GB2101797A - Treating radioactively contaminated ion-exchange resins - Google Patents

Abstract

In treating a radioactively contaminated ion-exchange resin to facilitate disposalor storage, a sludge of the resin has ammonia, other strongly alkaline material, or a mixture of these, added to it so as to raise the pH to at least 9, preferably 9-13, and this material is then dispersed by mechanical stirring in an unsaturated polyester resin, e.g. a resin comprising at least one unsaturated polyester and at least one ethylenically unsaturated copolymerisable monomer, together with suitable additives e.g. cross linking agents, and the mixture allowed to harden.

Description

SPECIFICATION Composition of matter containing radioactive material based on ion exchange resins The present invention has as an object particular compositions of matter suitable to be transformed into solid articles as well as a process for obtaining said solid articles. In particular, the present invention has as an object compositions of matter comprising dispersions of radioactive sludges in unsaturated polyester resins. More particularly the present invention has as an object dispersions of radioactive sludges based on ion exchange resins in unsaturated polyester resins.As to the processes for obtaining said solid articles starting form said compositions of matter, which processes also are among the objects of the present invention, they concern chemical and mechanical processes suitable to transform into stable dispersions the compositions comprising the aforesaid radioactive sludges and the unsaturated polyester resins and subsequently to convert said dispersions into solid articles which are mechanically resistant and contain the radioactive sludges in solidified and scarcely leachable form.

A further object of the present invention are said solid articles and their use for the disposal of said radioactive sludges in a physical form which is stable in time, whereby the risk of radioactivity leakages is considerably reduced.

Said solid articles are characterized by their stability to radiations and to chemical agents for very prolonged periods of time, by a low leachability if they are immersed in water or in aqueous solutions, and by high mechanical resistance, especially to impact and to compression, so that said articles may be transported and stored without the danger that the radioactive substances contained therein may leak out, even if accidents or natural calamities occur.

Processes for solidifying radioactive residues having low and medium radioactivity in the form of solid articles, obtained by solidifying said radioactive residues in cement or in particular resins or bitumen, are already known.

However the articles obtained by said processes have considerable defects. Thus for example the solidification in cement, which is obtained by preparing suspensions of cement and radioactive solutions and subsequent hardening cannot be carried out if the radioactive residues contain particular chemical substances which act as hardening retardants or inhibitors, such as for example the borates. Further, in the presence of other chemical substances, such for instance sulphates, the quality of the product obtained is not satisfactory.

Another drawback is due to the fact that the articles in cement thus obtained are not sufficiently resistant to leaching of the solidified radio-nucleids. As is known, cement normally has a highly porous structure through which water, which has accidentally come into contact with the article, may penetrate with the consequence that a part of the radioactivity leaks out.

Finally, the solidification in cement does not permit to reduce to a significant extent the volume of the residues to be conditioned, inasmuch as, in the major part of the cases, it is not possible to solidfy in cement more than 20%-30% of radioactive residue with respect to the volume of the final article.

The use has also been proposed of chemically hardened emulsion of urea-formaldehyde resins and radioactive residues.

Said process, while it does not permit to overcome to a satisfactory extent the aforesaid radioactivity leaching problem, has the further drawback that the articles thus obtained have low mechanical resistance, thus being subject to breakage into fragments in case of mechanical accidents, for instance during transport or storage operations. As to the process of solidifying in bitumen, in spite of the fact that it affords better results as to the resistance to leaching, it involves cost problems, since the solidification must be carried out at a temperature above 1 50"C in expensive mixing apparatus, as well as danger problems, especially in the case of the treatment of nitrate-containing solutions, in the presence of which oxidative reactions of the bitumen may occur, which reactions are sometimes explosive.Further, the mechanical characteristics of the product obtained by solidifying in bitumen are not wholly satisfactory, and become considerably worse as the storage temperature increases.

The art has directed its attention, for the purpose of solving said problems, to the use of unsaturated polyester or epoxy resins for the solidification of low and medium radioactivity residues in the form of solid articles, as described in published French patent application 7425298, and in the form of emulsion in polyester resins of the "water in oil" type, as described by Subramanian and others and disclosed in U.S.

patent 4.077.901.

