RU2790580C2 - Method for production of mineral-like matrix for immobilization of highly active waste - Google Patents

Abstract

FIELD: recycling.

SUBSTANCE: invention relates to recycling of highly active waste (HAW) by immobilization into mineral-like matrices (hereinafter – MLM) for use as part of radio-chemical production specialized on recycling of spent nuclear fuel. The method includes supply of liquid radioactive waste (hereinafter – LRW) and mineral-forming additives to a heated plate pelletizer. Dosing of LRW is carried out until reaching a set ratio of radionuclides and MLM components. The process is carried out at a temperature of a working surface of the pelletizer, not exceeding a melting or decomposition point of the least thermally stable metal nitrate included in LRW. Formed granules are continuously removed from the pelletizer by self-unloading and transmitted directly to a melting unit. Synthesis of the final product is carried out in a quasi-continuous mode by ingot extraction. The resulting compact material can be subjected to annealing for improvement of its physical and chemical properties.

EFFECT: invention allows for reduction in dust removal due to elimination of a dust-like fraction, when pelletizing granules, energy consumption, corrosion and abrasion wear of equipment, when curing LRW in MLM.

6 cl, 8 dwg, 1 tbl, 3 ex

Description

The field of technology to which the invention belongs

The invention relates to methods for processing HLW by immobilization in MPM, resistant to external influences and suitable for long-term controlled storage or final disposal.

State of the art

Known security document RU 2203512 C2 "Method of solidification of liquid radioactive waste and a device for its implementation" (FGUP "VNIINM named after A.A. Bochvar"), in which preliminary preparation of LRW and fluxing components is carried out for feeding into a cold crucible in order to obtain a vitrified forms. LRW is mixed with fluxing additives, the resulting product is dehydrated at a temperature of 110-160°C to a melt with a moisture content of about 20% in a compact once-through coil evaporator, after which the suspension is fed into a cold crucible. The disadvantage of this method lies in the moisture content of the material fed into the crucible. On the one hand, it is very low, which makes it difficult to transfer the melt through the pipeline and can cause blockage, on the other hand, it is high at the time of supply of the melt to the mirror, which requires additional energy to remove moisture.

An invention aimed at improving the quality of mixing waste with a glass former, which ensures continuous supply of the mixture to the melter without the formation of suspension plugs in pipelines, is described by RU 2432630 C2 "Method of preparing high-level liquid waste for vitrification" (JSC VNIINM). Nitric acid is first added to LRW, and then mixed with aqueous silica sol to obtain borosilicate glass with a silicon oxide content of 35-55 wt. %. Depending on the acidity, the resulting sol may not sediment from several days to several months, which makes it possible to transport it through pipelines without the risk of delamination. The disadvantage of this method also lies in the high moisture content of the material supplied to the melt mirror, exceeding 50%, which leads to unsustainable costs of the melter's thermal energy for dehydration of the charge and degrades the productivity of the process.

The method of supplying a watered charge to the melter can be used not only for HLW vitrification processes, but also for the synthesis of MPM; Radioactive waste (RW) containing 0.01 g / dm ³ uranium, 0.01 g / dm ³ plutonium, 0.01 g / dm ³ neptunium, 0.27 g / dm ³ zirconium and rare earth elements 0.8 g / DM ³ evaporated to a residual moisture content of 30-45 wt. %, after which it is mixed with oxides of titanium, calcium, zirconium, aluminum and barium with a total content of components, wt. %: evaporated RAO (in terms of oxides) - 20, TiO ₂ - 55, CaO - 8, ZrO ₂ - 8, Al ₂ O ₃ - 4 and BaO - 5, the resulting mixture is mixed with an ion exchange resin, taken in an amount of 6 wt . % by weight of the mixture, bring the moisture content of the obtained carbonaceous mixture to 12 wt. % and served on the surface of the melt of zirconolite and perovskite, having an operating temperature of 1500°C at a pressure below atmospheric pressure (the most stringent conditions). After the supply is completed, the mixture of carbon-containing charge and melt of zirconolite and perovskite is kept for 3-15 minutes (in the case of induction heating) or at least 1 hour (in the case of other types of heating) until a homogenized melt of the final product is obtained, after which it is cooled to form a monolithic Synroc ceramic with RAO included in it. The disadvantage of this method is the presence of a thermally unstable organic component, the rapid decomposition of which due to contact with the melt can be explosive and lead to the formation of foreign (metal, carbide or oxycarbide) inclusions in the matrix due to carbothermal reduction of its components.

