GB2123203A - Process for the preparation of concentrates with high salt concentrations from the waste solutions of nuclear power stations - Google Patents

Abstract

Radioactive waste solutions from nuclear power stations, which wastes contain organic materials, boric acid or borates and other inorganic salts, acids and alkalis, are prepared as concentrates using a process wherein the concentration of potassium ions in the waste is maintained at the lowest possible level. The organic material is oxidised with the stoichiometric amount of alkali metal permangate - sodium rather than potassium - in an acidic solution, and the products of this reaction are further oxidised so as to form a precipitate which is removed from the solution, and the solubility of the borate remaining in the solution is increased by adjusting the molar ratio of alkali to boron. The solution so formed is then subjected to a concentration step.

Description

SPECIFICATION Process for the preparation of waste solution concentrates with high salt concentrations of nuclear power stations This invention relates to a process useful for the preparation of waste solution concentrates with high salt concentrations arising from nuclear power stations. Particularly, the object of the invention is a process for increasing the concentratability of radio-active waste solutions of various origins, formed in nuclear energy power stations (arising from leakages and containing boric acid and/or borates; organic materials, e.g. oxalic acid and/or oxalates, citric acid and/or citrates, tartaric acid and/ortartrates; nitric acid, alkali permanganates and sodium hydroxide; and nitric acid, potassium hydroxide and/or sodium hydroxide arising from the regeneration of ion-exchange resins) in such a way that neither crystalline nor other solid materials are formed during concentration (evaporation or other operations) or during storage (at least at 20"C) of the concentrates.

The primary water cycle of the so-called pressurised water reactors (PWR) contains in general 4-12% of boric acid owing to the advantageous neutron-absorbing capacity of boron. The boric acid content of the primary cycle is regulated according to the condition of the heating elements and purified, the boric acid is recycled through the so-called boric acid recirculation cycle to the primary cycle.

Most nuclear power stations do not, however, have the technical capacity themselves for recovering and recycling boric acid passing into the waste water cycle from the primary cycle owing to various intentional or unintentional leakages. Thus, radioactive wastes containing boric acid accumulate, at the rate of several hundreds m3/year, and they are stored until decomposition of the isotopes of a short half-life (from a few months to one year) or until a final processing.

The storage, embedding in cement or bitumen etc. and burying of liquid radioactive wastes are costly and require much equipment. Thus, from the viewpoint of unit storage cost, unit cement or bitumen consumption as well as of unit cost of burying, it is more favourable to use a process wherein a more concentrated waste solution can be prepared. A process is known from Japanese Published Application No.

80-34397 (International Class G21 F9/14) in which waste solution containing boric acid is evaporated to dryness before the final solidification. However, most of the nuclear reactors work with a 'moist' process, i.e.

with solutions, for a variety of reasons, such as considerations of environment protection, avoidance of forming radioactive powders or dust, automatibility etc.

Two basic criteria should be taken into account concerning the concentration of radioactive waste solutions of low activity: a) The solution to be prepared should be as concentrated as possible, given that the storage capacity of a given power station is limited and given also the costs of the final solidification and burying.

b) The equilibrium concentration of the solubilities of the components should not be exceeded, as a precipitation of crystals or solid materials could cause faults or breakdowns in the equipment used for the concentration of the waste solution, or during pumping or storage.

Our process has been developed by extensively taking account of the above criteria; it does not require any separate or purpose-built equipment or operating unit, but rather it utilizes the pre-existing water treatment and storage apparatus, and by suitably chemically treating the waste solution according to the invention, a waste solution can be prepared that is nearly three times more concentrated than has hitherto been achievable by the traditional methods on the basis of the technological experiences of operators of nuclear power stations as well as of our own investigations.

A similar aim was disclosed in Soviet Patent Specification No. 654010 which is concerned with increasing the concentration of waste solutions of nuclear power stations containing oxalates and borates. It suggests the adjustment of the pH of the solution to 3.5-4.0 by using nitric acid, whereby a salt concentration of 130-150 g/dm3 is achieved in the concentrate.

As will be seen, the process according to this invention is more advantageous than this Soviet proposal partly from the point of view of corrosion, because the equipments made from metal, such as evaporators and storage vessels, are exposed to corrosion to a lesser degree owing to the use of a less acidic medium (pH > 5) and partly because a waste solution concentrate of approximately doubled salt concentration, i.e.

260-310 g/dm3 is prepared, depending on the boric acid and/or borate content.

