Method of handling radioactive solutions

Abstract

The invention relates to the field of environmental protection, more specifically to the field of processing radioactive waste, and can he used for the safe and effective handling of a large quantity of liquid radioactive waste of various activity levels that has been formed as the result of decontaminating protective equipment of boxes and chambers, and makes it possible to decrease the volume of stored waste by solidifying same and incorporating same into a ceramic matrix. For this purpose, radioactive solutions after decontamination of surfaces of protective equipment are evaporated as alkaline and acidic solutions containing sodium hydroxide, potassium permanganate, oxalic acid, and nitric acid until a solid residue forms, and are calcined, and the calcinate is mixed with components of a fusion mixture containing oxides of titanium, calcium, iron (III), zirconium, and manganese (IV) and aluminum in a specified ratio, and fused.

Claims

1. A method for handling radioactive solutions after decontamination of the surfaces of protective equipment, the method comprising the combined evaporation of alkaline and acidic solutions containing sodium hydroxide, potassium permanganate, oxalic acid and nitric acid to a solid residue, with subsequent calcination and mixing of the calcinate with the components of the fusion mixture containing oxides of titanium, calcium, iron (III) and aluminum in certain proportions, and the combined melting of all components to produce a ceramic matrix, wherein zirconium and manganese (IV) oxides are further introduced into the mixture of the calcinate and oxides of titanium, calcium, iron (III) and aluminum in the following component proportions, wt %: TABLE-US-00003 Calcinate of high-level waste 10.0-20.0 TiO.sub.2 53.0-57. CaO 9.0-11.0 Fe.sub.2O.sub.3 4.5-5.5 Al.sub.2O.sub.3 4.5-5.5 ZrO.sub.2 4.5-5.5 and the total content of MnO.sub.2 in the mixture does not exceed 10 wt %.

2. The method of claim 1, wherein the solid residue resulting from the evaporation of decontaminating solutions is calcined at 750 to 800 C. to fully decompose nitrates and carbonates.

3. The method of claim 1, wherein the calcinate is fused at 1,350 C. or higher for 1 hour to produce a monolithic fused ceramic material.

Description

(1) A method for immobilizing the high-level waste in a ceramic matrix (RF Patent No. 2315381 of May 22, 2006, Method for immobilizing high-level waste in a ceramic matrix, G21F 9/16) is the closest equivalent, having the greatest number of substantial features in common with the claimed invention.

(2) Said method comprises calcinating (denitrating) of high-level waste and mixing of the same with oxides (in the following proportions: calcinate of high-level waste10 to 20 wt %; TiO.sub.250 to 60 wt %; CaO7.5 to 12.5 wt %; MnO7.5 to 12.5 wt %; FeO.sub.32.5 to 7.5 wt %; Al.sub.2O.sub.32.5 to 7.5 wt %), heating the resulting mixture at a pressure not lower than atmospheric pressure to the maximum working temperature of 2,000 C., holding at the working temperature and cooling until a monolithic material is formed.

(3) Said method is designed to incorporate high-level waste with a high content of actinides and rare-earth elements, and is aimed at the immobilization and long-term geological storage of the actinide and rare-earth fraction of radioactive waste formed during the processing of spent nuclear fuel. The proposed method does not describe the possibility of incorporating high-salt waste containing significant amounts of alkali metal cations, including those containing components of acidic and alkaline solutions used to decontaminate protective equipment.

(4) The process is highly energy-consuming.

(5) The objective of this technical solution is to create a reliable and simple method for immobilizing non-process waste to produce a monolithic material suitable for long-term geological storage.

(6) To achieve this objective, a method is proposed for handling radioactive solutions after decontamination of the surfaces of the protective equipment, which method comprises the combined evaporation of alkaline and acidic solutions with subsequent calcination, the mixing of the calcinate with the components of the fusion mixture containing oxides of titanium, calcium, iron (III) and aluminum in certain proportions, and the thermal treatment and combined melting of all components to produce a ceramic matrix, characterized in that zirconium and manganese oxides are further introduced into the mixture of calcinate and oxides of titanium, calcium, iron (III) and aluminum in the following component proportions, wt %:

(7) TABLE-US-00001 Calcinate of high-level waste 10.0-20.0 TiO.sub.2 53.0-57.0 CaO 9.0-11.0 Fe.sub.2O.sub.3 4.5-5.5 Al.sub.2O.sub.3 4.5-5.5 ZrO.sub.2 4.5-5.5,

(8) and the total content of MnO.sub.2 in the mixture does not exceed 10 wt %.

