WO2008002282A1

WO2008002282A1 - METHOD FOR DECONTAMINATION OF LIQUID RADIOACTIVE WASTES (VARIANTS), AND Cs-SELECTIVE SORBENT

Info

Publication number: WO2008002282A1
Application number: PCT/UA2006/000048
Authority: WO
Inventors: Oleksiy Grigorovich Kotvitskyy; Oleksandr Dmitriyevich Podoltsev; Maksym Viktorovich Gachenko
Original assignee: Fos International S.A.
Priority date: 2006-06-29
Filing date: 2006-07-28
Publication date: 2008-01-03
Also published as: UA82581C2

Abstract

Method for decontamination of a liquid radioactive wastes includes two basic steps: (a) sorption of radioactive caesium ions by Cs-selective sorbent at temperature under water boiling point during time that is sufficient for reduction of said radioisotopes content under their maximum allowable concentration, and (b) precipitation of radioactive cobalt and other metal ions as fine-dispersed particles of insoluble in water carbonates and/or hydrated oxides MexCyn7H2O and removal of said precipitate. It is formed by addition into LRW of carbonate-ions source and synchronous thermal (at temperature under water boiling point), mechanical and electromagnetic treatment of obtained alkaline mixture within an induction heater equipped with at least one free-vibrating short-circuited electroconductive heating element. Said Cs-selective SORBENT based on potassium-cobalt hexacyanoferrate K2Co[Fe(CN)6], which is fastened on granular silica gel as a supporting medium.

Description

METHOD FOR DECONTAMINATION OF LIQUID RADIOACTIVE WASTES (variants),

AND Cs-SELECTIVE SORBENT

Field of the Invention

The invention relates to processes for decontamination of liquid media containing radioisotopes, and to chemical composition of Cs-selective sorbents. Such processes and sorbents can be used for: extraction of such long-living radioisotopes as caesium, cobalt, strontium etc from liquid radioactive waste (hereinafter LRW), which come of action of cooling systems of power and, to a lesser extent, research nuclear reactors usually, processing of extracted radioisotopes and other toxic incombustible admixtures into compact solid radioactive waste (hereinafter SRW) suitable for secure burial, and recovery of boric acid from acidic LRW as free-utilisable by-product.

Prior Art

Typical LRW of research nuclear reactors, which have water-cooling active zones, contain usually some radioactive products of nuclear reactions and water- soluble products of corrosion of metallic details of cooling systems.

Typical LRW of any atomic power plant (hereinafter APP) is presented as water solution of boric acid with admixtures of some radioactive products and other substances such as sodium and potassium chlorides and iron sulphate and/or nitrate [Radiochemistry in Nuclear Power Reactors (1996)

[http://www.nap.edu/openbook/N100156].

It is generally known that LRW of APP may exist as:

^initial (especially, acidic) LRW», which take away directly from a cooling system of active zone of an arbitrary nuclear reactor periodically, and «fu$ion cake» that is prepared from initial LRW and presented as conditionally solid radioactive mass containing crystalline boric acid (H₃BCVnH₂O), concentrated admixtures of radioisotopes, and ballast matters.

In U.S.A., initial LRW (together with all present radioisotopes) use, as a rule, for making of concrete tablets suitable for stationary burial. However even concrete is not absolutely safety means for immobilisation of LRW.

Thus, safekeeping and systematic radiation monitoring of each burial place are necessary.

Fusion cake is prepared by evaporation of non-constitution water from initial LRW. This process is used in APPs those are built according to soviet designs. For example, averaged fusion cake of APP equipped by soviet WWER-1000 reactor (see data of the «Atomproekt» Institute, Russia) comprises such basic ingredients as: borate 400 - 600 kg/m³ (25-37% by weight)

Na⁺, K⁺.ions .....300 - 400 kg/m³ (19-25% by weight) NO₃ ^". ions 280 - 340 kg/m³ (17-21% by weight)

SO₄ ^2" . ions 50-75 kg/m³ (3.5-5% by weight)

Cr ions 25-30 kg/m³ (1.6-2% by weight) crystal water 155-545 kg/m³ (10-34% by weight). Moreover, such fusion cake contains usually radioisotopes of ¹³⁷Cs in amount about 10⁷ Bq/kg, ¹³⁴Cs about 10⁷ Bq/kg and ⁶⁰Co about 10⁴ or more Bq/kg, and analytically definable admixtures of non-radioactive ions Fe²⁺, Fe³⁺, Mn²⁺, Mg²⁺, Ca²⁺, Ni²⁺, Cu²⁺ or other metals.

Fusion cake is stored in corrosion-resisting metallic containers, which are suitable, in principle, for transportation on railways or motor roads according to rules of radiation safety.

Unfortunately, many APPs dispose so excessive stock of fusion cake that it is dangerous for personnel and environment. Therefore, it is desirable: first, to exclude accumulation of fresh fusion cake by regular decontamination of initial LRW in order to reduce amounts of radioisotopes under their maximum allowable concentrations, reuse of boric acid in nuclear reactor cooling systems and bury of compact SRW and ballast matters only, and, second, to dissolve earlier accumulated fusion cake in water, and then decontaminate regenerated LRW as stated above. Traditionally, attempts of decontamination of initial and regenerated LRW are based on use of such sorbents which, after practically full exhaustion of their sorptive capacity, may be transformed into SRW (especially, by calcination and/or melting) and then buried.

