DE2652957A1 - PROCESS AND INCLUSION MEASURES FOR CONSOLIDATING RADIOACTIVE WASTE INTO A SOLID BODY - Google Patents

Description

PATENT LAWYERS:
Dipl.-Ing. W. COHAUSZ Dipl.-Ing. W. FLORACK Dipl.-Ing. R. KNAUF ■ Dr.-Ing., Dipl.-Wirtsch.-Ing. A. GERBER - Dipl.-Ing. HB COHAUSZ

NUCLEAR ENGINEERING COMPANY, INC. November 19, 1976

9200 Shelbyville Road

Louisville, Commonwealth of Kentucky U.S.A.

Method and containment compound for solidifying radioactive waste into a solid

The invention relates to a method and an inclusion mass for solidifying radioactive waste into a solid Body.

Urea-formaldehyde polymers are excellent auxiliaries to contain radioactive waste so that it can be deposited in a landfill. With such a deposit however, there is a risk that radioactive substances will be leached out of the solid inclusion mass if they are constantly or come into frequent contact with water. A number of methods have already been proposed to solve this problem been, in which additional process steps are provided or other plastics instead of or in addition to the Urea-formaldehyde polymer can be used.

For radioactive waste to be deposited, it must be enclosed in a solid mass that is resistant to leaching and in this respect complies with certain safety regulations «

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The invention is based on the object of a method for
Solidification of radioactive waste into a solid body
indicate in which a urea-formaldehyde polymer is used as the inclusion mass and which provides a product that is largely resistant to the leaching of radioactive constituents.

According to the invention, this object is achieved by mixing the radioactive waste with an aqueous urea-formaldehyde prepolymer, adding a hardener to the mixture, which
Urea-formaldehyde prepolymer is able to cure into a polymer that absorbs most of the radioactive waste in this way
includes that it cannot be leached by water and that the mixture is stirred until it solidifies.

Advantageous further developments of the invention are set out in the subclaims specified.

The method is also useful for trapping toxic chemical Waste so that it can be handled and disposed of without risk,

The process is suitable for the disposal of all forms of low-level radioactive waste. This waste can be in solid or liquid form and be of an inorganic or organic nature. If the material is in solid form, it should
expediently be comminuted to a grain size of 0.8 mm (20 ASTM mesh) or smaller. Liquids can be treated more easily according to the method, especially if they are aqueous liquids; you can solve,
contain dispersed or suspended solids. The radiating nuclides can be anions or cations (in complex or non-complex bonds), metals or organometallic
Compounds in real solution, as a dispersion or suspension

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be present. Nuclides in ion form are produced by. Water especially easily leached and the method of the invention is particularly useful for including such shapes in one Compound that is particularly resistant to leaching.

The concentration of the radiating nuclides can be from less than 1 ppm up to concentrations which are equal to the radiation tolerance of the enclosing polymer mass. This tolerance is included
duration of the system,

10
Tolerance is around 10 erg / g for the assumed food

The low-level radioactive waste can also contain various non-radioactive substances. Especially boric acid in concentrations up to saturation and beyond that up to mixtures containing solid boric acid crystals, as well as sodium sulfate in Concentrations up to saturation and beyond that up to mixtures containing solid sodium sulfate crystals (anhydrous or as decahydrate) can be tolerated. Since these substances occur frequently in radioactive waste and with certain Deposition systems cause difficulties is the tolerance of the method according to the invention such substances are particularly valuable.

According to the method of the invention, the radioactive waste brought into a storable form by being with an encapsulant capable of forming a synthetic resin body by the radioactive waste is immobilized, mixed, and then the containment agent is cured until the desired resin composition is obtained is preserved.

According to the method of the invention, the containment agent is a Pre-polymer made from urea and formaldehyde, which is used to enclose radioactive waste with other substances before it is used is treated.

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Urea-formaldehyde prepolymers are commercially available, and all these common products which cure to a high polymer with the addition of an acidic hardener let are suitable for the procedure. A preferred urea-formaldehyde prepolymer can be made from 1 to 2 moles of formaldehyde per mole of urea, preferably from 1.5 moles of formaldehyde and 1 mole of urea. Such urea-formaldehyde- Prepolymers that cure when acidic hardeners are added have already been proposed for containment of radioactive waste been. However, these inclusion masses tend to leach out when the solid block comes into contact with water, so that further Measures to solve the leaching problem are necessary. By the method according to the invention achieves that the inclusion mass and the polymer are considerably more resistant to leaching are.