The methods of the aforesaid two publications have proved to be unsuitable for solidifying radioactive materials constituted by ion exchange resins, which are frequently used for purifying process waters and condensation waters of thermonuclear power stations containing radioactive substances.

Usually said ion exchange resins are constituted by a mixture containing an excess of cationic resin with respect to the anionic resin.

The processes described in the aforementioned publications, are not usefully applicable in the case that the radioactive materials are constituted by sludges based on ion exchange resins, because said materials do not permit to obtain stable dispersion (emulsion and/or suspensions) of said sludges in unsaturated polyester resins. Mixtures of radioactive sludges of this type and unsaturated polyester resins are not capable of forming stable dispersions, even if subjected to violent stirring, but as soon as the mechanical stirring is discontinued, the phases became unmixed, whereby the aqueous radioactive material becomes separated from the organic unsaturated polyester resin phase.

Although the causes which cause such a behaviour are not known, one may assume that they are not merely due to the strongly acid pH of the solutions wherein cation exchange resins, for instance, are dispersed, inasmuch as the problem raised by the failure of the radioactive material to remain in dispersion is not solved by rendering the solution alkaline by means of aqueous caustic soda.

The Applicants have now surprisingly found, and this constitutes one of the objects of the present invention, processes which permit to obtain dispersions in unsaturated polyester resins of radioactive aqueous sludges based on ion exchange resins, which dispersions are stable in time, and successively to harden the resins containing said dispersions so as to obtain solid articles containing radioactive material in immobilized form.

Said compositions of matter, comprising the radioactive material based on ion exchange resins and unsaturated polyester resin, is therefore another object of the present invention.

A further object of the present invention are the articles thus obtained and their uses which permit to store, transport and dispose for long periods of time under safe conditions, the radioactive material contained in sludges based on ion exchange resins.

The process which is an object of the present invention, for solidifying radioactive residues contained in ion exchange resins having a pH below 7, is characterized in that it is effected by the following operations: a) the addition under suitable conditions, hereinafter specified, of one or more compounds having a basic pH to radioactive sludges based on ion exchange resins, in such a way that the pH of said sludges does not exceed 9, and is preferably form 9 to 13.

b) the dispersion in the product obtained by operation a, in unsaturated polyester resins, (comprising one or more unsaturated polyesters and one or more ethylenically unsaturated monomers which are copolymerizable with said polyesters, and conventional additives such as cross-linking agents) by mechanical stirring, so as to obtain a dispersion of three phases present (water, ion exchange resins and unsaturated polyester resins), which dispersion is stable in time and homogeneous; c) hardening the dispersion thus prepared by means of cross-linking agents, so as to obtain solid articles containing the radioactive material in solidified form.

As ion exchange resins, the commercial resins may be used or other equivalent ones.

Examples of said resins are those which are marketed by the firm Thermochimik under the registrated trademark "Powdex". Said resins are generally used in thermonuclear power stations, in the form of finely ground powders, for treating radioactive condensation waters. The aqueous sludges resulting from said treatments may be used as such for the solidification process, which is an object of the present invention.

The radioactive sludges used, which normally contain from 20 to 40% of radioactive solid material and from 60% to 80% of water, may also be previously concentrated so as to reduce the water content to below 60% down to 50%. Radioactive sludges in cation exchange resins or anion exchange resins, or their mixtures, may be used.

The alkalinization (phase a) consists in treating the radioactive sludges with ammonia (or compounds which give off ammonia) alone or together with strong alkaline bases, until a pH of 9-13 has been reached.

