The method of fractional purification of gases from hazardous chemical and radioactive substances formed during the dissolution of snf

 

(57) Abstract:

The invention relates to methods of treatment of radioactive and hazardous chemical gas emissions from the reprocessing of spent nuclear fuel. The method consists in the following sequence capture individual-harmful components, which capture each hazardous component is a preparatory operation for a subsequent allocation. Using modes and reagents, allowing to obtain high collection efficiencies on all impurity components: aerosols, nitrous gases, iodine-129, radiocarbon, radiocarbon, xenon, etc. the Claimed method is comprehensive, provides the minimum amount of secondary waste in a chemical form suitable for long term storage, disposal or possible use. 2 C.p. f-crystals, 2 Il.

The invention relates to methods of treatment of radioactive and hazardous chemical substances (HCHs) contained in the gas emissions from the reprocessing of spent nuclear fuel (SNF). SNF after shredding size of 5-10 cm is loaded into the apparatus, the solvent, where it is dissolved in hot nitric acid. The treatment of the, xidi nitrogen, tritium, iodine, radiocarbon, is a radioactive noble gases. Many of them are in different valence forms and chemical conditions. When delivering the mixture of the solvent carrier gas are formed of solid and liquid aerosols that may contain the full spectrum of harmful components, including uranium and plutonium. Required thorough cleaning gas emissions. To meet the requirements of NRB-86 and existing sanitary rules, can be removed from gases emitted into the atmosphere all harmful components with the maximum degree of cleaning. The most complete such requirements outlined in the Feasibility study of the RT-2 plant completed plant RT-2, inv. N 6156, 1994). In this paper, we construct the required degree of purification of gas emissions from aerosols, iodine-129, radiocarbon, radiocarbon, tritium, other radionuclides, as well as from nitrous gases other HCHs.

Closest to the claimed method is described in patent N 2711374 Germany, MKI G 21 F 9/02 from 21.09.78, According to this method, it is proposed to mix the exhaust of solvent gases from the carrier gas in a chemical form that is close to one of the radioactive substances, with subsequent their razdelid nitrogen, resulting from the disproportionation of higher oxides of nitrogen) narrows here to capture nitrous gases, iodine-129 and krypton-85. Schematic diagram of the purification of the prototype is shown in Fig. 1.

According to the prototype of the reactor solvent 1 gases containing water vapor, nitrogen, nitrogen oxides, nitrous acid vapour, xenon, krypton, and iodine, are directed into the condenser 2. Here is delayed part of the vapors, oxides of nitrogen and iodine. Next, the gases follow in the absorption column 3 and washed with nitric acid. Leaving the absorber column 3 gases pass through a condenser 4, where are exempt from the higher oxides of nitrogen, which through conductor 5 is returned to the beginning of the gas inlet 6. From the condenser 4 gases get into the separating column 7, which are separated by distillation into two fractions, one of which contains a radioactive substance with a carrier gas, and another faction that does not contain them, via the auxiliary adsorber 8 is exhausted through a chimney 9. The first fraction is served at the head of the separation column 10. From this column, the mixture of radioactive substances and the carrier gas through the conduit 11 is supplied to the base of the pillar 12 dual chemical processing. Xenon, separated in the, the formed nitric acid may be sent again in the solvent 1. Column 15 and a capacitor 20 connected in one system with columns 12-13, septic tank 14, are used for the treatment of nitrogen oxides to nitric acid, introduced here, namely the head of the column 13, the stoichiometric amount of oxygen.

Settling the product of the capacitor 2 and the absorption column 3 is subjected to desorption 17 and 18, and in the apparatus 19 gas free from iodine.

The main disadvantages of this process are:

- the great complexity of the process;

- lack in the first stages of purification from aerosols and dust that contaminates the entire system and the formation of secondary waste, liquid and solid;

- the necessity of introducing a carrier gas of nitric monoxide, which as described is formed by the disproportionation of higher oxides at the end of the scheme. This means that nitrogen oxides through the purification system is quite a lot, otherwise necessary additional generator of nitrogen oxides;

- no cleaning of radiocarbon;

- the need for accurate monitoring and dosage introduce third-party reagents, for example, oxygen;

- need stratenition for discharge from the gas oxides of nitrogen are given in number four). In the scheme not described method of allocation of iodine-129 from nitric acid, which involves the use of a solid sorbent, apparently, based on silver.

The task of the invention is:

- the simplification and cheapening of the process while enhancing the reliability of the circuit-gas purification from harmful components;

- capture the entire spectrum of harmful components, and potentially useful ingredients, for example, xenon;

- reduction of the amount of solid and liquid secondary waste gas cleaning with maximum return in the cycle is captured technological products, for example, nitrogen oxides - nitric acid;

- a comprehensive approach to the gas purification system with regard to the specific behavior of each component with the possibility of flexibility in the allocation and subsequent treatment of each individual hazardous ingredient.

