RU2124770C1 - Radioactive ash residue vitrification technique - Google Patents

Abstract

FIELD: recovery of ash residue products of combustion of solid radioactive wastes. SUBSTANCE: process involves mixing radioactive ash residue with powdered potassium permanganate, aluminum, and calcium-silicon alloy, initiating exothermic reaction in mixture, and cooling down the melt to monolithic state. EFFECT: reduced power requirement and time of process, increased amount of radioactive ash residue filling final product. 2 cl

Description

 The inventive method relates to the field of environmental protection, and more specifically to the field of processing of radioactive waste (RAW) by fixing them in a stable solid matrix. The most effectively claimed method can be implemented in the processing of ash residues (ZO) generated during the burning of solid radioactive waste (SRW).

 A known method of processing by melting ash of radioactive waste [1], including pre-melting in a metal induction heater glass-forming flux, feeding SRT ash into the melt, holding the mixture to form a glass-like material and cooling it to form a monolithic product sent for long-term storage.

 The disadvantages of this method are its inefficiency associated with increased energy intensity, and the complexity associated with the multi-stage process, as well as the large amount of secondary waste generated.

 A known method of processing ash using microwave radiation, when compounds of iron, calcium, silicon and / or boron are added to the waste, which increase the melting efficiency of ash. The resulting material is cooled and sent for burial [2].

 The disadvantage of this method is also uneconomical, associated with the consumption of electricity and the duration of the process.

 The closest in technical essence to the claimed method is a method of curing ash by mixing ash with an inorganic additive, subsequent heating and cooling of the obtained material [3].

The method includes mixing ash with an inorganic additive, which is used as a glass frit of the following composition, wt.%: 59 SiO ₂ ; 12 Na ₂ O; 15 B ₂ O ₃ ; 4.3 CaO; 7.5 ZnO; 2.2 K ₂ O, with an ash: frit ratio of 30-40: 60-70, heating the mixture by external energy supply to a temperature of 1100-1250 ^o C, holding at this temperature for 2 hours and cooling. Withstanding the specified process parameters, get a glass-like material suitable for long-term storage.

The disadvantages of this method are:
uneconomy associated with increased energy intensity of the process;
- a long course of the process, which is accompanied by increased formation of secondary waste requiring re-processing;
- low filling of the matrix with radioactive waste - 30-40 wt.%.

The advantages of the proposed method are:
- reduction of energy costs due to the conduct of the process without external supply of energy;
- reduction of the process in time;
- increasing the ash content of the radioactive waste of the final product.

These advantages are achieved due to the fact that in the inventive method, the ash residue from the burning of combustible radioactive waste is mixed with exothermic components in a ratio of ingredients, wt.%:
Ash residue - 50-56
Potassium Permanganate - 20-22
Aluminum (powder) - 4-6
Silicocalcium - 18-22
A mixture of the ash residue and exothermic components is placed in a steel container, ignited, initiating an exothermic reaction between the components, during which the ash residue and reaction products pass into the melt, after the melt is cooled.

 The reaction proceeds in a narrow zone and spontaneously propagates through the substance in the form of a combustion wave. A high temperature develops in the combustion wave, the formation of new compounds and the generation of heat sufficient to form a melt from the components of the mixture.

The process of converting the ash residue to the glassy state is carried out due to the heat generated during the redox reaction between silicocalcium, aluminum and potassium permanganate. The reaction is characterized by a high thermal effect, takes place in the combustion mode and provides a rise in temperature in the combustion wave to 1700 ^o C.

The choice of silicocalcium as the main exothermic component is explained by the fact that it has been established that silicocalcium is able to form aluminosilicates such as Ca (Al ₂ Si ₂ ), CaMgSi ₂ O ₆ , Ca ₃ (Si with Al, Mg and Si as constituent components of the ash residue) ₃ O ₉ ), which are easy to melt and are well compatible with the glass phase, which helps to improve the quality of the final product by increasing its homogeneity.

 A decrease in silicocalcium content of less than 18 wt.% Will lead to an increase in the process temperature and the release of reaction products together with radionuclides into the environment. The increase in the content of silicocalcium over 22 wt. % will lead to incomplete participation in the process and to obtain a final product unsuitable for long-term storage.