According therefore to the present invention there is provided a process for-preparing a highly concentrated waste solution from a solution of radioactive waste from nuclear power station, which waste contains organic materials, boric acrid and/or borates, and other inorganic salts, acids and alkalis, the process comprising rnaintaining the concentration of potassium ions in the waste solution at the lowest possible level, oxidizing the organic materials with a stoichiometric quantity of an alkali permanganate, in an acidic medium and thus removing them from the solution, at the sametime oxidizing the decomposition products of the permanganate so as to transform them to a precipitate, separating the precipitate from the solution, increasing the solubility of boric acid and/or borates by adjusting the molar ratio of alkali to boron, and subjecting the resulting solution to a concentrating operation.

Preferred embodiments of the invention will now be described with the aid of the drawing in the only Figure of which a schematic illustration is given of the principle of the system for processing nuclear power station waste solutions of low activity.

Aqueous solutions arising from various decontamination and regeneration stages are received in the settling vessel 1. The waste solution passes through a filter 2 to a concentrating unit 3 which may be an evaporator operating at atmospheric or reduced pressure, or it may be a device of another type such as the more recently introduced reverse osmosis type of device. The concentrate is allowed to dwell in a storage tank 4 until completion of the decay of the short half-life isotopes. Then the dissolved salts are fixed in a solid matrix by various solidification processes, (i.e. by cementation, embedding in bitumen etc.) and buried.

An impdrtant feature of the process according to the preferred embodiment of the invention consists in that in the waste solution in the settling tank 1 various appropriate and easily realizable chemical reactions are caused to take place so as to bring the solution to a physico-chemical condition which exhibits much more favourable properties for concentrating without precipitation or separation of solid materials or crystals, than in its original state (a concentrate having a nearly five times higher concentration can be prepared) and this concentrate also remains thermodynamically stable at the storage temperature (at least 20or), i.e. no crystal formation or solid material separation occurs.

The advantages of a more concentrated solution have been demonstrated above in relation to the unit costs of solidification, transport and burying. Here we wish merely to emphasize that we know of cases where a nuclear power station had to be shut down because of the limited capacity of the storage tank 4, and such shut-downs evidently also have conomic consequences.

Afurther advantage of our preferred embodiment is that the concentratability of the concentrate may further be enhanced by inserting a boric acid recovery stage between the units 3 and 4. By using the process of the preferred embodiment of the invention, the physicochemical condition of the waste solution is also optimal for the recovery of boric acid through a cellulose-acetate membrane, see Japanese Patent Specification No. 77-09684.

The Figure (Figure 1 ) illustrating the principle of the treatment system for waste solutions of low activity from nuclear power stations utilizes the following reference letters: A: Radioactive waste solutions of various origin B: Settling tank C: Filter D: Evaporator E: Concentrate F: Storage tank for concentrate G: Solidification.

The main features of the process of the preferred embodiment of the invention are summarized as follows.

1) The concentration of K ions in the waste solution is maintained at as low a level as possible, suitably below 10-2 mol/dm3, by using a decontaminating agent containing sodium permanganate instead of potassium permanganate for decontamination of the equipments and an appropriate sodium compound is always used instead of a potassium compound in all other cases where the use of a potassium compound is not indispensable for other reasons.

This method is significantly more favourable than the recovery of K ions and K±compounds from large amounts of waste solutions in order to achieve the given concentration of less than 10-2 mol/dm3 of K ions.

A reduction of the K ions in the waste solution is necessary because the solubility of potassium borates (K2O.5B2O2.8H2O) is approximately one-third of that of the corresponding sodium borates (Na2O.5B203.8H20) 2) The elimination of organic materials, mostly of oxalic acid and/or oxalates, citric acid and/or citrates and tartaric acid andlor tartrates and simultaneously also the elimination of permanganates from the waste solution collected in the settling tank 1 proceeds as described below.

The elimination of organic materials is of advantage because, of all the components of the waste solutions, oxalates and borates have the lowest solubility and they exert a mutually unfavourable effect on each other's solubility (Linke, W.F.: Solubilities of Inorganic and Metal-Organic Compounds; American Chem. Society, Washington, D.C., 1958).

2.1) In the waste solution containing organic material, in most cases oxalic acid and/or oxalates, a strong mineral acid, suitably nitric acid, or a salt thereof and a strong alkali, suitably sodium hydroxide or a salt thereof, an alakali permanganate, suitably sodium permanganate, and boric acid and/or borates, the molar ratio of strong bases (suitably sodium hydroxide) to strong acids (suitably nitric acid) is adjusted by adding a strong acid (suitably nitric acid) or a strong alkali (suitably sodium hydroxide) in such a way that, according to paragraph 2.2 below, and including the alkali permanganate to be added to the waste solution, the molar ratio of the strong monovalent alkali (sodium hydroxide) to the strong monovalent acid (nitric acid} amounts to 1::1, i.e. the salt formation of the strong acid with the strong alkali becomes complete.