At the same time, in practice, methods are used that provide for the supply of completely dehydrated material to the melter. An example is RU 2531637 C2 "Method of processing nitrogen-containing liquid aqueous waste using calcination and vitrification" (Areva NS). Preliminary evaporation and calcination of nitrate LRW to obtain an oxide product is carried out in a rotary dryer at 400°C, after which the calcinate is loaded into a cold crucible and melted to obtain a compact glass block.

In a similar way, LRW can also be prepared for the synthesis of MPM, which is shown in RU 2164716 C1 “Method of solidifying liquid radioactive waste and a device for its implementation” and RU2160937C1 “Monolithic block for immobilization of liquid radioactive waste” (SSC RF “VNIINM named after A.A. Bochvar). Processed wastes are dehydrated and calcined in a high-temperature spray dryer at 600-800°C, which provides a pure oxide product, which is sent together with powdered mineral-forming components to IPCT. Melting is carried out at a temperature of 1250-1800°C, the resulting mineral-like product may consist of phases of aegirine, jadeite, aegirine-augite, arfvedsonite, orthite, sherlite, andradite, lovchorrite. The disadvantages of this approach include abrasive wear of the equipment caused by contact with a solid material, and an increased load on the local gas cleaning system due to intense dust entrainment caused by ascending flows from the melter cut. Also, in the case of using rotary calciners, there is a problem of corrosion damage to the hot surfaces of equipment in contact with LRW, which are nitric acid solutions.

Patent RU2212069C2 “Method of curing solutions of long-lived radionuclides” (FGUP “PA Mayak”) is devoted to the solution of the problem of reducing dust entrainment during the transportation of material and its remelting in order to obtain MPM. LRW is contacted with a crystalline sorbent based on zirconium and titanium dioxide and then evaporated to dryness at a temperature of 90-130°C to obtain a given saturation with radionuclides in one or more contacts of the solid material and the solution. As a result of superstoichiometric sorption, a free-flowing intermediate product is formed, which can be transferred into matrices consisting of one or more silicon-free minerals of zirconium and titanium of the type (Zr, Me)O ₂ (stabilized zirconia), (Ti, Me)O ₂ (rutile ), BaAb(Ti ₆ , Me)O1 ₆ (hollandite), Ca(Me, Ti)O ₃ (perovskite), Ca(Zr, Me, Ti ₂ )07 (zirconolite), their mixtures and others. The sorbent saturated in this way can be remelted by IPCT and does not contribute to dust formation. Significant disadvantages include the high cost and limited availability of inorganic sorbents based on titanium and zirconium oxides, the cyclical nature of the process, the complexity of automating transport operations for transferring the original sorbent to the reaction vessel and extracting the saturated intermediate from it (prototype).

Disclosure of invention

The objectives of the invention are to reduce dust entrainment, energy consumption, corrosive and abrasive wear of equipment during solidification of LRW in MPM. This is achieved by the fact that, in contrast to the known technical solutions, processed LRW are dosed at a certain rate into a heated plate pelletizer together with powdered mineral-forming components in the form of oxides and/or their multicomponent systems. In the pelletizer, in addition to the formation of granules, the continuous evaporation of moisture and other volatile components of LRW is carried out, the scull layer formed on the working surface of the pelletizer protects it from abrasive wear and corrosion by acid solutions. The resulting granules are characterized by high mechanical strength and resistance to abrasion, providing good transportability and the absence of a dusty fraction, which increases the load on gas cleaning systems both during transportation and when fed to the melting unit. Obtaining a compact MMM is carried out in a quasi-continuous mode by remelting the granules, unloading the compact fused material - by pulling the ingot. The sequence of operations for obtaining MPM for LRW immobilization is shown in Fig. 1.

Description of drawings

The invention is illustrated by the following figures:

Fig. 1 - Scheme of the process of immobilization of HLW in the MRM, where ... - operations are not mandatory, they are performed if necessary.

Fig. 2 - Scheme of an experimental stand for the synthesis of a granular precursor, where

1 - peristaltic pumps;

2 - solution injectors;

3 - regulating autotransformer;

4 - container for LRW simulator solution;

5 - container for auxiliary reagents;

6 - plate pelletizer with a heated plate;

7 - vector frequency converter.

Fig. 3 - X-ray diffraction pattern of a compact alloyed material based on orthorhombic titanate and rutile (33 wt.% Nd2O3).

Fig. 4 - Dynamics of neodymium leaching from a compact alloyed material based on orthorhombic titanate (33 wt.% Nd2O3).

