GB2159320A - Concentrating radioactive waste solutions from nuclear power stations - Google Patents

Abstract

To a radioactive waste solution obtained by eliminating decontaminating agents, e.g. oxalic acid/oxalates, citric acid/citrates; tartaric acid/tartrates and permangantes, from nuclear power station waste, there is added, depending on the composition, sodium hydroxide and/or potassium hydroxide or a strong mineral acid, e.g. nitric acid, so that the molar ratio of alkali hydroxide/boric acid in the solution is brought into the range 0.8 to 1.5 preferably 1.0, then the so-treated solution is evaporated to a salt concentration of 310 to 900 g/dm<3>. The solution treated contains inorganic salts, mainly borates and nitrates, and may contain boric acid; nitric acid, and alkali metal hydroxides particularly sodium hydroxide and/or potassium hydroxide. The concentration of potassium nitrate in the solution before evaporation should not exceed 220 g/dm<3>. The product is stable, at least at 20 DEG C, and may be stored for 1 year or directly embedded.

Description

SPECIFICATION Process for concentrating of radioactive waste solutions from nuclear power stations The invention relates to a process for the chemical treatment of radioactive waste solutions of nuclear power stations and for the preparation of concentrates from the solutions.

Particularly the object of the invention is a process for the preparation of a concentrate from a waste solution obtained by the elimination of decontaminating agents, containing among others oxalic acid and/or oxalates, citric acid and/or citrates, tartaric acid and/or tartarates and permanganates and which solution consists of inorganic salts mainly of boric acid and/or borates, nitric acid and/or nitrates, sodium hydroxides and/or potassium hydroxide.Depending on the composition of the solution sodium hydroxide and/or potassium hydroxide or a strong mineral acid, suitably nitric acid is added to the solution for adjusting alkali hydroxide/boric acid molar ratio into the 0.8-1.5 range, suitably to 1.0, taking into consideration that the concentration of the potassium nitrate, formed in acid-based reaction should not exceed the 220 g/dm3, then the chemically treated solution is evaporated to a salt concentration of 310-900 g/cm3, depending on the composition of the starting waste solution.

Due to the chemical treatment neither foaming nor precipitation of crystals occurs at evaporation and furthermore from the concentrate prepared according to our invention nor precipitation of crystals and/or solid material takes place during storage at least at 20"C.

According to the state of art a process is known from Japanese Published Application No.

80-34397, in which waste solution containing boric acid is evaporated to dryness before solidification. However, in most nuclear power plants for a variety of reasons such as avoiding the formation of radioactive dust or powders, susceptibility to automation, etc a "moist" process is used i.e. working with solutions is preferred.

According to the process described in Soviet Patent No. 654010 the pH of the waste solution containing boric acid and/or borates is adjusted to 3.5-4.0 by using nitric acid and then the salt concentration of the obtained solution is increased to 130-150 g/dm3. The process according to our invention is more advantageous then the method according to the SU No.

654010 partly from the point of view of corrosion, that is using alkali medium, in our process equipment is exposed to corrosion to a lesser degree, and partly because the waste solution concentrate obtained in our process has a salt concentration of 310-900 g/dm3, which is much higher than that achieved by the mentioned Soviet process.

The Hungarian Patent Application No. 1876/82 claims a process suitable for the treatment of waste solutions containing K+ ions, in a concentration below 10-2 mol/dm3, furthermore boric acid and/or borates. The achieved maximum salt concentration is 1 50 g/dm3. On the other hand, in the process according to the present invention there is no limit to the concentration of K+ ions and as a result, even in solutions with K+ ion content higher than 10-2 mol/dm3, salt concentration can be increased to 310-900 g/dm3.

Most of the nuclear power stations store waste solutions until completion of the decay of the short half-life isotopes and until the final processing. Considering specific storage costs, it is more advantageous to store radioactive waste solutions when concentrated solution can be prepared. When concentrating low activity solutions the most concentrated solutions should be achieved since the storage capacity of a nuclear power station is limited and because in this case the specific cost of solidification or burying is also smaller. However, while increasing concentration the equilibrium concentrations defined by the solubilities of the components should not be exceeded, because after supersaturation crystal precipitation occurs that may cause breakdowns in the equipment used for concentrating the waste solution, or during pumping or storage.

