RU2059306C1 - Filter cleaning gaseous products of breakdown at atomic power plant - Google Patents

Abstract

FIELD: atomic power engineering. SUBSTANCE: filter is designed in particular for cleaning of gaseous products from radioactive aerosols and volatile forms of radioactive iodine and from organic volatile compositions of iodine. Filter is composed of heat insulation case with inlet and outlet unions of cleaned gaseous atmosphere inside which perforated partitions are installed which carry layers of filtering adsorbing material with diminishing size of granules in each subsequent layer in way of movement of gaseous products. First layer of filtering adsorbing material includes catalyst to burn hydrogen. Size of granules of first layer is 2.0-4.0 mm and of the last layer 50.0-200.0 mkm. Value of filtering surface of last layer exceeds that of surface of first layer by 1.5-3 times. Thermally stable and moisture resistant sorbent based on titanium dioxide is used as filtering adsorbing material. EFFECT: enhanced cleaning efficiency. 1 dwg

Description

 The invention relates to devices that are used in systems for cleaning gaseous products of a nuclear reactor accident from radioactive substances, in particular for cleaning a gas-vapor medium discharged from under a protective shell of a reactor (containment) in order to prevent releases of radioactive substances through shell leaks in excess of the permissible value beyond design basis accidents with reactor core (fuel) melting.

A well-known Filtra-MVSS system, developed by two Swedish companies, is designed to filter a gas mixture with a flow rate of up to 13 kg / s with a cleaning coefficient from 500 to 10000 [1] The device is a vessel (scrubber) with a volume of about 400 m ³ , inside of which there are many Venturi pipes and 270 m ^{3 of an} aqueous solution of sodium thiosulfate stabilized to pH 10 with an addition of soda are poured. At the exit of the scrubber, a gravel filter is installed as the second stage. The gas-vapor mixture is passed through venturi tubes, which are placed below the solution level, and is cleaned of aerosols and molecular iodine.

 However, this device does not clean the organic form of iodine (methyl iodide). In addition, it has a high cost due to the complexity of the design.

 Siemens company (Germany) developed the Multiventuri Skrubber-Aerosol Filter Combination system [2]. This device includes a scrubber with Venturi pipes and a filter layer of metal fiber. Due to this, the cleaning coefficient for aerosols increased to 10,000, and for molecular iodine is 100. The diameter of the device is 5.0 m, height about 12 m, productivity 13.5 kg / s.

 The disadvantages of the device are low efficiency due to the low degree of purification from the organic form of iodine (≅ 5%), the amount of which is up to 20% of all gaseous forms, as well as the relatively high cost due to the large size.

The first operational emergency waste treatment device was built at Barseback NPP in Sweden. A gravel filter made of reinforced concrete with a diameter of 20 m, a height of 40 m and a volume of 10,000 m ^{3 was used here} . It loaded 15,000 tons of quartz gravel with a grain size of 25-35 mm. The purification coefficient, including molecular iodine, is 1000.

 However, this device does not provide cleaning from the organic form of iodine and has a high construction cost.

 Closest to the invention is a filter for adsorption of iodine from spent radioactive gases [3], which is a housing with fittings for the inlet and outlet of the working medium. Inside the case, perforated partitions are installed. On the partitions placed layers of filtering material of zeolite, arbitrary particle size distribution. To maintain the temperature of the filter material, heaters are installed in the filter housing.

 The disadvantages of this filter are the potential explosiveness of the filter, low efficiency due to the almost complete breakthrough of the organic form of iodine through the zeolite, and also due to dusting during the filtration process, which is associated with the low mechanical strength of the zeolite. In addition, zeolite is not applicable for the purification of gases with a high moisture content, which is typical for emergency discharges of nuclear power plants.

 The use of a zeolite of arbitrary particle size distribution in the filter material of the filter reduces the reliability and service life of the filter due to possible clogging with aerosols.

 The advantages of the invention in comparison with the prototype are higher filter efficiency due to the purification of the vapor-gas radioactive mixture from the volatile organic form of iodine (methyl iodide), increased filter life due to the prevention of clogging of the adsorbent granules by aerosol particles, as well as the possibility of using the filter to clean the gas-vapor mixture radioactive media containing aerosols of various dispersion and a high moisture content. In addition, the advantages of the filter for cleaning the gaseous products of an accident at a nuclear power plant include a reduction in the level of hydrogen explosiveness of the filter.

 These advantages are provided due to the fact that in the filter, which includes a housing with fittings for entering and exiting the working medium, perforated partitions are installed, with the help of which bulk filter material is placed and fixed, which is a heat-resistant highly active with respect to gaseous forms of iodine and moisture-resistant sorbent on based on titanium dioxide, moreover, this sorbent in the filter is placed in at least two layers and granules of the sorbent in each subsequent layer along the medium are smaller than in the previous one, moreover, Lednov downstream sorbent medium has a filter surface layer is 1.5-3 times larger than the corresponding surface of the other layers.

 In order to increase the efficiency of the filter in capturing gaseous forms of iodine by eliminating vapor condensation and increasing the temperature of the process, it is equipped with heaters and thermal insulation.

 Hydrogen may be present in the vapor-gas medium removed from the containment; therefore, an explosive concentration of hydrogen may form during steam condensation. Considering that an increase in temperature increases the efficiency of trapping gaseous forms of iodine, and the conditions of aerosol deposition are improved during catalytic reactions, a catalyst for burning (recombining) hydrogen is placed in the first layer of the sorbent.

