RU2203512C2 - Method and device for immobilizing liquid radioactive wastes - Google Patents

Abstract

FIELD: immobilizing liquid radioactive wastes. SUBSTANCE: liquid radioactive wastes undergo pre- immobilization treatment, then they are mixed up with fluxing dopes to produce phosphate glass blocks, concentrated, and heated; mixture is melted, melt is poured into tanks, cooled down, and blocks are formed from them. In the process concentration is effected by evaporating wastes in continuous flow at 110-160 C and melting is made at 1000-1800 C with direct exposure to induction filed at frequency of 0.3-1.8 MHz. Device for immobilizing liquid radioactive wastes has tank 1 holding liquid radioactive wastes, tank 2 holding fluxing dopes, concentration apparatus incorporating cylindrical case 5, tubular spiral coil 6, and cylindrical separator 7. Melting unit has mixing adapter 8, metal melting apparatus 9, drain assembly 11, and partition 12. Gas-cleaning unit has steam and acid vapor condenser 16, nitric oxide neutralizer 18, coarse and fine gas filters 17. Device is also provided with coolant producer 19, vacuum pump 20, automatically controlled valves 21, and remote connectors 22. EFFECT: enhanced environmental safety in keeping radioactive wastes in storage for long time. 9 cl, 1 dwg, 2 tbl

Description

 The invention relates to the protection of the environment, and in particular to the technology of solidification of liquid radioactive waste that is generated during the regeneration of spent nuclear fuel, as well as during other radiochemical and metallurgical processes.

 Ensuring human life in an environmentally friendly environment is impossible without solving the problem of the neutralization of liquid radioactive waste (hereinafter LRW) generated during the processing of nuclear fuel. One of the main tasks is to significantly reduce the volumes (concentration) of radioactive waste, as well as transfer them into a form convenient for reliable long-term storage (500-1000 years), during which the decay of radioactive isotopes occurs. So, when reprocessing irradiated nuclear fuel, LRW is formed containing components of various technological solutions and corrosion products of technological devices containing radionuclides - fission fragments, nuclear fuel residues and transuranic elements. Currently, the bulk of LRW is accumulated and stored in special storage tanks and only a small part of LRW is vitrified.

 The storage of radioactive waste without reprocessing does not meet modern environmental safety requirements and is a possible source of technogenic radiation accidents. The transfer of LRW into compact solid forms suitable for transportation, storage and disposal will significantly reduce this risk.

 As matrices for incorporating high-level waste, a wide range of high-temperature compositions is known, including glass-like and crystalline materials.

A known method of curing LRW, including their evaporation to 40-80% of residual moisture, mixing with a clay substance containing cement, molding a mixture, drying granules at 150 ^o C, calcination at 800 ^o C and firing at 1400 ^o C for 10-20 hours to obtain a substance such as ceramics (German patent 2726087, class B 03 9/06, 1978). The known method does not allow to obtain highly radioactive waste (hereinafter HLW) in a sufficiently chemically stable and mechanically strong matrix.

 A known method of solidification of liquid high-level waste, including the evaporation of liquid HLW, the direction of the spray dryer to obtain a powdery material, then calcination in the furnace to decomposition of nitric acid, vitrification in the melting chamber of the furnace.

 Here is the installation for implementing this method, which consists of a collection of LRW, a supply tank, an evaporator, a spray dryer, annealing and melting chambers, combined in a single unit. (Japanese Application 60-24440, CL G 21 F 9/16, publ. 12.06.85).

 The method described in this patent is the closest to the claimed one and is selected as a prototype. The disadvantage of this method is that the used case-type evaporator does not allow for deep concentrated concentration, for which the authors are forced to use an additional apparatus - a spray dryer. In addition, the case-type melter used has a very limited resource due to direct contact of the aggressive melt with the melter material.

 A device for the vitrification of radioactive waste containing ion-exchange resins, including a container for LRW, a capacity of a dehydration apparatus, a dehydration apparatus connected to a separator and a condenser, a mixer tank connected to a dehydration apparatus, a container with glass-forming agents and a LRW tank (RF patent 2115182, class G 21 F 9/16, published 09.09.1997). The mixer tank is connected to a storage tank, which is connected through an eddy apparatus to an induction furnace to produce a vitrified mass. The induction furnace contains a "cold" crucible with a movable inductor. The drain device consists of a drain pipe with a water-cooled jacket, a top cover and a water-cooled drain valve. At the same time, a water-cooled shirt of the drain pipe is built into the body of the cold crucible. The outlet of the furnace is connected in series with the filtration system, the condenser of the filtration system, the capacity of the filtration system, an absorption unit, a heater, a catalytic reactor and a condenser of a catalytic reactor.

