Patent Number: 047327057
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The current invention concerns a process for the improvement of the stability properties of solidified radioactive ion exchange resin particles, wherein the resin particles are embedded in a mixture containing an inorganic and/or organic binding agent, which is then left to harden. 2. The Prior Art In most nuclear power plants organic ion exchange resins in the form of beads or powder are used for the cleaning of the various water circulation systems. In the following, the beads as well as the powder particles of the ion exchange resins are designated as resin particles. The ion exchange resin particles act to retain general impurities in the water circulation systems, and also radionuclides. In this manner, the activity of the circulation systems can be kept within limits. Active ion exchange resins also accumulate in the reprocessing plants. The use of ion exchange resins almost always is carried out in mixed bed processes, i.e., mixed anion and cation exchange resins. Only fresh resins in the OH' or H' form are used in each case, so that no foreign ions are introduced into the circulation systems. The ion exchange resins have to be replaced each time when their capacity has been exhausted by charging with general impurities when they can no longer accept any activity. The replaced ion exchange resins are to be considered mildly to medium active radioactive waste which has to be disposed of. For a final storage, but even for transport, radioactive waste has to be generally solidified, whereby, for security reasons, varying demands are made with regard to the solidified waste. This includes sufficiently high compressive strength, a good water resistance, sulfate resistance and the lowest possible leach rate. For the solidification of radioactive ion exchange resins, the resin particles are embedded into inorganic and/or organic binding agents, such as cement, bitumen or plastics for the formation of a so-called matrix. It is desired to accommodate the greatest possible amount of waste within a certain matrix volume. The swelling and shrinking behavior of organic ion exchange resins is responsible for the fact that the matrix, after solidification, is possibly not water resistant. For this reason the cement solidification for such resins is often regarded with skepticism. In fact, such a matrix may develop cracks during later storage in water, or even decay, if not special techniques are used during solidification. With regard to the described facts, the amount of resin for the solidification of resin particles has usually been limited to about 20 kg of dry resin particles per 100 liters of matrix, whereby the resulting compressive strength was a little above 20 N/mm.sup.2. In addition, if the solidification of the cement mixture takes place under water, the matrix also becomes water resistant, unless it is not dried between. With a higher share of ion exchange resins in the matrix, the compressive strength decreases below 10 N/mm.sup.2. But even such a matrix can, under certain circumstances, remain stable at water storage, if there is no drying beforehand. However, if test bodies of such cement solidification procedures are conditioned, e.g., in air with 20% relative humidity, whereby drying causes a weight loss of up to 25%, they are no longer stable for water storage. Their compressive strength decreases already considerably during the drying process, whereby shrinkage tears appear. During subsequent water storage, the test pieces decay in most cases within hours or a few days, or at least large tears appear. SUMMARY OF THE INVENTION It is the object of the current invention to create a process by which the stability properties of solidified radioactive ion exchange resin particles is considerably improved and thus also makes it possible to give greater amounts of resin into a matrix volume. The invention is based on the knowledge that by a suitable treatment of the radioactive ion exchange resin particles before or during the solidification process, the swelling and shrinking properties of the resin particles can be improved in such a manner that the resulting solid matrix can, with approximately the same compressive strength, not only contain a considerably increased amount of ion exchange resin, but also have good water resistance and stability after drying. By the treatment according to the invention, the ion exchange resin particles can be brought into a stable state in which they have, compared to untreated resin particles, a reduced swelling capability and possibly also a smaller volume. The invention will be now explained in more detail by way of examples, and compared to the state of the art. Depending on the type of nuclear reactor, two different types of ion exchange resins are used in Switzerland, which are, mainly powder resins such as the Powdex resins from Graver Water Conditioning Co., U.S.A in boiling water reactors, and almost exclusively bead resins such as the Lewatit resins from Bayer/Leverkusen, FRG in pressurized water reactors. The following examples are based on tests with bead resins of the last mentioned type. However, the results with powder resins are almost the same. Used as anion exchange resin was the type Lewatit M-500 and as cation exchange resin the type Lewatit S-100, both from Bayer/Leverkusen, FRG. The radioactive ion exchange resins taken from the water circulation systems of a nuclear power plant were, in addition, charged as follows: anion exchange resin M-500 with approx. 