CS210438B1 - Method of denitrification, solidification and fixation of radioactive solutions or suspensions - Google Patents

Abstract

The method is intended for the treatment of radioactive solutions containing nitrates, which are waste products from the operation of nuclear reactors. The purpose of the invention is to limit toxic nitrous exhalations during volume reduction and stabilization of these wastes into a form intended for permanent storage. This is achieved by mixing mixed waste solutions containing nitrates at pH values of 6 to 9 with ammonium sulfate, the solution is concentrated and denitrified at temperatures of 100 to 300 °C, and the remaining mixture is then solidified by the addition of calcium hydroxide.

Description

(54) Method of denitrification, solidification and fixation of radioactive solutions or suspensions

The method is intended for the treatment of nitrate-containing radioactive solutions which are omitted when operating nuclear reactors. The purpose of the invention is to reduce toxic inhalation emissions by volume reduction and stabilization of these wastes into a form intended for permanent disposal.

This is accomplished by mixing the mixed nitrate-containing waste solutions at pH 6-9 with ammonium sulfate, concentrating the solution and denitrifying at 100-300 ° C, and then adding the calcium hydroxide to consolidate the remaining mixture.

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The invention relates to a method for denitrification, solidification and fixation of radioactive solutions or suspensions, containing mixtures of nitrates and other fission cation salts with corrosion products, resulting in low or medium radioactivity from nuclear reactor operations.

Maintaining a nuclear reactor in satisfactory operation requires a very precise operating mode, resulting in a variety of nuclear waste with different radioactivity characteristics. According to the International Atomic Energy Agency (AAAE), the following wastes were classified according to radioactivity into five categories:

with activity

1. Low-level waste 37 kBq / m ³

2. 37 kBq to 37 MBq / m ³

3 · Moderately active waste 37 MBq to 3.7 GBq / m ³

4. 3.7 GBq to 370 TBq / m ³

5. ^* Highly active waste 370 TBq / m ³

In the nuclear reactor, the fission process generates thermal energy, which is transferred to a suitable cooling medium and converted into electricity. In this process, the construction materials are subjected to thermal and neutron radiation, and the active particles of the coolant are gradually enriched; also small amounts of fission products and fuel cell pores pass into the cooling ring.

In order to prevent contamination of the coolant beyond the permissible value, ion filters are immediately included in the cooling circuit, thus blocking the risk of environmental contamination. After some time, the ion exchangers become saturated and must be regenerated and cleaned of active corrosion and fission products. This results in a liquid effluent, which is the main source of low- and intermediate-level waste from nuclear power plants and the main salt component of which is the nitrate anion. Low-level and intermediate-level waste is generated in nuclear power plants, as well as in the decontamination and cleaning of buildings and equipment facilities, in the repair or replenishment of equipment, in the storage of spent or partially spent fuel cells in storage pools; radioactive materials are used, especially from laboratories or workplaces where, for example, contaminated work clothes are cleaned, etc.

High-level waste, on the other hand, is generated exclusively in operations where already spent nuclear fuel is recovered to produce uranium and plutonium. Here, very high activities, which are particularly problematic, necessitate incorporation into mechanically, chemically, thermally and radiologically stable materials, such as incorporation into glass melts, e.g. Certificate No. 202109 and Certificate No. 202611.

Although high-activity waste fission products account for more than 99.9% of all radionuclide waste, they account for less than 1% of total waste. However, the highest volumes are reached by low and medium radioactive solutions, both from auxiliary operations of fuel cell regeneration and from nuclear power plant operation in particular. These wastes in the form of sludge, concentrates and ion exchangers must be transferred to a stabilized product for final disposal. The fixatives used are primarily required to form homogeneous, solid and not readily dispersible products which do not contain free water and in which the radionuclides are tightly bound. With regard to low- and intermediate-level waste, they are not subject to strict aspects of thermostability and radiation stability, as with high-level waste.

Several different techniques are currently used to solidify and fix low- and medium-level radioactive waste. In particular, the simplest method used is the fixation of medium or low-active residues after water decontamination by mixing them with cement or other silicates, e.g. No. 1,485,625. The silicate version, however, always results in an approximately doubling of the volume of radioactive residues, resulting in products which are not sufficiently resistant to water and especially to saline solutions.

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Similar difficulties are burdened by the otherwise relatively unpretentious and widely applicable process of fixing with lime and, for example, fly ash, as disclosed, for example, in US Patent 3,785,840.

However, both of these difficulties are eliminated by the very often used incorporation of concentrates into bitumen, for example as described in the FRG. Patent Application 2,784,098 according to which the concentrated waste is granulated or granulated and encapsulated with bitumen or other thermoplastic material. All water is evaporated and the salt residues are completely covered with bitumen. The product is filled into the drums while hot and allowed to solidify while cooling. Bitumenized products have, depending on the salt content of the concentrate, only about 1/3 to 1/6 of the volume, as compared to equivalent cement blocks. Also, their leachability is about 10 to 10 times lower than for eilatate blocks. Here, for example, the nitrate content does not interfere with the possibilities of incorporation here, but the contents of readily degradable substances must always be avoided. Similar properties can be achieved for products whose solid radioactive phase is encapsulated with organic polymers, for example, according to German Patent Application Publication No. 2,851,888. In general, solid wastes can also be provided by an encapsulation technique which entails the sealing of any of these products or even a solidified product into a corrosion resistant vessel.