 A disadvantage of the known device is that by design it is intended for the treatment of medium- and low-level waste and cannot be used for high-level waste, since most devices (LRW tank, dehydration device capacity, dehydration device, heterogeneous LRW tank, heterogeneous LRW dispenser , storage tank, feeder) is equipped with electric motors that are rapidly destroyed in the radiation fields created by HLW. In working with HLW, all equipment should be replaced remotely in the event of a malfunction, which the known device does not provide. In addition, the disadvantages of the known device are the high residual moisture content of the resulting concentrate and the frequency of the concentration operation, which reduces the overall performance of the device.

 The closest in technical essence to the claimed device is a device for vitrification of LRW, including a container of homogeneous LRW, a container of additives, a mixer container equipped with a dispenser, a dewatering apparatus (rotary calciner), a dust collector, a container with a glass former, a lock dispenser for glass former, a metal crucible with a fixed inductor, having a bottom drain pipe, equipped with a cooling jacket and its own inductor, as well as an exhaust gas neutralization unit, including triad-deduster, filter system condenser, filter system capacitor capacity and gas purification (A. S. Nikiforov, V. V. Kulichenko, M. I. Zhikharev. Neutralization of liquid radioactive waste. M: Energoatomizdat, 1985, p. 85, 93 , 94).

 The known device operates as follows.

LRW from the tank of homogeneous LRW, as well as additives for calcination, are fed into a mixer tank equipped with a dispenser, from where the resulting mixture is fed to a dehydration apparatus, consisting of a rotating pipe, inside which there is a device for grinding solid calcine. The dehydration apparatus is heated by a resistance furnace, divided into four zones. The waste gases from the dewatering apparatus are sent to a dedusting device, where the dust extracted from the gas is collected in a dust collector and sent to a mixer tank. Dust-free exhaust gases are sent to the condenser of the filtration system and to the gas filtration system and released into the atmosphere. The condensate from the condenser of the filtration system is collected in the capacitor of the filter system and sent to the tank of homogeneous LRW. Glass-forming agents from a container with glass-forming agents are supplied together with a liquid sludge batcher together with LRW calcine exiting the dehydration apparatus to a metal crucible with a fixed inductor. The first portion of LRW calcine and glass former is used to create a starting melt in a metal crucible, after which subsequent portions of LRW calcifier and glass former are fed to the melt surface, and the exhaust gases from the metal crucible are sent to a dehydration apparatus. The melt of the finished radioactive glass obtained at 1100-1150 ^o C, is carried out periodically after the accumulation of 120 kg of glass in the crucible through the bottom drain pipe. To ensure the reliability of the drain, the latter is carried out with the inductance of the bottom drain pipe turned on. At the end of the drain, after about 5 kg of the glass melt remains in the crucible, the bottom drain pipe inductor is turned off and water is supplied to the cooling jacket of the bottom drain pipe, due to which a glass plug is formed, and a single LRW vitrification cycle is completed, after which the above cycle is repeated.

 The disadvantages of the known device are the increased danger of radioactive contamination due to the complexity and insufficient reliability of the equipment operating in conditions of intense radiation, as well as the use of bulky and complex equipment for dehydration of LRW and the use of a melter with heating the melt from its wall, which (due to direct contact with melt) quickly corrodes. In addition, the device has low productivity due to the return of condensate from the gas purification system to the tanks of the initial solution for recycling, as well as due to the lack of melt mixing in the crucible and the frequency of the melting process. In the known device there are no conditions for the remote replacement of used-up contaminated apparatuses.

 The technical task of the invention is to provide a method for curing LRW of various compositions, suitable for the remote manufacture of glass and mineral-like blocks, providing environmentally safe storage of radioactive substances for a long time. The technical task of the invention also lies in the creation of high-performance, economical, small-sized equipment with an increased resource and with remotely removable and replaceable modules.

