HU193209B - Method for increasing the concentratability of nuclear power station radioactive waste solutions - Google Patents

Abstract

The invention relates to a method for increasing the concentration of accumulated in nuclear power plants radioactive waste solutions. More particularly, the invention relates to a process for the treatment of the waste solution obtained after the removal of the oxalic acid and / or oxalates, citric acid and / or citrates, permanganates and other decontaminating agents, which in addition to other inorganic salts is primarily boric acid and / or borates, nitrates and / or nitric acid and sodium hydroxide and / or potassium hydroxide. For the purposes of the invention, depending on the composition of the solution by addition of sodium hydroxide and / or potassium hydroxide or - if the solution is more alkaline than desired - by adding nitric acid, the molar ratio between alkali metal hydroxide and boric acid, taking into account that the concentration of the Acid-base reaction forming potassium nitrate in the concentrate obtained after evaporation must be below 220 g / dm3, to a value between 0.8 and 1.5, suitably set to 1 and then by a concentration process, suitably by evaporation a 310- Concentrate containing 900 g / dm3 salts. The concentrate prepared according to the invention is storable at a temperature of at least 20C, neither crystals nor other solids separate out.

Description

The present invention relates to a process for the chemical treatment and concentration of radioactive waste solutions in nuclear power plants.

More particularly, the present invention relates to a waste solution obtained after the elimination of oxalic acid and / or oxalate, citric acid and / or citrate, permanganate and other decontaminating agents from the waste solution to be processed, which waste solution includes, in addition to other inorganic salts, boric acid and / or and sodium hydroxide and / or potassium hydroxide, depending on the composition of the solution, by adding sodium hydroxide and / or potassium hydroxide or, if the solution is more basic, by adding nitric acid, the molar ratio of alkali hydroxide to boric acid is adjusted to 1.0- In the range of 1.5, preferably to 1.0, provided that the concentration of potassium nitrate in solution in the concentrate after evaporation does not exceed 220 g / dm ³ and after a favorable molar ratio has been adjusted by evaporation, a concentrate with a salt concentration of 310-900 g / dm ³ is prepared, depending on the composition of the stock solution. As a result of the chemical treatment, neither foaming nor crystallization occurs during evaporation and no crystals and / or solids are precipitated from the concentrate produced by the process at a storage temperature of at least 20 ° C.

Although the prior art (Japanese Patent 80 34, 397) discloses that a boric acid waste solution is evaporated to dryness prior to final consolidation, it is known in most nuclear power plants for various reasons (environmental considerations - prevention of radioactive dust formation, automation and others) work with wet process, ie solutions.

According to US Patent No. 654010, the boric acid and / or borate containing waste solution is acidified to pH 3.5-4.0 with nitric acid and concentrated to a salt concentration of 130-150 g / dm ³ . On the one hand, our process is more corrosive than the above-mentioned Soviet process because it works in an alkaline medium, thus making the equipment less corrosive, and on the other hand, it produces a concentrated concentrate of 310-900 g / dm ³ salt.

Hungarian Patent Application No. 185,105 discloses a salt concentration of up to 310 g / dm ³ and a K ⁺ concentration greater than 10 ' ² mol / dm ^{3 when} working with solutions containing potassium and less than 10' ² mol / dm ³ K7. up to 150 g / dm ^{3 of} salt for concentration of the solutions, however, our process does not require strict restrictions on the K * concentration, so that solutions with an iC concentration of more than 10 ' ² mol / dm ³

It is concentrated to a salt concentration of 310-900 g / dm ³ .

