Patent Number: 045307830
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention refers to a composition of matter based on particular unsaturated polyester resins, particularly suitable for solidifying liquid or solid low or medium level radioactive wastes. The present invention further refers to articles obtained by cross-linking aqueous emulsions comprising said composition of matter and radioactive material. Said solid articles are characterized by their stability to radiation and chemical agents for highly prolonged periods of time, by low susceptibility to leaching if immersed in water or in aqueous solutions, and by high mechanical resistance, in particular impact and compression resistance, whereby said articles may be transported and stored without danger that the radioactive substances therein contained may be released therefrom, even in case of accidents or natural calamities. The present invention further refers to a process suitable for obtaining the articles based on the aforesaid composition of matter. 2. Prior Art Processes for solidifying said low and medium level radioactive wastes in the form of solid articles are known, which articles are obtained by immobilizing said radioactive wastes in concrete, cement or in particular in resins or bitumen. The articles obtained by said methods, however, have considerable drawbacks. Thus, for instance, the solidification by concrete or cement which is obtained from the suspension of cement and radioactive solutions and successive hardening, cannot be carried out in the case that the radioactive wastes contain particular chemical substances which act as retardants or inhibitors in the hardening phase, as for instance borates. Further in the presence of other chemicals such as for example the sulphates, the quality of the articles obtained is not satisfactory. Another drawback is due to the fact that the pieces in concrete or cement thus obtained are not sufficiently resistant to leaching of the radionuclides contained therein. This occurs because, as well known, concrete and cement normally have highly porous structure through which water, which has accidentally come into contact with such a piece, may penetrate into the piece, whereby a part of the radioactivity is leached therefrom. Finally, the solidification by concrete or cement does not permit significantly to reduce the volume of the residues to be treated, in as much as, in majority of the cases, it is not possible to solidify by concrete or cement more than 20%-30% of the radioactive waste with respect to the volume of the final product. The use of urea-formaldehyde resin emulsions and radioactive wastes, chemically hardened, has also been proposed. Said process, while it does not permit to overcome the aforementioned problem of the leaching of radioactivity, has, further, the drawback that a piece thus obtained has low mechanical resistance, so that there is a danger that product may become cracked in the case of mechanical accidents, for example during the transport and storage operations. As to the process of solidification by bitumen, in spite of the fact that it offers better results with reference to the resistance to leaching, cost problems are associated thereto, since the solidification must be carried out at a temperature above 150.degree. C. in expensive mixing apparatus. Furthermore, safety problems, especially in the case of the treatment of solutions containing nitrates, may occur which may give rise to bitumen oxidation, which sometimes involves explosive reactions. Finally, the mechanical characteristics of the product obtained by solidification in bitumen are not wholly satisfactory, and considerably deteriorate as the storage temperature increases. In view of the solution of said problems, the art has directed its attention to the use of unsaturated polyester or epoxy resins for the solidification of low and medium level radioactive wastes in the form of solid articles, as described in French Patent Application No. 7425298, and in the form of emulsions in polyester resins of the "water in oil" type, as described by Subramanian and others and as reported in U.S. Pat. No. 4,077,901. In every case, however, it has been noted that insufficient attention has been devoted to the correct choice of an unsaturated polyester resin which should yield optimum result as concerns the most significant characteristics which solid articles incorporating radioactive substances must present. The literature nearly always discloses for this purpose conventional polyester resins based on phthalic anhydride, maleic anhydride and glycols, and in some cases vinyl ester type resins are proposed. No particular effort has been devoted to discover resins suitable to the purpose of obtaining the optimum characteristics required to the solid articles nor resins more suitable to the preparation of emulsions of an unsaturated polyester resin and a radioactive solution or suspension. It is known that unsaturated polyester resins may in general be emulsified with water in the presence of suitable surface active agents or electrolytes dissolved in the water itself or even without their help. In suitable conditions emulsions may be formed both