EP0182172B1 - Process for improving the stability properties of solidified radioactive ion exchange resin particles - Google Patents

Description

The present invention relates to a method for improving the stability properties of solidified radioactive ion exchange resin particles, in which method the resin particles are embedded in a mixture containing inorganic and / or organic binders, which is then allowed to harden.

In most nuclear power plants, organic ion exchange resins in the form of small balls or in powder form are used to clean the various water circuits. In the following, both the balls and the powder particles of the ion exchange resins are referred to as resin particles. The ion exchange resin particles have the task of retaining general impurities in the water circuits, but also radionuclides. The activity of the circuits can thus be kept within limits. Active ion exchange resins are also obtained in reprocessing plants. The ion exchange resins are almost always used in the mixed bed process, i.e. Anion and cation exchange resins mixed. Only fresh resins in the OH 'or H' form are used, so that no foreign ions are introduced into the circuits. The ion exchange resins must be replaced when their capacity is exhausted by loading them with general impurities or when they are no longer able to take up activity. The exchanged ion exchange resins are to be regarded as weakly to moderately active radioactive waste that has to be disposed of.

For final storage, but also for transport, radioactive waste generally has to be solidified, with various requirements being placed on the solidified waste for safety reasons. This includes a sufficiently high compressive strength, good water resistance, sulfate resistance and the lowest possible leaching rate. When radioactive ion exchange resins are solidified, the resin particles are embedded in inorganic or / and organic binders such as cements, bitumen or plastics to form a so-called matrix. The aim is to accommodate the largest possible amount of waste in a given matrix volume. The swelling and shrinking behavior of organic ion exchange resins is to blame for the fact that the matrix may not be water-resistant after it has solidified. The cement hardening of such resins is therefore often viewed with skepticism. In fact, such a matrix can crack or even disintegrate when it is later stored in water if special techniques are not taken into account during solidification.

In view of the situation described, when solidifying ion exchange resins in a cement matrix, the amount of resin has generally been limited to about 20 kg of dry resin particles per 100 liters of matrix, resulting in a compressive strength of slightly more than 20 N / mm ² . If the cement mixture is also cured under overcoated water, the matrix also becomes water-resistant, provided that it is not dried in between. With a higher proportion of ion exchange resin in the matrix, the compressive strength drops to below 10 N / mm ² . Such a matrix can also remain stable when stored in water, as long as no drying takes place beforehand. But if test specimens of such cement consolidation, for example in air with 20% rel. Conditioned moisture, with a weight loss of up to 25% due to drying, they are then no longer stable when stored in water. Their compressive strength drops considerably during the drying process, whereby shrinkage cracks form. During subsequent water storage, the test pieces usually disintegrate within hours to a few days, or at least large cracks appear.

DE-A-31 02 473 discloses a method in which the radioactive resin particles are treated with additives and / or thermally before their embedding in order to separate anion and cation resins. However, the influencing of the source factor was not controlled, but only the aforementioned separation was aimed for.

Furthermore, from FR-A-25 02 382 a method is known which describes the detachment of the activity from the organic resins and their binding to inorganic materials for the purpose of solidifying radioactive resin particles. A simple method for stabilizing the resin against swelling and shrinking does not come from this either.

It is an object of the present invention to provide a method by which the stability properties of solidified radioactive ion exchange resin particles are considerably improved and thus also enables a larger amount of resin per matrix volume unit to be solidified.

The method found as a solution to this problem is defined in claim 1. Appropriate developments of the method according to the invention result from the dependent claims and the following explanatory description.

The invention is based on the knowledge that the swelling and shrinking properties of the resin particles can be improved by a suitable treatment of the radioactive ion exchange resin particles before or during the solidification process in such a way that the resulting solidified matrix not only considerably more ion exchange resins with an approximately constant compressive strength may contain, but also has good water resistance after drying. The treatment according to the invention allows the ion exchange resin particles to be brought into a stable state in which they have a reduced swelling capacity compared to untreated resin particles and possibly also have a smaller volume.

The invention is explained in more detail below with the aid of examples and compared with the prior art.

Depending on the type of nuclear reactor, two different types of ion exchange resins are used in Switzerland, namely mostly powder resins in boiling water reactors, e.g. the Powdex resins from Graver Water Conditioning Co.USA, and almost exclusively spherical resins in pressurized water reactors, e.g. the Lewatit resins from Bayer / Leverkusen, FRG. The following examples are based on tests with spherical resins of the latter type. However, practically the same results can also be seen with powder resins. The Lewatit M-500 was used as the anion exchange resin and the Lewatit S-100 as the cation exchange resin, both from Bayer / Leverkusen FRG. The radioactive ion exchange resins taken from the water circuits of a nuclear power plant were also loaded as follows: M-500 anion exchange resin with about 200 g of boric acid (H ₃ B0 ₃ ) per liter of resin; Cation exchange resin S-100 with 4 g lithium per liter resin.

