RU2121722C1 - Method of deactivating iron-carbon alloys - Google Patents

Abstract

FIELD: radiation protection. SUBSTANCE: composition containing vanadium pentoxide and mixture of sodium, potassium, and sulfur-containing salts is applied on surface of polluted material. Amount of vanadium pentoxide constitutes 25 to 75% of total weight of composition, K/Na atomic ratio ranges from 0.3384 to 1.8144, and sulfur content in salt mixture is 29.33-33.84 wt %. Covered surface is heated for 10-30 min mainly at 850-950 C, after which resulting scale is removed from the surface. Operation, if necessary, is repeated. EFFECT: increased purification efficiency. 3 cl, 1 tbl

Description

 The invention relates to a technology for processing materials with radioactive contamination.

 A known method of decontamination of metal, which consists in its heating in the presence of a substance containing sodium salts of acids, and cleaning the surface (as.with. USSR N 730156, G 21 F 9/28, 1984).

 However, the known method does not allow to obtain compact waste without additional operations, for example, evaporation.

 The closest to the invention in technical essence and the achieved result is a method of deactivating iron-carbon alloys, including heating them in the presence of a mixture of salts, which include sodium, potassium and sulfur, and cleaning the surface of the alloys from scale (Application DE N 3418207, G 21 F 9 / 28, 1985).

 However, in the case of a high degree of radioactive contamination, the known method is not effective enough and requires a significant investment of time.

 The aim of the invention is the creation of the most effective environmentally friendly method for the decontamination of iron-carbon alloys, the final result of which is to obtain easily disposed waste.

 To achieve this result in a method for deactivating iron-carbon alloys, including heating them in the presence of a substance consisting of the first group of ingredients in the form of a mixture of sodium, potassium and sulfur salts, and cleaning the surface of the alloys from scale, vanadium pentoxide in an amount of 75-85 % by weight of the total amount of the substance in the following ratio of atomic fractions of the ingredients of the first group (potassium: sodium) = 0.3384 - 1.8144, while the sulfur content is selected in the range of 29.93 - 33.84% by weight in this group.

 Heating is carried out at temperatures of 850-950 degrees Celsius for 10-30 minutes, after cleaning the surface, the alloy is reheated in the presence of the aforementioned substance, the surface is cooled.

 The following arguments can be made to prove the feasibility of a patent. The process of decontamination of contaminated iron-carbon alloys is based on the corrosion of the surface layer of the alloys and the chemical interaction of radionuclides with the substance. This effect is achieved by the fact that when the surface of the alloys is heated, the substance enters into chemical interaction with the chemical elements of the surface layer of the alloys, including radionuclides (for example, cobalt). In this case, fusible melts are formed, which also contribute to the acceleration of corrosion of the surface of alloys. The resulting new substances form a new non-metallic phase containing radionuclides, scale, which, after cooling, is easily separated from the alloy.

Additionally, the introduction of potassium as one of the ingredients in a deactivating substance determines the production of low-melting melts. Although potassium sulfates have a higher complete decomposition temperature ( _mp = 1084 ^° C) than sodium sulfates ( _mp = 884 ^° C), a mixture of potassium and sodium pyrosulfates formed during deactivation has a eutectic temperature of 408 ^° C and 396 ^° C, respectively [ one]. Due to the rapid formation of the melt, the time of its interaction with the decontaminated surface increases, and this helps to improve the conditions of the decontamination process.

 The effect of the potassium to sodium ratio in the substance on the results of the high-temperature deactivation process was determined experimentally (table 1).

Using sulfur as one of the ingredients of the substance allows, starting from a temperature of 550 ^o C, to carry out the process of chemical corrosion of the surface of iron-carbon alloys due to:
- the implementation of reactions forming sulfides, sulfates, sulfites and pyrosulfates of Fe, Cr, Ni, etc. These processes are carried out according to the following scheme:
6K ₂ SO ₄ + Fe ₂ O ₃ = 2K ₃ Fe (SO ₄ ) ₃ + 3K ₂ O (1)
6Na ₂ SO ₄ + Fe ₂ O ₃ = 2Na ₃ Fe (SO ₄ ) ₃ + 3K ₂ O (2)
3K ₂ SO ₄ + Cr ₂ O ₃ = Cr ₂ (SO ₄ ) ₃ + 3K ₂ O (3)
K ₂ SO ₄ + NiO = NiSO ₄ + K ₂ O (4)
- the formation of sulfites, sulfates, sulfites of radionuclides, for example, Co, etc.

