RU2165111C2 - Method for decontamination of parts and apparatuses contaminated by radionuclides - Google Patents

Abstract

FIELD: procedure of washing and decontamination of surfaces of equipment contaminated by radioactive substances, applicable in various operations at radiochemical plants. SUBSTANCE: washing baths with parts or apparatuses to be decontaminated are alternately filled with solutions of oxidizing and complexing agents, they are heated and cured at a heating temperature. Only part of the space of the washing bath or apparatus equal to 0.25 to 0.5 of its volume is filled with the solutions and heated to the boiling point. The method consists of several cycles of treatment. In each cycle heating is conducted by steps with disconnection of heat supply after every 10 to 15 min for 3 to 5 min with a total duration of each cycle within 1.5 to 2 hours. Another method of heating of the decontaminating solution is provided: gradual heating at a rate of 10 to 15 C per minute up to the beginning of the boiling process. EFFECT: enhanced cleaning coefficient by 3 to 40 times, reduced consumption of solutions by 35% and reduced time of decontamination by 1.5 times. 3 cl, 4 tbl

Description

 The invention relates to techniques for washing and decontaminating surfaces of equipment contaminated with radioactive substances, and can be used when carrying out repairs on parts and apparatuses of radiochemical plants that process irradiated fuel elements (TVEL) in order to regenerate fissile materials.

 According to current industry standards, all radiochemical equipment must undergo periodic inspection and, if necessary, restoration repair, to ensure reliability and trouble-free operation of the equipment during the next overhaul cycle. Before examination by visual and instrumental methods, it is required to clean the working surfaces of the parts and walls of the apparatus (tanks) from radioactive contamination accumulated in the form of corrosion products, deposits of labor-insoluble substances embedded in the pores of the metal, sorbed films, including radionuclides.

 Many methods of cleaning and decontamination are known for the neutralization of equipment contaminated with radionuclides of nuclear reactors and radiochemistry.

 Mechanical removal of sediments containing radionuclides from flat or cylindrical surfaces (German Application N 3404575 G 21 F 9/28. Publication 14.08.85 N 33). But mechanical cleaning is not effective for removing sorbed films and is used only as a preliminary operation, for example, for descaling heat exchangers from refrigerators with a low level of radioactivity.

 A method is described in which a gel, for example glycerophosphate, is applied to the working surfaces, is kept for a certain time, and then the gel is washed off with water (French Application N 2380624, G 21 F 9/30. Publication 13.10.78 N 41). This method is suitable only in cases where there is direct access of stripping operators to the surfaces to be cleaned, i.e. with a relatively low background of radioactive radiation. The method is also not effective enough to remove radionuclides adsorbed and introduced into the pores.

 Hydromonitor cleaning methods have a high deactivating ability when the jets of chemical agents emitted through special nozzles not only perform chemical cleaning, but also have a shock-mechanical effect, increasing the cleaning efficiency of radionuclides adsorbed and incorporated into metal (Japanese Application No. 56-54600, G 21 F 9/28. Publication 12/26/81). However, this method has found industrial application so far only for hollow tanks without complex internal devices, since elements of instrumentation sensors, airlifts, plates, partitions, etc., can be damaged during jet washing. In addition, internal devices create a "shadow effect", which reduces efficiency decontamination.

 There are developments of a method for decontamination of tanks for liquid radioactive media, including the use of a vaporous solvent, its condensation on the treated surface and the removal of waste solutions (Russian Patent N 2022379, G 21 F 9/34. Publication 10/30/94,) A significant disadvantage of this method is that that the supplied steam and the resulting condensate are used at the same time: after one contact, the solution is discharged and cannot be used again in clean steam production equipment, which will reduce efficiency and increase p flow rate of solutions and the amount of liquid radioactive waste (LRW). In steam methods, dense deposits in the bottom of the apparatus are not sufficiently removed. Therefore, steam decontamination methods have not been widely used on the equipment of radiochemical plants, they can only be used for equipment such as tanks with a low level of contamination with radionuclides.

There is a so-called two-way method for deactivating the treatment with aqueous solutions of oxidizing and co-complexing reagents, which alternately treat surfaces contaminated with radionuclides. In this case, each of the solutions is heated to +95 ^o C and incubated for 1.5 hours. The solution is poured into the washing bath to the working volume and the decontaminated parts and units are immersed in it. In most cases, the radiochemical apparatus (reactors, monzhus, settling tanks, etc.) have a very high level of radioactivity and cannot be delivered to the washing site without first decontamination on the spot. Then the two-way method is used by directly alternatingly pouring oxidizing and complexing solutions directly into the working cavity of the apparatus until the working volume is filled, which itself serves as a washing bath, the solutions are heated to +95 ^o C and kept for 1.5 hours (Prototype. A.s N 730156, G 21 F 9/28. Publication 10.30.84, Bull. N 40). Washing cycles are repeated up to 10 or more times to ensure the achievement of sanitary repair standards.