The most noticeable results of this kind, are attained in development of ion- exchange sorbents based on such complex compounds, as: potassium-nickel hexacyanoferrate that may be prepared by (as a rule, repeated) impregnation of silica gel with solutions of Ni(NO₃)₂ and K₄Fe(CN)₆ (Mimura et a/. Selective removal of caesium from highly concentrated sodium nitrate neutral solutions , by potassium nickel hexacyanoferrate{\\)-\oaύeύ silica gel / Solvent Extraction and Ion Exchange. Vol. 17; Issue 2; PBD, March 1999); hexacyanoferrates of transitional metals or quinquivalent antjmony hydroxide that are located on granular polymeric (in particular, phenol-formaldehyde) supporting medium (J. Narbutt, A. Bilewicz, B. Barto Composite ion exchangers: Prospective nuclear applications I Journal of Radioanalytical and Nuclear Chemistry. Vol. 183, No. 1, pp. 27-32).

However such ion-exchange sorbents allow selecting from water solutions the caesium radioisotopes only.

Therefore, more complicated processes and means are needed for decontamination of such APP's LRW in which the radioisotopes ¹³⁴Cs, ¹³⁷Cs and ⁶⁰Co are presented jointly (and, as a rule, together with ions of other metals).

So, radioisotopes ⁶⁰Co and ¹³⁴Cs may be jointly concentrated by cetyl pyridinechloride, separated by flotation and then extracted by hexacyanoferrate (II) cobalt (II) as sorbent (M. Aziz, Sh. G. Beheir Removal Of ⁶⁰Co and ¹³⁴Cs from radioactive process waste water by flotation I Journal of Radioanalytical and Nuclear Chemistry.

1995, Vol. 191 , No.1 , pp. 53-66).

However, content of caesium and cobalt radioisotopes is very small even in LRW regenerated from fusion cake. Thus, their joint concentration by flotation and following sorption is acceptable in research laboratory, while their separate extraction is preferable in industry.

Naturally, special means are needed thereto.

Such method for LRW decontamination, which is most near to proposed below method, is known already (Szabolcs Szoke, Gyόrgy Patzay, Laszlό Weiser

Development of selective cobalt and sesium removal from the evaporator concentrates of the PWR Paks I Radiochimica Acta, Vol. 91 , 2003 N°3, pp. 229-232). This method comprises of: evaporation of water from low-concentrated LRW to obtaining of such more strong (and suitable for long-term storage) solution, in which contents of ¹³⁴Cs, ¹³⁷Cs and ⁶⁰Co radioisotopes are increased substantially, extraction of radioactive caesium from said solution by a Cs-selective sorbent based on such ion-exchange material as potassium-nickel hexacyanoferrate (II), extraction of radioactive cobalt from said solution by an absorbent carbon, processing of used sorbents into compact SRW for following burial.

A processing line for carrying out of described method comprises of: at least one source of initial LRW (said source is usually a buffer collector of LRW that is effluent from a cooling system of a nuclear reactor's active zone), at least one source of concentrated LRW (said source is either a vaporiser connected directly to said collector of initial LRW, or a depository of beforehand prepared fusion . cake equipped at the output with a mixer for dissolution of said cake into concentrated LRW), at least one filled by a Cs-selective sorbent adsorber that is connected, in particular, to the source of concentrated LRW, at least one apparatus for extraction of radioactive cobalt and other metal ions (namely, adsorber filled by an absorbent carbon), which is connected to said "Cs- adsorber", pumps and a pipe manifold for transmission of all above-mentioned LRW and fluid intermediates, by- products and target products of their decontamination, and a means for processing of used sorbents into compact SRW for its following burial. However this process and known means for its carrying out are extremely uneconomical over great heat losses during water evaporation from initial LRW. Moreover, described process does not provide effective decontamination even if concentrated LRW is prepared by dissolution of fusion cake. In fact, sorptive capacity of hexacyanoferrate potassium-nickel decreases if liquid medium contains such metal ions as Fe²⁺, Fe³⁺, Mn²⁺, Mg²⁺, Ca²⁺, Co²⁺, Ni²⁺, Cu²⁺, and absorbent carbon is not selective as sorbent in principle. Addition into LRW of such complexing agents as citrate-ions, oxalate-ions or ethylendiaminetetraacetic acid complicates process of LRW-decontamination substantially but does not exclude negative action of said cations upon adsorption of caesium ions. And, finally, known process does not provide for recovery of boric acid as valuable free-utilisable by-product.

Summary of the Invention

The invention is based on the problem, by change of means, conditions and order of extraction of long-living radioisotopes from low-concentrated water solutions, to create such method and such sorbent which would provide effective decontamination of both initial LRW and acidic LRW prepared by dissolution of fusion cake.