200 g of boric acid (H.sub.3 BO.sub.3) per liter resin; cation exchange resin S-100 with 4 g lithim per liter resin. By treating the anion resin particles with a polysulfide it was surprisingly possible to induce the resin particles to strong shrinkage with simultaneous water expulsion. After drying at room temperature and subsequent washing of the thus treated resin particles, they showed a swelling factor of only 1.5 as compared to 2.0 before the treatment. The swelling factor is here defined as the quotient from the settled volume of the resin particles in water-moistened swollen state and the settled volume of the same resin particles in dry state. When, after the polysulfide treatment, the anion resin particles were treated for about 24 hours at a temperature of 50.degree. C., the swelling factor even dropped to near 1.0, which means that the resin particles then do not swell anymore at all and shrink during washing and drying. If, during the treatment with polysulfide, a vulcanization agent, e.g. a xanthate, was also added to the anion resin particles, the swelling factor dropped to between 1.0 and 1.1, even at room temperature. It was possible, not only by treatment with polysulfides, to greatly reduce the swelling factors of anion resin particles, but also by ion exchange with special organic acids or anion-active organic compounds. Named as such can be mono and polyfunctional carboxylic acids, their salts and their derivatives, such as stearic acid, acrylic acid, natural and modified root resins, sebacic acid, etc.; sulfuric acid mono-esters, such as lauryl sulfate; sulfonates, such as vinyl-sulfonate; phosphoric acid mono and di-esters, such as stearic phosphates, butyl phosphates, nonyl phosphates. These substances block the hydrophilic groups of the anion resins and can, in part, still be cross-linked. For the anion resin, a thermolysis process also proved as suitable as the treatment by addition of polysulfide or of another of the above cited compounds. The splitting off of amines from anion resins at higher temperatures is generally known. The producers of resins issue clear warnings against too high temperatures, as those would endanger the ion exchange properties. However, it has now been discovered that this so far undesirable phenomenon can be used for the improvement of the stability properties of solidified radioactive ion exchange resin particles. If anion resin particles are heated for extended time to 150.degree. C., preferably in an air stream while stirring, amines are split off, primarily trimethylamine. With such a process, the resin particles shrink strongly and lose their swelling and shrinking properties. By adding alkali or earth alkali hydroxides, the decomposition temperature can even be slightly lowered. The duration of the thermal treatment depends on the treatment temperature. The higher the temperature, the shorter the treatment time can be. The temperature for thermolysis can be chosen in the range from between 50.degree. C. and 250.degree. C., preferably between 100.degree. C. and 200.degree. C., whereby the duration of treatment can be, e.g., in the range of between 24 hours and down to 1/2 hour. It is also possible to split off amines from anion resin particles which have previously been treated with sulfides or polysulfides by an additional heat treatment within the limits as described above. Surprisingly, this is already possible successfully at a temperature below 100.degree. C., e.g., between 70.degree. and 80.degree. C. This low decomposition temperature affords essential technical advantages, especially for the gas purification. By a subsequent oxidation with H.sub.2 O.sub.2 it is possible to produce from the original anion resins even cation-active resins, which can disintegrate oxidatively more easily. It was possible to equally strongly reduce the swelling and shrinking properties of cation resin particles with very specific cation or cation active compounds as that of anion resin particles. This was done by the addition of substances from the following groups: primary, secondary, tertiary or quartenary basic amines which have, per molecule, either one, two or more amine groups, whereby the organic groups can be additionally cross-linked; basic organic phosphonium compounds; basic organic sulphonium compounds. A similar effect is shown by Ba.sup.++ and Fe.sup.