 The described two-way method is used to decontaminate equipment at all radiochemical plants of Minatom nuclear plants both for sequential cleaning of the entire chain of equipment mounted in canyons and for washing removable parts at specialized desorption sites in washing bathtubs.

 Two-way method is taken as a prototype.

The disadvantages of the prototype in the first place include a relatively small degree of decontamination in one treatment cycle, since the method will not allow heating to the boiling temperature of the working volumes of solutions in bathtubs and devices because of the danger of solutions entering the blow-off lines, their filling and overload, violations normal operation of air cleaning systems (traps, multicyclones, filters, etc.) and the release of radionuclides into the atmosphere. Therefore, for washing by filling the baths and devices with exposure at +95 ^o C, it is necessary to carry out a large number of cycles, requiring in some cases 20 to 30 days to wash to repair standards. In addition, to fill the full volumes or working volumes (~ 80% of the total volume of the bath or apparatus), a large amount of washing reagents is consumed, which leads to the formation of the same large volumes of LRW that require mandatory neutralization and disposal.

 The objective of the invention is to increase efficiency and reduce the duration of decontamination.

 The problem is solved in that in a method for decontamination of parts and apparatuses contaminated with radionuclides, including pouring solutions of oxidizing or complexing reagents into washing baths with parts or heating devices, heating them and holding them at a heating temperature, fill a portion of the cavity of the washing bath or apparatus equal to solutions 0.25 - 0.5 of its total volume, and heating is carried out to the boiling point.

 This goal is also achieved by the fact that heating in each cycle is carried out stepwise with a deviation of heat supply and mixing every 10 - 15 minutes for 3 - 5 minutes with a total cycle duration of 1.5 - 2.0 hours.

Heating decontamination solutions is carried out at a speed of 10 - 15 ^o C per minute until intense boiling.

The boiling of the poured detergent solution significantly intensifies the process in the washing bath or in the lower part of the apparatus filled with a solution of an oxidizing or complexing agent (liquid phase), as compared to a simple exposure to immersion in a solution at t = +95 ^o C.

 An important role is played by the high intensity of heating, which increases the convective flow rates of the liquid medium and its abrasive effect, especially in the presence of solid particles of deposits.

 Also increases the efficiency of desorption of surfaces in all areas above the solution. Spray solutions, aerosols and vapors intensively wash the walls, lid and all internal structural elements, vapors condense in colder areas, condensates flow down, creating constant contact with fresh portions of desorbing agents. In the processes of condensation-evaporation in each area above the solution, the effects of simple chemical washing and the influence of the mass transfer process characteristic of the “dew points” are summarized, when, in addition to chemical reactions, electrochemical effects appear due to increased uneven aeration and the formation of a large number of anode-cathode sections. Thus, atmospheric oxygen begins to take an active part in the desorption of radionuclides. For this reason, the volume not filled with solutions should be as large as possible, which ensures an increase in the decontamination efficiency. But in the bottom part of the apparatus there may be dense precipitation, which is better loosened and removed from the surfaces of the equipment by flows of boiling solution. Experiments show that the optimal ratio of the liquid phase of the stripping solution is 0.25 - 0.5 of the total volume of the apparatus.

 The stepwise mode of conducting intense heatings can also in some cases increase the efficiency of decontamination, since brief interruptions in heating, mostly with hot steam, and mixing give time to complete the condensation and runoff of already saturated condensates. There are conditions for access of fresh portions of aerosols and vapors, which increases the mass transfer coefficients during transitions from solid (metal equipment) to liquid phases. Thus, it provides a peculiar shock-swaying temperature regime of the solution-vapor effect on the contaminated surface radionuclides, which increases the decontamination efficiency.

 Experience shows that for a break in heating and stirring, 3-5 minutes is enough every 10-15 minutes of washing with a full cycle time of 1.5-2.0 hours.

 In a washing bath, all processes are similar. A certain number of units of parts, for example, on which there is more deposits and more corrosive wear, is immersed in a solution poured in an amount of 0.25 - 0.5 of the total volume of the bath, and other parts are placed in the gas phase above the solution. All the positive effects of intense convection during boiling, condensation, evaporation, etc. described above have a similar effect during decontamination in a washing bath. In the process of decontamination in different cycles, the upper and lower parts can be interchanged, i.e. rearrange from the gas phase to the solution and vice versa. Such a permutation, as a rule, increases the efficiency of the removal of radionuclides.

 Large-sized units and assemblies (vessels for washing external surfaces) are immersed in a washing solution, which is partially filled in a bathtub in 0.25 - 0.5 volumes below the solution level, and 0.5 - 0.75 of the height of the washing object remain in the gas (air) phase. In this embodiment, decontamination is determined similarly to the processes described above.