The main features of the process of the present invention are summarized as follows. The alkali hydroxide boric acid molar ratio in a radioactive waste solution from a nuclear power station obtained by the elimination of decontaminating agents including amongst others oxalic acid and/or oxalates, citric acid and/or citrates, tartaric acid and/or tartarates and permanganates or in a waste solution which is originally free of decontaminating agent and consists of inorganic salts, mainly of boric acid and/or borates, nitric acid and/or nitrates and sodium or potassium hydroxide, is adjusted by the addition of alkali hydroxide, suitably sodium and/or potassium hydroxide, or, if the solution is more alkaline than required, by the addition of a strong mineral acid, suitably nitric acid, into the range 0.8 ~ alkali hydroxide/boric acid ~ 1.5, suitably to 1.0, taking into consideration that in the concentrate with 310-900 g/dm3 salt concentration prepared from the obtained solution-favourably by evaporation-the concentration of potassium nitrate-formed in an acid base reaction-should not exceed 220 g/dm3, i.e.

in solutions containing potassium the alkali-hydroxide ratio is adjusted by sodium hydroxide addition. The elimination of decontaminating agents is carried out according to the Hungarian Patent Application No. 1876/82 (British patent application No. 8315559). After eliminating decontaminating agents the alkali hydroxide/boric acid molar ratio is adjusted either in the waste solution to be concentrated before the concentrating operation or in the concentrating operation suitably evaporation, in the evaporator while controlling alkali hydroxide/molar ratio continuously and preparing a stable concentrate with a salt concentration of 310-900 g/dm3.

As a result of the applied chemical treatment, such as elimination of decontaminating agents and adjustment of alkali hydroxide/boric acid molar ratio, no crystals or/and solid material precipitate during storage at 20"C, i.e. stable concentrate can be prepared with a salt concentration of 310-900 g/dm3 depending on the composition of the starting solution.

Solubility of boric acid in water and in water containing dissolved neutral salts, e.g. sodium nitrate, is 48 g/dm3, nevertheless, the solubility of alkali borates, such as sodium or potassium borates-in the solution with the defined alkali hydroxide/boric acid ratio, expressed in boric acid is 340 g/dm3 at 20"C.

The advantage of the process according to our invention is that it does not require any special equipment, it can be carried out in pre-existing water treatment and storage facilities and it can be achieved by the suitable chemical treatment so that the concentrate has a salt concentration of 310-900 g/dm3, which concentrate can be embedded and/or cemented without evaporation after a regular 1 year storage. This way one step of the technology, i.e. the evaporation before cementing, can be omitted and at the same time much more waste solution can be stored as concentrate in a given storage capacity.

Example 1 The composition of a water solutionconsidered as a reference solution -alculated on the basis of the technological description of a nuclear power station is characterized without specifying the radioactive elements and their activity as follows: g/dm3 Oxalate expressed as oxalic acid 1.8 Borate expressed as boric acid 8.04 Sodium nitrate 13.6 Potassium permanganate 0.32 Sodium hydroxide 0.8 Potassium hydroxide 0.06 According to our process decontaminating agents are first eliminated from the reference solution by the method described in Hungarian Patent Application No. 1876/82 and then, after adjusting alkali hydroxide/boric acid molar ratio to 1:1, concentrate with 850 g/dm3 salt concentration is prepared by evaporation.

Using the process of the invention 2.7 ml nitric acid of 65% by weight concentration and 0.95 g KMn04 is added to each liter of the reference solution and then air is bubbled through the solution for 1 hour. Thereafter 5.20 g NaOH is added to each liter of the solution, the solution is filtered, the filtrate is evaporated until a concentrate with 850 g/dm3 salt concentration is obtained. During subsequent storage at least at 20'C no crystals and/or solid material precipitated.