Most nuclear power plants store the radioactive waste solutions that are generated until short-lived isotopes are decomposed and finalized. The more concentrated the waste solution concentrate is, the more advantageous the storage of liquid radioactive waste solutions is in terms of specific storage costs. Concentration of low-level radioactive waste solutions should preferably result in the production of the most concentrated waste solution concentrate because of the limited storage capacity available at a given power plant and the reduction in specific costs of final solidification and burial, but may not balance the solubility of the components. exceed because crystalline precipitation due to over saturation may cause malfunctions in the equipment for concentrating the waste solution and during pumping and storage

The essence of our procedure is summarized below. Nuclear waste solution in solution prepared after the elimination of oxalic acid and / or oxolate, citric acid and / or citrate, tartaric acid and / or tartrate, permanganate and other decontaminants, or in a waste solution free of decontaminants and containing in particular boric acid and other inorganic salts containing borates, nitrates and / or nitric acid or potassium and / or sodium hydroxide, depending on the composition of the solution, the molar ratio of alkali hydroxide / boric acid is added by addition of alkali hydroxide, preferably sodium hydroxide and / or potassium hydroxide, or if the solution is more basic than the desired value by addition of a strong mineral acid, preferably nitric acid, within a range of 1.0: galgal hydroxide / boric acid to 5 µg, preferably 1.0, taking into account that the resulting solution is preferably concentrated by by evaporation - tt The concentration of potassium nitrate formed in the acid concentration of 310-900 g / dm ³ does not exceed 220 g / dm ³ , so that the molar ratio of alkali hydroxide / boric acid is adjusted by the addition of sodium hydroxide in potassium solutions . Elimination of decontaminating agents is described in art. Hungarian Patent No. 5,198,198. The molar ratio of alkali hydroxide / boric acid, after elimination of the decontaminating agents, is adjusted in the waste solution to be concentrated and / or concentrated, preferably during evaporation, and the molar ratio of alkali hydroxide / boric acid is continuously controlled and concentrated in the evaporator. preferably by evaporation, a stable concentrate is prepared from the waste solution having a salt concentration of-310-900 g / dm ³ . Our chemical treatment - elimination of decontaminating agents and alkali hydroxide / boric acid

Adjustment of -2193209 molar ratio - as a result no crystals and / or solids are precipitated when stored at concentration temperature and / or at least 20 ° C, ie a stable concentrate at 20 ° C with a salt concentration of 310-900 g / dm ^3, depending on the stock solution can be produced.

The solubility of boric acid in water and in aqueous solutions of neutral salts, such as aqueous sodium nitrate, is 48 g / dm ³ , but the solubility of the alkali borates, sodium and / or potassium borates, in the molar ratio of alkali hydroxide / boric acid we set. 340 g / dm ³ expressed as boric acid.

The advantage of our process is that it does not require any additional equipment, but produces a concentrate with a salt concentration of 310900 g / dm ³ , which can be implemented in any water treatment and storage facility at any nuclear power plant and is directly immersed after the prescribed one year or can be cemented. This eliminates a technological step - pre-cementing - while allowing significantly more waste solution concentrates to be placed at the same tank capacity.

1. Application Example

The composition of the waste solution calculated as a reference solution based on the technological description of a nuclear power plant, without characterizing the radioactive elements and their activity, is as follows:

g / dm ³

Oxalate content expressed as oxalic acid 1,80 Borate content expressed as boric acid 8,04

Sodium nitrate content 13.60

Potassium permanganate content 0.32

Sodium hydroxide content 0.80

Potassium hydroxide content 0.60

The above reference solution was first decontaminated according to the procedure described in Hungarian Patent No. 185,105 and then? According to our process, the molar ratio of alkali hydroxide to boric acid was adjusted to 1: 1 and concentrated by concentration as described below to a salt concentration of 850 g / dm ³ :

In the process according to the above reference solution of 2.7 ml per liter of 65 wt% nitric acid and 0.88 g of _KMnO4 cent was added and air was bubbled into the solution for 1 hour. Thereafter, 5.2 g of sodium hydroxide per liter were added and the solution was filtered. The filtrate had a sodium nitrate content of 15.9 g / dm ³ , a potassium nitrate content of 1.1, a borate content of 1.1, expressed as boric acid, and a sodium hydroxide content of 8.04, and a potassium hydroxide content of 0.9. The filtrate was evaporated to give a stable concentrate having a salt concentration of 850 g / dm ³ , i.e. no crystals and / or solids were precipitated from the concentrate when stored at 20 ° C.