First variant of solution of said problem consists in that a method for decontamination of LRW includes following steps: separation of portion of initial LRW for decontamination, addition of carbonate-ions source into said portion of initial LRW to assignment of alkaline reaction, synchronous thermal, mechanical and electromagnetic treatment of obtained mixture in an induction heater equipped with at least one such short-circuited electroconductive heating element that is capable to mechanical vibration under act of alternating electromagnetic field, separation of such fine-dispersed sediment of water-insoluble carbonates and/or hydrated oxides of cobalt radioisotope and other non-alkaline metallic admixtures of general formula Me_xO_y./?H₂O, which is created as a result of said addition of carbonate- ions and said synchronous thermal, mechanical and electromagnetic treatment, treatment of mother waters, which are residuary after separation of said sediment, by Cs-selective sorbent during time that is sufficient for extraction of caesium radioisotopes up to level under their maximum allowable concentrations, processing of used sorbent and said Me_xOyOH₂O sediment into compact SRW for following burial. Formation of said fine-dispersed sediment containing radioactive cobalt and other non-alkaline metals at temperature under water boiling point using said induction heater, and separation of this sediment in combination with following treatment of obtained mother waters by Cs-selective sorbent allows (as it would be disclosed and explained in examples below): first, to decrease of content of each radioactive component of LRW up to level under its maximum allowable concentration surely, second, to eliminate overgrowing of internal surfaces of any apparatuses or pumps and pipelines by radioactive sediments practically. Second variant of solution of said problem consists in that a method for decontamination of LRW includes following steps: separation of portion of acidic initial LRW of APP containing boric acid, products of corrosion of a cooling system of a power nuclear reactor and at least such radioisotopes as ¹³⁷Cs, ¹³⁴Cs and ⁶⁰Co, treatment of said LRW portion by Cs-selective sorbent at temperature no more than 6O⁰C during time that is sufficient for extraction of any caesium radioisotope up to level under its maximum allowable concentration, addition of carbonate-ions source into LRW practically decontaminated from radioisotopes ¹³⁷Cs and ¹³⁴Cs to assignment of alkaline reaction, synchronous thermal, mechanical and electromagnetic treatment of obtained mixture in an induction heater equipped with at least one such short-circuited electroconductive heating element that is capable to mechanical vibration under act of alternating electromagnetic field, separation of such fine-dispersed sediment of water-insoluble carbonates and/or hydrated oxides of cobalt radioisotope and other non-alkaline metallic admixtures of general formula Me_xCy/7H₂O, which is created as a result of said addition of carbonate- ions and said synchronous thermal, mechanical and electromagnetic treatment, partial evaporation of practically decontaminated mother waters to receipt of concentrated solution of boric acid with admixtures of mineral salts, cooling of said solution to fallout of boric acid sediment, separation of practically decontaminated boric acid sediment for following utilization, vacuum evaporation of mother waters to receipt of practically decontaminated dry mixture of salts, and processing of used sorbent and said Me_xO^nH₂O sediment into compact SRW for following burial.

It allows to decrease power inputs for LRW decontamination substantially and to obtain practically decontaminated boric acid as valuable by-product.

Third variant of solution of said problem consists in that a method for decontamination of LRW includes following steps: separation of portion of fusion cake, regeneration of liquid radioactive wastes by dissolution of said fusion cake's portion with acidified water in circulation regime through flow-type induction heater supporting temperature under water boiling point, cooling of prepared solution to sedimentation of boric acid practically decontaminated from ⁶⁰Co radioisotope and containing admixture of ¹³⁷Cs and ¹³⁴Cs radioisotopes, separation of boric acid sediment, wash-out of admixtures from it within a closed scrubbing circuit provided extraction of ¹³⁷Cs and ¹³⁴Cs radioisotopes on Cs-selective sorbent and drying of washed sediment to constant mass, addition of carbonate-ions source to supernatant, that are residuary after separation of boric acid sediment, to assignment of alkaline reaction, synchronous thermal, mechanical and electromagnetic treatment of obtained mixture in an induction heater equipped with at least one such short-circuited electroconductive heating element that is capable to mechanical vibration under act of alternating electromagnetic field, separation of fine-dispersed sediment of water-insoluble carbonates and/or hydrated oxides of cobalt radioisotope and other non-alkaline metallic admixtures of general formula Me_xCyOH₂O, which is created as a result of said addition of carbonate- ions and said synchronous thermal, mechanical and electromagnetic treatment, at least double dilution of mother waters residuary after separation of said sediment, treatment of prepared dilute solution by Cs-selective sorbent at temperature under water boiling point during time that is sufficient for extraction of caesium radioisotopes up to level under their maximum allowable concentrations, vacuum evaporation of decontaminated from caesium radioisotopes solution to receipt of practically non-radioactive dry mixture of salts, and processing of used sorbent and said Me_xCynH₂O sediment into compact SRW for following burial.

It provides for effective processing of present fusion cake's stocks into compact SRW suitable for secure burial, recovery of boric acid and return of decontaminated water into technological cycle of APP.

The first additional feature consist in that water obtained by vacuum evaporation of decontaminated from caesium radioisotopes solution is used for dissolution of next portions of fusion cake, or for said dilution of mother waters. It decreases demand of fresh water for decontamination of LRW.

Next additional feature consists in that decontaminated from caesium radioisotopes warm solution, before it's vacuum evaporation, circulates no less than two hours at temperature under water boiling point through an induction heater equipped with at least one such short-circuited electroconductive heating element that is capable to mechanical vibration under act of alternating electromagnetic field. It allows transforming residue of iron, aluminium and other metal ions, which are corrosion products of cooling system of any power nuclear reactor, into water-insoluble form.