For this purpose it is possible to add gaseous or aqueous ammonia to the sludges until the said pH has been reached. Since this treatment often involves an excessive dilution with water and the consequent necessity of using large amounts of polyester resins for solidifying the sludges, it is normally preferred to use ammonia together with a strong bases. Strong bases are basic derivatives of sodium, potassium, calcium and magnesium, such as for example oxides, hydroxides or carbonates of said metals, optionally in mixture the ones with the others. Said compounds may be added as such to the sludges or preferably they may be added in aqueous solution (for instance 5-50% aqueous caustic soda). The addition of ammonia and of the strong bases may be effected in any technically valid succession.It is however preferred to add firstly the strong bases until a pH below 9 has been reached, and thereafter to add concentrated aqueous ammonia of about 32% until the pH becomes 9-13. The amount of ammonia to be used is at least 0.2 parts, preferably 0.6 parts, of 32% aqueous ammonia solution, per 100 parts of sludges containing 50% of solid material. If the concentrations of ammonia and sludges are different, the ratio is corresondingly varied. The strong bases and the ammonia are preferably gradually added to the well stirred sludges.

The phase b of the process, consisting in dispersing the thus transformed sludge in unsaturated polyester resins, is carried out by using an efficient stirring system which permits to disperse the sludges in minute particles. A turbine, a mechanical stirrer of the helical, anchor or another type or a static mixer, is conveniently used for this purpose. The stirring must be efficient taking also into account the considerable viscosity of the sludge, and has to be continued until a stable and finely subdivided dispersion of the three phases present (water, solid radioactive sludge material and unsaturated polyester resin) has been obtained.

The minimum ratio of sludge to unsaturated polyester resins is not critical, inasmuch as it is possible to dilute the sludge to any desired extent in a great excess of unsaturated polyester resin. For economical reason, however, it is not advisable to suspend excessively small amounts of sludge in a great excess of unsaturated polyester resins, but it is preferred to use possibly high ratios of sludge to unsatureted polyester resin. The maximum limit of such ratio is constituted by the inversion point, beyond which the dispersion of the aqueous phase in the organic phase comprising the unsaturated polyester resin changes to a dispersion of the organic phase in the aqueous phase (of the "oil-in-water" type). Beyond said limit the solid articles obtained after the subsequent hardening are no longer suitable for the purpose of the present invention.

Said maximum limit varies according to the water content of the sludge and to the particular type of unsaturated polyester resins used. As a rule, one may indicate a limit ratio of sludge to unsaturated polyester resins of about 60:40, preferably about 50:50.

As a general rule a large number of types or mixtures of unsaturated polyester resins may be used for the purpose of the present invention. It is preferred to use resin types having a medium or high unsaturation degree and a reduced hydrophilic character. Particularly good results are obtained by using polyester resins comprising an unsaturated polyester of type A or its mixtures with a polyester of type B, the unsaturated polyester of type A and the polyester of type B being described hereinafter.

Said preferred resins comprise: 1 - an unsaturated polyester (A) obtained by polycondensation of: (a) maleic anhydride and/or maleic acid and/or fumaric acid; (b) isophthalic acid and/or terephthalic acid; (c) neopentyl glycol; (d) optionally one or more conventional glycols, such as for example, ethylene glycol, 1,2-propylene glycol, the compound (c) of said unsaturated polyester (A) being present in an amount of at least 50% by mols with respect to the total of components (c) and (d); 2 - an unsaturated polyester (B) obtained by polycondensation of: (a) maleic anhydride and/or maleic acid and/or fumaric acid; (b) isopropylidene-bis (phenyloxy-propanol-2); (c) optionally one or more conventional glycols, such as for instance ethylene glycol, 1,2-propylene glycol, neopentyl glycol, the compound (b) of said unsaturated polyester (B) being present in an amount of at least 50% by mols with respect to the total of compounds (b) and (c); 3 - an ethylenically unsaturated liquid monomer capable of copolymerizing with unsaturated polyesters (A) and (B) chosen among styrene, vinyltoluene and methylmethacrylate alone or in a mixture with each other; 4 - optionally polymerization initiators, accelerators, glass fibers and other conventional additives and fillers; the ratio by weight of components (A) and component (B) being comprised between 100:0 and 20:80, preferably between 80:20 and 40:60.

The mixture of the components herein before specified may occur in any technically suitable order, for instance, by following the succession indicated hereinbefore, or by mixing the solution of the individual polyesters prepared by dissolution in monomer (3). The amount of monomers (3) with respect to the total of the composition of matter is suitably chosen between 15% and 70%, preferably between 25% and 50%.

