RU2559205C2 - Method of conditioning radioactive wastes of heat-insulating materials - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: in claimed method to obtain magnesia compounds as binding material used is caustic magnesite powder, for instance of grade PMK-87, tempering of materials is realised with magnesium chloride solution, process of magnesia cement mixture is carried out in the following succession. 85÷90 l of MgCl₂·6H₂O solution with density 1.12÷1.15 g/cm³ are supplied into metal 200-litre barrel by means of dosing device. After that, 165÷175 kg of preliminarily crushed heat-insulating material (HIM) with particle size to 1 mm are dosed into barrel with "a" solution in portions with weight 9÷10 kg. After that, 60÷65 of caustic magnesite powder are supplied into barrel, containing mixture "b", in portions with weight 6÷7 kg with constant mixing until homogeneous mixture is obtained.

EFFECT: possibility of obtaining strong water-resistant compounds with high degree of filling heat-insulating materials at rate of leaching caesium-137 from obtained compounds 5-10 times lower than set permissible limit for cement compounds without necessity to add sorption additive.

2 tbl, 2 ex

Description

The invention relates to the field of environmental protection, and more specifically to a technique for conditioning radioactively contaminated wastes of porous fiber insulation materials (TIM) by incorporation into magnesia cement.

During operation, repair work and decommissioning at existing nuclear power plants and other nuclear energy and industry facilities, a large amount of TIM radioactive waste (fiberglass, mineral wool, basalt fiber) is generated, the treatment of which comes mainly to temporary storage .

The bulk of TIM removed during the repair process usually contains activity not exceeding 3.7 · 10 ⁵ Bq / kg for beta-active nuclides and an order of magnitude less for alpha-active nuclides. Contamination of TIM is characterized mainly by the presence of radionuclides such as cesium-134, 137, cobalt-60 and strontium-90. It was experimentally established that, in contrast to other types of building materials (concrete, brick, plaster, etc.), radioactive contaminants sorbed on the surfaces of TIM are characterized by great heterogeneity and are mostly weakly fixed, and, therefore, can pass into groundwater if flooding of storages.

A significant amount and lack of effective methods for processing TIM create serious problems during their storage. The most common way to reduce such waste is by cold pressing.

It is known to use for the processing of radioactive waste TIM at nuclear power plants of domestic cold-pressing plants "Briquette" based on hydraulic presses. Compressed briquettes with dimensions of 400 × 400 × 400 mm are tied with wire and sent for burial. The coefficient of waste reduction with this processing method does not exceed a value of 3 [Babenko Yu.K. Status of RW management at NNPP and prospects for their reliable isolation in the future. - In the book: Collection of reports of the All-Union Scientific and Technical Meeting "Problems of Radioactive Waste Management and Environmental Protection" (ZAES June 17–21, 1991), Moscow, 1992. - S.13 ÷ 16; Barbuzova N.T. Features of SRW handling, methods of their processing and temporary storage at ZAES. - In the book: Collection of reports of the All-Union Scientific and Technical Meeting "Problems of Radioactive Waste Management and Environmental Protection" (ZAES June 17–21, 1991), Moscow, 1992. - S.13 ÷ 16].

It is known to use at a nuclear power plant in England for pressing TIM radioactive waste into bales and directly compressing plants with a plunger driven by compressed air with a force of 8.5 and 10.5 kN. When pressed into bales, the volume of waste was reduced by 2.5 times, and when compressed, by 5 times. Compressed briquettes were placed in barrels and poured with cement [Klyuchnikov A.A. and others. Radioactive waste of nuclear power plants and methods of handling them. - K .: Institute for Safety Problems of NPPs of NAS of Ukraine, 2005. - P.280 ÷ 283].

A known method of processing radioactive waste TIM at the Kursk NPP using a hydraulic press with a nominal force of 960 kN. With its help, TIMs with a volume of more than 4.5 · 10 ⁴ m ³ were pressed into steel barrels with a capacity of 200 l [V. Nikitenko Recycling of low-level waste, including heat-insulating materials, by melting in electric furnaces, see Appendix 16 to the memorial note on the international meeting on radioactive waste and spent nuclear fuel management, June 18–23, 2007, Bohunice NPP, Piestany, Slovakia] .