An additional feature for each above-mentioned variants of LRW decontamination consists in that the carbonate-ions source is selected from group, consisting of sodium hydrocarbonate, sodium carbonate, lithium hydrocarbonate, lithium carbonate and potassium carbonate. It allows picking out of accessible means for LRW saturation by carbonate-ions.

And, finally, said problem is solved in that Cs-selective sorbent is based on potassium-cobalt hexacyanoferrate K₂Co[Fe(CN)₆], which is precipitated on granular silica gel as a supporting medium. Such sorbent has high sorptive capacity that is weakly dependent from presence of cobalt and other non-alkaline metal ions in LRW.

An additional difference consists in that said ingredients are in the ratio (% by weight): potassium-cobalt hexacyanoferrate...20 - 40 granular silica gel 60 - 80. Such Cs-selective sorbent may be easy prepared as described below.

Each person skilled in art comprehends that neither selection of any specific mode of carrying out of the invention by arbitrary combination of basic inventive idea with said additional features nor described below preferable embodiments of the invention are not confined the measure of rights based on claims. The best Embodiments of the Invention

The invention will now be explained by: detailed description of making and composition of Cs-selective sorbent (further FC-M), generalized description of basic process of LRW decontamination together with brief description of required manufacturing equipment, and examples of carrying out the invention taking into consideration specific initial chemical compositions of LRWs of varied origin.

Making and composition of FC-M ion-exchange Cs-selective sorbent Said sorbent, which is meant for selective extraction of Cs⁺-ions from dilute solutions, was prepared in laboratory environment using such reagents as: large granular silica gel, sixhydrate cobalt chloride CoCI₂*6H₂O (having density 1.92 g/cm³), yellow bloody salt K₄[Fe(CN)₆] (having density 1.85 g/cm³), 3% aqueous ammonia NH₃, and 0.1 M aqueous solution of nitric acid HNO₃.

Moreover, laboratory beakers produced by Czech firm «BOMEX" and having volume of 5.0 litres and 0.2 litre are used.

Silica gel granules were introduced into five-litre beaker up to level about 2.5 I, poured over by 3% aqueous ammonia up to level that was exceeded the hard phase level about 1 cm, and obtained mixture was kept under a closure during one hour. Then excess of aqueous ammonia was poured out, and granules were rinsed by distilled water to disappearance of ammonia smell. 380 g of cobalt chloride were added to moist granules under intensive mixing. When cobalt chloride was dispersed in all volume evenly, obtained mixture was matured during 24 hours and washed from residue of unreacted cobalt chloride, whereupon 370 g of yellow bloody salt were added into the beaker by small portions under intensive mixing. Obtained mixture was matured during 24 hours and washed from residue of unreacted yellow bloody salt. Then granules were poured by 0.1 M aqueous nitric acid, kept during 30 minutes and rinsed by distilled water.

Obtained product was placed on a filtration paper and dried airing at room temperature.

Chemical composition of FC-M sorbent of all separate laboratory parties was corresponded to the data mentioned in the table 1. Table 1

Chemical composition of FC-M ion-exchange Cs-selective sorbent

Regardless of concrete chemical composition of separate parties the FC-M sorbent is characterized by such properties: granule's size of basic fraction 0.1-0.5 mm operating range of pH 1-12 possible admixture of background ions to 200 g/l (totally NaCI+KCI) maximally possible operating temperature 150°C.

It is recommended to keep FC-M sorbent in impermeable containers (for example, in polyethylene sacks) at temperature in the range 18-25°C. All obtained parties of FC-M sorbent were mixed up prior experimental decontamination of initial and regenerated LRW from Cs⁺-radioisotopes. Averaged theoretical exchange capacity of obtained product was no less than 1.7 gram- equivalent/g (counting on the K⁺-ions).

Basic process of LRW decontamination according to the invention Generally, any method for decontamination of LRW according to the invention provides two basic steps meant for removal of radioisotopes, namely:

(a) sorption of radioactive caesium ions by Cs-selective sorbent at temperature under water boiling point during time sufficient for decrease of content of caesium radioisotopes up to level under their maximum allowable concentrations; and (b) formation and separation of fine-dispersed sediment of water-insoluble carbonates and/or hydrated oxides of cobalt radioisotope and other non-alkaline metallic admixtures of general formula Me_xO_y./?H₂O, which is created as a result of: addition of carbonate-ions source into LRW, and synchronous thermal (at temperature under water boiling point), mechanical and electromagnetic treatment of obtained alkaline mixture in an induction heater equipped with at least one such short-circuited electroconductive heating element that is capable to mechanical vibration under act of alternating electromagnetic field.

Said steps (a) and (b) may be taken in direct (as stated above) or in inverse order in various embodiments of the invention.

Described above sorbent FC-M is preferred for extraction of caesium radioisotopes in all embodiments of the invention. Each portion of this sorbent must be processed into SRW (usually by heating at temperature no less than 200⁰C) before exhaust of its sorptive capacity.

Above-mentioned sediment of carbonates and/or hydrated oxides of metals must be separated from mother waters and processed into SRW too.

The carbonate-ions source is selected usually from group comprising sodium hydrocarbonate, sodium carbonate, lithium hydrocarbonate, lithium carbonate and potassium carbonate. The most available in the market sodium carbonate, and lithium carbonate, that facilitate sorption of caesium radioisotopes, are preferable.