Preferably said monomer is styrene. As stabilizers of the resins one may use for instance hydroquinone or tert.-butyl cathecol. The radicalic polymerization initiator comprises one or more peroxides or hydroperoxides and optionally a protomer (accelerator). A system of polimerization initiators which is particularly convenient comprises benzoyl peroxide and N,N-disubstituted aromatic amine, such as for instance N,N-dimethyl-aniline, N,N-dimethyl-p-toluidine or N,N-di-(hydroxyethyl)aniline or polymers containing N,N-dialkyl-substituted aromatic groups. The promoter is added to the resin possibly at a late moment, so as not to induce premature resin cross-linking processes. It is convenient to add the promoter shortly before the end of the stirring (phase b).

The phase c of the process, consisting in hardening the stable suspension obtained from phase b by radicalic cross-linking, occurs through the action of the radicalic initiation system. The hardening is conveniently effected at a temperature between 0 and 50"C, but preferably at about room temperature. The temperature in question is the temperature of the beginning of the hardening process. Subsequently, because the hardening reaction is exothermic, the material spontaneously heats to considerably above said temperature.To avoid excessive overheatings, which would imply the danger of the cracking of the articles, especially if large volumes of material are hardened, it is advisable suitably to balance the amount of catalytic system and the amount of stabilizers, in such may that the hardening will proceed at a moderate speed and with thermal peaks which are below 90"C.

In order to improve the mechanical characteristics of the articles obtained, it may be advantageous to add to the unsaturated polyester resin reinforcing fillers, such as glass fibers, or fillers which confer self-extinguishing properties, such as aluminium hydroxide.

The compositions of matter which form a further object of the present invention, comprise a fine and homogeneous dispersion, which is stable for long periods of time (at least a number of hours) even after the mechanical stirring has been discontinued, essentially constituted by: 1 - radioative aqueous sludges based on ion exchange resins in the state of finely subdivided powders, as dispersed phase, and 2 - a continuous phase comprising at least one polyester resin, as hereinbefore defined, to which conventional additives have been added, such as stabilizers, antioxidants, and radicalic polymerization initiator systems, these last preferably comprising one or more poroxides and/or hydroperoxides, preferably benzoyl peroxide, and one or more promoters (accelerators), preferably N,N-dimethylaniline, N,N-dimethyl p-toluidine or N,N-di-(hydroxyethyl) aniline or polymers containing N,N-dialkyl substituted aromatic groups, the weight ratio ofthe aqueous sludges (1) to the continuous phase (2) being equal or lower than 50:50 and the ratio of peroxide and/or hydroperoxide to the promoter being comprised between 50:1 and 3:1, preferably between 20:1 and 5:1, and the ratio of unsaturated polyester resins to the peroxide and/or hydroperoxide being comprised between 200:1 and 20:1, preferably between 100:1 and 30:1.

The solid articles containing solidified radioactive sludges based on ion exchange resins, which are a further objects of the present invention, are characterised by: 1 - high resistance to the leaching of the incorporated radioisotopes.

A solid article containing aqueous solutions and/or emulsions and/or dispersions, in the form of small particles occluded in the solid article, according to what is described in the present invention, looses a minimum amount by weight of the radioactive and non-radioactive ions therein contained, when it is leached with de-ionized water for 72 hours at 99"C according to the "Soxhlet Leach - Test Procedure for Testing of Solidified Radioactive Waste" or when it is leached with de-ionized water for 300 days at 23"C according to the method described in the following examples.

2 - High compression strength.

The compression strength (10% deformation) is always higher than 50 kg/cm2 and may attain values of about 170 kg/cm2 (measured according to the ASTM D 695 norm).

3 - High impact resistance.

4 - Resistance to radiations. A block of solid material as hereinbefore defined, when subjected to radiation with radioactive Co with a radiation density of about 2-105 rad/hours for a total absorbed dose of 5.6.108 Rads, undergoes a weight loss in the order of approximately 0.1%, or less of the initial weight of sample, and does not exhibit substantial alterations of its mechanical characteristics.

The following examples are illustratives and are not intended to limit the scope of the present invention.