This problem is solved in that in the claimed method of solidification of liquid radioactive waste, including their preparation, concentration and mixing with fluxing additives, heating the resulting mixture, melting, draining the melt in the tank, cooling the melt and forming blocks, concentration is carried out by evaporation of LRW in a continuous stream at a temperature of 110-160 ^o C, and melting is carried out at a temperature of 1000-1800 ^o C with direct exposure to an induction field with a frequency of 0.3-1.8 MHz on the melt.

 In particular, the technical problem is solved by the fact that in the manufacture of blocks of phosphate glass, the mixing of waste with fluxing additives is carried out before the concentration stage.

 In particular, the technical problem is solved in that the mixture is melted at a melt viscosity of from 20 to 80 dPa • s and the melt electrical resistance is from 0.1 to 1.5 Ohm / cm.

 The problem is also solved by the fact that in the device for solidification of liquid radioactive waste (including a container for liquid radioactive waste, a concentration apparatus, an induction melter equipped with a drainage unit, a container with fluxing additives, equipped with a dispenser, a mixer, an exhaust gas purification unit, a piping system and fittings), the concentration apparatus is made in the form of a cylindrical body, inside of which a cylindrical separator is coaxially located, between the separator and the housing a spiral tubular coil is laid, one end of which is connected with a container of liquid radioactive waste and the other with a separator, the separator is communicated in the upper part with an exhaust gas treatment unit, the induction melter is made in the form of a water-cooled crucible with side walls and a bottom transparent for alternating electromagnetic field and bottom through the mixer communicated with the concentration apparatus and the flux dispenser.

 In particular, the technical problem is solved in that the tubular coil is made of two interconnected in the upper part of the tubular spiral sections - ascending, communicating with the capacity of radioactive waste, and descending, communicating with the separator.

In particular, the technical problem is solved in that the inner diameter of the tubular coil is selected from the following ratio:
d = k • Q • 10 ^-2 ,
where d is the diameter of the coil in cm,
k is a coefficient of 0.15-0.3,
Q is the productivity of the device in the initial solution in l / h.

 In particular, the technical problem is solved by the fact that the side walls and the bottom of the body of the water-cooled crucible consist of a set of parallel water cooling tubes sealed with corrosion-resistant dielectric ceramics.

 In particular, the technical problem is solved in that the distance between the opposite walls of the crucible is chosen from 0.21 to 0.55 m

 In particular, the technical problem is solved by the fact that the concentration apparatus and induction melter are made in the form of remotely disassembled and replaceable modules.

The process of concentrating LRW by evaporation in a continuous stream at a temperature of 110-160 ^o C in the evaporator allows you to intensify heat transfer, increase the productivity of the device due to faster dehydration of the solution in one pass through the apparatus. Concentration of LRW in a once-through evaporator allows one to obtain a concentrate in the form of a melt with a moisture content of less than 20%.

The melting process at a temperature of 1000-1800 ^o With allows you to get blocks of any glass-like and crystalline materials with high chemical resistance, capable of holding radionuclides for a long time in a burial environment.

 The melting process with the direct influence of the induction field on the melt with a frequency of 0.3-1.8 MHz allows the interaction of the induction field with the melt in the optimal mode with minimal energy and time. Carrying out the process at an induction field frequency below 0.3 MHz does not allow the heat energy necessary to melt the starting material to be released. Carrying out the process at a frequency above 1.8 MHz leads to unreasonably high energy costs and, as a result, to an increase in the cost of the final product.

 Carrying out the melting process at a melt viscosity of 20 to 80 dPa • s provides electrodynamic mixing of the melt in the crucible and thereby reduces the process time and increases productivity. When the melt viscosity is less than 20 dPa • s, the volatility and ablation of radionuclides sharply increase, the thickness of the skull layer on the crucible walls decreases, which is undesirable. At a melt viscosity of more than 80 dPa • s, electrodynamic mixing of the melt practically ceases, which leads to a significant increase in the process time and a decrease in productivity.

 The implementation of the melting process with an electrical resistance of the melt from 0.1 to 1.5 Ohm / cm allows you to optimize the melting process. So an increase in electrical resistance of more than 1.5 Ohm / cm dramatically reduces the performance of the device, and a decrease in electrical resistance below 0.1 Ohm / cm leads to local overheating of the melt, which negatively affects the disposal of radionuclides.