Any flow-type induction heaters, which are suitable for heating of LRW and other fluid media on any steps of decontamination process, must be made according to invention disclosed 19 January 2006 in the International publication WO 2006/006946 A1 and UA Patent 75778.

Every such heater is equipped with at least one short-circuited electroconductive- heating element that is capable to mechanical vibration under act of alternating electromagnetic field. Therefore, synchronous thermal, mechanical and electromagnetic treatment of suspension within any such induction heater eliminates precipitation of any water- insoluble salts on internal surfaces of this heater, pumps and pipelines practically.

Available in the market high-speed centrifuges can be used for separation of fine- dispersed sediment of Me_xOy^H₂O, but use of micronic filters, those are equipped with (especially, ultrasonic) means for periodic evacuation of sediment, is preferable thereto. Containers-vaporisers, which are made from alternated layers of stainless steel and depleted uranium and able to save mechanical durability and geometric form at repeated heating of their external shells at temperature over 25O⁰C, are meant for receipt, accumulation and burial of SRW. Each container-vaporiser is suitable for accumulation and burial of about 20 kg

SRW.

1. Decontamination of LRW of research nuclear reactor

Experimental system for decontamination of LRW of WWR-M research nuclear reactor (Institute of nuclear researches, Ukraine) was mounted. This system comprises of: a collector of initial LRW having volume 180 litres; a flow-type induction heater equipped with at least one short-circuited electroconductive-heating element that is capable to mechanical vibration under act of alternating electromagnetic field; a high-speed centrifuge provided at least 17 000 revolutions per minute; a storage tank having volume 300 litres; two filled by FC-M sorbent ninth-litre adsorbers; an intermediate collector of decontaminated water having volume 70 litres. Initial LRW had pH under 5 and was contaminated by 30-40 mg/l iron ions and

30^40 mg/l aluminium ions (in separate portions) and, on average, by such radioisotopes as ⁶⁰Co in amount 2.03*10^~11 g/l (8.5*10³ Bq/I) and ¹³⁷Cs in amount 5.59*10^~9 g/l (1.8*10⁵ Bq/I). These amounts and radioactivity indexes were defined by usual gamma-spectrometric analysis. Sodium hydrocarbonate or carbonate was added into SRW to assignment of pH more than 8 (but no more than 11) after each next filling of said collector up to level about 150 I.

Prepared mixture was pumping through said induction heater at temperature in the range 46-60⁰C and circulation rate about 2 litres per minute to appearance of fine- dispersed water-insoluble flakes of carbonates and hydrated oxides of ⁶⁰Co, iron and aluminium. These flakes, which were arisen out of addition of said carbonate-ions source and following synchronous thermal, mechanical and electromagnetic treatment of said mixture, not to precipitate onto internal surfaces of the induction heater and other devices. Prepared suspension was centrifugalized in order to separate sediment containing concentrated radioisotope of ⁶⁰Co, iron and aluminium. Accumulated radioactive sediment was removed from centrifuge and, after vacuum drying, buried.

Warm mother waters were transferred into said storage tank and then gradually pumped through said adsorbers in order to extract of ¹³⁷Cs radioisotope by FC-M sorbent.

Decontaminated water was poured into said intermediate collector in order to make up gamma-spectrometry, and then poured out in sewage system without dilution, as residual activity was no more than 2.39 Bq/I for ⁶⁰Co and 4.67 Bq/I for ¹³⁷Cs.

Described method was used for decontamination of 1050 I LRW from ⁶⁰Co and

137, Cs.

2. Decontamination of LRW containing boric acid

Example 2.1. Decontamination of initial LRW of APP containing boric acid in concentration no less than 27 g/l.

1.75 m³ of acidic initial LRW of APP having pH about 5 was taken for decontamination.

27 g/l of boric acid, 42.3 g/l of sodium ions, 10.1 g/l of potassium ions, 0.9 g/l of chlorine ions, 17.5 g/l of sulphate-ions, 34.8 g/l of nitrate-ions and 1.1 g/l of carbonate- ions, and such defined by gamma spectroscopy radioisotopes as ¹³⁷Cs (1.2*10⁹ Bq/I), ¹³⁴Cs (2.02*10⁷ Bq/I), and ⁶⁰Co (4.56*10⁶ Bq/I) are contained in this portion of LRW.

Decontamination was carried out using mentioned below equipment.

At first, selected portion of LRW was pumped once at rate about 20 litres per minute through six heated at temperature no more 6O⁰C adsorbers containing totally 96 kg of Cs-selective FC-M sorbent. Partly decontaminated in that way LRW had contained 4.0 Bq/I of ¹³⁷Cs, less than 0.6 Bq/l of ¹³⁴Cs and 4.2.10⁶ Bq/I of ⁶⁰Co. These data (and known sorptive capacity of FC-M sorbent) allow to state that 96 kg of said sorbent would be sufficed for decontamination of about 3000 m³ of LRW.

Practically decontaminated from Cs-radioisotopes solution containing boric acid and above-mentioned cations and anions was poured into mixer having volume 2.5 m³. Further, 6 kg of sodium carbonate (Na₂CO₃) were added into this mixer at intensive stirring during 12 minutes.

Alkaline solution having pH » 9 was pumping during 50 minutes through above- mentioned induction heater at temperature no more than 6O⁰C and circulation rate about 1.5 litres per minute. In that way, all components of solution were thermally, mechanically and electro-magnetically treated in alternating electromagnetic field. Said treatment was interrupted as fine-dispersed particles of water-insoluble carbonates and/or hydrated oxides of general formula Me_xO_y*A7H₂O (mainly such metals as iron and cobalt) were caused turbidity of solution.