The main disadvantage of all of the above methods of compaction of TIM by cold pressing is the low degree of fixation of radioactive substances by the surface of heat-insulating materials.

To condition TIM waste, it is necessary to organize additional processes, such as cementing or bitumen, whose matrices provide reliable isolation of radioactive waste from the environment. In all countries of the world with developed nuclear energy, cementing is considered the easiest and cheapest way to cure LRW, SRW and still bottoms, which does not require heating and sophisticated equipment for mixing the ingredients. Common disadvantages of the cementing method are the relatively low degree of inclusion of waste in cement (not more than 30%) and the presence of a large amount of water in the cured product, which together leads to an increase in the volume of final products stored up to 2 times [Kozlov P.V. ., Gorbunova O.A. Cementation as a method of immobilization of radioactive waste. - Ozersk: RIC VRB FSUE "PO" Mayak ", 2011, - S.9].

The identification of effective binders and the development of matrix compositions suitable for the curing of RW of various types, eliminating the above disadvantages, continues to be an urgent task. Air-conditioned, that is, suitable for long-term storage forms of radioactive waste, during storage should have high chemical stability, low rate of leaching of radionuclides with water, mechanical, thermal, radiation resistance. The specified requirements are rigidly fixed in regulatory documents [GOST R 51883-2002 and NP-019-2000]. According to these requirements, cement compounds must satisfy the following acceptable limits of quality indicators: leaching rate of cesium-137, the compounds of which have the highest solubility in water, not more than 1 · 10 ^-3 g / (cm ² · day); mechanical strength (ultimate compressive strength) of at least 4.9 MPa (50 kg / cm ² ); resistance to prolonged stay in water - for 90 days; frost resistance - not less than 30 cycles of freezing / thawing without reducing the permissible compressive strength.

A known method of curing heat-insulating materials contaminated with radionuclides, we have chosen for the prototype, by including in Portland cement with a sorption additive Cambrian clay. The crushed TIM is mixed with 400 grade Portland cement at a water-cement ratio of 0.7 and an clay-cement ratio of 0.1. The manufacture of cement compounds is recommended to be carried out in 200-liter metal barrels on a modular cementing unit developed by specialists of FSUE GP NITI named after A.P. Alexandrova. The density of the resulting compounds is about 1.8 g / cm ³ [Oleinik MS, Epimakhov V.N., Korablev N.A. etc. Air conditioning of heat-insulating materials contaminated with radionuclides. Sixth scientific and technical meeting “Problems and prospects for the development of chemical and radiochemical control in nuclear energy” (Atomenergoanalitika - 2011), September 13-15, 2011 Pinery. Abstracts of reports. SPb .: VVM Publishing House, 2011. - P.23 ÷ 25].

The disadvantages of this method of processing TIM should include:

- low degree of filling with cement compound waste (up to 8%);

- a significant increase in the final volume of the compound in comparison with the original volume of TIM (up to 7 times);

- increased rate of leaching of cesium-137 from compounds during the first 150 days in water (> 1 · 10 ^-3 g / (cm ² · day), which leads to leaching of most of the radionuclides (up to 80 ÷ 85%) during this period time;

- the need for introducing into the matrix composition an additional ingredient in the form of a sorption additive.

The problem to which the invention is directed, is to develop a method for conditioning radioactive waste of TIM with obtaining chemically stable and mechanically strong compounds that meet the requirements of reliable isolation of radioactive waste from the environment, suitable for safe long-term storage or disposal, and allowing:

- increase the degree of filling compounds with waste;

- receive compounds without increasing the initial volume of radioactive waste;

- reduce the leaching rate of ¹³⁷ Cs by at least 5-10 times, from the moment the compounds come into contact with the aqueous medium, which will allow localizing most of the radionuclides (up to 95%) in the cured product;

- to obtain compounds with a cesium-137 leach rate of ≤1 · 10 ^-3 g / (cm ² · day), acceptable according to the requirements of regulatory documents, without inclusion of a sorption additive in the matrix composition.