of the "water in oil" type and of the "oil in water" type. The preparation of resin/water emulsions is well known to persons skilled in the art, as described for instance in U.S. Pat. Nos. 3,442,882 and 3,792,006. The unsaturated polyester resins heretofore proposed for the purpose of solidifying solid or aqueous low and medium level radioactive wastes, have been proved not to be suitable, both as concerns the characteristics required from the solid articles obtained therefrom and the emulsifying of the radioactive wastes in the resins themselves. SUMMARY OF THE INVENTION It has now been surprisingly found, and this is one of the objects of the present invention, that compositions of matter based on a particular polyester resin comprising an unsaturated polyester (I) alone or admixed with a different unsaturated polyester (II) in specified composition ratios, not only satisfied the requirements of an easy emulsifying with the aforesaid radioactive wastes, but also present optimum characteristics of the solid articles obtained after hardening said emulsions. An aspect of the present invention is therefore particular compositions of matter suitable for solidifying radioactive wastes, said compositions being formed by unsaturated polyester resins comprising: an unsaturated polyester (I) 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 e.g. ethylene glycol, 1,2-propylene glycol, component (c) of said unsaturated polyester (I) being present in an amount of at least 50% by mols with respect to the total of components (c)+(d); an unsaturated polyester (II) obtained by polycondensation of: (a) maleic anhydride and/or maleic acid and/or fumaric acid; (b) isopropylidene-bis-(phenyleneoxy-propanol-2); (c) optionally one or more convention glycols, such as for example ethylene glycol, 1,2-propylene glycol, neopentyl glycol, component (b) of said unsaturated polyester (II) being present in an amount of at least 50% by mols with respect to the total of the components (b)+(c); an ethylenically unsaturated liquid monomer (III) capable of becoming copolymerized with unsaturated polyesters (I) and (II), chosen from among styrene, vinyltoluene and methylmethacrilate alone or in admixtures with each other; polymerization initiators, accelerators, glass fibers and other conventional additives and fillers; the weight ratio of components (I) and (II) being comprised between 100:0 and 20:80, preferably between 80:20 and 40:60. As to unsaturated polyester (I) and (II) hereinbefore specified, even if some of the components of the compositions of matter hereinbefore defined should be individually and generally known for other uses, nevertheless only the use of compositions comprising components (I) or, preferably, components (I) and (II) in the particular weight ratio hereinbefore and hereinafter specified, confers to the composition those qualities which render them particularly suitable of the purpose of the present invention. If the two unsaturated polyesters hereinbefore defined are individually used for the purposes of the present invention, the results obtained are less satisfactory than those obtained by using their combination. Surprisingly, when the two aforesaid unsaturated polyesters are employed in admixture within the ratio ranges indicated as preferential, an overall optimum of the aforesaid characteristics is achieved both in the process phase of solidifying the radioactive wastes, and as to the properties the solid articles which contain said radioactive wastes solidified therein. The mixture of the components of the compositions of matter hereinbefore specified may occur in any technically suitable order, for instance by following the succession indicated hereinbefore or by mixing the solutions of the two individual polyesters prepared by dissolution thereof in the ethylenically unsaturated monomer. The amounts by weight of the ethylenically unsaturated monomer with respect to the total of the composition of matter is suitably chosen between 15% and 60%, preferably between 25% and 50%. A further object of the present invention is a process for emulsifying said mixtures which form the aforesaid composition with the aforesaid radioactive wastes of an aqueous solution of suspension and hardening said emulsion under particular condition to form solid articles. By this process all types of radioactive wastes may be solidified, except those defined as "high level wastes" residues produced in installations for the spent nuclear fuel reprocessing. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Said radioactive wastes may be in the form of aqueous solutions and/or suspensions of solid material in water (sludges) and/or emulsions of liquid material in water, or may be solid. In this latter case they must be previously suspended or dissolved in water. Said solutions and/or suspension and/or emulsions of radioactive material in water are then emulsified with the hereinbefore defined compositions of matter by means of a stirring system preferably constituted by mechanical stirrer or by a turbine or by a static mixer. The ratio of the composition of matter based on unsaturated polyester resins and the radioactive material to be solidified is not critical as concerns the upper limit, inasmuch as it is possible to emulsify small amounts of radioactive wastes with a strong excess of resins. It is however convenient for economical reasons to use possibly small ratios of resin to radioactive waste. The lower limit of said ratio is 40:60 by weight, preferably 50:50 by weight. In the majority of cases no particular agents are required to facilitate the emulsifying and to stabilize the emulsion in time, unless it should be necessary to store the emulsion for long periods of time. In this last case commercial emulsifying agents are conveniently used. The hardening of the emulsion obtained is carried out by copolymerization of the olefinic double bonds of the unsaturated polyesters and of the ethylenically unsaturated monomer by the action of polymerization initiators. For this purpose the composition of the matter based on unsaturated polyesters contains a polymerization initiator system, preferably constituted by conventional peroxide and optionally by co-initiators and/or accelerators. Examples of peroxides which may be employed are: benzoyl peroxide, tert.butyl peroxide, methylethylketone peroxide, cyclohexanone peroxide and acetylacetone peroxide. Examples of accelerators are: cobalt salts, N,N-dimethylaniline, N,N-diethylaniline, N,N-disubstituted polymeric amines. During the hardening, due to the exothermicity of the copolymerization process (in the order of 80-100 cal/g) a considerable amount of heat develops which is not easily removed especially when the amount of emulsion to be hardened is considerable and in particular if the thickness of the hardened block is large. In the case that the temperature within the emulsion should rise above 100.degree. C. due to the evolution of reaction heat, water may become vapor, thus resulting in rupture of the emulsion and cracking or breaking up of the article to be obtained. It is therefore necessary to assure that the temperature within the emulsion should never rise above 100.degree. C., and preferably should remain below about 90.degree. C. This may be obtained by suitably dosing the catalytic system and the inhibitors, according to known techniques which prolong the hardening process in time. The emulsifying and the subsequent hardening may suitably occur in 2 different vessels or they may occur in the same vessel. The vessel in which the hardening occurs may be a drum of iron or of any other material destined to contain the radioactive block to be obtained, both during transport and during storage. A convenient solution may consist in using a drum made of resin reinforced with glass fibers; in this way the drum and the hardened block to be contained therein will form a single body after the polymerization of the emulsion. It is preferable to employ a composition of matter which contains fibers so as to increase the mechanical resistance of the radioactive block obtained. To this purpose, cut glass fibers, called "chopped-strand" are conveniently used. The solid blocks containing the material in the form of occluded aqueous particles may embody up to 60% by weight of radioactive aqueous solutions or suspension with respect to the total weight of the piece. Said solid blocks obtained by the process according to the present invention are particularly suitable for transportation and storage of the radioactive waste thanks to a series of particular characteristics: (1) high resistance to leaching of the incorporated radioisotopes. A solid piece containing aqueous solutions and/or emulsions and/or dispersions, in the form of small particles blocked therein, according to what is described in the present invention, must lose only minimum amounts by weight of the radioactive and non-radioactive ions contained therein, when it is leached with deionized water for 72 hours at 99.degree. C. according to SOXHLET LEACH TEST PROCEDURE "for testing of Solidified Radioactive Waste", and when it is leached with deionized water for 300 days at 23.degree. C. according to the method described in Example 20. (2) High compression strength. The compression strength is higher than 50 kg/cm.sup.2 and may reach values above 170 kg/cm.sup.2 (measured according to the norm ASTM D 695). (3) High impact resistance. (4) Radiation resistance. A block of solid material as hereinbefore defined, when irradiated by a Co source at the radiation dose of about 2.10.sup.5 rad/hours for a total absorbed dose of 5.6.10.sup.8 Rads, undergoes a loss of weight in the order of 0.1% approximately or less, of the initial weight of the sample and does not show substantial alterations of its mechanical characteristics. In order to further reduce a possible leaching of water-soluble ions of the radioactive material of the solid block, it is convenient to add to the water/radioactive material system, before emulsifying it with the hereinbefore defined compositions of matter, a substance which is capable of sequestring the water-soluble radioactive ions present, in the form of slightly soluble complexes. The invention is illustrated by the following examples, which have no limitative function. The parts (indicated by the symbol "p") are to be understood as parts by weight, unless otherwise specified.