Patent Number: 055457983
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT Preliminary Comments: These figures show the preferred treatments of radioactive ion-exchange resins. The embodiments described are consistent with treatments now used for ion-exchange resins contaminated by water carried by alloyed metal tubing through certain nuclear reactors at power stations. For teaching the method of this invention, the figures show chemically or physically important stages of the treatments: For each figure, on a local scale the stage order indicated is substantially followed, although the stage times may be almost simultaneous, as is discussed later. On the bulk scale the stages are reached at different times as the solid resin depolymerizes over a period of time because only the resin surface is exposed to reaction. FIG. 1: This figure shows the preferred embodiment for treatment to reduce the burial volume for radioactive ion-exchange resin when barium hydroxide is the anchor material supplied. FIG. 1 Stage 1: A sealable container with stirrer is supplied an aqueous slurry of barium hydroxide anchor material and radioactive ion-exchange resin, e.g., from operations of a BWR nuclear-electric power station. Barium anchoring ions, Ba.sup.++, load hydrogen ion sites of the ion-exchange resin, thereby anchoring the cation-exchange groups, here sulfonate groups, and decaying atoms, which may be on the resin or in aqueous solution. First-treated resin is formed. Excess anchoring ions also remain, and water is drained off for recycle to more aqueous slurry. FIG. 1, Stage 2: By heating the stirred container to a suitable temperature, e g., to 150.degree. C., water vapor is driven off and is collected for recycle to the BWR turbine generator. FIG. 1, Stage 3: By further heating toward 300.degree. C., a series of reactions take place. (i) Heat and further anchoring ions, perhaps with the assistance of water, attack the bonds between the anchored cation-exchange groups, here sulfonate groups, and the organic portion of the first-treated resin; the attack converts sulfonyl groups to firmly bonded radioactive material such as radioactive BaSO.sub.4, i.e., synthetic barite mineral, that is at least in part chemically freed from organic material. (ii) The attack also releases organic polymer residue that is at least in part freed from anchored sulfonate groups and their attached decaying atoms. (iii) The heated organic polymer residue is also allowed to depolymerize, at least in part, and barium compounds may catalyze the depolymerization. (iv) Vaporization of the depolymerized resin allows the organic material to be removed from the firmly bonded radioactive material and be collected elsewhere. FIG. 1, Stage 4: The condensed organic vapor may need final purification, e.g., washing with dilute acid to remove contaminants such as traces of radioactive material or hazardous material or trimethyl amine from anion resin that may have been present. FIG. 1, Stage 5: The firmly bonded radioactive material goes to storage or burial, and the condensed organic material goes to nonradioactive disposal. FIG. 2, Stage 1: The system is supplied radioactive ion-exchange resin that has been roughly dried consistent with power station policy, e.g., by squeezing it and pumping vapor from it. This resin is placed in a separation contain-along with bonding material (also called anchor material) which, in this preferred embodiment, is a mixture of sodium hydroxide and potassium hydroxide. Aqueous hydroxides form immediately. Other materials comprising oxides could also be used in powder or liquid form, or in other form which could make firmly bonded radioactivity of the next stage. FIG. 2: This figure shows the earlier preferred embodiment to reduce the burial volume for radioactive ion-exchange resin when sodium hydroxide-potassium hydroxide is the bonding (i.e., anchor) material supplied. FIG. 2, Stage 2: The hydroxide solution brings strong ionic environments around both the exposed radioactive ions and the ion-exchange structures attached to the resins. Many surface radioactive ions will move into the hydroxide-solution region--there the radioactive ions are surrounded by ionic fields which bond them more firmly than nonionic organic regions of the resin can do it. Also, the inorganic ion-exchange groups bonded to the organic resin become subject to strong bonding from the ionic aqueous phase. If thermal agitation breaks organic bonds, the originally ion-exchange groups will remain with an ionic aqueous phase or other largely ionic phase. Ion exchange will lead to some removal of interior decaying atoms out to hydroxide solution. However, completing Stage 2 will require conversion of the resin to a different form which gives the hydroxide access to the interior of the solid resin. Heating and various decomposition