Prepared suspension was strained through a filter having averaged openings' size no more than 1 micrometers. Accumulative sediment was thrown down periodically into an intermediate receiver, and obtained filtrate was poured into collector having volume 6 m³.

In that way, about 3.1 litre of pasty sediment containing 17.9 g of hydrated iron oxides and 8.90*10^"4 g of ⁶⁰Co carbonate was obtained. This sediment was loaded into above-mentioned container-vaporiser and transformed into 19.98 g of SRW that was suitable for accumulation and following burial.

Water vapour was condensed and returned into above-mentioned mixer.

Practically decontaminated filtrate containing micro-admixture of radioisotopes (namely: 4.3 Bq/I of ¹³⁷Cs, about 0.5 Bq/I of ¹³⁴Cs and about 2.4 Bq/I of ⁶⁰Co) was poured into a vaporiser having volume 2 m³, that was connected to a liquid-packed vacuum-pump by productivity of 35 m³/h. Then filtrate was acidified to assignment of pH*δ and evaporated partly under residue pressure no more than 40 millimetres of mercury (53 mbar) to receipt of 350 litres solution containing 150 g/l of boric acid.

This solution was poured into a crystallizer having volume 2.5 m³ where greater part of boric acid (36 kg calculating on dry matter) was fallen out as sediment after cooling at temperature 10⁰C. This sediment of boric acid was practically fully decontaminated from ⁶⁰Co and ¹³⁴Cs radioisotopes and contained about 65 Bq/kg of ¹³⁷Cs (i.e. under its maximum allowable concentration substantially). Damp boric acid was dried and conveyed to storage.

Mother waters were vacuum-evaporated to obtaining of 185.45 kg dry mixture, which was conveyed to storage. This mixture had contained 9.00 kg of boric acid, 63.50 kg of sodium ions, 15.00 kg of potassium ions, 1.35 kg of chlorine ions, 43.75 kg of sulphate-ions, 52.20 kg of nitrate-ions, 1.65 kg of carbonate-ions and admixture of ¹³⁷Cs, ¹³⁴Cs and ⁶⁰Co at level under their maximum allowable concentrations, which are established by in full mentioned below «Ukrainian Norms of Radiation Safety».

Residue water obtained in this experiment in amount about 1.5 m³ was poured out into sewage system of APP.

Example 2.2. Decontamination of 360 kg fusion cake prepared by evaporation non-constitution water from initial LRW of Zaporozhian APP (Ukraine)

This portion of fusion cake had contained 111.6 kg of boric acid (H₃BO₃), 80.28 kg of sodium ions, 19.8 kg of potassium ions, 0.009 kg of iron ions, 1.8 kg of chlorine ions, 34.2 kg of sulphate-ions, 64.0 kg of nitrate-ions and crystallisation water as for the rest to 360 kg. Moreover, such radioisotopes as ¹³⁷Cs at level of 5.18.10⁹ Bq (1.439.10⁷ Bq/kg), ¹³⁴Cs at level of 8.68.10⁷ Bq (2.41*10⁵ Bq/kg) and ⁶⁰Co at level of 1.96.10⁷ Bq (5.44*10⁴ Bq/kg) were presented as admixtures to said fusion cake according to gamma-spectrometric analysis.

Decontamination was carried out using mentioned below equipment.

Said portion of fusion cake was dissolved in 700 litres of 0.01% aqueous nitric acid during 50 minutes at temperature about 90⁰C and circulation rate about 15 litres per minute by means of flow-type induction heater having power consumption no more than 20 kWh.

159.00 g/l of boric acid, 114.70 g/l of sodium ions, 28.30 g/l of potassium ions, 2.57 g/l of chlorine ions, 48.85 g/l sulphate-ions and 94.42 g/l nitrate-ions were presented in obtained solution having pH no more than 5.

This solution was poured into an intermediate thermally-insulated tank having volume about 2 m³, in which temperature no less than 60⁰C was supported, and then pumped into a cooled crystallizer having volume about 2.5 m³. After cooling at temperature 1O⁰C greater part (86 kg) of boric acid was fallen out as sediment that was practically decontaminated from radioisotope of ⁶⁰Co, but containing noticeable admixtures of ¹³⁷Cs and ¹³⁴Cs ions. Further - damp sediment of boric acid was conveyed to a rinse device composed of two flow-type vessels having volume of 300 I and 100 I accordingly, five filled by FC-M sorbent heated adsorbers, each of which had volume of 16 I, and pump, and acidic supernatant having pH about 5 and containing Na⁺, K⁺, Cl^", SO₄ ^2", NO₃ ^" ions and radioisotopes of ¹³⁴Cs, ¹³⁷Cs and ⁶⁰Co was poured into a tank-mixer.

Boric acid was washed within the rinse device by 13 litres of technical water that was circulated three- times at rate about 5 litres per minute through above-mentioned adsorbers heated at temperature about 6O⁰C.

Washed boric acid was removed from the rinse device and dried up to constant mass of 85 kg. This by-product was chemically clean and, according to results of radiological research, appeared ¹³⁷Cs-radioactivity at level only 60-70 Bq/kg, while admixtures of ¹³⁴Cs and ⁶⁰Co radioisotopes were absent practically. Therefore, recovered in that way boric acid can be advertising for free sale at chemical market.