Fig. 5 - X-ray diffraction pattern of a compact fused material with the pyrochlore structure Nd2ZrTiO7 (~62 wt.% Nd2O3).

Fig. 6 - Dynamics of neodymium leaching from a compact fused material with the pyrochlore structure Nd2ZrTiO7 (62.3 wt.% Nd2O3).

Fig. 7 - X-ray diffraction pattern of a compact fused material with pollucite CsAlSi2O6 and tausonite SrTiO3 structures.

Fig. 8 - Dynamics of cesium and strontium leaching from compact fused material with pollucite CsAlSi2O6 and tausonite SrTiO3 structures.

In FIG. 1 shows the sequence of operations for obtaining a compact MSM; in fig. 2 shows a diagram of a granular precursor production unit; in fig. 3 shows a X-ray diffraction pattern of an orthorhombic titanate matrix (33 wt % Nd ₂ O ₃ ); in fig. 4 shows the dynamics of leaching of neodymium (radionuclide simulator) from a matrix based on orthorhombic titanate (33 wt.% Nd ₂ O ₃ ); in fig. 5 shows the X-ray diffraction pattern of the MSM of the target composition Nd ₂ ZrTiO ₇ ; in fig. 6 dynamics of neodymium leaching (simulator of radionuclides) from MPM of target composition Nd ₂ ZrTiO ₇ ; in fig. 7 shows the X-ray diffraction pattern of MPM based on structural analogues of pollucite and tausonite; in fig. Figure 8 shows the dynamics of cesium and strontium leaching from MPM based on structural analogues of pollucite and tausonite.

Implementation of the invention

Processed LRW, which is a nitric acid solution containing fission products, is continuously fed into a heated plate pelletizer together with powdered mineral-forming components. The composition and amount of inactive solid components is selected individually based on the isotopic composition of the waste, the required degree of inclusion of radionuclides, the target final form of HLW, the expected heat release and other requirements. If necessary, auxiliary substances - binders, acidity correctors - can be supplied to the pelletizer through separate lines with an independently controlled flow rate. The temperature of the working surface of the pelletizer should not exceed the melting or destruction point of the least stable metal nitrate, which is part of the processed high-level waste (not more than 400°C). The scull layer formed on the working surface of the pelletizer protects it from abrasive wear and corrosive attack by acid solutions. In the process of granulation, intensive evaporation of residual moisture, nitric acid vapors, stripping and destruction of organic components (if any), formation and enlargement of granules removed from the pelletizer by self-unloading directly into the melting unit take place simultaneously. Obtaining a compact fused material is carried out in a high-frequency induction furnace, followed by drawing the ingot.

The effectiveness of the proposed method is confirmed by the results of tests carried out on inactive simulators of fractionated HLW formed during the processing of spent nuclear fuel: REE-actinide fraction; cesium-strontium fraction. To obtain granules, an installation based on a disk pelletizer was used, the scheme of which is shown in Fig. 2.

The strength of the granules was tested by destructive analysis using an IPG-1M strength meter (JSC UNIKHIM s OZ, Russia). Experiments on the remelting of granules into compact matrices were carried out in a high-frequency induction furnace with a cold crucible VCHI-80, the main characteristics of which are presented in Table 1.

The resulting matrices were analyzed using a Dron-4M X-ray diffractometer (PO Burevestnik, USSR), data interpretation was carried out using the Match! (Cryallimact Gmbh., Germany). Hydrolytic stability was determined according to the method given in GOST 29114-91 “Radioactive waste. A method for measuring the chemical stability of solidified radioactive waste through long-term leaching. Example 1

The objective of the experiment was to synthesize a matrix based on orthorhombic titanate for immobilization of rare-earth-actinide containing ~3 wt. _{% Nd2O3} . The granules were obtained by rounding the mixture with 5 wt. % ZrO ₂ +95 wt. % TiO ₂ with acidified nitrate solution 200 g/DM ³ Nd ³⁺ .

The compressive strength of the granules, determined as the arithmetic average of 10 measurements, was 3.61 MPa, which makes it possible to organize the transfer and their loading into the melter without the risk of destruction and the formation of significant amounts of dust.

The diffraction pattern of the compact fused material is shown in FIG. 3. As can be seen, the product is represented by a composition consisting of a non-stoichiometric phase, structurally identical to the compound Nd _1.68 Ti ₂ O _6.52 , and titanium dioxide in the form of rutile.

