RU2332732C1 - Method of decontamination of radiation-alloyed silicon - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: in consecutive two-stage processing with alkaline oxidative solution, which contains sodium hydroxide, and with solution of oxalic acid, on both stages oxidative solutions based on hydrogen peroxide are used. The second stage is carried out not later than 5÷30 minutes after the end of the first stage. On the first stage it is preferable to use oxidative solution with the following composition: 10-50 g/l NaOH+20÷30 g/l H₂O₂, on the second stage - oxidative solution with composition: 20÷50 g/l H₂C₂O₄+20÷30 g/l H₂O₂.

EFFECT: possibility of silicon decontamination at room temperature and obtaining silicon suitable for use in any branch of industry.

3 cl, 1 tbl

Description

The claimed invention relates to the deactivation of doped silicon and may find application in its industrial production in power reactors.

The fundamental basis of the method of radiation doping of silicon is the conversion of silicon-30 nuclei into phosphorus-31 under the influence of thermal neutrons, in the water-cooled channels of a nuclear reactor, for example RBMK-1000. As a result of neutron irradiation, silicon ingots acquire a large volume and surface radioactivity. Volumetric radioactivity is mainly due to the unstable isotopes ³¹ Si and ³² R. Surface radioactivity is due to the adsorption of silicon impurities of ²⁴ Na, ⁵¹ Cr, ⁵⁴ Mn, ⁵⁹ Fe, ⁶⁰ Co and others from the water in the irradiation channel. To obtain usable radiation-doped silicon, decontamination of its surface is required. The decontamination process is complicated by a number of features characteristic of the surface of single-crystal silicon. First of all, in air, silicon is coated with a continuous film of silicon oxide (SiO ₂ ). Silicon oxide gives the surface increased sorption activity, as a result of which various radioactive contaminants from the environment are deposited and retained on it. In addition, silicon oxide has hydrophobic properties, is poorly wetted by water and difficult to dissolve in acid solutions, which makes deactivation difficult. Radionuclides adsorbed by the surface diffuse into the surface layer of silicon, which leads to an increase in volumetric activity.

A known method of decontamination (RF Patent No. 2119688), which consists in treating the surface of silicon ingots with an alkaline oxidizing solution of 10 ÷ 100 KHON + 0.5 ÷ 5 g / l X ₂ S ₂ O ₈ , where X is an alkali metal.

The disadvantages of this method are:

- low decontamination efficiency;

- the formation on the surface of silicon, after decontamination, of secondary deposits, consisting mainly of sodium silicate (water glass), which leads to a deterioration in the semiconductor characteristics of doped silicon and necessitates its mechanical processing after deactivation;

- increased processing temperature (80 ÷ 100 ° C).

There is also a known method of decontamination, which consists in treating surfaces with solutions of nitric and oxalic acids with the addition of fluorides (Ampelogova N.I. et al. Decontamination in nuclear energy, M., Energoatomizdat, 1982, p. 124). The method is effective for removing sparingly soluble radioactive oxides from the surface of stainless steel.

The disadvantage of this method is that it cannot be used for deactivation of silicon in nuclear power plants due to the high corrosivity of the reagents and the inability to process liquid radioactive waste generated during decontamination using standard technology.

The closest analogue is the method of decontamination (Ampelogova N.I. et al. Deactivation in nuclear energy, M., Energoatomizdat, 1982, p.135, 183-185), which consists in a sequential two-stage treatment of the silicon surface with an oxidizing solution of alkaline permanganate first potassium followed by a solution of oxalic acid. Using this method allows you to effectively deactivate the surface of carbon and stainless steels, zirconium, brass and bronze.

The disadvantages of this method are:

- low efficiency of silicon deactivation, especially with deep pollution, which does not allow to obtain a material suitable for unlimited use;

- the need for increased processing temperature (70 ÷ 100 ° C), which increases the cost of doped silicon;

- deterioration in the semiconductor characteristics of doped silicon after deactivation due to the formation of manganese compounds on its surface.

The problem solved by the invention is to increase the efficiency of the method of decontamination, reducing labor costs.

The essence of the present invention lies in the fact that in the method of deactivating radiation-doped silicon by sequential two-stage treatment with an alkaline oxidizing solution containing sodium hydroxide and a solution of oxalic acid, it is proposed to use oxidizing solutions based on hydrogen peroxide in both stages, and to carry out the second stage no later than 5 ÷ 30 minutes after the end of the first. In addition, it was proposed at the first stage to use an oxidizing solution of the composition: 10 ÷ 50 g / l NaOH + 20 ÷ 30 g / l H ₂ O ₂ , and at the second stage an oxidizing solution of the composition: 20 ÷ 50 g / l H ₂ C ₂ O ₄ + 20 ÷ 30 g / l H ₂ O ₂ .