Rinse water may be reused many times to saturation of it by such amount of potassium ions, which can hinder effective sorption of ¹³⁴Cs and ¹³⁷Cs radioisotopes, and then poured into above-mentioned tank-mixer together with supernatant from the crystallizer of boric acid. During 15 minutes at stirring 2 kg of sodium carbonate (Na₂CO₃) was added into supernatant being found in said tank-mixer in order to assignment pH about 9. Prepared alkaline solution was pumping during 50 minutes at rate about 15 litres per minute through above-mentioned induction heater provided temperature about 6O⁰C to appearance of turbidity. It was testified fallout of water-insoluble carbonates and/or hydrated oxides of general formula Me_xO_y*/7H₂O of such metals as iron and cobalt preferably. <

Prepared suspension was strained through a filter having averaged openings' size no more than 1 micrometers. Accumulative sediment was thrown down periodically into an intermediate receiver, and obtained filtrate was poured into a collector having volume 6 m³.

In that way, about 3.0 litre of pasty sediment containing 18.0 g of hydrated iron oxides and 8.92*10^"4 g of ⁶⁰Co carbonate was obtained. This sediment was loaded into above-mentioned container-vaporiser and transformed into 20.00 g of SRW that was suitable for accumulation and following burial. Water vapour was condensed and returned into above-mentioned collector.

Liquid had being within said collector was diluted by technical water in 2.5 times. Obtained solution had pH under 9 and was contained 14.6 g/l of H₃BO₃, 47.1 g/l of sodium ions, 11 !3 g/l of potassium ions, 1.03 g/l of chlorine ions, 19.5 g/l of sulphate- ions, 37.8 g/l of nitrate-ions, 1.6 g/l of carbonate-ions, and admixture of ¹³⁷Cs and ¹³⁴Cs radioisotopes.

Dilute solution was pumped once during 90 minutes at rate about 20 litres per minute through six heated at temperature no more than 6O⁰C adsorbers containing 96 kg of FC-M Cs-selective sorbent totally. Output solution was contained 8.37 Bq/I of ¹³⁷Cs and 8.38 Bq/I Of ⁶⁰Co, whereas ¹³⁴Cs-isotope was not determined. These data (and known sorptive capacity of FC-M sorbent) allow to state that 96 kg of said sorbent would be sufficed for decontamination of about 50 metric tons of fusion cake.

Practically decontaminated solution had volume about 1.75 m³. It was pumping minute during 2 hours 25 minutes through above-mentioned induction heater at circulation rate about 20 litres per in order to warm up at temperature no more than 70⁰C. Warmed solution was poured into a tank-vaporiser that had volume 2 m³ and connected to liquid-packed vacuum-pump by productivity of 35 m³/h, and finally evaporated under residue pressure no more than 40 millimetres of mercury (53 mbar).

Water vapour was directing into a condenser through a separator of salts' particles. Obtained in that way pure water may be used for dissolution of next fusion cake portions or dilution of filtrate (but water obtained as a result of above-described experiment in volume of 1.52 m³ was poured out into sewage system).

Dry precipitate in amount 225 kg was discharged from said tank-vaporiser and conveyed on storage. This precipitate was contained 25.00 kg of boric acid, 80.28 kg of sodium ions, 19.80 kg of potassium ions, 1.80 kg of chlorine ions, 34.20 kg of sulphate- ions, 64 kg of nitrate-ions, 1.32 kg of carbonate-ions, and such admixtures of ¹³⁷Cs, ¹³⁴Cs and ⁶⁰Co radioisotopes, each of which was under level of respective maximum allowable concentration.

Industrial Applicability According to the Ukrainian norms of radiation safety (NRBU-97-97) - total cobalt- and caesium-radioactivity, which is admissible for pouring out any water into a sewage system, must not exceed 87 Bq/I; radioactivity of solid materials of 1st class, which may be used in building industry beyond all bounds, must not exceed 370 Bq/kg; radioactivity of solid materials of 2nd class, which are meant for industrial building

(including road-building), must be in the range of 370 Bq/kg to 740 Bq/kg; radioactivity of solid materials of 3rd class, which are meant for other construction work outside any settlements, must be in the range of 740 to 1350 Bq/kg.

(see: 3EIPHMK Baxcπi/iBMx oφiLpm/ix MaτepiariiB 3 caHiτapm/ιx i πpoTMeπiflewiiMHMX πι/ιτaHb. Bi/iflamna oφiujwHe y flee'flTn τowιax. TOM 7, Maσπ/ma 1 - KI/IΪB: MiHiσrepcTBo oxopom/i 3flopoB'a yiψaϊHW, 1998, c.153-271; in English: COLLECTION of important official materials about sanitary and antiepidemic questions. Official Edition in nine volumes. Volume 7, Part 1 - Kiev: Ministry of Public Health of Ukraine, 1998, pp.153-271). As it is shown in foregoing examples, any product of decontamination of arbitrary

LRW according to the invention has radioactivity under lowermost established by NRBU limit.