This is necessary for eliminating the oxalic acid at the appropriate pH value (pH < 7) according to paragraph 22 while only minimally increasing the salt concentration of the waste solution by the addition of the chemical.

2.2) In the waste solution, the ratio of oxalic acid and/or oxalates to the alkali permanganate (suitably sodium permanganate) is adjusted by adding the alkali permanganate (suitably sodium permanganate) or oxalic acid in order to assure a molar ratio of the oxalic acid and/or oxalate to the alkali permanganate (suitably sodium permanganate) of 2.5:1. Thus, the stoichiometric molar ratio is provided for the pentavalent reaction of the permanganate in the course of the oxidation of oxalic acid in a mildly acidic medium. The elimination of oxalic acid proceeds according to the following reaction in a mildly acidic medium (5 < pH7) (assured by the present process, according to paragraph 1.1): 2MnO4 + 5(COO)22-+16H+ = 2Mn2+ + lOCO2 + 8H2O (1) 2.3.After accomplishment of the requirements described above in paragraphs 2.1 and 2.2, then depending on the temperature (approximately 20 C) and concentration conditions, oxalic acid is reacted with sodium permanganate under stirring by blowing in air. The end of the reaction is indicated by the disappearance of the purple colour. The removal of carbon dioxide, the oxidation of Mn2+ ions and the formation of the manganese precipitate are promoted by the injection of air. Under these conditions, Mn2+ ions are transformed into a readily settling manganese precipitate under the effect of the oxygen in the air.

According to certain authors, the composition of the precipitate is Mn(O)OH (Erdey, L.: Bevezetés a kémiai analizisbe, (Introduction to Chemical Analysis), Budapest, 1951).

3. After the colour of the solution has disappeared, the molar ratio of sodium hydroxide to boron is adjusted to within a range of 0.17-0.25, suitably to 0.21, whereby an optimal solubility of boric acid and/or borates is assured and at the same time the complete separation of the manganese precipitate is promoted.

The solubility in water of boric acid is 48 g/dm3 (at 20"C)in the presence of neutral salts, e.g. sodium nitrate, while the solubility of borates characterized by the molar ratio of 0.21 of sodium hydroxide to boron as expressed in boric acid under identical conditions is 160 g/dm3.

4. Thereafter the stirring (air injection) is terminated and settling, filtering or centrifuging of the manganese precipitate take place.

It is expedient to carry out the chemical treatment and settling described in paragraph 2 in the settling tank 1. In principle, the order of the operations indicated under 2.1 and 2.2 can be reversed though the sequence given in the above description is more expedient. It can be achieved by the chemical treatment described above that the waste solution can be evaporated to a salt concentration of 120-150 g/dm3 instead of 60-80 g/dm3 while satisfying the condition that neither crystals nor other solid materials are precipitated during evaporation or subsequent sotrage in the tank 4 at at least 20"C. Where, however, sodium permanganate is used not only for the elimination of oxalic acid described under paragraph 2.2, but also for decontaminating, in place of potassium permanganate, and thus the K+ ion concentration in the waste solution in the settling tank 1 is reduced, then a concentrate containing 260-310 g/dm3 of salt can be prepared in the presence of a K+ concentration of 10-2 mol/dm3 or less, depending on the boric acid and/or borate concentration, in such a manner that no solid material will separate out at at least 20"C.

A concentrate of 400 g/dm3 salt concentration can also be prepared in the case of a low boric acid concentration or where recovery of boric acid and/or of borates is applied, under the conditions described above.

The invention will be further described with the aid of the following Examples given by way of illustration but not of limitation.

Example I The composition of waste solution in the settling tank 1 of a nuclear power station, calculated on the basis of its technological description, is characterized as follows: gidm3 Oxalate content expressed as oxalic acid 0.36 - 1.8 Borate content expressed as boric acid 3.10 - 8.04 Sodium nitrate content 0.85 - 13.6 Potassium permanganate content 0.05 - 0.32 'Sodium hydroxide' content 0.24- 0.8 'Potassium hydroxide' content: max. 0.06 Sodium and potassium hydroxide have been given in quotation marks, because in aqueous solutions they form oxalates with oxalic acid and borates with boric acid.