To solve the problem in a method for conditioning radioactive waste of heat-insulating materials, including the preparation of a cement mixture by dosing into a metal 200-liter barrel of a grinder, pre-crushed radioactive material, mineral binders, mixing the ingredients using a disposable mixer in a barrel until a homogeneous mixture is obtained, exposure to air before turning into a monolith, mixing of the ingredients is carried out with a solution of chloride magnesium, and astringent powder is caustic magnesite powder, for example PMK-87.

The essence of the proposed method for conditioning radioactive waste of thermal insulation materials is that the process of preparation of magnesia cement mixture is carried out in the following sequence:

a) 85 ÷ 90 l of MgCl ₂ · 6H ₂ O solution with a density of 1.12 ÷ 1.15 g / cm ^{3 is} fed into the barrel using a metering device;

b) 165 ÷ 175 kg of TIM with particle sizes of up to 1 mm are dosed in a barrel with solution “a” in portions of 9 ÷ 10 kg each with constant stirring;

c) 60 ÷ 65 kg of caustic magnesite powder is metered in portions of 6–7 kg in a barrel containing mixture “b” with constant stirring until a homogeneous mixture is obtained.

The description of the proposed method for conditioning TIM is illustrated by two examples.

Example 1. Determination of the maximum degree of filling compounds TIM. The experimental technique. Inorganic TIM used at nuclear power plants are mineral and glass wool, which is a material consisting of the thinnest (5 ÷ 6 microns) flexible fibers. Mineral fibers are obtained from silicate melt of rocks (basalt, perlite, andesite, granite, etc.) and blast furnace slag or mixtures thereof. The chemical composition of thermal insulation materials is given in table 1.

Table 1 The chemical composition of thermal insulation materials Material type Chemical composition,% (by weight) SiO ₂ Al ₂ O ₃ Na ₂ O Cao MgO Bao Fe ₂ O ₃ Other Fiberglass 55-59 2-5 11-15 16-22 6-10 - Mineral wool 38-50 5-16 - 20-35 4-9 1-10 - Basalt 49 11,2 - 12,4 4.05 1.43 16.3 6.19

The filaments are pulled from the respective melts by blowing the melt with a gas jet or centrifugal force while feeding the melt to the rotating disks. Such fibers have high tensile strength, chemical resistance, low sound and heat conductivity.

The most contaminated fragments of mineral wool piercing mats of the M-100, M-150 and M-200 brands, which for a long time were located on the surfaces of the equipment of the multiple forced circulation circuit of the Leningrad NPP, were used as samples. TIM fragments were crushed, sieved, and material samples with particle lengths less than 1 mm were used for experiments.

Compounds were prepared as follows: weighed TIM samples were transferred to porcelain cups, then portland cement M-400 and Cambrian clay were introduced in portland cement compounds in the amount of 1.5 g (10% by weight of cement), and for magnesia compounds, weighed portions of magnesite powder caustic grade PMK-87, containing at least 87% magnesium oxide in its composition [GOST 1216-87. Caustic magnesite powders]. Portland cement was shut with industrial water. Caustic magnesite powder was shut with a solution of MgCl ₂ · 6H ₂ O with a density of 1.27 g / cm ³ and additionally, if necessary, technical water was introduced (see Table 2). The contents of the cups were intensively mixed until a homogeneous plastic dough was obtained, which was then used to fill cylindrical split molds (d = 30 mm) made of polystyrene. After a day, the compounds were removed from the molds and dried at room temperature for 8–10 days until a constant final mass of the compound was reached. Next, measurements were taken of the height of the compound, and the degree of filling of TIM compounds was calculated. The experimental results are shown in table.2.