The ratios indicated hereinbefore and hereinafter, are by weight, unless otherwise specified.

Example 1 Preparation of a type A resin 1390 g of neopentyl glycol 552 g of diethylene glycol 1494 g of isophthalic acid 883 g of maleic an hydroxide are introduced into a 6 liter flask.

The temperature is gradually brought to 200"C within 3 hours while the mixture is maintained under stirring under an inert gas (for instance pure nitrogen). The mixture is maintained at said temperature for another 7 hours. The volatile condensation products are removed in a pure nitrogen stream through a deflagmator kept at 100 Candthrough a condenser. At the end, the mixture is cooled to 130and 0.41 g of hydroquinone are added. The temperature is further lowered to 1200C. 1 975 g of styrene are added, and the mass is cooled down to room temperature while it is always stirred in a nitrogen stream. In this way a type A resin is obtained.

Preparation of a type B resin A type B resin is prepared by proceeding in a way analogous to that followed for the preparation of the type A resin, using the following materials: 1376 g of isopropylidene-bis-(phenyloxy-propanol-2) 392 g of maleic anhydride 0.41 g of hydroquinone 1750 g of styrene The two resins are mixed in a ratio of 70 parts by weight of type A resin and 30 parts by weight B type resin.

Sludges containing radioactive material Acommercial cation exchange resin "Powdex PCH" in powderform, marketed byThermochimik Corporation, and a commercial anion exchange resin "Powdex CPN" in powder form of the same firm, are mixed in a ratio of 2:1 and are then used for the ion exchange of an aqueous sodium chloride solution containing radio-nucleide Cs'37 having activity of 200 ttCi as a tracer. Once the ion exchange has been completed, an aqueous sludge is obtained, which is centrifugated for the purpose of reducing its water content to 60% (solid content 40%). It has a pH of 4.5.

40% aqueous soda is then added until a pH of 8.5 is reached and subsequently 32% aqueous ammonia is added under good stirring until a pH of 11.6 has been reached.

Preparation of the composition of matter and of an article 1,2 g of 50% benzoyl peroxide paste are added to 120 g of the mixture of the two, type A and type B, resins in a 70:30 ratio. The mixture thus obtained is poured into a beaker and is kept under stirring by means of a helical stirrer having a speed of rotation of 900 r.p.m., 80 g of sludges prepared as hereinbefore described are added by means of a dosing pump at a rate of 25 mI/min. 0.4 g of N,N-dimethylamine-p-toluidine are further added during the last 30 sec.

A stable dispersion is formed, which is cast into a large test tube and is allowed to harden.

The gel time of the mass is of 24 min.

When 48 hours have passed and the article has become completely hardened, test samples, suitably cut from the article, are subjected to compression test (according to ASTM D 695) and to leaching in the cold, whereby the following results are obtained: compression strength (at 10% deformation) 115 kg/cm2 (initial) elastic modulus under compression 3600 kg/cm2.

A leach test is then carried out using the method hereinafter described, which gives the following results: Na+ leached: 4.9% of the Na+ present in the sludge leaching rate: R = 6-10-3 g cm2/day Irradiation with radioactive Co with a density of about 2105 rad/hours for a total absorbed dose of 5.6-103 rad, undergoes a weight loss of 0.1% of the initial weight.

Leaching procedure An emulsion prepared as hereinbefore described is cast, before it polymerizes, into a polyethylene container having a diameter of 50 mm and a length of 55 mm. The material is allowed to polymerize and an article having a diameter of 50 mm and a height of 50 mm is obtained. The test sample thus obtained is subjected to a leach test, as hereinafter specified.