Both in this Example and in Example 2 described below, the basic stipulation for concentrating the solution was that no solid material should separate during evaporation or storage at at least 20"C.

Under this stipulation, a concentrate of 60-80 g/dm3 salt concentration could be prepared from a reference waste solution by evaporation at atmospheric pressure. However, by using the process according to the invention and using the chemical treatment, from the reference waste solution a concentrate of 120-150 g/dm3 salt concentration could be prepared, under identical conditions. Thus, the concentratability of the waste solution was significantly increased; this is because oxalic acid and/or oxalates, being only slightly soluble components, as well as the alkali permanganates were simultaneously eliminated from the waste solution. Moreover, the solubility of the borates was also considerably increased by adjusting the molar ratio of sodium hydroxide to boron to an optimum value, whereby a waste solution concentrate of nearly doubled concentration was prepared.Should the solution become overconcentrated, then the solid phase is represented by the composition K2O.5B203.8H2O.

Example 2 The composition of the reference waste solution of Example 1 was modified by replacing potassium permanganate sodium permanganate. From the viewpoint of the waste solution, this corresponded to a technological change wherein sodium permanganate instead of potassium permanganate is used as decontaminating agent. There are no technological obstacles to this change, since essentially, K+ ions were replaced by Na ions and the Na+ ions do not represent an interference factor during the decontaminating operation.

By using the process of the invention for the reference waste solution modified as described above, under conditions identical with those given in Example 1, a concentrate of 260-310 g/dm3 salt concentration was prepared. The concentratability of the solution was defined in this case by the solubility of sodium borate (150-160 g H3BO2/dm3 of waste solution as expressed as boric acid, at at least 20"C). Should overconcentrating occur, the composition of the solid phase that separates is Na20.5B203.10H2O. According to paragraph 3, the ratio of sodium hydroxide to boron was adjusted in order to increase the solubility of boric acid by forming the most suitable sodium borate. (The solubility of boric acid in water, in the presence of neutral salts, e.g. sodium nitrate, is 48 g H3BO3/dm3 at 20"C.) It should be noted that by reducing the molar ratio of boric acid and/or borate in the waste solution to be processed (when the leakage rate is low or where the boric acid is recovered from the waste solution), concentrates with 400 g/dm3 or even higher concentrations can be prepared by using the process of the invention.

Claims (8)

CLAIMS ~

1. A process for preparing a highly concentrated waste solution from a solution of radioactive waste from nuclear power station, which waste contains organic materials, boric acid and/or borates, and other inorganic salts, acids and alkalis, the process comprising maintaining the concentration of potassium ions in the waste solution at the lowest possible level, oxidizing the organic materials with a stoichiometric quantity of an alkali permanganate, in an acidic medium and thus removing them from the solution, at the same time oxidizing the decomposition products of the permanganate so as to transform them to a precipitate, separating the precipitate from the solution, increasing the solubility of boric acid and/or borates by adjusting the molar ratio of alkali to boron, and subjecting the resulting solution to a concentrating operation.

2. A process as claimed in claim 1 in which the K+ concentration of the waste solution is kept below 10-2 mol/1 000 ml by using in the power station a decontaminating agent containing an alkali permanganate excluding potassium permanganate and/or by using a nonpotassium alkali permanganate to oxidize the organic materials andlor by using a non-potassium alkali hydroxide or salt, for the regeneration of ion-exchange resins that may be utilized and in the course of other uses of alkalis or salts.

3. A process as claimed in claim 1 or 2, in which the waste solution is agitated by the injection of a gas containing oxygen, and the removal of carbon dioxide from the solution is promoted, simultaneously the decomposition products of permanganate are oxidized to transform those to a precipitate, and the precipitate is separated from the solution.

4. A process as claimed in any preceding claim in which the increase in the solubility of boric acid and/or borates, amounting to at least 160 g of H3BO311000 ml of the waste solution concentrate expressed as boric acid at 20 C, is effected by adding an alkali hydroxide, preferably sodium hydroxide.

5. A process according to claim 4, wherein the amount of alkali hydroxide is added so as to adjust the molar ratio of alkali hydroxide to boron to 0.17 to 0.25, preferably to 0.21.

6. A process according to any preceding claim, wherein the molar ratio of organic material, preferably oxalic acid and/or oxalates, to alkali permanganate is adjusted to 2.5:1.

7. A process according to claim 1 substantially as herein described with reference to either of the Examples.

8. Nuclear power station waste solution whenever made by the process claimed in any preceding claim.