 According to the available studies, the filter, in which all of the above structural elements will be used, will satisfy the requirements of high-safety nuclear power plants with a VVER-1000 reactor and at a capacity of up to 20 kg / s provide a coefficient of purification from aerosols and gaseous forms of iodine, including methyl iodide from 1000 to 10,000.

 The drawing shows a filter for cleaning gaseous products of an accident at a nuclear power plant in the context.

 The filter contains a housing 1 with a fitting 2 for entering the working medium and a fitting 3 for leaving the working medium. Inside the case, partitions 4 with openings for the passage of the working medium are installed. Bulk filter material 5 is placed and fixed on the partitions 4. As a bulk filter material 5, a heat-resistant and moisture-resistant sorbent based on titanium dioxide is highly active with respect to gaseous forms of iodine. The sorbent in the filter is placed in at least two layers and the granules in each subsequent layer are smaller than in the previous one. The last layer of sorbent 6 with a thickness of 10-40 mm from granules of 50-200 microns in size to increase the filtration surface is placed in the filtering elements 7 made in the form of glasses, the side surface of which is made of two coaxially arranged partitions with holes.

 A layer of sorbent 6 is placed between the partitions. Due to this, the filtering surface becomes 1.5-3 times larger than the similar surface of other layers. Heaters 8 are installed inside the housing 1, before and after the filter elements, and the outer surface of the housing is covered with heat insulation 9. In the first layer of the filter material 5 along the medium, a catalyst 10 for burning hydrogen is placed.

 The filter works as follows.

 During normal operation of the nuclear power plant, the filter is disconnected from the reactor containment and is in constant standby mode. In an accident accompanied by melting of the reactor core (fuel), the pressure in the protective shell rises to a dangerous value and the filter is connected to the protective shell and ventilation pipe using bursting membranes or shut-off devices (not shown).

 The gas-vapor mixture enters through the nozzle 2 into the filter, is sequentially filtered through the bulk filtering material 5, the sorbent layer 6 and the mixture purified from radioactive impurities is discharged through the nozzle 3 and the ventilation pipe into the atmosphere. Gaseous forms of iodine are sorbed in the pores of the sorbent, and aerosols are deposited on its surface.

 The number of sorbent layers, the thickness of each layer and the size of the granules are taken based on the size and number of aerosol particles.

 Due to the fact that in each subsequent sorbent layer, the granules are smaller than in the previous one, larger aerosol particles are predominantly held in the first layers along the medium, and smaller particles arrive in subsequent layers in smaller quantities. The smallest particles are held in the last layer of the sorbent 6, located in the filtering elements 7, where the smallest fractions of the sorbent are used and the filtration rate is significantly lower than in other layers.

 The size of the granules in the first layer of the sorbent along the medium depends on the nature and size of the aerosol particles and is selected in such a way that a strong autofiltration layer of aerosol particles does not form in this layer, due to which the filter may clog.

 For example, it was experimentally established that for effective cleaning and preventing clogging of the filter for aerosols with solid particles of an average size of 0.4-0.7 microns, the size of the granules in the first layer of the sorbent along the medium should be 4-2 mm.

 Thus, with a relatively small filter size and the amount of sorbent, efficient cleaning of all types of radioactive contaminants is provided, including the organic form of iodine and fine particles, and filter clogging is prevented, while achieving all the previously listed filter advantages compared to the prototype.

In normal plant operation mode using the heaters 8, 9 and insulation is provided sorbent temperature 20-30 ^{° C} higher than the saturation temperature of water vapor in the containment at the time of inclusion. This eliminates the condensation of steam on the sorbent and in the filter housing, which leads to an increase in the activity of the sorbent in relation to the gaseous forms of iodine.

 Due to the placement of a catalyst for burning hydrogen in the first layer of the sorbent, the filter temperature increases significantly, which increases the activity of the sorbent with respect to the gaseous forms of iodine and increases the efficiency of aerosol capture due to the temperature difference between the catalyst and the gas stream, and also reduces the hydrogen explosion hazard.

 Based on the studies and calculations, it is expected that a filter with a diameter of 4.5 m and a height of 5.5 m made according to the invention will satisfy all the requirements of nuclear power plants with a WWER-1000 power unit and provide a cleaning coefficient of at least 1000 at a flow rate of up to 20 kg / s (70 t / h).

Claims (1)

 FILTER FOR CLEANING GAS-PRODUCED ACCIDENT PRODUCTS OF NUCLEAR POWER PLANE ACCIDENTS, including a thermally insulated body with fittings for the entrance and exit of a cleaned gaseous radioactive medium, perforated partitions installed inside the body with layers of granular filtering adsorbent material placed on them, as well as heating elements that differ in quality adsorbent filtering material used heat-resistant and moisture-resistant sorbent based on titanium dioxide with a decrease the size of the granules in each subsequent layer located in the direction of the cleaned gaseous radioactive medium, and in the first layer of the cleaned gaseous radioactive medium there is a catalyst for burning hydrogen, the granule size of the first layer is 2 4 mm, the last layer is 50 200 μm, and the size of the filtering surface of the last layer is 1.5 to 3.0 times greater than the size of the filtering surface of the first layer.

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