Comparative example: The same reference solution was treated according to Hungarian Patent Application No. 1876/82 as described below: To each litre of the solution 2.7 ml nitric acid of 65% by weight concentration and 0.85 g NaMnO4 is added then air is bubbled through the solution for 1 hour. Thereafter 0.23 g NaOH is added to each litre of the solution, then the solution is filtered. By evaporation of the filtrate a concentrate with a salt concentration of maximum 1 50 g/dm3 could be prepared because at higher concentrations precipitation of crystals and/or solid material could not be avoided during storage at least at 20"C.

Example 2 Composition of a radioactive waste solution from a working nuclear power station-without specifying the radioactive elements and their activity-is as follows: g/dm3 Boric acid 1.7 Sodium nitrate 0.8 In accordance with the claimed process 1.10 9 NaOH is added to each litre of waste solution and a concentrate of 715 g/dm3 salt concentration is prepared by evaporation. No crystals and/or solid material precipitated from the concentrate during evaporation and storage at least at 20"C.

Comparative example: The same waste solution was treated according to Hungarian Patent Application No. 1 876 82 as described below: To each liter of the solution 0.23 g NaOH is added. From the solution a concentrate of maximum 240 g/dm3 salt concentration can be prepared by evaporation in order that no precipitation of crystals and/or solid material should occur during storage at least at 20"C.

Example 1 Composition of a radioactive waste solution from a working nuclear power plant--without specifying the radioactive elements and their activity-is as follows: g/dm3 Boric acid 1.46 Sodium nitrate 0.16 Potassium nitrate 1.95 Nitric acid 1.33 Processing of the waste solution is carried out in a nuclear power station according to our invention. 1.8 kg NaOH is added to each m3 of the waste solution and a stable concentrate with a salt concentration of 530 g/dm3 is prepared by subsequent evaporation. No precipitation of crystals and/or solid material takes place in the evaporator or during storage at 20"C.

Comparative example: According to the nuclear power station's instructions of technology a concentrate with a salt concentration of 60-80 g/dm3 only is prepared from the same waste solution.

Claims (5)

1. A process for concentrating of radioactive waste solution from nuclear power stations, characterized in that to the waste solution obtained by the elimination of decontaminating agents containing, among others, oxalic acid and/or oxalates, citric acid and/or citrates, tartaric acid and/or tartarates and permanganates and which waste solution consists of inorganic salts, mainly of boric acid and/or borates, nitric acid and/or nitrates, alkali hydroxides, especially sodium hydroxide and/or potassium hydroxide, depending on the composition of the solution, sodium hydroxide and/or potassium hydroxide or a strong mineral acid, suitably nitric acid, is added, adjusting alkali hydroxide/boric acid molar ratio into the 0.8-1.5 range, suitably to 1.0, increasing the solubility of boric acid and/or borates, expressed in boric acid equivalent to 340 g/dm3 and from the chemically treated waste solution a concentrated, i.e. 310-900 g/dm3 salt concentration, and stable solution at the evaporation temperature and at least at 20"C storage temperatures, is prepared by a concentrating operation, suitably by evaporation.

2. A process as claimed in claim 1 in which first the decontaminating agents are eliminated from the waste solution, then the alkali hydroxide/boric acid ratio is adjusted by adding nitric acid or sodium hydroxide and/or potassium hydroxide to the solution in such an amount, that the concentration of potassium nitrate, formed in an acid-base reaction is maximum 220 g/dm3 in the final concentrated solution.

3. A process as claimed in claim 1 and 2, in which alkali hydroxide/boric acid ratio is adjusted in the waste solution before evaporation and/or in the evaporator.

4. A process for concentrating radioactive waste solutions from which decontaminants have been removed, which process comprises adjusting the pH of the waste solution (if necessary) so that the molar ratio of alkali metal hydroxide to boric acid (or borate salts expressed as boric acid) lies in the range of from 0.8 to 1.5 and concentrating the solution until the salt concentration of the solution lies in the range of from 310 to 900 g/dm3.

5. A process as claimed in any one of claims 1 to 4, which is substantially as hereinbefore described in any one of Examples 1 to 3.