The complex system of gas purification is as follows: on the way vibronic gases are a few steps for trapping aerosols, nitrogen oxides, iodine, tritium, carbon-14, radiocarbon and xenon.

The method is illustrated by the scheme shown in Fig. 2.

In the reactor-restorilonline NO to NO2.

The air is intensively served at the time of peak excretion of nitrogen oxides and iodine, the allocation of which is correlated to each other. The gas flow after the removal of aerosol cooling and condensation of water vapor, acid reflux condenser and apparatus aerosol cleaning 2, 3, 4, 5 is fed to the first absorption column 6. Column irrigated weak nitric acid with the addition of 2-5 wt.% of hydrogen peroxide. Hydrogen peroxide under these conditions serves to oxidation resulting from the dissolution of nitrogen oxides of nitrogen in nitric acid, thereby prevents the secondary formation of nitrogen oxides by the reaction:

2HNO2---> H2O + NO2+ NO.

The reaction proceeds instantaneously. At the same time, the peroxide oxidizes volatile form of iodine (J2in the non-volatile Iodate (JO3-).

After saturation of the absorber up to 4-6 m/l nitric acid with iodine is displayed in a separate thread and processed for separation in the node 11. The release of iodine is effected either by Stripping with air at a temperature of 70-80oC in the presence of 2 wt.% peroxide of hydrogen, or by using the method of extraction with an organic solvent, for example benzene or RED-4 in the presence of the waste, convenient for disposal or long-term storage, and nitric acid after special preparation is used in technology.

The gas flow after the removal of the main part of the nitrogen oxides and iodine contains residual amounts of these substances, all carbon-14 in the form of CO2radiocarbon and stable xenon. He arrives on clearing the second column 7, containing 2-4 M solution of sodium hydroxide with the addition of 2-5 wt.% reductant, such as urea. In the second column 7 is dolabellane oxides of nitrogen and iodine, the capture of radiocarbon. The wash solution of this column is open to a residual concentration of NaOH, 0.2-0.5 M/l, then processed for allocation in the solid phase of carbon-14 (BaCO3or CaCO3and iodine to node 12. After removal from the solution of carbon and iodine he supported a concentrated NaOH solution to 2-4 M/l and re-enters the cleaning cycle gas emissions. Thus, the absorption column 7, performing a major role in the capture of radiocarbon, is guarded for iodine and nitrous gases. In the case of an emergency on the head speed gas purification system she doesn't release iodine and nitrogen oxides in the ventilation.

Thus, the advantages of the proposed method are as follows:

- cleaning gas emissions integrated approach, i.e., the capture of each component serves as a preparation for the subsequent selection;

- there is no need to introduce the carrier gas is nitric oxide, which is necessary specially to receive;

every harmful component is displayed in a separate faction, providing convenience and ease further handling with the aim of disposal or use;

in the process used cheap available reagents, renunciation of scarce silver with maintaining a high degree of gas purification from iodine;

- due to mnogoyadernosti cleaning increases the degree of purification of gas emissions, reduces the likelihood of emergency ibruprofen.

Example 1. 0.5 kg chopped into pieces by a length of ~ 5 cm of spent nuclear fuel of VVER-1000 with a burnup of ~ 40 MW/suttas of uranium loaded into the machine-solvent total volume of 10 L. After a leak test of the entire system in the solvent gave ~ 3 l 8 M nitric acid and slowly raised the temperature of the solution to 100oC. In the gas purification system were located consistently a reflux condenser, aerosol filter, column absorber with HNO3+H2O2(2-5 wt. %), absorption column with NaOH+(NH2)2CO (~ 0.3 M/l) and the absorption site for trapping of xenon and krypton by the method of selective absorption on freon-13. For the kinetics of gas evolution was monitored using flow meters, and the rate of dissolution of SNF - content in the exhaust gas of krypton-85.

In the process of dissolution of the apparatus of the solvent in the gas purification system has been vapor-gas stream comprising nitrogen oxides (up to 70% at the maximum), water vapor, nitric acid, components of air, carbon dioxide, about 0.1. %, iodine-129 (with an average concentration of ~ 100 mg/m, xenon (maximum of ~ 0.1 to about. %), krypton (85Kr) (maximum ~ 0,01% vol.). The basic amount released during the dissolution of nitrogen oxides, iodine, xenon and krypton of nabludalos oxygen with a flow rate of ~ 100 l 1 o'clock The total gas flow at the output of the installation was equal to ~ 1 m3/H. With this mode of dissolution of nitric acid solution was removed in the gas phase 98% iodine from its original content in spent nuclear fuel.