One improvement is that nitric or phosphoric acid is used as the hardener. Experimental results show that When using this hardener, the hardened urea-formaldehyde polymer has a higher resistance to leaching is obtained. A further improvement in the inclusion mass is obtained by treating the mass with a cation exchanger or a chelating agent prior to adding the radioactive waste and curing.

The ion exchanger can be one of the many commercially available products. However, a preferred material is a polysulfonated one Polystyrene, which is used in the form of the free acid or in the form of a salt, for example the sodium salt can be. In other words, a preferred ion exchanger is polystyrene sulfonic acid or an alkali metal salt Polystyrene sulfonic acid. The effectiveness of the ion exchanger in the treatment of the urea-formaldehyde prepolymer

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by preconditioning the ion exchanger with water significantly increased prior to mixing in the urea-formaldehyde prepolymer. This preconditioning is best done in carried out in such a way that equal amounts of ion exchange resin and water are left to stand for about 1 week, but it goes without saying that that other amounts of water and shorter times are also effective.

Another excellent form of inclusion mass consists of the aforementioned urea-formaldehyde prepolymer, the one Contains a small amount of a chelating agent. In general, any chelating agent that can be used with the material is compatible. Preferred chelating agents which are particularly effective in the inclusion of polyvalent ions are ethylenedinitrile tetraacetic acid, Nitrile triacetic acid, dodecylamine dimethylene phosphonic acid and their chemical analogues.

To carry out the containment process, those described above are used Containment agents simply added to the radioactive waste in the amounts and ranges specified below, the total amount of water being kept in the specified ranges, regardless of whether the water comes from the Containment agent or from the containment agent and radioactive waste. After mixing it becomes an acidic hardener added, which lowers the pH so much that the mixture hardens to a solid resin mass.

The method according to the invention for solidification of radioactive Waste to a solid generally consists of the following steps in the order given: First, an inclusion mass according to the invention is prepared, which mainly consists of an aqueous liquid Phase consists of urea-formaldehyde prepolymer; these

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Inclusion compound is mixed with radioactive waste and then with a to cure the urea-formaldehyde prepolymer added acidic hardener, which is able to form a cured polymer, which the main part of the radioactive waste in a form resistant to water leaching. Technically, the hardener is with the Mixture stirred, so that a uniform hardening and an essentially uniform dispersion of the radioactive waste occurs in the polymer mass.

Urea-formaldehyde synthetic resins have been commercially available from numerous sources for years. These synthetic resins are made by reacting urea and formaldehyde in molar ratios between about 1: 1 and 1: 4, usually between 1: 1.5 and 1: 2.5, manufactured and formed urea-formaldehyde prepolymers, In other words, solid urea and an aqueous solution of formaldehyde react with each other brought and result in a synthetic resin syrup, which becomes a thermosetting plastic is curable. These resins are available in syrup form and sometimes also in spray-dried form, which can be redispersed in water to form a dispersion with the desired solids content.

It has been found that best results are obtained with a formaldehyde to urea ratio at the lower end of the commercial scale, Mi-in other words, the mole ratio of formaldehyde to urea should be between about 1: 1 and 2: 1, preferably between about 1.5 moles of formaldehyde to 1 _f 0 mole of urea. In addition, the prepolymer is prepared from about 35 to 60% should contain _r preferably 40%, water. The end product contains more water due to the added additives and radioactive waste, so that the final ratio of water and synthetic resin solids in

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the final dispersion preferably about 2.5: 1 to about 5.1 parts of water per part by weight of synthetic resin and preferably about 3 to 5 parts by weight of water per part by weight of synthetic resin solid.

As is known, the urea-formaldehyde prepolymer can contribute Ambient temperature can be cured by an acid catalyst to a thermoset. In general, any acid can which has a dissociation constant between about 10 and 10 in water. The amount of catalyst required depends on the strength of the acid and the nature of the mass to be hardened. For example, fabrics tend to like Boric acid, to inhibit polymerization; in this case, therefore, a larger amount of catalyst is required to the to achieve the same curing time. In general, any acid present in the dispersion can have a pH of about 5 or lower generated, can be used. Both inorganic acids, such as hydrochloric acid, sulfuric acid, phosphoric acid, Nitric acid, or acidic salts such as bisulfates, as well as organic ones Acids, such as strong carboxylic acids or variously substituted carboxylic acids, such as chloroacetic acid, as crosslinking ones Catalysts are used. Nitric acid and phosphoric acid are best, as these acids appear to be a dense one form packed inclusion resin which is considerably more resistant to leaching than the resin obtained with other acids.