 The use as a concentration apparatus of a spiral direct-flow evaporator, which does not have mechanical moving parts, increases the reliability and service life of the entire device.

 The implementation of the tubular coil of the evaporator with the ascending and descending sections allows to reduce the dimensions of the device by increasing the surface and intensifying heat transfer and to ensure its remote dismantling, transportation for storage in standard containers and thereby reduce the environmental hazard of the exhausted device.

 The choice of the diameter of the coil depending on the performance of the device according to the initial LRW solution allows to optimize the concentration of LRW, increase productivity and thereby reduce the overall dimensions of the device while maintaining high performance.

 The use of a crucible in an induction melter, the side walls and the bottom of which are drawn from water cooling tubes sealed with corrosion-resistant dielectric ceramics, reduces the energy lost to the melt and, due to the formation of a skull from the cooled melt on the inner surface of the crucible, eliminates corrosion of the melter material, which significantly increases the resource and melter safety.

 The minimum distance between the walls of the crucible 0.21 m is determined by ensuring industrial productivity. Crucibles less than 0.21 m wide do not provide optimal use of electrical power. When the distance between the walls of the crucible is more than 0.55 m, the central part of the melt bath is poorly heated. In addition, the maximum melt bath size of 0.55 m is limited by the size of the containers into which the spent crucibles are placed.

 The flue gas cleaning unit is a single chain of apparatuses connected to a vacuum pump, which creates a vacuum in the entire apparatus chain.

 The use of a complex of remotely replaceable direct-flow evaporator and induction melter devices allows to reduce the material consumption of the device and to reduce the amount of contaminated solid waste after the devices have reached the end of their life.

 The drawing shows a General view of the device.

 A device for the solidification of liquid radioactive waste includes the following apparatus units: LRW preparation unit containing a container for LRW 1, containers with fluxing additives 2 of solution 3, intermediate flux storage devices 4; the LRW concentration unit includes a concentration unit (direct-flow evaporator) consisting of a cylindrical body 5, a spiral tubular coil 6 and a cylindrical separator 7; the melting unit includes an adapter-mixer 8, the metal melter is a “cold” crucible 9 with a fixed inductor 10, a drain unit 11 and a partition 12 separating the cooking 13 and storage 14 zones, the melt receiver 15; gas treatment unit includes a condenser of water vapor and acid 16, a neutralizer of nitrogen oxides 18, filters for coarse and fine purification of gas 17. Low-level LRW from the capacitor 16 and the neutralizer 18 are processed according to a separate standard scheme. The device is equipped with a coolant generator 19, a vacuum pump 20, automatically adjustable valves 21, remote connectors 22.

A device for curing LRW works as follows. To prepare a monolithic block, the waste located in the LRW tank 1 is transferred to the tank of the initial fluxed solution 3, mixed with fluxing additives located in the tank 2 with compressed air, then the fluxed waste is supplied under pressure of compressed air or by means of a metering pump through automatically controlled valves 21 to the concentrating apparatus (once-through evaporator) at the input of the spiral tubular coil 6 located in the cylindrical body 5. Before starting the supply of the LRW solution, it is preheated to temperature 110-160 ^o With the coolant from the generator 19 is fed into the cavity between the cylindrical body 5 and the separator 7 of a direct-flow evaporator for heating the coil, after heating it, the initial fluxed LRW solution enters the coil, where it occurs sequentially as the product passes through an upward spiral and then along a downward spiral, its heating to 110-160 ^o C and evaporation before turning into a salt melt. Next, the product enters the separator 7, where the vapor-gas phase and the salt melt are separated due to the centrifugal effect and a sharp decrease in the flow rate. Grain salt is directed by gravity along the walls of the separator to a water-cooled melting crucible 9 (in the production of phosphate glass blocks) or to a mixing adapter 8 (if necessary, further fluxing of the initial LRW in the case of obtaining a borosilicate glass or mineral-like composite). The vapor-gas phase from the separator 7 enters the gas treatment system of the exhaust gas neutralization unit 16, where it is cleaned of harmful impurities in a chain of apparatuses connected to a vacuum pump 20. From the reservoir 2 through intermediate accumulators 4, which operate alternately, which avoids the penetration of radionuclides from the apparatuses into a clean zone, in the adapter-mixer 8 receives fluxing additives in the amount necessary to obtain the final product in the form of a monolithic block of a certain composition and structure. The quantitative ratios of melt and fluxing additives are set by the speeds of the ingredients, which ensure that a given matrix is obtained after the melting process. The starting or reheating of the cooled melt in the crucible is carried out by placing an electrically conductive starting material into the crucible through the adapter 8 from the accumulator 4 and applying a high-frequency electromagnetic field to the crucible.