In the proposed method, the following distinctive features were used:

Sign 1 - deactivation of radiation-doped silicon is carried out by a sequential two-stage treatment with oxidizing solutions based on hydrogen peroxide, and the treatment with the second solution should be carried out 5-30 minutes after the first treatment.

Sign 2 - for the treatment at the first stage, an oxidizing solution containing 10 ÷ 50 g / l NaOH + 20 ÷ 30 g / l H ₂ O _{2 was used} .

Sign 3 - for acid treatment, an oxidizing solution containing 20 ÷ 50 g / l H ₂ C ₂ O ₄ + 20 ÷ 30 g / l H ₂ O _{2 was used} .

In order to justify the compliance of the claimed combination of features of the invention with the criteria of "novelty", ″ inventive step ", we present the following:

On the basis of 1. As follows from the data given in the table, the processing of doped silicon only with oxidizing solutions using only the first or second stages (experiments No. 3, 4) does not provide the necessary degree of deactivation. In addition, when doped silicon is treated only with an oxidizing solution containing sodium hydroxide and hydrogen peroxide, secondary deposits consisting of sodium silicate are formed on the surface (experiments No. 4-7). Only sequential treatment with oxidizing solutions leads to a positive effect: first, an oxidizing solution containing sodium hydroxide and hydrogen peroxide, and then an oxidizing solution containing oxalic acid and hydrogen peroxide, and the time interval between treatments should not exceed 30 minutes. Otherwise, a film of secondary deposits remains on the surface (experiment No. 14), which affects the semiconductor characteristics of doped silicon. It is not possible to start processing silicon with an acid solution earlier than 5 minutes due to the need for silicon extraction and transportation operations.

According to characteristic 2. The experimental results shown in Table 2 show that the optimal concentration of reagents in an alkaline oxidizing solution is 10 ÷ 50 g / l for NaOH, and 20 ÷ 30 g / l for Н ₂ O ₂ , with a positive effect observed only with the combined presence of sodium hydroxide and hydrogen peroxide in the solution. When silicon is treated with solutions containing only sodium hydroxide or hydrogen peroxide, the degree of removal of radioactive contaminants is insignificant and amounts to 80% and 40%, respectively. At concentrations of NaOH less than 10 g / L and H ₂ O ₂ less than 20 g / L, the decontamination efficiency is sharply reduced. Increasing the concentration of NaOH more than 50 g / l and H ₂ O ₂ more than 30 g / l does not give an additional effect (experiment No. 12) and is not economically feasible.

According to characteristic 3. From the data given in Table 3, it follows that the minimum concentration of oxalic acid and hydrogen peroxide in an acidic oxidizing solution is 20 g / L. An increase in the concentrations of H ₂ C ₂ O ₄ more than 50 g / L or H ₂ O ₂ more than 30 g / L does not give an additional effect on the removal of radionuclides from the surface of doped silicon (experiment No. 12).

Example. For decontamination, a batch of radiation-doped silicon ingots irradiated with a thermal neutron flux (fluence of 3.58-4.29 · 10 ¹⁷ n · cm ^-2 ) was used in the RBMK-1000 reactor, with surface contamination from 250 to 310 Bq / cm ² . To ensure radiation safety and eliminate volumetric radioactivity, the irradiated silicon ingots in the irradiation container were immersed in the pre-reactor pool and kept for a period corresponding to at least 10 half-lives of ³¹ Si and ³² P. radionuclides. After exposure for 5 days and decay volumetric activity of the ingots of silicon was unloaded from the irradiation container, placed in lodges that prevented the mutual contact of the ingots, and was decontaminated to remove surface activity. The washing of the surface radioactivity of silicon was carried out in separate baths in two stages: the first stage at room temperature for 60 minutes in an alkaline oxidizing solution of the claimed composition. This treatment is designed to remove the bulk of the radioactive contamination along with a film of silicon dioxide. The second stage of washing was carried out at room temperature for 60 minutes in an acidic oxidizing solution. Processing with this solution can further increase the decontamination efficiency and remove sodium silicate formed after exposure to an alkaline oxidizing solution from the surface of a silicon ingot.

After washing the ingots in decontamination solutions, silicon was washed with clean water and dried.

The deactivation efficiency was determined by the degree of remote activity β,%:

β = (A _beg- A _con ) / A _beg · 100,%

where A _beg , A _con - specific activity before and after treatment, Bq / cm ²

and the coefficient of deactivation Cd:

Cd = A _start / A _con .