By treating the anion resin particles with a polysulfide, it was surprisingly possible to induce the resin particles to shrink considerably while at the same time allowing water to escape. After drying at room temperature and subsequent washing of the resin particles treated in this way, they had a swelling factor of only 1.5 to 2.0 prior to the treatment. The swelling factor is defined here as the quotient of the shaking volume of the resin particles in the water-wet, swollen state and the shaking volume of the same resin particles in the dry state.

If the anion resin particles were dried at a temperature of 50 ° C for approx. 24 hours after the polysulfide treatment, the swelling factor even decreased to around 1.0, which means that the resin particles then no longer swell and shrink when washed and drying.

If, during treatment with polysulfide, the anion resin particles still contain a vulcanizing agent, e.g. a xanthate was added, the swelling factor dropped to 1.0 to 1.1 even at room temperature.

The swelling factor of anion resin particles could not only be greatly reduced by treatment with polysulfides, but also by ion exchange with special organic acids or anion-active organic compounds. Examples include: Monofunctional and polyfunctional carboxylic acids, their salts and their derivatives, e.g. Stearic acid, acrylic acid, natural and modified root resins, sebacic acid, etc .; Sulfuric acid mono-esters, e.g. Lauryl sulfate; Sulfonates, e.g. Vinyl sulfonates; Phosphoric acid mono- and di-esters, e.g. Stearyl phosphates, butyl phosphates, nonylphenol phosphates. These substances block the hydrophilic groups of the anion resins and can then also be partially crosslinked.

In the case of the anion resin, a thermolysis process also proved to be as suitable as the treatment by adding polysulfide or another of the abovementioned compounds. The elimination of amines from anion resins at higher temperatures is generally known. The manufacturers of the resins expressly warn against excessive temperatures, as this affects the ion exchange properties. However, it has now been found that this previously undesirable phenomenon can be used to improve the stability properties of solidified radioactive ion exchange resin particles. If anion resin particles are heated to 150 ° C. for a long time, preferably in an air stream with stirring, amines split off, primarily trimethylamine. The resin particles shrink considerably and lose their swelling and shrinking capacity. The decomposition temperature can be slightly reduced by adding alkali and alkaline earth hydroxides. 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 the thermolysis can be selected in the range from 50 ° C to 250 ° C, preferably between 100 ° C and 200 ° C, the treatment time e.g. can range from 24 hours down to 1/2 hour.

Anion resin particles, which have previously been treated with sulfides or polysulfides, can also be removed by additional heat treatment within the temperature limits described above. Surprisingly, this can already be done successfully at a temperature below 100 ° C, for example at 70-80 ° C. This low decomposition temperature offers significant technical advantages, especially in exhaust gas cleaning. Subsequent oxidation with H ₂ 0 ₂ can then be used to produce cation-active resins from the original anion resins, which are more easily oxidatively degradable.

The swelling and shrinking capacity of cation resin particles could be reduced by ion exchange with very specific cations or cation-active compounds as much as that of anion resin particles. This was done by adding a substance from the following groups: primary, secondary, tertiary and quaternary basic amines which have either one, two or more amine groups per molecule, it being possible for the organic radicals to be additionally crosslinkable; basic organic phosphonium compounds; basic organic sulfonium compounds. Ba ⁺⁺ and Fe ^H salts have a similar effect. Some of the ionic compounds that sterically fit into the ion exchange resins are so strongly bound to the resins that they no longer exchange these ions in the environment of a cement lye or in highly mineralized groundwater and the resins therefore remain stable in volume. This adhesive strength could often be determined by retrofitting Improve heat treatment even more, whereby the swelling factor could be reduced again.

In practice, mixtures of anion and cation resin particles are mostly used. In order to achieve a reduction in the swelling factor for both types of resin particles at the same time, either one of the treatments described for the anion resin particles and one of the treatments described for the cation resin particles should be used, or the mixture of anion and cation To add resin particles a compound that contains both anion and cation-active components, ie effective anions and cations or anion-active as well as cation-active components. The volume ratios and the swelling factors of mixtures of anion and cation resins treated in this way are proportional to the mixture ratio from the data of the individual components and can therefore be calculated for mixtures in advance if the data for the individual components are known.