The relative proportion of the catalyst used depends on the desired rate of crosslinking. For a gelation At room temperature within a period of 5 minutes after the addition of the catalyst, the addition of 2% by volume is (based on the combined volume of the urea-formaldehyde prepolymer and radioactive waste) a 4.2N aqueous solution of a mineral acid catalyst is best. It can

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also correspondingly larger amounts of a more dilute acid catalyst be used; for example 4% by volume of a 2.1N acid solution. The use of such dilute solutions can be advantageous if there are corrosion problems or difficulties in handling the acid. To note is that the preferred amounts given above for the acid catalyst for the inclusion of a neutral radioactive Waste apply. Alkaline wastes require correspondingly larger amounts of crosslinking catalyst while one in the case of acidic waste, correspondingly smaller quantities can be used.

Particularly good inclusion masses are obtained by combining the urea-formaldehyde prepolymer with an ion exchange resin. This significantly increases the ability of the finished polymer mass to retain radiating cationic elements against the leaching attack of groundwater. In general, any of the many commercially available cationic exchange resins will be effective, but a polysulfonated polystyrene, which may be in the free acid form or as a salt of this acid, will work best. An ion exchange resin which is particularly advantageous in the process according to the invention has the formula RSO ₃ M, where R is a plastic such as sulfonated polystyrene and M is hydrogen or an alkali metal.

Ion exchange resins can be mixed in as either solids or liquids. In the case of solids, it depends Effectiveness of the additive does not depend significantly on the particle size, at least not in the grain size range between 0.8 and 0.074 mm (ASTM mesh size 20 to 200). However, the effectiveness of the ion exchange resin depends on the existing one Amount from. At concentrations of 1.0% by weight, based on

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On the urea-formaldehyde prepolymer, an essential occurs Reinforcement of the retention of Cesium-137 and Sodium-22. At even higher concentrations up to and including 65% by weight a correspondingly greater gain is observed. A favorable concentration range lies between about 2 to 8 weight percent.

Another improvement in the use of an ion exchange resin is to mix the ion exchange resin into the urea-formaldehyde prepolymer with water preconditioning. Preconditioning the ion exchange resin is best done by mixing the resin with a the same amount of water and let the mixture stand for about a week. Then the ion exchanger still wet mixed with the urea-formaldehyde prepolymer. Although the stated concentration and time span for preconditioning is considered optimal, better results are also obtained when the cation exchanger preconditioned with about 0.1 to 10.1 parts by weight of water and the mixture is left to stand for more than 12 hours.

Thus, mixing the urea-formaldehyde prepolymer with an ion exchange resin provides an improved inclusion mass obtained, and particularly good results are obtained when the ion exchange resin is conditioned beforehand with water.

An improved inclusion mass is also obtained by blending chelating agents into the urea-formaldehyde prepolymer. Preferred chelating agents that are particularly effective in trapping polyvalent ions are ethylene dinitrile tetraacetic acid, Nitrile triacetic acid, dodecylamine methylenephosphonic acid and their chemical analogues.

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The minimum concentration of these chelating agents depends on the concentration of those present in the radioactive waste emitting cations. For solutions of radioactive waste with a cation content of 10 M, the effective is Chelating agent concentration 0.01% by weight, based on the urea -Formaldehyde prepolymer, but larger amounts can be used if desired.

From the above description it can be seen that by treating urea-formaldehyde prepolymers with Chelating agents improved inclusion masses can be obtained. It was also shown that the addition of Ion exchange resins can improve the properties of the inclusion masses, and it will be understood that, if desired, Can use chelating agents and ion exchange resins together.

The preferred method of applying the containment mass depends on the physical and chemical nature of the radioactive Waste off. In the case of wastes that mainly consist of aqueous solutions, the urea treated with an additive -Formaldehyde * - prepolymer (1 part by volume) with radioactive waste (1-3 parts by volume) to a well homogenized mixture stirred. With further stirring, the crosslinking acid (4.2 nj 2 vol%, based on the total mixture) becomes aqueous Solution added and stirring continued until gelation occurs, which occurs at room temperature after about 2 to 4 minutes the case is.

Mixed radioactive waste, which consists partly of solid materials and partly of aqueous solutions, can be similar Way to be processed. Solids can also be treated in a similar manner or mixed with water prior to inclusion will. Strongly acidic radioactive waste can be less or less

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do not need any additional crosslinking catalyst. Alkaline waste must be sufficient before the addition Amount of crosslinking catalyst can be adjusted to a pH of 7 with mineral acid.