Comparative Example: The above reference solution was also treated according to Hungarian Patent Application No. 185,105, which is incorporated herein by reference, as follows:

Per liter of solution of 2.7 ml of 65 wt% nitric acid and 0.85 g of ₄ namn cent was added and air was bubbled into the solution for 1 h and per liter of 0.23 g of NaOH was added and the solution was filtered. By evaporation of the filtrate, a stable concentrate with a salt concentration of up to 150 microns / dm ^{3 was} obtained, i.e. no crystals and / or solids were precipitated from the concentrate upon storage at 20 ° C.

2. Application Example

The chemical composition of the radioactive waste solution generated in the operating nuclear power plant - without the characterization of the radioactive elements and their activity is as follows:

g / dm ³

Boric acid content 1.7

Sodium nitrate content 0.8

To the above waste solution was added 1.6 g of KOH per liter and evaporated to give a stable concentrate having a salt concentration of 715 g / dm ³ , i.e. crystalline and / or solid from the concentrate during evaporation and storage at 20 ° C. does not trigger.

Comparative Example: The above waste solution is described in Example 185.105. Hungarian Patent Application Serial No. 4,197, was treated as follows:

0.23 g of sodium hydroxide per liter was added to the waste solution, and the solution was concentrated to provide a stable concentrate having a salt concentration of up to 240 g / dm ³ , i.e. no crystalline and / or solid precipitated from the concentrate when stored at 20 ° C.

3. Application Example

The chemical composition of the radioactive waste solution generated by the operating nuclear power plant - without characterizing the radioactive elements and their activity - is as follows:

g / dm ³

Boric acid content 1.46

Sodium nitrate content 0.16

Potassium nitrate content 1.95

Nitric acid content 1.33

During the processing of the waste solution at the nuclear power plant, 1.8 kg of sodium hydroxide per m ³ was added to the above waste solution and a concentrated concentrate of 530 g / dm ³ salt was evaporated and the concentrate was stored neither in the evaporator nor at 20 ° C. no crystalline and / or solid precipitates at temperature.

comparative example:

Based on the technological instructions of the nuclear power plant, without chemical intervention, the above waste solution is concentrated by evaporation to a salt concentration of 60-80 g / dm ³ , which is still salt-free at a storage temperature of 20 ° C.

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4. Application Example:

The chemical composition of the radioactive waste solution generated by the operating nuclear power plant, without characterizing the radioactive elements and their activity, is as follows:

g / dm ³

Boric acid content 6.2

Sodium hydroxide content 4.0

Potassium hydroxide content 5.6

To the above waste solution was added 7.14 cm ^{3 of} 65% nitric acid per liter and evaporated to give a stable concentrate having a salt concentration of 608 g / dm ³ , ie crystallized from the concentrate during evaporation and storage at 20 ° C. and / or no solid precipitates. comparative example:

The above waste solution was treated as follows according to Hungarian Patent Application No. 185,105:

12.79 cm ^{3 of} 65% nitric acid per liter were added to the waste solution and the solution was concentrated to a stable concentration of up to 207 g / dm ^{3 of} salt, ie no crystalline and / or solid precipitated during storage at 20 ° C. Who.

Claims (2)

CLAIMS 1. A process for increasing the concentration of C-preconditioned wastewater solutions of a nuclear power station containing predominantly borates, boric acid, nitric acid and / or nitrates, alkaline hydroxide, in particular sodium and / or potassium _5- hydroxide, to 20 ° C. and / or solids-free, 10 to 310-900 g / dm ^{3 of} salt, characterized in that the molar ratio of alkali hydroxide / boric acid in the waste solution is adjusted to 1.0-1.5, preferably 1.0, sodium hydroxide and / or 15 / or by addition of potassium hydroxide or strong mineral acid, preferably nitric acid, and the waste solution is concentrated to a concentration of boric acid and / or borates in the resulting concentrate of not more than 340 g / dm ³ , or increasing the concentration of potassium nitrate to 220 g / dm ³ or less. Process according to claim 1, characterized in that the alkali hydroxide / boric acid The molar ratio of 25 in the waste solution to be concentrated is adjusted before and / or during evaporation.