A series of tests were carried out in each case with cation resin particles of the Lewatit S-100 type and anion resin particles of the Lewatit M-500 type and with a mixture of 50% by weight of Lewatit S-100 and 50% by weight of Lewatit M-500, to determine the comparative volumes of resin particles in water-wet, swollen and dry states, as well as the swelling factor, for resin particles without treatment and for resin particles after one of the treatments described above. The results obtained are summarized in Table I.

The comparison volumes given in Table 1 (liters / kg) are each the specific vibrating volume in liters of an amount of 1 kg of dry ion exchange resin particles in the H or OH form, the specific vibrating volume once for the wet, swollen resin particles and once for the dry resin particles is listed. The swelling factor is the quotient wet volume by dry volume.

The initial state of the resin particles was always the H or

OH form. The resin particles were treated with solutions which exclusively contained the substances listed in Table I. The quantities of the treatment solutions were each sufficiently large so that the resins could be fully loaded in accordance with their maximum capacity. Unless otherwise noted, the resin particles were each treated with the relevant solution for 1/2 hour at 50 ° C., then cooled to 20 ° C., after which stirring was continued at 20 ° C. for 1/2 hour before being filtered off and the resin particles were washed with distilled water. To determine the specific shaking volume of the dry resin particles, the latter were dried in vacuo at 40 ° C. until their water content was less than 1% by weight.

Where a heat treatment at 160 ° C is mentioned in Table I, it was a drying and subsequent heating to 160 ° C for 2 hours.

The characteristic values listed in Table 1 under Nos. 1 to 3 relate to untreated ion exchange resins. Experiments Nos. 4 to 61 were carried out with cation resin particles and Experiments Nos. 62 to 83 with anion resin particles. Nos. 84 to 103 relate to experiments with a mixture of 50% by weight of cations 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 2.10 and 2.24 with a specific vibrating volume in the wet, swollen state of 2.50 to 3.23 liters per kg of dry substance. Furthermore, Table I shows that the swelling factor can be considerably reduced to or almost to 1.0 by suitable treatment of the resin particles. All those types of treatment that result in a swelling factor of less than 1.7 are of interest in practice. The information in Table I regarding the wet volume of the treated resin particles is also important. The smaller the specific wet volume, the greater the amount of resin particles that can be solidified in a given volume. It is therefore preferable to select a type of treatment which results in an optimum between the lowest possible swelling factor and at the same time the lowest possible specific wet volume. In order to determine the effects of the reduction of the swelling factor on the stability properties of solidified radioactive ion exchange resin particles, the comparative investigations described below were carried out on the one hand on a known standard cement hardening of untreated resin particles and on the other hand on a cement hardening of resin particles treated to reduce the swelling factor.

In both cases, a mixture of 50% by weight of Lewatit S-100 cation exchange resin and 50% by weight of Lewatit M-500 anion exchange resin was used, e.g. is generated as radioactive waste in the Gösgen nuclear power plant in Switzerland.

a) Cement hardening of the untreated resin particles

60 parts by weight of the untreated resin particle mixture with 50% water content, ie fully swollen, were mixed with

100 parts by weight of artificial Portland cement with a high silicate content, designation CPA 55 HTS, manufactured by Ciments Lafarge France, F-92214 St. Cloud, (corresponds to the French standard NF P 15301, December 1978, and the American standard ASTM as Type V, quality "low alkali cement"),

40 parts by weight of hydraulic Nettetaler trass according to DIN 51043, manufactured by Trass-Werke Meurin, Andernach / Rhein, FRG, Kruft plant,

10 parts by weight of calcium hydroxide, Ca (OH) ₂

4.2 parts by weight of superplasticizer (naphthalene-formaldehyde condensate), called Sikament, manufactured by Sika AG, CH-8048 Zurich, and

30.8 parts by weight of water.

The mixture according to the above recipe was allowed to harden with an overcoating of water. The resulting solidified matrix had the key figures shown in Table 11.

b) pretreatment of the resin particles

• 36,0 Gew.-Teile BaS₄-Lösung mit 72,4 Gew.% Wassergehalt (Festkörper trocken 27,6 Gew.%) und 43,35 Gew.-Teile Ba(OH)₂ . 8H₂0.

63.65 parts by weight of the resin particle mixture with 16.4% by weight water content (dry solids 83.6% by weight) were mixed with the following additives to form a thin paste:

• 36.0 parts by weight of BaS ₄ solution with 72.4% by weight water content (dry solids 27.6% by weight) and 43.35 parts by weight Ba (OH) ₂ . 8H ₂ 0.