The invention is described in more detail with the aid of the following examples. In the examples, containment effectiveness was determined by measuring the persistence of selected radionuclides determined against leaching by water. In a quick test, a test specimen is immersed in a water bath and by a slowly flowing water flow around room temperature. The test specimen has a cylindrical shape with a diameter of 22 mm and a length of 50 mm. Before immersing the test body in the Leaching bath and then at regular intervals the radiation emission spectrum of the test body is determined with a chess detector and multi-channel analyzer. For everyone in the Polymer mass entrapped radionuclide, the percent reluctance at the time of measurement was calculated. With radionuclides with a relatively short half-life, the natural decay was taken into account by means of a correction.

A less aggressive, generally preferred, leach test will only use a circular base of the cylindrical Test specimen exposed to the leaching effect of a slowly flowing stream of water at room temperature. This less aggressive test is more beneficial as it gives a more accurate assessment of the relative merits of different basic weights Composition allows.

To check the containment of radioactive waste different radionuclides are used, their type, chemical form and other data are given in the following table are;

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Tabel

Nuclide chemical form

Na-22 O
CO Co-57
Co-60 ^, Sr-85 he" Cd-109 Cs-137

Na ⁺ in Q.01-m H ₃ SO ₄

++
Co in 0.01-m H ₀ SO

Co (NHg) 5H9O in

Cationic
Molality Approx.
JiCx / ml activity
1
s. ml /, ο χ. ι vj 0.01 370 -1 9
8.2 χ 10 0.004 148 8.8 10 ^IU 0.06 2220 9.9 χ ΙΟ " ¹² 0.02 740 7.0 χ 10 0.02 740 3.4 χ 10 " ⁹ 0.04 1480

Sr ⁺⁺ in 0.01-m H ₂ SO ₄ ^Ä "'"" ^1Z ■ - -« - ^ - "

Cd ⁺⁺ in 0.01-m H ₂ SO ₄ Cs in 0.01-m H ₃ SO ₄

cn ro co

EXAMPLE 1

It became an aqueous solution of a urea-formaldehyde prepolymer produced in which the molar ratio of urea to formaldehyde was 1: 1.5. The prepolymer contained about 65% of a mixture of methylolurea and dimethylolurea in addition to a small amount of condensation products and 35% water. 49 ml of this solution were with an aqueous Solution (144 ml) mixed with the Na-22, Co-57 and Cs-137 of the in the specific concentrations and specific activities given in the table above.

To this mixture was added a saturated aqueous solution of sodium hydrogen sulfate (4.0 ml; 4.2 N) with vigorous stirring. The viscous mixture was poured into a Nalgen vial and solidified into a solid gel after 4 minutes. To allowing the sample to cure for 48 hours in the sealed container The vial was unsealed and the sample in the vial leached with room temperature tap water. After leaching for 300 hours, the content of retained cesium-137 was 34.0%, of sodium-22 28.0% and of cobalt-57 56.0%.

EXAMPLE 2

20 ml of the urea-formaldehyde prepolymer described in Example 1, but which contained 0.01 wt .-% dodecylamine-dimethylene phosphonic acid, was with 60 ml of an aqueous solution mixed the sodium-22, cobalt-57 and cesium-137 in the specific concentrations given in the table above and activities included. The mixture was washed with saturated aqueous sodium hydrogen sulfate solution (1.6 ml; 4,2-n) crosslinked and treated as in Example 1. After 300 hours

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According to the alkaline solution, the amount of cesium-137 retained was 40.0%, Sodium-22 55.0% and Colbalt-57 72.0%. From the above comparison it can be seen that the retention of trapped radioactive material by the Chelating agent is increased in the leach test.

EXAMPLE 2A

The procedure of Example 2 was followed with ethylene dinitrile tetraacetic acid repeated as a chelating agent. Similar good results were obtained.

EXAMPLE 2B

The procedure of Example 2 was carried out with nitrile triacetic acid repeated as a chelating agent. A similar improvement in containment was observed.

EXAMPLE 3

The procedure of Example 1 was repeated using aqueous nitric acid (4.0 ml; 4,2-n) was used as a crosslinking catalyst. After leaching for 300 hours, the amount was retained Cesium-137 39.5%, Sodium-22 52.0% and Cobalt-57 45.2%. From these results it can be seen that the use of nitric acid as a crosslinking catalyst is too unexpected gave better results than when using sodium hydrogen sulfate.