In order quantitatively to define the properties of resistance to leaching, the amount of the radioisotopes released is calculated as a quantity called "leaching-rate", the value which in the present case is found to be 5.1-10-5 cm days for Cuss37. The method used is the following: a test sample of the article containing radioactive residues solidified therein according to the method described in the text, which test sample is constituted by a block of material having a cylindrical shape and a total geometrical surface between 10 and 200 cm2, is placed into a polytretafluoroethylene or polypropylene container, in such a way that the sample is suspended by means of threads covered with one of the two aforesaid materials, so as not to touch the surface of the container.The container is filled with de-ionized water so as completely to cover the test sample, which latter is surrounded at each point by at least 1 cm of thickness of de-ionized water. The size of the vessel and the amount of de-ionized water should be chosen in such a way that the volume ratio of de-ionized water to the area of the total geometrical surface of the test sample be within the range from 0.08 to 0.12 m. The container is sealed and maintained for 300 days at 23"C + 1"C, in such a way that the water does not undergo any mechanical stirring.The de-ionized water, after a certain contact time, is substituted with fresh de-ionized water with the following frequency: once a day for the first seven days, twice a week for the second week, once a week for the third, fourth, fifth and sixth week, and thereafter once a month the remaining time up to 300th day.

The individual fractions of water used are collected and the pH, the sodium ion and the Cos'37 are determined with the usual analytical chemistry and radioactivity methods.

The results of the leach test must be expressed for every component by the leaching rate (speed) R defined by: R= a/ (Ab F t, S) wherein: R ' = leaching rate (speed) in m/s of the i-th component during the n-th leaching period; a ' = radioactivity in s- or mass in kg, leached during the n-th leaching period, of the i-th component leached; A, = specific radioactivity in 1 -1 or concentration by weight initially present in the test sample; F = exposed surface of the sample in m2; tn = duration of the n-th leaching period in sec; S = mass volume of the test sample in kg/m3.

A0 and anshould be corrected by taking into accountthe decay time ofthe radionucleid under consideration.

For the most suitable occluding materials, the leaching rate Rn becomes constant after a certain number of continuous renewals of the leaching solution, as appears from a diagram illustrating Rn as a function of time.

Example 2 An article of the same type of Example 1 is prepared by the procedure described in Example 1 and using the same materials as in Example 1, except that the resin used is exclusively of the A type (the type B resin being absent).

The article is subjected to compression test according to the ASTM D 695 norm, and to a cold leach test (as described in Example 1), and furnishes the following results: - compression strength (at 10% deformation): 137 kg/cm2 - (initial) elastic modulus under compression: 4000 kg/cm2 - resistance to leaching of the Na+ present in the sludge leached Na+: 5.0% - leaching rate: R = 6.310-39 cm2/day Result of the irradiation test with radioactive Co, with a radiation density of about 2 105 rad/hours for a total absorbed dose of 5.6.108 rad: weight loss 0.08% of the initial weight.

Example 3 The same sludge of Example 1, kept under good stirring, is brought to a pH 11.6 by addition of about 7 g of concentrated 32% aqueous ammonia. A sludge is obtained which has a solid content of approximately 36-37%.

120 g of mixture of the two resins of the two type A and B resins in 70:30 ratio, described in Example 1, is used for the solidification process.

1.2 g of 50% benzoyl peroxide paste is added.

The mixture thus obtained is poured into a beaker, is kept under stirring with a helical stirrer having a rotation speed of 900 r.p.m. and 90 g of sludges, rendered alkaline with ammonia as hereinbefore described, are added by means of a dosing pump at a rate of 25 ml/min. 0.4 g of N,N-dimethylamino-p-toluidine are further added during the last 30 sec.

A stable dispersion is formed, which is cast into a large test tube and allowed to harden.

The gel time of the mass is 26 min. After 48 hours have passed and the article has completely hardened, test samples suitably cut off the articles are subjected to compression test (according to ASTM D 695 norm) whereby the following results are obtained: - compression strength (at 10% deformation) 95 kg/cm2 - (initial) elastic modulus under compression 2700 kg/cm2

Claims (12)

1. Process for solidifying radioactive residues contained in ion exchange resins, characterized by the fact that it is effected by the following operations: (a) addition of ammonia and one or more strong alkaline bases to radioactive sludges based on ion exchange resins, in such a way that the pH is raised to a value of at least 9, preferably from 9 to 13; (b) dispersion of the product obtained as described under (a), in unsaturated polyester resins, comprising one or more unsaturated polyester and one or more ethylenically unsaturated monomers copolymerizable with said polyesters, as well as conventional additives, such as cross-linking agents, by mechanical stirring, so as to obtain a dispersion of the three phases present (water, ion exchange resins and unsaturated polyester resins) which is stable in time and homogeneous;; (c) hardening, by means of cross-linking agents, of the dispersion thus prepared, so as to obtain a solid article incorporating the radioactive material in solidified form.