The cation resin was loaded with Ba ⁺⁺ and the anion resin with S ₄ ^- . The borate detached from the anion resin was precipitated with further Ba ++ as an insoluble barium metaborate. As a result of these reactions occurring during the mixing together, heat was released, which caused the mixture to warm itself from room temperature to approx. 50 ° C. Then the mixture was kept at 50 ° C for a few hours. The cement solidification took place about 24 hours after the pretreatment described. In the meantime, the mixture was further stirred to prevent the solids from settling and the formation of larger crystals. Loss of water due to evaporation during this time was compensated for by adding more water. The resin particles pretreated in this way had the key figures listed in Table I under No. 89.

c) cement hardening of the pretreated resin particles

A previously prepared mixture consisting of was added to the mixture with the pretreated ion exchange resin particles described in section b)

100 parts by weight of artificial Portland cement of the same quality as described in section a), and

40 parts by weight of hydraulic Nettetaler trass of the same quality as described in section a).

Initially, only so much of the latter mixture was added and stirred in homogeneously until a thick paste was formed which, however, still ran together on its own. Then an addition of

2.5 parts by weight of concrete admixture to improve the cement tightness and strength, called Sperrbarra Plus OL, supplied by Meynadier + Cie AG, CH-8048 Zurich.

When this concrete admixture was added, the slurry became noticeably thinner. The rest of the Portland cement / trass mixture could then also be added with constant stirring. The porridge that was finally formed had a just pumpable consistency and was mixed homogeneously for a further 10 minutes. Mixed air bubbles were removed by vibration. After about 2 hours the mixture had solidified sufficiently thixotropically so that it could be overlaid with water for curing. The hardening by the setting of the cement started after 5 to 6 hours, which was recognizable by an increase in temperature. The resulting solidified matrix had the key figures listed in Table II.

d) Comparison of the properties of the solidified matrix with untreated or treated ion exchange resin particles

Table II compares the corresponding key figures of the solidified matrix produced in accordance with section a) (prior art) and the matrix produced in accordance with sections b) and c).

It can be clearly seen from Table 11 that the described pretreatment of the ion exchange resin particles according to the invention for the purpose of reducing the swelling factor achieves two significant advantages over the prior art. The main advantage can be seen in the fact that the water resistance of the solidified matrix is guaranteed even if the matrix up to constant weight at 20% rel. Moisture is dried and then stored again in water, whereas with the matrix according to the prior art the water resistance is only guaranteed as long as no intermediate drying takes place. The other advantage is that in a given matrix volume, e.g. 100 liters, a considerably larger amount of resin particles, namely 35.1 kg versus 22 kg dry matter of the starting resin particles, can be included. This noticeably facilitates the disposal and disposal of the radioactive waste ion exchange resins. As a further advantage of the new process, it should be mentioned that the other properties of the solidified matrix, in particular the compressive strength and the sulfate resistance, are not impaired by the pretreatment of the ion exchange resin particles according to the invention.

It is clear that numerous other substances than those listed in Table I can be used for the pretreatment of the ion exchange resin particles to be consolidated and that the formulations for cement consolidation, which are only described by way of example, can be modified. The ion exchange resin particles pretreated according to the invention with a reduced swelling factor are not only suitable for cement hardening, but can also be hardened just as well by means of bitumen or plastics.

Claims (4)

1. Process for improving 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 mixture is thereafter left to harden, characterized in that the radioactive ion exchange resin particles are altered before or during the solidification process by at least one ion-active additive and optionally by an additional thermal treatment such that their swelling and shrinking behaviour is reduced to a swelling factor of below 1.7, whereby the swelling factor is equal to the quotient from the settled volume of resin particles in water-moistened, swelled state and the settled volume of the same resin particles in dry state, and whereby the additive for treatment of anion resin particles is chosen from the group consisting of: inorganic and organic mono- and polysulfides, sulphuric acid monoester, sulphonates and phosphoric acid mono- and diesters, and whereby the additive for treatment of cation resin particles is chosen from the group consisting of: primary, secondary, tertiary and quartenary basic amines, which may also contain two or more amino groups, basic organic phosphonium compounds, basic organic sulphonium compounds, barium⁺⁺ and iron++ compounds.

2. Process according to claim 1, characterized in that the resin particles are subjected, for the purpose of altering their shares of anions, to a heat treatment at a temperature in the range between 50°C and 250°C, preferably between 100°C and 200°C.

3. Process according to claim 1, characterized in that the resin particles, treated with the additive, are subjected afterwards to a heat treatment at a temperature in the range between 50°C and 250°C, preferably between 100°C and 200°C.

4. Process according to any of claims 2 and 3, characterized in that during the thermal treatment a gaseous material, preferably air, is conveyed as transport means through or across the resin particles for the purpose of carrying off volatile decay products.