K ₂ SO ₄ + CoO = CoSO ₄ + K ₂ O (5)
Na ₂ SO ₄ + Co ₂ O ₃ = CoSO ₄ + K ₂ O (6)
K ₂ S + CoO = CoS + K ₂ O (7)
Na ₂ S + CoO = CoS + Na ₂ O (8)
During the initial oxidation, which is protective in the first stage, Fe, Cr, Ni are predominantly oxidized. The resulting oxides interact with sulfur, which leads to the destruction of the surface layer of the base alloy, i.e. scale is formed containing radionuclides.

- the formation of melts of Na ₂ SO ₄ and K ₂ SO ₄ , starting from a temperature of 396 ^o C.

The use of a group of substances from mixtures containing potassium, sodium, and sulfur for high-temperature deactivation predetermined the boundaries of the ratio characterizing them (Na: K). So, the minimum value of this ratio of atomic fractions of potassium and sodium - 0.3118 characterizes the substance (NaHSO ₃ + K ₂ S), and the upper value of the ratio characterizes the substance (Na ₂ S + KHSO ₃ ). The total sulfur content in these substances will be 29.93 and 33.84% by weight, respectively. Other sulfur-containing substances having a ratio of (Na: K) within the indicated limits can also be used for high-temperature decontamination (table 1).

The use of vanadium pentoxide as one of the ingredients of the substance allows, starting from a temperature of 625 ^o C, the process of so-called “catastrophic” corrosion of the surface of iron-carbon alloys due to the resulting Na ₂ O • V ₂ O ₄ • 5V ₂ O ₅ melts, which provide high oxygen diffusion rates to the oxidation boundary of the alloys. CrVO ₄ with a eutectic temperature of 635 ^o C, as well as COVO ₃ and NiVO ₃ compounds with melting points of 705 ^o C and 720 ^o C, respectively, are also formed [1].

At temperatures above 800 ^o C, the corrosion process is determined by the dissolution rate of the formed oxide layer of the alloy in the liquid phase, consisting of vanadium and sodium oxides with the formation of Fe ₂ O ₃ • V ₂ P ₅ at a temperature of 816 ^o C. Thus, for the 12X18H9T alloy this determines an increase in the corrosion rate from 2.8 • 10 ^-5 to 6.4 10 ^-5 g / cm ² • s, at a temperature of 800 ^o C, which ensures intensive formation of scale containing oxidized radionuclides.

 The use of metallic vanadium is impractical because it will not be able to interact with other reagents of the substance under these conditions until it is oxidized to vanadium pentoxide, which will require additional time and energy consumption.

 The boundaries of the amount of vanadium pentoxide used in the substance for decontamination are determined by the optimal results of the experiments shown in table 1.

The experimental results are shown in table 1, show that the optimum temperature conditions for the processing of iron-carbon alloys are their exposure at a temperature of 850-950 ^o C for 10-30 minutes.

Up to 850 ^o C, these processes have a low speed, which leads to an increase in processing time.

At heating temperatures above 950 ^o C, the effectiveness of the decontamination process is reduced due to the slowdown of the corrosion process due to decomposition and sublimation of the ingredients.

 The method of decontamination of a steel or cast iron surface contaminated with radionuclides is as follows. A layer of substance is applied to the decontaminated surface of metal radioactive waste. Then they are heated. After cooling and cleaning the surface from a layer of scale containing radionuclides, a pure metal is obtained. Solid waste is compacted and disposed of.

 An example implementation of the method.

Samples of radioactive waste from steel with a pollution level of 3.3 • 10 ⁴ - 4.9 • 10 ⁵ Bq / o together with a decontaminating substance were subjected to oxidative heating for 10-30 minutes in the temperature range of 700-1000 ^o C. As substances mixtures of the following substances were used:
Na ₂ SO ₄ , K ₂ SO ₄ , V ₂ O ₅ , K ₂ S, Na ₂ S, NaHSO ₃ , KHSO ₃ .

 The experimental results are shown in the table.

Claims (3)

 1. The method of deactivation of iron-carbon alloys, including heating them in the presence of a substance consisting of the first group of ingredients in the form of a mixture of sodium, potassium and sulfur salts, and cleaning the surface of the alloys from scale, characterized in that the substance is additionally introduced vanadium pentoxide in an amount of 75 - 25% by weight of the total amount of the substance with a ratio of atomic fractions of the ingredients of the first group of potassium: sodium = 0.3384 - 1.8144, while the sulfur content is selected within the range of 29.33 - 33.84% by weight in this group. 2. The method according to claim 1, characterized in that the heating is carried out at a temperature of 850 - 950 ^o C for 10 to 30 minutes  3. The method according to claim 1, characterized in that after cleaning the surface, the alloy is reheated in the presence of the aforementioned substance, cooled and cleaned.

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