(9) The solid residue resulting from the evaporation of decontaminating solutions is calcined at 750 to 800 C. to fully decompose nitrates and carbonates.

(10) The calcinate is fused with the fusion mixture at 1,350 C. or higher for 1 hour to produce a monolithic fused ceramic material.

(11) The presence of zirconium and manganese dioxides in the fusion mixture reduces the ceramic melting temperature, which makes the process less energy-consuming.

(12) The manganese oxide content of the fusion mixture varies depending on the actual content of potassium permanganate in process solutions after decontamination in terms of manganese.

(13) A manganese dioxide content in the fusion mixture of above 10% is inexpedient, since it raises the melting temperature of the mixture.

(14) If the zirconium dioxide content is below the lower limit, the phase composition of the ceramic material may change.

(15) It is inexpedient to increase the content of zirconium dioxide in the fusion mixture to above 5.5%.

(16) When said process operations are implemented, the solutions after decontamination can be evaporated to salts, and after the heat treatment thereof to form oxides, they can be mixed with the components of the fusion mixture to produce a ceramic material, which in turn can be obtained by fusing all the components to form a monolithic material suitable for the long-term geological storage of radioactive waste.

(17) Decontaminating solutions of two compositions, acidic (containing 5% HNO.sub.3, 0.5% H.sub.2C.sub.2O.sub.4 and 0.5% EDTA) and alkaline (containing 0.5% KMnO.sub.4 and 5% NaOH), were mixed together in equal volume proportions. The resultant solution was evaporated at 95 C., and after the water had evaporated, the temperature was raised to 350 C. The resultant residue was mixed with the components of the ceramic fusion mixture containing oxides of titanium, manganese (IV), aluminum, iron (III), calcium, and zirconium in such a way, that the composition of the resulting ceramic material conformed to the component proportions listed in Table 1. Due to the presence of hygroscopic sodium hydroxide in the residue resulting from the evaporation of decontaminating solutions (melting point: 323 C.), the product does not become perfectly dry after evaporation and heat treatment at 350 C. and does not require additional introduction of moisture-rich products to prevent dust formation. This reduces the emission of aerosols when the product is mixed with the fusion mixture and simplifies the process conditions for melting the target ceramic material.

(18) The manganese oxide content may be adjusted depending on the actual content of potassium permanganate in process solutions after decontamination in terms of manganese. If the acidic and alkaline solutions are not uniformly mixed, and if the potassium permanganate fraction in the process radioactive solutions is increased or reduced, the composition shown in Table 1 can be adjusted by reducing or increasing the MnO.sub.2 content in the initial fusion mixture.

(19) After mixing the solidified solutions with the fusion mixture components, the entire composition is heat-treated at 800 C. to decompose nitrates and carbonates, and the resulting composition is then melted at 1,350 C. for 1 hour. According to the X-ray phase analysis data, the melted material obtained after cooling to room temperature is formed by the main phase with the murataite structure (with the cubic crystal lattice parameter a=14.630.01 A) and the additional phase with the perovskite structure. The leaching rates of the main components (cations) of the matrix is in the range of 10.sup.6-10.sup.7 g/cm.sup.2 per 2.4 hours when the material is leached in hot bidistilled water at 90 C. (samples are taken on the 3rd, 7th and 14th days).

(20) TABLE-US-00002 TABLE 1 Compositions for producing ceramic material Component Content, mass fraction % TiO.sub.2 55.00 MnO.sub.2 8.94 CaO 10.00 AL.sub.2O.sub.3 5.00 Fe.sub.2O.sub.3 5.00 ZrO.sub.2 5.00 Waste (dry residue resulting from 11.06 evaporation of decontaminating solutions)