Claims

^ gCLAI MS

1. Method for decontamination of liquid radioactive wastes comprises of: separation of portion of initial liquid radioactive wastes for decontamination, addition of carbonate-ions source into said portion of initial liquid radioactive wastes to assignment of alkaline reaction, synchronous thermal, mechanical and electromagnetic treatment of obtained mixture in an induction heater equipped with at least one such short-circuited electroconductive heating element that is capable to mechanical vibration under act of alternating electromagnetic field, separation of such fine-dispersed sediment of water-insoluble carbonates and/or hydrated oxides of cobalt radioisotope and other non-alkaline metallic admixtures of general formula Me_xCy/?H₂O, which is created as a result of said addition of carbonate- ions and said synchronous thermal, mechanical and electromagnetic treatment, treatment of mother waters, which are residuary after separation of said sediment, by Cs-selective sorbent during time that is sufficient for extraction of caesium radioisotopes up to level under their maximum allowable concentrations, processing of used sorbent and said Me_xCVnH₂O sediment into compact solid radioactive wastes for following burial.

2. Method of decontamination of liquid radioactive wastes comprises of: separation of portion of acidic initial liquid radioactive wastes of atomic power plant containing boric acid, products of corrosion of a cooling system of a power nuclear reactor and at least such radioisotopes as ¹³⁷Cs, ¹³⁴Cs and ⁶⁰Co, treatment of said LRW portion by Cs-selective sorbent at temperature no more than 6O⁰C during time that is sufficient for extraction of any caesium radioisotope up to level under its maximum allowable concentration, addition of carbonate-ions source into practically decontaminated from radioisotopes ¹³⁷Cs and ¹³⁴Cs liquid radioactive wastes to assignment of alkaline reaction, synchronous thermal, mechanical and electromagnetic treatment of obtained mixture in an induction heater equipped with at least one such short-circuited electroconductive heating element that is capable to mechanical vibration under act of alternating electromagnetic field, separation of such fine-dispersed sediment of water-insoluble carbonates and/or hydrated oxides of cobalt radioisotope and other non-alkaline metallic admixtures of general formula Me_xCynH₂O, which is created as a result of said addition of carbonate- ions and said synchronous thermal, mechanical and electromagnetic treatment, partial evaporation of practically decontaminated mother waters to receipt of concentrated solution of boric acid with admixtures of mineral salts, cooling of said solution to fallout of boric acid sediment, separation of practically decontaminated boric acid sediment for following utilization, vacuum evaporation of mother waters to receipt of practically decontaminated dry mixture of salts, and processing of used sorbent and said Me_xOyDHaO sediment into compact solid radioactive waste for following burial.

3. Method of decontamination of liquid radioactive wastes comprises of: separation of portion of fusion cake, regeneration of liquid radioactive wastes by dissolution of said fusion cake's portion with acidified water in circulation regime through flow-type induction heater supporting temperature under water boiling point, cooling of prepared solution to sedimentation of boric acid practically decontaminated from ⁶⁰Co radioisotope and containing admixture of ¹³⁷Cs and ¹³⁴Cs radioisotopes, separation of boric acid sediment, wash-out of admixtures from it within a closed scrubbing circuit provided extraction of ¹³⁷Cs and ¹³⁴Cs radioisotopes on Cs-selective sorbent and drying of washed sediment to constant mass, addition of carbonate-ions source to supernatant, that are residuary after separation of boric acid sediment, to assignment of alkaline reaction, synchronous thermal, mechanical and electromagnetic treatment of obtained mixture in an induction heater equipped with at least one such short-circuited electroconductive heating element that is capable to mechanical vibration under act of alternating electromagnetic field, separation of fine-dispersed sediment of water-insoluble carbonates and/or hydrated oxides of cobalt radioisotope and other non-alkaline metallic admixtures of general formula Me_xOyHH₂O, which is created as a result of said addition of carbonate- ions and said synchronous thermal, mechanical and electromagnetic treatment, at least double dilution of mother waters that are residuary after separation of said sediment, treatment of prepared dilute solution by Cs-selective sorbent at temperature under water boiling point during time that is sufficient for extraction of caesium radioisotopes up to level under their maximum allowable concentration, vacuum evaporation of decontaminated from caesium radioisotopes solution to receipt of practically non-radioactive dry mixture of salts, and processing of used sorbent and said Me_x0_y*nH₂0 sediment into compact SRW for following burial.

4. Method of claim 3 wherein water obtained by vacuum evaporation of decontaminated from caesium radioisotopes solution is used for dissolution of next portions of fusion cake, or for said dilution of mother waters.

5. Method of claim 3 wherein decontaminated from caesium radioisotopes warm solution, before it's vacuum evaporation, circulates no less than two hours at temperature under water boiling point through an induction heater equipped with at least one such short-circuited electroconductive heating element that is capable to mechanical vibration under act of alternating electromagnetic field.

6. Method of claim 1 , or of claim 2, or of claim 3 wherein said carbonate-ions source is selected from a group, consisting of sodium hydrocarbonate, sodium carbonate, lithium hydrocarbonate, lithium carbonate and potassium carbonate.

7. Cs-selective sorbent based on potassium-cobalt hexacyanoferrate K₂Co[Fe(CN)₆], which is precipitated on granular silica gel as a supporting medium.

8. Sorbent of claim 7 wherein said ingredients are in the ratio (% by weight): potassium-cobalt hexacyanoferrate...20 - 40 granular silica gel ...60 - 80.