++ salts. These ionogenic compounds, which sterically fit into the ion exchange resins, are, in part, so tightly bound to the resins that, in the solution of a cement mixture or in highly mineralized ground waters, they no longer exchange these ions and thus the resins remain stable as to volume. This adhesion could often be still improved by a subsequent heat treatment, whereby the swelling factor was still further reduced. In practice, mixtures of anion and cation resin particles are used in most cases. In order to attain, simultaneously, a reduction of the swelling factor for both types of resin particles, one of the described treatments for the anion resin particles as well a one of the described treatments for the cation particles is to be used, or a compoud should be added to the mixture of anion and cation resin particles which contains anion as well as cation active components, and thus effective anions and cations or anion active as well as cation active components. The volume ratios and the swelling factors of the thus treated mixtures of anion and cation resins are composed proportional to the mixture ratio from the data of the individual components, and can thus be precalculted for mixtures when the data of the individual components are known. A series of tests was done with each of the cation resin particles of the type Lewatit S-100 and anion resin particles of the type Lewatit M-500 as well as with a mixture of 50% by weight of Lewatit S-100 and 50% by weight of Lewatit M-500, in order to determine the comparative volumes of the resin particles in water-moistened, swelled state and in dry state, as well as the swelling factor, which was done for resin particles without treatment and for resin particles after one of the treatments described above. The results obtained are displayed in table I. The comparative volumes cited in table I (liter/kg) are, in each case, the specific settled volume in liters of an amount of 1 kg dried ion exchange resin particles in the H or OH form, whereby the specific settled volume is cited once for the wet, swelled resin particles and once for the dry resin particles. The swelling factor is the quotient of wet volume over dry volume. The original state of the resin particles always was the H or OH form. The resin particles were treated with solutions which contained only the substances stated in table I. The amounts of the treatment solution were usually sufficient that a complete charging of the resins according to their maximum capacity was made possible. Where nothing else is stated, the resin particles were treated, in each case, for 1/2 hour at 50.degree. C. with the stated solution, then cooled to 20.degree. C., and stirring continued for another 1/2 hour at 20.degree. C., before filtering and washing the resin particles with distilled water. To determine the specific settled volume of the dried resin particles, the latter were dried in a vacuum at 40.degree. C. until their water content was less than 1% by weight. Where a heat treatment at 160.degree. C. is mentioned in table I, this refers to a drying and subsequent heating to 160.degree. C. for 2 hours. The values cited in table I under nos. 1 to 3 refer to untreated ion exchange resins. The tests no. 4 to 61 were done with cation resin particles and the test no. 62 to 83 with anion resin particles. The information under no. 84 to 103 refer to tests with a mixture of 50% by weight of cation and 50% by weight of anion resin particles. It can be seen from table I that the untreated ion exchange resin particles have a swelling factor between 2.1 and 2.24 at a specific settled volume in a wet, swollen state of 2.5 to 3.23 liter per kg dry substance. Table I shows also that the swelling factor can be substantially reduced to or nearly to 1.0 by a suitable treatment of the resin particles. Of interest in practice are all those types of treatment which result in a swelling factor of less than 1.7. However, also of importance are the statements in table I concerning the wet volume of the treated resin particles. The smaller the wet volume, the greater is the amount of resin particles which can be solidified in a given volume. Thus, a type of treatment should preferably be used which provides an optimum between the lowest possible swelling factor and, simultaneously, the smallest specific wet volume. In order to determine the effect of the reduction of the swelling factor on the stability properties of solidified radioactive ion exchange resin particles, the comparison tests, described below, were executed, on the one hand, on a known standard cement solidification of untreated resin particles and, on the other hand, of a cement solidification for the reduction of the swelling factor of treated resin particles. Used as base was in both cases a mixture of 50% by weight of cation exchange resin of the type Lewatit S-100 and 50% by weight of anion exchange resin of the type Lewatit M-500, as it is obtained, e.g., from the Swiss nuclear power plant Goesgen as radioactive waste.