The test results are shown in the table.

From the data given in the table, it is seen that the use of the invention allows deactivation of silicon at room temperature with a degree of activity removal (β) of about 100%, and secondary deposits do not form on the silicon surface that worsen its semiconductor properties, and to obtain silicon suitable for use in any industry. At the same time, the reagents used are widely used in nuclear power plants and are easily processed using standard technology with other liquid radioactive waste. The effectiveness of the deactivation of radiation-doped silicon according to the proposed method and method - the closest analogue is illustrated by the examples shown in the table.

Table No. p / p Decontamination Method Temperature,
° C The degree of remote activity,% Note one four 5 6 7 one 1. 50 g / l NaOH + 5 g / l KmnO ₄ , τ - 1 h 90 90 traces of manganese on the surface 2. 10 g / l H ₂ C ₂ O ₄ , τ - 1 h (closest analogue) (10) 2 1.100 g / l NaOH + 20 g / l KmnO ₄ , τ - 2 h 90 95 traces of manganese on the surface 2. 30 g / l H ₂ C ₂ O ₄ , τ - 2 h (closest analogue) (twenty) 3 50 g / l H ₂ C ₂ O ₄ + 30 g / l H ₂ O ₂ twenty 80 (5) four 100 g / l NaOH + 30 g / l H ₂ O ₂ twenty 99.45 traces of sodium silicate on the surface (182) 5 50 g / l NaOH + 30 g / l H ₂ O ₂ twenty 99,4 traces of sodium silicate on the surface (167) 6 10 g / l NaOH + 30 g / l H ₂ O ₂ twenty 98.94 traces of sodium silicate on the surface (96) 7 5 g / l NaOH + 30 g / l H ₂ O ₂ twenty 96,42 traces of sodium silicate on the surface (28) 8 1. 10 g / l NaOH + 20 g / l H ₂ O ₂ twenty 99.94 no trace of sodium silicate detected 2. 10 g / l H ₂ C ₂ O ₄ + 20 g / l H ₂ O _{2 the} time interval between acid and alkaline treatments is 15 minutes (1670) 9 1. 50 g / l NaOH + 30 g / l H ₂ O ₂ twenty 99.95 no trace of sodium silicate detected 2. 50 g / l H ₂ C ₂ O ₄ + 30 g / l H ₂ O _{2 the} time interval between acid and alkaline treatments is 15 minutes (2000)

Table continuation

one four 5 6 7 10 1.5 g / l NaOH + 30 g / l H ₂ O ₂ twenty 99 traces of sodium silicate on the surface 2. 15 g / l H ₂ C ₂ O ₄ + 30 g / l H ₂ O _{2 the} time interval between acid and alkaline treatments - 15 min (one hundred) eleven 1. 50 g / l NaOH + 10 g / l H ₂ O ₂ twenty 99 traces of sodium silicate on the surface 2. 50 g / l H ₂ C ₂ O ₄ + 10 g / l H ₂ O _{2 the} time interval between acid and alkaline treatments is 15 minutes (one hundred) 12 1. 70 g / l NaOH + 40 g / l H ₂ O ₂ twenty 99.94 no trace of sodium silicate detected 2. 70 g / l H ₂ C ₂ O ₄ + 40 g / l H ₂ O ₂ time interval between acid and alkaline treatments - 30 min (1670) 13 1. 50 g / l NaOH + 30 g / l H ₂ O ₂ twenty 99.95 no trace of sodium silicate detected 2. 50 g / l H ₂ C ₂ O ₄ + 30 g / l H ₂ O ₂ time interval between acid and alkaline treatments - 5 min (2000) fourteen 1. 50 g / l NaOH + 30 g / l H ₂ O ₂ twenty 99.92 traces of sodium silicate on the surface 2. 50 g / l H ₂ C ₂ O ₄ + 30 g / l H ₂ O ₂ time interval between acid and alkaline treatments - 45 min (1250)

Claims (3)

1. The method of decontamination of radiation-doped silicon by sequential two-stage treatment with an alkaline oxidizing solution containing sodium hydroxide and a solution of oxalic acid, characterized in that at both stages use oxidizing solutions based on hydrogen peroxide, and the second stage is carried out no later than 5-30 minutes after ending first. 2. The method according to claim 1, characterized in that in the first stage an oxidizing solution of the composition is used: 10 ÷ 50 g / l NaOH + 20 ÷ 30 g / l H ₂ O ₂ . 3. The method according to claim 1, characterized in that in the second stage an oxidizing solution of the composition is used: 20 ÷ 50 g / l H ₂ C ₂ O ₄ + 20 ÷ 30 g / l H ₂ O ₂ .