SUMMARY OF THE INVENTION The present invention provides a process for denitrifying, solidifying and fixing radioactive solutions or suspensions containing mixtures of nitrates and other fission cation salts with corrosion products resulting in low or medium radioactivity from plants, nuclear reactors, and characterized in that waste mixed solutions containing sodium nitrate at a concentration corresponding to half of the total dissolved mass is mixed with ammonium sulphate at a pH equivalent to nitrate anion at pH 6-9, then the solution is concentrated and denitrified by conversion at temperatures of 100 to 3 ° C and reaching 38 to 25% wt. Sodium sulphate, the mixture is mixed with a stoichiometric amount of calcium hydroxide to a sulphate content at temperatures of 30 to 120 [deg.] C., whereupon the gradually solidifying mixture is filled in a known manner into non-corrosive containers which are optionally encapsulated and stored in storage sites.

The process of the invention utilizes the thermal conversion of sodium nitrate with ammonium sulfate or ammonium sulfate and the following, completely safe degradation to nitrous oxide, according to the reaction scheme

NaNO-j + (NH ₄ ) ₂ SO _{4 -} 1,., =, .., 100, -7, .10.0,.,. 9 _> 2 N ₂ O + Na ₂ SC> ₄ + 4 HgO

This makes it possible to decompose relatively easily, for example, the alkaline or caustic earth nitrates present, otherwise they are difficult to denit at high temperatures to produce nitrous emissions. However, the method according to the invention does not produce toxic nitrous emissions, but only non-toxic nitrous oxide; the resulting solid waste may be free-flowing and fixed to cement or bitumen. Preferably, however, solid waste can be fixed within the gypsum matrix within certain limits of the sodium sulfate concentration, since the conversion of gypsum, i.e. the formation of CaSO ₄ , can be utilized. 0.5 HgO and its hydration to a solid CaSO ₄ fixative. 2 HgO. For example, at a sodium sulfate concentration of 38-25 wt. and at temperatures of 30 to 120 ° C, such favorable circumstances arise in the aqueous medium by mixing with calcium hydroxide that the conversion of the resulting calcium sulphate, especially when mixed with sodium hydroxide and other admixtures, most often borates and compounds of corrosive products, e.g. solidifying gypsum mixtures that meet the requirements for low- and medium-level radioactive fixed wastes, in containers suitable for storage.

An advantage of the process according to the invention is in particular that, when concentrated and solidified, the mixture generally provides products that do not overflow and do not form eutectic melts, eliminate any handling of dry and dusty products which are the greatest source of environmental contamination. diameter 250 ° C; the necessary additive to the product is added only in the minimum necessary ratio, so that the volume reduction during solidification is characterized by an extremely high reduction factor; the process can be carried out in conventional, completely conventional equipment of ordinary material, while the end product has the character of meeting the requirements for special storage.

The following non-limiting examples illustrate the invention and, by varying, adapting, or combining them, the advantages of the present invention can also be well realized.

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Example 1

A model solution of moderately active nuclear waste water containing 220 g NaNOp 28 g NagCrO4, 56 g NagB2O4, 25 g NagCO4, 31 g NaOH, organic fly (Stavelic acid) 40 g in 1000 cm ^{3 of} water; amount of suspended solids 5 - 10 g, total concentration of solution 40% by weight, pH of solution 9.

250 µl of said solution was mixed with 58.3 g of ammonium sulfate and the solution was evaporated at 100 to 300 ° C to give a total of 92.7 g of the original dry matter of 100 g + 58.3 g. reduced form Cr ⁺⁺⁺ ) neither the presence of nitrates nor nitrites or ammonia was indicated. The mixture is poured over the Ca (OH) ₂ suspension formed by mixing 17.5 g of CaO with 88 cm ^{3 of} HgO and at 30 ° C a solidification mixture of 51 cm ³ , i.e. with a reduction factor of 1/5, allowed to freely set in a polyethylene vessel. and encapsulated by melting the polyethylene lid.

Example 2

In a similar manner, the same solution of Example 1 was treated when the pH of the solution was 6. The mixture was concentrated with the same ratio of ammonium sulfate and denitrified at 100 ° C.

The mixture was then heated to 120 ° C and mixed with 17.5 g of CaO with the addition of Ca (OH) ₉ .

163 cm HgO, left to solidify.

Claims (1)

BEFORE THE INVENTION A method for denitrification, solidification and fixation of radioactive solutions or suspensions containing mixtures of nitrates and other fission cation salts with corrosion products resulting in low or medium radioactivity from nuclear reactor operations, characterized in that waste mixed solutions containing sodium nitrate at a concentration corresponding to half All dissolved masses are mixed with ammonium sulphate in an equivalent ratio to the nitrate present at a pH of 6-9, then the solution is concentrated and denitrified by heating to 100-300 ° C to reach 38-25% by weight. at 30 DEG-120 DEG C., the mixture is solidified by the addition of calcium hydroxide in an amount corresponding to the stoichiometry of the sulfates present in the mixture.