2. Process according to claim 1, characterized by the fact that the radioactive sludges of stage (a) contain from 20 to 50% of radioactive solid material.

3. Process according to at least one of the preceding claims, characterized by the fact that the alkalinizing treatment of stage (a) is effected by adding ammonia, preferably gaseous or aqueous, to the sludges kept under stirring until a pH of at least 9, preferably from 9 to 13, has been reached.

4. Process according to claims 1 -3, characterized by the fact that at least a strong alkaline base comprising an hydroxide, oxide or carbonate of a metal chosen among sodium, potassium, calcium and magnesium, is used together with the ammonia.

5. Process according to claims 1 to 3, characterized by the fact that initially the strong base is added until a pH below 9 has been reached, and then aqueous ammonia, concentrated to 32% approximately, is added until a pH from 9 to 13 is reached.

6. Process according to claims 4 or 5, characterized by the fact that the strong alkaline base is caustic soda and that it is added in the form of an aqueous solution having a concentration between 5 and 50% by weight.

7. Process according to one of the preceding claims, characterized by the fact that the ammonia added in the stage (a) is added as such, preferably in the form of an aqueous solution, or is added in the form of a compound capable to release ammonia under the reaction conditions.

8. Process according to one of the preceding claims, characterized by the fact that the maximum limit of the weight ratio of sludge to unsaturated polyester resins is 60:40, preferably 50:50.

9. Process according to one of the preceding claims, characterized by the fact that the unsaturated polyester resin employed is a resin comprising: (1) an unsaturated polyester (A) obtained by polycondensation of: (a) maleic anhydride and/or maleic acid and/or fumaric acid, (b) isophthalic acid and/or terephthalic acid; (c) neopentyl glycol; (d) optionally one or more conventional glycols, such as for instance ethylene glycol, 1,2-propylene glycol, the component (c) of said (A) unsaturated polyester being present in an amount of to at least 50% by mols with respectto the total components (c) and (d); (2) an unsaturated polyester (B) obtained by polycondensation of:: (a) maleic anhydride and/or maleic acid and/or fumaric acid; (b) isopropylidene-bis (phenyoxy-propanol-2); (c) optionally one or more conventional glycols, such as for instance ethylene glycol, 1,2-propylene glycol, neopentyl glycol, the component (b) of said unsaturated resin (B) being present in an amount of at least 50% by mols with respect to the total components (b)+(c); (3) an ethylenically unsaturated liquid monomer capable of copolymerizing with unsaturated polyesters (A) and (B) chosen among styrene, vinyl toluene and methymethacrylate, alone or in mixture with each other; (4) optionally polymerization initiators, accelerators, glass fibers and other conventional additives and fillers; the weight ratio of component (A) to component (B) being comprised between 100:0 and 20:80, preferably 80:20 and 40:60.

10. Process for solidifying radioactive residues contained in ion exchange resins, according to what has been described and exemplified.

11. Compositions of matter characterized by the fact that they comprise a fine and homogeneous dispersion which is stable for long periods of time (at least a number of hours) even after the mechanical stirring has been discontinued, essentially constituted by: (1) aqueous sludges based on radioactive ion exchange resins, in the state of finely subdivided powders, as dispersed phase, and (2) a continuous phase comprising at least one unsaturated polyester resin, to which conventional additives have suitably been added, tha weight ratio of aqueous sludges (1) to the continuous phase (2) being equal or lower than 50:50 while the ratio of peroxide and/or hydroperoxide to the promoter is comprised between 50:1 to 3:1, and the ratio of unsaturated polyester resin to peroxide and/or hydroperoxide being comprised between 200:1 and 20:1.

12. Solid articles containing solidified radioactive sludges based on ion exchange resins, obtained by the process according to claims 1 to 10.