From the analysis of the data presented in table 2, it follows that:

- when cementing TIM samples according to the prototype method with portland cement of grade M-400 (experiment 1), the final volume of the compound increases by 7.5 times compared to the initial volume of waste;

- when magnesia cementation of TIM samples according to the proposed method, the final volume of the compound decreases by 1.20 ÷ 1.25 times compared with the original volume of waste (see experiment 14-17). These results were obtained for TIM samples with a bulk density of 1 g / cm ³ (loose, loose material);

- an increase in the degree of filling of Portland cement compounds TIM> 7.5% (experiments 2 and 3) leads to their formation in the water after 7 ÷ 10 days to form precipitation. In this case, a visual deterioration in the surface quality of the samples is observed. Similar results were obtained by the employees of FSUE VNIINM named after Bochvara, according to their conclusion, the filling of portland cement compounds of TIM can not exceed 8% [Promotional brochure “Compacting radioactive thermal insulation by melting in an induction furnace with a cold crucible (IPCT) and the prospects for using high-temperature methods for processing RAO NPPs, Moscow , 2006, C.15];

table 2 Matrix compositions and basic indicators of compounds during the curing of TIM according to the prototype method and the proposed method Experiment Number The composition of the matrix mixture The main indicators of compounds Mass TIM, g Binder mass The amount of added water, cm ³ The final volume of the compound, cm ³ The final mass of the compound, g The degree of filling of the compound TIM,% one* 1,5 fifteen 6.0 10.8 19.9 7.5 2 3.0 fifteen 7.0 13,2 23.8 12.7 3 4,5 fifteen 7.5 15,5 28,4 15.8 four** 4,5 10 / 5,3 - 8.9 19.2 22.6 5 6.0 10 / 5,6 - 10.3 21.3 28.6 6 7.5 10 / 6.2 - 11,2 24.5 30.6 7 9.0 10 / 6.5 - 12,4 26.1 34.5 8 10.5 10 / 6.7 - 13.8 27,2 39 9 12 10 / 7.0 - 14.3 25.5 52 10 13.5 10 / 7.8 - 14.6 26.5 53 eleven fifteen 10 / 8.0 1,0 15.8 27,2 55.5 12 fifteen 10 / 5,6 3,5 15.8 28,4 52 13 fourteen 10 / 5.0 4.0 17,2 31 45 fourteen fourteen 8 / 4.0 4.0 14.5 26.5 62 fifteen fourteen 6 / 3,5 4.0 12.5 21.8 64 16 fourteen 5 / 3,5 3,5 11.5 20.6 68 17 fifteen 5 / 3,5 4.0 12.0 21,4 70 *) in experiments 1–3, Portland cement was used as a binder; **) in experiments 4–17, magnesia cement was used as a binder, with the PMK-87 mass in grams indicated in the numerator and the volume in cm ³ of MgCl ₂ · 6H ₂ O solution with a density of 1.28 g / cm ³ , selective sorbent was not added.

- with the same mass of cured TIM, the final volume of the compound based on PMK-87 (experiment 4) is 1.7 times less than the volume of the Portland cement compound obtained by the prototype method (experiment 3);

- with a fixed amount of PMK-87 in the cement mixture (10 g) and an increasing content of TIM from 4.5 to 13 ÷ 15 g in the sample, the degree of filling of compounds with radioactive waste increases to 53 ÷ 55.5% (see experiments 10 ÷ 13 );

- the maximum degree of filling compounds with radioactive waste is achieved when the ratio of the mass of TIM to the mass of binder in the matrix mixture in the ratio (2.5 ÷ 3): 1 and is 65 ÷ 70% (see experiments 14 ÷ 17).

Example 2. The main factor determining the safety of disposed radwaste is their leaching, determined by the removal of radionuclides from them.

Cambrian clays of the Leningrad region are not inferior in sorption capacity to bentonite clays, traditionally used as selective sorbents for radionuclides. To obtain porous granular sorbents with high sorption ability, clays are subjected to heat treatment and chemical modification by treatment with acids or alkalis. In the preparation of compounds from Portland cement, granular (2 × 2 mm) samples of Cambrian clay of the Koporskoye field calcined at a temperature of 750 ÷ 850 ° C were used as a sorption additive, which prevented clay from swelling in water while maintaining its sorption properties. Chemical modification of calcined clay granules was carried out by treatment with hot (50 ÷ 95 ° C) 0.5 M HCl and NaOH solutions for 5 ÷ 10 hours [V. Epimakhov et al. Chemical modification of granular selective sorbents for radionuclides based on calcined Cambrian clay. Fifth scientific and technical meeting "Problems and Prospects for the Development of Chemical and Radiochemical Controls in Nuclear Energy" (Atomenergoanalytics - 2009) September 22-24, 2009 Sosnovy Bor. Abstracts of reports. - SPb .: VVM, 2009. - S.7 ÷ 9].