Oxides of nitrogen and iodine was located in the absorption column, irrigated with water containing 2 wt.% of hydrogen peroxide. The mode of operation of the column provided the ratio of gas/liquid 250: 1 at room temperature. Absorption of nitrogen oxides was carried out at 99% and about 99% of the iodine entering the head of the column. The remaining amounts of these components were entered in the column with the alkaline absorber (4 M NaOH + 0.3 m/l (NH2)2CO., where he Golovlevs, forming sodium nitrate and sodium iodide. Here was located the radiocarbon in the chemical form of sodium carbonate.

After alkaline washing the exhaust gas to the drying and cleaning of liquid aerosols and then went into the system by selective absorption of radiocarbon and xenon freon-13. At first absorbtsionnoi column of this node was allocated stable xenon, and the other carried out the capture of krypton (85Kr).

The resulting secondary liquid waste is then processed to identify harmful sostavlyauschuu, containing iodine was served on a separate platter column, where the method of countercurrent air were Stripping iodine. When the mode column: feed ratio of gas/liquid below 100, the temperature of the solution equal to 70oC, the content of hydrogen peroxide is not lower than 2 wt.%, iodine was doing 98%. Rolling in the gas phase iodine caught in 2-4 M/l NaOH solution, which is then sent for processing to obtain a solid composition CuJ + Cu, convenient for long-term controlled storage.

The purified nitric acid were sent to special training and re-used in the processing of spent fuel.

From alkaline absorber 2nd column after developing it to a residual concentration of 0.2-0.5 M/l of NaOH was isolated carbon-14 in the chemical form BaCO3. The output of ~ 99%. Also cleaned the solution accumulated in it iodine-129 (with repeated use), distinguishing it either in the form of elemental iodine, either in the form of a solid low-solubility compounds.

Selected at the final stage of purification of xenon and krypton were separated, cleaned of components of air (N2O2, and was kompiliroval in cylinders under pressure.

Carried out, thus the former is>/BR>iodine-129 > 102,

the carbon-14 > 20,

for aerosols > 107,

the krypton-85 > 102,

the tritium - 102,

the oxides of nitrogen < 7102.

Example 2. At the pilot dissolution of spent fuel, consisting of a reactor-solvent total volume of 25 l systems software agents, air and so on, gas stand, including a reflux condenser, aerosol filters, absorption columns: 1-I for trapping nitrogen oxides and iodine; 2-I - for capture of radiocarbon, absorption column for trapping of xenon and radiocarbon control systems, analysis and control system tahaliyani them with liquid nitrogen, dissolved four parties SNF weight 7,1 kg (0.5; 2,7; 1,9; 2.0 kg). Chopped into pieces of fuel loaded into the reactor in acid HNO - 8 M/l, volume of 15 l, have sealed and raised the temperature of the solution to a boil. The process of dissolution, control and management was carried out similarly. The control process carried out during the dissolution of the takeoff device samples the output in the gas phase of krypton-85 (continuous), on samples taken from all gas purification apparatus after the dissolution. Analysis was performed on all volatile and kazooba the AE using activated charcoal brand SKT-2B. Heating of the coal by adsorption of nitrogen oxides were observed, their content before adsorption columns was at the limit of detection sensitivity of the chromatographic method.

The release of iodine from forming in column 1 of the regenerated nitric acid was carried out by the method of extraction in RED-4 after restore iodic acid with hydrazine. The organic solvent was removed > 99% iodine.

In the conducted experiment achieved the following efficiencies:

aerosols > 108,

iodine-129 > 102,

carbon-14 - 20,

krypton-85 ~ 103,

nitrogen oxides ~ 103,

1. The method of fractional purification of gases from hazardous chemical and radioactive substances formed during the dissolution of spent fuel, including the capture of aerosols and dust, nitrous gases, radioiodine and radioactive noble gases (RBG), characterized in that the cleaning is performed in the sequence in which the capture of each component is a preparatory operation for a subsequent allocation, after the capture of aerosols and dust they hold joint capture nitrous gases and iodine in acid absorber, containing added nicoleb of radiocarbon with the purification of a gas stream from nitrous gases and iodine alkaline absorber, containing added nicoleabrahamse reductant in the amount of 2 to 5 wt.%, for example, urea or hydroxyamine and capture RWB, in particular, xenon and krypton with a final clean of all impurities is conducted by the method of selective absorption or adsorption on solid sorbents.

2. The method according to p. 1, characterized in that above the surface of the solution SNF at the time of peak excretion nitrous gases and iodine additionally insure flow of gas absorber, fully saturated with oxygen.

3. The method according to p. 1, characterized in that the fractionation produced during the process of gas purification secondary waste in the form of nitrogen and iodine acids carried out by the method of Stripping iodine in 70 - 80oC and with the addition of a weak reductant, such as hydrogen peroxide, in amounts of 1 to 2 wt.% or extraction of elemental iodine after recovery adnovate acid, for example, hydrazine, and from the alkaline solution with a residual concentration of 0.2 - 0.5 M/l of NaOH produce radiocarbon and radioed in the form of a solid chemical compounds.

 

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