic remote replacement with the help of manipulators. To increase the mechanical strength of chamotte, it was proposed to heat treat for 3 hours at 1350-1500 ° C. Heat treatment of chamotte in the indicated temperature range leads to an increase in compressive strength by 1.5-3.4 times and the content of the amorphous phase by 12-15 wt.% . At the same time, the specific surface area decreases by 8.3-22.1 rel.% And open porosity by only 2.7-10.5 vol.%, While the values of these characteristics remain high.

To increase the geometric surface, it is proposed to produce a chamotte filter in a cylindrical form, concave from the ends with concentric recesses on them. The proposed shape of the filter has a working surface of ~ 1.3 times more than a simple cylindrical shape.

To increase the cesium filter capacity by a factor of 2, it is proposed to use both of its end surfaces. After saturation with cesium vapor of one hemispherical surface, the filter is proposed to be rotated around its horizontal axis by 180 °, and then continue to capture cesium vapor by another hemispherical surface.

Compared with the prototype, the advantage of the inventive filter is its higher cesium capacity. Another advantage of the proposed filter is the availability and relatively low cost of the source material - porous lightweight chamotte brand ШЛ-0,4, as well as the ease of manufacturing from it a filter of the necessary configuration by machining.

The rationale for the use of the inventive filter for high-temperature chemisorption of vapors of cesium isotopes are the following examples and tables.

Example 1

Filters (mass ~ 12 g, volume ~ 30 cm ³ ) made of chamotte (open porosity 84.0 vol.%, Specific surface area 101.0 m ² / g, compressive strength 1.1 MPa, amorphous phase content 80 wt.%), checked in static mode in the laboratory.

As sources of cesium vapors, cesium-137 doped with cesium-137 was used, the calcination of which at 900 ° C and above ensures high volatility of its vapors.

The cesium nitrate crucible was placed in a stainless steel beaker, on which two filters were installed: the lower (working) one for trapping cesium vapor, and the upper (control) one for detecting their leakage through the lower filter.

The assembly "glass-crucible with cesium nitrate-two filters" was placed in a resistance furnace, which was heated to 900 ° C. After 24 hours exposure at 900 ° C, one end surface of the working filter sorbed ~ 2 g of Cs or ~ 0.07 g of Cs / cm ^{3 of the} filter. In terms of the radioactive isotopes of cesium, these values are ~ 55 Ci (2 TBq) / filter and ~ 2 Ci (0.07 TBq) / cm ³ filter, respectively.

After the working filter flipped 180 ° around the horizontal axis, its other end surface sorbed approximately the same amount of cesium in 24 hours. Thus, the total amount of adsorbed cesium was ~ 4 g / filter or 0.13 g of Cs / cm filter. In terms of cesium isotopes, these total values are ~ 110 Ci (4 TBq) / filter or ~ 3.7 Ci (0.14 TBq) / cm ³ filter.

According to x-ray phase analysis, cesium was fixed in the working filter in the crystalline phases CsAlSiO ₄ and CsAlSi ₂ O ₆ (pollucite).

No cesium-137 was detected in the control filter, i.e. the escape of its vapor through the working filter was absent.

Example 2

The filters were checked as described in the example, but the chamotte was heat treated at 1350 ° C for 3 hours.

Heat treatment of chamotte led to an increase in compressive strength by 1.5 times (from 1.1 to 1.6 MPa) and the content of the amorphous phase by 12 wt.% (From 80 to 92) and to a decrease in the specific surface by 8.3 rel. % (from 101.0 to 92.6 m ² / g) and open porosity of 2.7 vol.% (from 84.0 to 81.3). The results of the trapping of cesium vapor filters at 900 ° C for 48 hours are similar to those described in example 1.

Example 3

The filters were checked as described in example 1, but the fireclay was heat treated at 1500 ° C for 3 hours

Heat treatment of chamotte led to an increase in compressive strength by ~ 3.4 times (from 1.1 to 3.7 MPa) and an amorphous phase content of 15 wt.% (From 80 to 95) and to a decrease in specific surface area by 22.1 rel. % (from 101.0 to 78.7 m ² / g) and open porosity of 10.4 vol.% (from 84.0 to 73.6).

The results of the trapping of cesium vapor filters at 900 ° C for 48 hours are similar to those described in example 1.

The effect of heat treatment of chamotte on the main characteristics of the filters is given in table 1

Table 1 The effect of heat treatment of chamotte on the main characteristics of the filters T, ° C The limit of compressive strength, MPa Specific surface, m ² / g Open porosity, vol.% 1300 ^*) 1,1 101.0 84.0 1350 1,6 92.6 81.3 1400 1.9 86.6 78.8 1450 2,4 82,4 75.8 1500 3,7 78.7 73.6 1550 11.0 53.0 64.7 1600 17.4 26.1 31.3 ^*) 1300 ° C - chamotte firing temperature during its industrial production

From table 1 it can be seen that heat treatment of chamotte at 1350-1500 ° C for 3 hours led to an increase in compressive strength by 1.5-3.4 times and a decrease in specific surface area by 8.3-22.1 rel. % (from 101.0 to 78.7 m ² / g) and open porosity of 2.7-10.4 vol.%. Heat treatment of chamotte at 1550-1600 ° C was accompanied by an increase in compressive strength by 10-16 times, but uneven shrinkage in height, cracking, and a decrease in specific surface area by 1.9-3.9 times and open porosity by 19.3-52, 7 vol.%.

The effect of heat treatment on the phase composition of the chamotte is presented in Table 2.

table 2 The effect of heat treatment on the phase composition of chamotte T, ° C Phase composition, wt.% amorphous phase α-SiO ₂ (cristobalite) α-SiO ₂ (quartz) 3Al ₂ O ₃ · 2SiO ₂ (mullite) 1300 * ⁾ 80 10 5 5 1350 92 one 2 5 1400 95 0 0 5 1450 1500 ^*) 1300 ° C - chamotte firing temperature during its industrial production

From table 2 it is seen that an increase in the temperature of heat treatment of chamotte from 1300 to 1350 ° C leads to an increase in the content of the amorphous phase by 12 wt.% (From 80 to 92), and an increase in temperature from 1350 to 1500 ° C - only by 3 mass. % (from 92-95). Cristobalite and quartz are gradually amorphized during heat treatment. The content of mullite in chamotte in the studied temperature range does not change.

As can be seen from the examples and tables, the inventive aluminosilicate filter can increase the capacity for cesium vapor by 8-11 times. In addition, the filter is made of affordable and relatively cheap material - porous lightweight chamotte grade ШЛ-0.4 industrial production.

Claims (4)

1. Aluminosilicate filter for high-temperature chemisorption of vapors of cesium isotopes formed during the heat treatment of cesium-containing radioactive materials, characterized in that the filter is made of a material with a spongy, disordered structure, specific surface area up to 101 m ² / g, open porosity up to 84 vol.% And the content of the amorphous phase to 95 wt.%. 2. The aluminosilicate filter according to claim 1, characterized in that the filter is made of porous chamotte of industrial production. 3. The aluminosilicate filter according to claim 2, characterized in that the filter is made of porous chamotte, previously heat-treated at 1350-1500 ° C for 3 hours 4. The aluminosilicate filter according to claim 2 or 3, characterized in that the filter is made in a cylindrical shape, concave from the ends with concentric recesses on them.