The amount of sorption additives from the processed granules of Cambrian clay, introduced during the experiments according to the prototype method, amounted to 10% by weight of Portland cement grade M-400 in the matrix mixture.

Determination of the leaching rate of cesium-137 from compounds obtained by curing TIM with Portland cement (see table 2, experiment 1) and magnesia cement (see table 2, experiments 15–17) was carried out according to GOST 29114. The compounds were placed in conical flasks and filled with distilled water, which was the leached medium. The experiments were stopped when the activity output for equal periods of time became constant (limit of measurement accuracy ± 10%). The activity of the cesium-137 leachate samples was determined on a gamma-spectrometric setup with a sensor made of highly pure germanium. The leaching rate (R), expressed as the ratio of the activity of radionuclides that went into water within 1 day from 1 cm ^{2 of the} surface of the sample, to the specific activity of the solid material was calculated by the formula:

where: a is the activity of cesium-137, converted to leach, for a certain time interval, Bq;

m is the mass of the sample, g;

A _o - specific activity of cesium-137 in the original sample, Bq;

s is the open "geometric" surface of the sample, cm ² ;

ν is the length of the leaching period, days.

For Portland cement compounds obtained by the prototype method (experiment 1), an increased leaching rate of cesium-137 was observed for 100 ÷ 120 days in water. The decrease in the activity of samples of leachate during this time occurred exponentially. Acceptable under standard documents cesium-137 leach rate (≤1 · 10 ^-3 g / (cm ² · day) to attain these values after 100 ÷ 120 days immersion in water. The percentage activity removed from the cesium-137 Portland cement compounds for the first 100 ÷ 120 day of testing was 80-85%. The data obtained are consistent with the data obtained by the authors of the prototype method.

The leaching rate of cesium-137 from samples of magnesia compounds (experiments 15–17) in the time interval of 30–120 days remained virtually unchanged and amounted to (1–2) · 10 ^-4 g / (cm ² · day). The percentage of remote activity of cesium-137 from magnesia compounds for 120 days of testing was no more than 4 ÷ 4.5%. This figure is almost 20 times less compared to the prototype method.

The proposed method for conditioning radioactive waste of porous fiber insulation materials allows to obtain durable waterproof compounds with a high degree of filling of TIM up to 70%. The leaching rate of cesium-137 from the resulting compounds without adding a sorption additive is 5–10 times lower than the established allowable limit for cement compounds ≤1 · 10 ^-3 g / (cm ² · day).

The proposed method can be carried out on the same equipment as when using Portland cement. The method is industrially applicable, since PMK-87 and caustic magnesite powders of other grades are produced on an industrial scale. It should be borne in mind that when testing the TIM conditioning technology using a specific cementing unit, it will be necessary to match the dimensions and design of the mixer with the speed of its rotation in the barrel, at which a uniform plastic dough will be obtained without stopping mixing of the mixer (or breakage) due to possible critical excess viscosity of the magnesia mixture.

Claims (1)

A method for conditioning radioactive waste of heat-insulating materials, including the preparation of a cement mixture by dosing into a metal 200-liter barrel of a grinder, pre-ground radioactive heat-insulating material, mineral binders, mixing the ingredients using a disposable mixer in a barrel until a homogeneous mixture is obtained, exposure to air up to conversion into a monolith, characterized in that the mixture is mixed with a chloride solution agnya, binder is caustic magnesia powder, and process of preparation of a cement mixture is carried out in the following sequence:
a) 85 ÷ 90 l of MgCl ₂ · 6H ₂ O solution with a density of 1.12 ÷ 1.15 g / cm ^{3 is} fed into the barrel using a metering device;
b) 165 ÷ 175 kg of TIM with particle sizes of up to 1 mm are dosed in a barrel with solution “a” in portions of 9 ÷ 10 kg each with constant stirring;
c) 60 ÷ 65 kg of caustic magnesite powder is metered in portions of 6–7 kg in a barrel containing mixture “b” with constant stirring until a homogeneous mixture is obtained.