The results of hydrolytic stability tests in two parallel experiments are presented in FIG. 4. As can be seen, on the 28th day of the test, the neodymium leaching rate decreases to a level of ~5.1×10 ^-9 g/(cm ² × day), which significantly exceeds the requirements for matrices during plutonium immobilization (≤10 ^-7 g/ (cm ² × day)).

Example 2

The objective of the experiment was the synthesis of MPM for the immobilization of the REE-actinide fraction structurally identical to pyrochlore with the formula Nd ₂ ZrTiO ₇ containing ~62 wt. _{% Nd2O3} . The granules were obtained by rounding a mixture consisting of 60.5 wt. % ZrO ₂ and 39.5 wt. % TiO ₂ with an acidified solution of neodymium nitrate 400 g/DM ³ Nd ³⁺ .

The compressive strength of the granules, determined as the arithmetic average of 10 measurements, was 3.27 MPa, which makes it possible to organize the transfer and their loading into the melter without the risk of destruction and the formation of significant amounts of dust.

The diffraction pattern of the compact fused material is shown in Fig. 5. As can be seen, the synthesized MPM is represented by REE zirconates with the pyrochlore structure Nd ₂ Zr2O ₇ and the oxygen-deficient analog NdZrC _3.05 . The results of testing hydrolytic stability in two parallel experiments, presented in Fig. 6 show that on the 28th day of exposure, the neodymium leaching rate decreases to a level of <1.5×10 ^-8 g/(cm ² × day), which exceeds the requirements for matrices for plutonium immobilization (<10-7 ^g /( cm ² × _С ut)).

Example 3

The objective of the experiment was to synthesize a composite matrix consisting of structural analogs of pollucite CsAlSi ₂ O ₆ and tausonite SrTiO ₃ for immobilization of the cesium-strontium fraction. The granules were obtained by pelletizing a mixture of powders containing 17.6 wt. % TiO ₂ , 57.6 wt. % SiO ₂ , and 24.8 wt. % Al ₂ O ₃ , with a nitrate solution containing 104 g/dm ³ Cs ⁺ and 25.6 g/dm ³ Sr ²⁺ .

The compressive strength of the pellets, determined as the arithmetic average of 10 measurements, was 3.29 MPa, which makes it possible to organize the transfer of pellets and their loading into the melter without the risk of destruction and the formation of significant amounts of dust.

The diffraction pattern of the compact fused material is shown in FIG. 7. Target phases structurally identical to pollucite CsAlSi ₂ O ₆ and tausonite SrTiO ₃ have been identified. The results of measurements of hydrolytic stability in two parallel experiments, presented in Fig. 8 show that the leaching rates on the 28th day of exposure are ~1.02×10 ^-7 and ~4.48×10 ^-8 g/(cm ² × day) for strontium and cesium, respectively, which is significantly lower than the requirements for final forms for HLW immobilization.

Claims (6)

1. A method for producing mineral-like matrices (MLM) for immobilization of high-level waste (HLW), in which liquid radioactive waste (LRW) is continuously fed into a heated tray pelletizer together with powdered mineral-forming components, and the waste is fed until the required ratio of radionuclides and inactive solid components is reached , the composition of which is selected based on the isotopic and chemical composition of the waste, the required final form of HLW and the planned heat release, and the temperature of the working surface of the pelletizer is maintained at a level not exceeding the melting or decomposition temperature of the least thermally stable metal nitrate, which is part of the LRW, the agglomerated product is continuously removed from pelletizer by self-unloading and transferred directly to the melting unit or to intermediate storage for hardening, further remelting is carried out in an induction furnace, followed by unloading of the compact fused material by bending of the ingot, after which the compact material can be annealed. 2. The method according to claim 1, characterized in that the processed LRW, which are nitric acid solutions containing fission products, is continuously fed into a heated plate pelletizer together with powdered mineral-forming components in the form of oxides and / or pre-obtained oxide systems of the required composition. 3. The method according to claim 1, characterized in that the amount of inactive solid components is selected individually based on the chemical and isotopic composition of the waste, the required degree of inclusion of radionuclides, the target final form of HLW, and the expected heat release. 4. The method according to claim 1, characterized in that, if necessary, auxiliary substances - binders, acidity correctors - can be supplied to the pelletizer through separate lines with independently controlled flow rates. 5. The method according to claim 1, characterized in that the remelting of the agglomerated product to obtain a compact MPM is carried out in a high-frequency induction furnace with a cold/hot crucible. 6. The method of claim. 1, characterized in that the compact fused material can be subjected to annealing to increase its crystallinity, hydrolytic stability and strength.

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