EXAMPLE 3A
The procedure of Example 3 was repeated using instead

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the nitric acid aqueous phosphoric acid (4 _r 0 ml; 4,2-n) was used ver. Leach tests showed that phosphoric acid also increased radioactive waste retention compared to sodium hydrogen sulfate.

EXAMPLE 4

The procedure of Example 1 was repeated with the following two modifications. First was polystyrene! Polysulfonic acids which had been preconditioned for a week with the same amount of water by weight, the urea-formaldehyde prepolymer added before mixing with the aqueous radionuclide solution. The urea-formaldehyde prepolymer contained 2.8% by weight of this additive. Then the polymer system crosslinked with aqueous phosphoric acid (4.0 ml; 4.2-n) instead of sodium hydrogen sulfate. After leaching for 300 hours, the retained proportion of cesium-137 65.0%, of sodium-22 53.2% and cobalt-57 45.2%. From this example it can be seen, that the retention of radioactive substances, in particular the retention of cesium-137, through the use of the ion exchange resin is increased.

EXAMPLE 5

The procedure described in Example 1 was repeated with the following modifications;

The aqueous radionuclide solution contained the isotopes cobalt-60, Strontium-85 and Cadmium-109 in the specified in the table Concentrations and Activities.

2. The urea-formaldehyde prepolymer was mixed before mixing with the radionuclide solution, the sodium salt of a poly

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styrene polysulphonic acid was added, which had been preconditioned for a week with the same amount of water by weight. There. Urea-formaldehyde prepolymer contained 3.4% by weight of this Additive.

3. The polymer system was washed with aqueous nitric acid (4.0 ml; 4,2-n) networked.

After leaching for 300 hours, the amount of retained cobalt-60 was 96.0%, that of strontium-85 was 100% and that of cadmium-109 96.0%. Example 5 illustrates the excellent results obtained on ion retention on leaching through the use of pretreated ion exchange resins: nd the preferred crosslinking catalyst (nitric acid) were obtained.

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Claims (13)

Expectations Process for solidifying radioactive waste to form a solid body, characterized in that the radioactive waste is mixed with an aqueous urea ^ formaldehyde prepolymer, and a hardener is added to the mixture which contains the urea-formaldehyde prepolymer
capable of hardening to a polymer which encloses most of the radioactive waste in such a way that it cannot be leached by water, and the mixture is stirred until it solidifies. 2. The method according to claim 1, characterized that nitric acid or phosphoric acid is used as hardener, 3. The method according to claim 1 or 2, characterized in that that the urea-formaldehyde prepolymer is used in the liquid phase. 4. The method according to any one of claims 1 to 3, characterized characterized in that the urea-formaldehyde prepolymer prior to mixing with the radioactive waste a chelating agent is added. 5. The method according to claim 4, characterized in _{that f} that the chelating agent dedecyclamine-dimethylene phosphonic acid _r ethylene ^ dinitrile-tetraacetic acid,
Nitrile triacetic acid or an ion exchange resin is added _r 6. The method according to claim 5, characterized 30 309 U / Be - 2 - 709827/0571 draws that polysulfonized polystyrene is used as the ion exchange resin. 7. The method according to claim 6, characterized in that the ion exchange resin is polystyrene sulfonic acid or an alkali metal salt of polystyrene sulfonic acid is used. 8. The method according to any one of claims 5 to 7, characterized in that the ion exchange resin prior to addition to the urea-formaldehyde prepolymer is conditioned with water. 9. The method according to claim 8, characterized in that that the ion exchange resin by mixing with 0.1 to 10.0 parts by weight of water and more is preconditioned as standing for 12 hours. 10. The method according to claim 8, characterized in that that the ion exchange resin by mixing with about an equal part by weight of water and Allowing to stand for about a week is preconditioned. 11, containment compound for low-level radioactive waste, characterized in that it consists of a urea-formaldehyde prepolymer, which consists of Conversion of urea and formaldehyde in a molar ratio of about 1: 1 to 2: 1 in water has been and 25-6Ο wt .-% water, based on the total weight of the prepolymer, and at least about 0.01% by weight, based on the weight of the inclusion mass, of a chelating agent. 709827/0571 12. Inclusion compound according to claim 11, characterized that the chelating agent dedecylamine dimethylene phosphonic acid, Ethylene dinitrile tetraacetic acid, Nitrile tr! Acetic acid or an ion exchange resin, where the ion exchange resin is present in an amount of 2-8% by weight of the mixture. 13. Inclusion compound according to claim 12, characterized that the ion exchange resin is polystyrene sulfonic acid or an alkali salt of polystyrene sulfonic acid is. 709827/057 1