 Interacting with the electromagnetic field, the starting material is heated and melts the charge around it, forming a molten bath. In this case, the electrical resistance of the melt decreases, reaching a value at which thermal energy is generated directly in the melt. The upper limit of the electrical resistance of the melt is recommended 1.5 Ohm / cm with a melt viscosity of 20-80 dPa • s, at which electromagnetic mixing of the melt occurs. A further decrease in the viscosity of the melt leads to a thinning of the skull layer and an increase in heat loss. The finished mixture of ingredients from the adapter-mixer 8 is fed into the crucible on the surface of the formed melt. In the melting chamber, the residual water and nitric acid are sequentially evaporated, the components of LRW are calcined and decomposed, nitrogen oxides are distilled off, melting and matrix synthesis are performed. The finished melt is poured through a water-cooled drain unit 11 into a container 15 and cooled.

 As LRW to be cured and disposed of, solutions were taken after regeneration of spent nuclear fuel from power reactors and separation of valuable components (uranium, plutonium, REE, etc.). The principle of selecting the composition of the matrix composition for incorporating radionuclides is based on the maximum use of macro components of LRW for the synthesis of the future matrix by melting.

 In the table. 1 shows the technological modes of the manufacturing process of blocks from various types of materials.

 In the table. 2 provides information on the specific and total productivity of the device depending on the parameters of the device, the diameter of the tubular coil and the internal size of the melter. Samples 1, 3, and 5 were processed in rectangular crucibles. Samples 2, 4 and prototype processed in crucibles of circular cross section.

 Unit capacity is given in liters per hour of the processed LRW solution. The specific productivity of the device was calculated as the performance of the LRW solution, referred to the area of the mirror of the melting chamber, that is, the dimensions of the apparatus were taken into account.

 As can be seen from the above examples, a wide range of radioactive waste was subjected to reprocessing, while the type of matrices was built with the maximum use of LRW components, which increases the efficiency of the process for producing compositions.

 As can be seen from the data shown in tables 1 and 2, the device performance is from 50 to 200 l / h for the initial LRW solutions, in contrast to the prototype, which was 18 l / h, which is due to the advantages of the proposed method and device. The parameters are specified in the source data and confirmed by testing in the given examples (tab. 1, 2). In this case, the overall dimensions of the device, established by the maximum size (width, height) of any of the blocks (concentration unit, melting unit) to be replaced, correspond to the internal dimensions of standard containers sent for intermediate controlled storage and disposal. The prototype device does not provide for interchangeability of individual blocks. The device of the present invention is equipped with special remote connectors (drawing, item 22), and its weight corresponds to the performance of standard manipulators, which makes it possible to remotely mount and dismantle it with movement to containers sent to storage.

The melter used in the prototype is large, it is not designed for remote mounting and dismounting and, together with hardened radioactive glass, remains for long-term storage at its place of work. In addition, its productivity in the processed fluxed solution is 18 l / h, and the specific productivity in the solution of such a device is 2.5 l / dm ² • h. The proposed device according to a two-stage scheme for curing LRW using a "cold" crucible and with dimensions corresponding to the parameters of standard containers has a capacity for the initial HLW solution from 50 to 200 l / h. Moreover, its specific productivity is in the range from 8 to 10 l / dm ² • h, that is, 4 times higher than that of the prototype (see table. 2). The small dimensions of the device according to the proposed method also mean low material consumption of the apparatus for installation.

 In addition, the performance of the device is limited exclusively by the specified dimensions acceptable for the manipulators and the container, and not the method used. The increase in the size of the device with a water-cooled crucible can significantly increase the overall performance of the device as a more highly efficient.

Thus, the proposed invention allows:
- To process a wide range of liquid LRW, with virtually no restrictions on their composition and the presence of corrosive components (iron, nickel, sulfur, etc.) in the compositions.