 The choice as the second component of the reducing agent of aluminum is due to the large heat release during its oxidation. When the aluminum content is less than 4 wt. % heat dissipation in the system decreases, as a result of which the mixture does not melt, the process proceeds sluggishly, and an increase in its content in excess of 6 wt.% leads to an increase in heat generation and the explosive flow of the process.

Potassium permanganate plays the role of an oxidizing agent. A decrease in the content of KMnO _{4 of} less than 20 wt.% In the composition of the mixture will lead to a decrease in the process temperature, which will not ensure the melting of all components of the mixture and the formation of a glass-like product, resulting in a porous material that is not suitable for long-term storage. An increase in its content in excess of 22 wt.% Will significantly increase heat generation, which will lead to spraying of the melt and a dangerous process.

 The introduction of the ash residue from the burning of SRW of more than 60 wt.% Into a mixture with exothermic components will lead to a decrease in the process temperature, since the individual components of the ash, including silica, alumina and hydroxylaptite, are refractory and the heat generated by the reaction will not be enough to completely the interaction of all components of the mixture, resulting in unsatisfactory quality of the final product. When the ash residue is introduced into the mixture in an amount of less than 50 wt.%, The process temperature will increase, which will lead to a dangerous process flow due to an increase in the entrainment of radionuclides.

 The method is implemented as follows.

3 kg of a mixture of the composition, wt.%: Silicocalcium - 20, Al-4, KMnO ₄ - 20. ash-56, is loaded into a steel container, and an exothermic reaction is initiated using a flame-retardant cord or electric discharge. Then the container with the final product is cooled by naturally lowering the temperature and sent for disposal.

The proposed method, using silicocalcium, aluminum and potassium permanganate as energy-releasing reagents, allows to obtain a monolithic product, the structure of which is represented by amorphous glass phase and crystalline inclusions (Ca ₂ P ₂ O ₇ , KAlSiO ₄ , Ca ₂ SiO ₄ , Na ₃ Ca ₆ ( PO ₄ ) and others). The water resistance of the product for Na ⁺ is 10 ^-6 - 10 ^-7 g / cm ² • days, for ¹³⁷ Cs - 10 ^-6 - 10 ^-7 g / cm ² • days, which fully meets the requirements. The filling of the final product with the components of the radioactive waste is 50-60 wt.%.

As a result of tests, it was found that compared with the prototype
- for its implementation, the method does not require an external supply of energy;
-time of implementation of the method is reduced by 3-4 times;
- the degree of filling of the final product with radioactive waste increases by 2-2.5 times.

LITERATURE
1. Japanese application N 4-50558 B ₄ MKI ⁵ : G 21 F 9 / 30.9 / 32, op. 02/28/85.

 2. Japanese application N 55076029-A, MKI G 21 F9 / 32, C 22 B 7/00, H 05 B 6/64, op. 07.06.80.

 3. Management of Alpha-Contaminated Wastes. IAEA. Vienna, 1981, p. 339-354, C.R. Palmer, G. B. Mellinger, J.M. Rusin.

Claims (2)

1. The method of vitrification of the radioactive ash residue, comprising mixing the ash residue with powder components, ensuring its transformation into a glassy state, heating the mixture to form a glass melt and cooling it to obtain a monolithic product, characterized in that as powder components, providing the transfer of the ash residue into glassy state, use a mixture of potassium permanganate, aluminum and silicocalcium with a total ratio of ingredients, wt.%:
Radioactive ash residue - 50 - 56
Potassium Permanganate - 20 - 22
Aluminum - 4 - 6
Silicocalcium - 18 - 22
and the mixture is heated by initiating an exothermic reaction between the components of the mixture of the radioactive ash residue, potassium permanganate, aluminum and silicocalcium.  2. The method according to claim 1, characterized in that the initiation of an exothermic reaction between the components of the mixture of the radioactive ash residue, potassium permanganate, aluminum and silicocalcium is carried out by setting the mixture on fire or passing an electric discharge through the mixture.