 Compared to cleaning methods with steam compositions, in the proposed method, stripping steam is used repeatedly in each cycle until it is saturated with radionuclides, since steam is generated from condensates flowing down, which makes it possible to economically use volumes of stripping solutions. In the proposed method, an extremely saturated moist steam is obtained containing a large proportion of aerosols and sprays, which increases the efficiency of decontamination by flowing condensates.

 The effectiveness of decontamination according to the proposed method will be higher than the simple sum of the effects of soaking in solutions and steam exposure, since the complex interaction includes new mechanisms that increase washing and desorption, namely, increasing convection, abrasive exposure, spray, condensation - re-evaporation, creating multiple uneven aeration zones. The effectiveness of the proposed method is higher than the simple amount of the prototype and steam processing by the method-analogue, which confirms its inventive step.

 The proposed method provides high efficiency in the washrooms at the decontamination sites of equipment that is already being dismantled (valves, pumps, instrumentation sensors, etc.) and in radiochemical devices (reactors, monguses, sedimentation tanks, etc.) at the place of their installation before dismantling, when the cavity of the device itself is a washbasin to deactivate itself.

 An example implementation N 1.

 Laboratory tests were carried out on bellows of glandless valves after dismantling after operation for 4 - 5 years in the scheme of a radiochemical plant.

 For comparative tests, routine detergent solutions were used.

Solution N 1
NaOH - 1.0 mol / L
KMnO ₄ - 0.5 g / l
Solution N 2
HNO ₃ - 1.0 mol / L
H ₂ C ₂ O ₄ - 1 g / l
According to the prototype method, the bellows were immersed in a solution poured into the bath and heated to +95 ^o C. According to the proposed method, 0.25 - 0.5 alternately 1 and 2 solutions were poured into the washing bath, one bellows was loaded under the mirror of the solution, and the other the bellows were hung on a FUM tape over the solution at a distance of 20-50 mm from the solution mirror and the solution was boiled. In both cases, the duration of the experiments was 1.5 hours.

 Test data are given in tables.

 As can be seen from table 1, in both stripping solutions according to the claimed method, an increase in the coefficients of surface cleaning by 4-30 times is achieved.

 This is also confirmed by the chemical analysis of washing solutions after a washing cycle for the content of radionuclides, the data of which are shown in table 2.

 As can be seen from the data in table 2, in most comparative analyzes, a greater transfer of radionuclides into the stripping solution is achieved by the claimed method, compared with the prototype.

 Example 2

 Pilot operations were carried out on two reactor apparatuses (AD-6004 / 1,2) in the processing scheme of irradiated blocks (TVEL) of industrial nuclear reactors at the radiochemical plant (RCP) of the Siberian Chemical Combine. Summary results of industrial tests are given in table 3.

 Tests have shown that the number of treatment cycles is reduced by 35%, the flow rate of solutions and the time of decontamination are halved.

 Example No. 3.

 Comparative corrosion tests of structural materials have been conducted. Comparative data are given in table 4.

 As can be seen from table 4, with all modes of decontamination, the typical structural material of radiochemical equipment 12X18H10T is characterized as "persistent".

 The claimed method of decontamination can be used in the preparation of radiochemical equipment for inspection and repair according to the PPR and KPR schedules at all radiochemical plants of the Ministry of Atomic Energy at plants processing irradiated blocks of industrial and dual-purpose nuclear reactors, as well as at plants processing fuel elements of energy and transport nuclear installations ( nuclear icebreakers, submarines, mobile stations of the Arbus type, etc.).

 The method proposed by the invention is also applicable for washing equipment in the production of processing technological discharges and storage of liquid and solid radioactive waste, in the production of radioactive isotopes and "labeled atoms".

 The basic principles of the intensive washing process can be used for various purposes in the chemical and biochemical industry, in the production of the ministries of defense, ammunition and weapons.

Claims (3)

 1. The method of decontamination of parts and apparatuses contaminated with radionuclides, including alternating treatment by pouring solutions of oxidizing and complexing agents into washing baths with parts or deactivating apparatus, heating and holding them at a heating temperature, repeating processing cycles, characterized in that the solutions supplement part of the cavity washing bath or apparatus, equal to 0.25 - 0.5 of its full volume, and heating is carried out to a boiling point.  2. The method according to claim 1, characterized in that the heating in each cycle is carried out stepwise with the heat supply and mixing turned off every 10-15 minutes for 3-5 minutes with a total duration of each cycle of 1.5-2.0 hours. 3. The method according to claim 1, characterized in that the heating of the decontamination solutions is carried out at a rate of 10 - 15 ^o C per minute until intense boiling.

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