 - Maximize the use of liquid LRW components to create the structure of a monolithic solidified waste block.

 - To receive radioactive waste in the form of compounds capable of retaining radionuclides for a long time in a burial environment.

 - Increase the specific productivity of the device due to a more effective method of curing LRW (table. 2).

 - Use simple devices without moving working bodies and thereby increase reliability and increase the overhaul life of the device.

- To synthesize a variety of materials at temperatures of 1000-1800 ^o C (table. 1), while it becomes possible to synthesize analogs of minerals, high specific productivity of synthesis is achieved, due to the formation of a skull between the walls of the melter and the aggressive melt of the material, the life of the apparatus is increased.

 - Reduce the dimensions of the device and, accordingly, reduce the material consumption and cost of the device, make it remotely replaceable, and after running out of life, delete it for intermediate storage and disposal due to efficient preliminary concentration and high-temperature synthesis of materials with high specific productivity.

 - Reduce the amount of secondary radioactive waste.

 The present invention solves the problem of curing LRW of various compositions in glass and mineral-like blocks, providing environmentally safe storage of radioactive substances for a long period, made in an economical, small-sized device with high performance and with remotely removable and replaceable modules.

 Most effectively, the present invention can be applied for the curing of highly radioactive waste in order to immobilize their toxic and radioactive components and subsequent disposal in geological formations.

 The use of induction melters allows the synthesis of materials (including glass and mineral-like), suitable for conditioning (curing) of almost all classes of HLW. Also, this technology allows the use of compact technological equipment, remotely removed for disposal after running out of resources.

 Implementation of the development will ensure an environmental effect and prevent the risk of radiation accidents and technological disasters associated with the storage of liquid HLW at radiochemical production sites.

Claims (9)

1. The method of solidification of liquid radioactive waste, including their preparation, concentration, mixing with fluxing additives, melting the mixture, draining the melt in a tank and manufacturing blocks, characterized in that the concentration is carried out by evaporation in a continuous stream at 110-160 ^o C, and melting carried out at 1000-1800 ^o With direct exposure to the induction field with a frequency of 0.3-1.8 MHz on the melt.  2. The method according to claim 1, characterized in that in the manufacture of blocks of phosphate glass, the mixing of waste with fluxing additives is carried out before the concentration stage.  3. The method according to claim 1 or 2, characterized in that the melting of the resulting mixture is effected by an induction field on a melt having a viscosity at a working temperature of 20 to 80 dPa • s and an electrical resistance of 0.1 to 1.5 Ohm / cm.  4. A device for the solidification of liquid radioactive waste, including a container for liquid waste, a concentration apparatus, an induction melter equipped with a drainage unit, a container with fluxing additives, equipped with a flux dispenser, a mixer, an exhaust gas purification unit, a piping system and valves, characterized in that the concentration apparatus is made in the form of a cylindrical body, inside of which a cylindrical separator is coaxially located, and a spiral tubular coil is located between the separator and the body, the end of which is connected with the capacity of liquid radioactive waste, and the other with the separator, while the separator is connected in the upper part with the flue gas cleaning unit, in addition, the induction melter is made in the form of a water-cooled crucible with side walls and bottom transparent to the alternating electromagnetic field and through the mixer communicated with the concentration apparatus and the flux dispenser.  5. The device according to claim 4, characterized in that the spiral tubular coil is made of two interconnected in the upper part of the tubular spiral sections - ascending, communicating with the capacity of liquid radioactive waste, and descending, communicating with the separator. 6. The device according to p. 4 or 5, characterized in that the inner diameter of the spiral tubular coil is selected from the following ratio:
d = k • Q • 10 ^-2 ,
where d is the diameter of the coil in cm;
k is a coefficient of 0.15-0.3;
Q - the performance of the device according to the initial solution, l / h  7. The device according to claim 4, characterized in that the side walls and the bottom of the body of the water-cooled crucible are made of a set of parallel water cooling tubes, sealed with corrosion-resistant dielectric ceramics.  8. The device according to claim 4 or 7, characterized in that the opposite walls of the water-cooled crucible are located at a distance of 0.21 to 0.55 m  9. The device according to claim 4, characterized in that the concentration apparatus and induction melter are equipped with remote connectors and are made in the form of remotely removable and replaceable modules.

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