stages as follow are used to give that access. FIG. 2, Stage 3: Heating of the hydroxide-resin mixture is carried out in a portion of the separation container. The heating, assisted by catalytic and chemical action of the hydroxide, causes (i) depolymerization of much of the resin to form organic liquid solution which is largely immiscible with water, (ii) separation of much of the decaying atoms and much of the ion-exchange portion the resin into aqueous ionic solution, and (iii) formation of some resin residue mixed with some trapped decaying atoms, which mixture is immiscible with either the aqueous or the organic phase. FIG. 2, Stage 4: In this embodiment the physical separation of the decaying atoms from the organic material is primarily by vapor transport. The vaporization and subsequent condensation in another region of the separation container moves major portions of the nonradioactive material where it can be collected and be moved on toward disposal. The vapor transport is assisted by water vaporization with condensation at a collection region of the separation container. The steam acts as a carrier gas (steam distillation). Other carrier gases can also be used for transport of organic vapor to the collection. If the separation container is hermetically sealed, reduced system pressures can assist the vapor transportation. The reduced pressures lower the boiling points for the vapors evolved, and vapor transport is sharply increased by boiling. While vaporization is preferred, in come cases other techniques such as aqueous-organic solvent extraction may also usefully be used. FIG. 2, Stage 4A: The vaporized organics are condensed and held for further vapor condensation as a result of other techniques. FIG. 2, Stage 4B: Here material not decomposed by depolymerization is subjected to pyrolysis by heating. Some pyrolysis is essentially inevitable as corollary to the heating for depolymerization. The depolymerization and pyrolysis in some ways blend into one another: However, the depolymerization refers more to breaking the bonds formed by the original polymerization of reactants, while pyrolysis refers more to breaking miscellaneous bonds, as in charring paper. FIG. 2, Stage 5: Here carbonaceous material, carrying the hydroxide residues, has now largely altered chemically. FIG. 2, Stage 6: Material from Stages 4 and 5 may be combined. They move separately or together to monitoring for possible environmental contaminants. FIG. 2, Stage 8: The nonradioactive organics are monitored. If they pass the monitoring they are ready for release, possibly to recycle and possibly to nonradioactive disposal. FIG. 2, Stage 9: The radioactive material goes to radioactive disposal in smaller volume than it would have had in current technology. FIG. 2, Stage 10: The nonradioactive material, in this case free of chemical hazards as well, is disposed of or is recycled. FIG. 3: This figure shows how the essentials of this preferred embodiment in FIG. 2 may be usefully be expanded or altered. All stages retain their meanings as in FIG. 2. Primarily the stages not included in FIG. 2 are discussed below: FIG. 3, Stage 4C: The carbonaceous material and decaying atoms which might have moved to disposal may also be oxidized primarily to carbon dioxide, but moisture and other molecules may be released during oxidation. This oxidation can remove most of the remaining carbon, but inorganics such as oxides, hydroxides, carbonates, sulfates, etc., will remain, holding the decaying atoms. FIG. 3, Stage 4D: Other techniques may be used instead of vaporization to separate radioactive and nonradioactive portions of the original radioactive ion-exchange resin. For example, as the material sits after depolymerization and corollary initial polymerization, three regions at least will be present, i.e., a liquid organic phase, a liquid aqueous phase, and solid residuals from the depolymerization. In effect, a rough solvent extraction already has been achieved by the depolymerization. The separation already may be adequate to provide easy separation of radioactive and nonradioactive materials. Radioactive aqueous liquid can be poured off and dried with radioactive solids then move in small volume to radioactive disposal. And organic liquid decanted before drying off the water can move to nonradioactive disposal. FIG. 3, Stage 7: Once the larger organic molecules are condensed, nonradioactive carbon dioxide can be collected at another collection site in a separation container. The two sites are not distinguished in the figure but they normally will be separate. Production of gases such as carbon dioxide should be minimized in early stages of the resin destruction to avoid producing large amounts of gases which are difficult to collect and monitor before they are prepared for disposal. By conceptual design, residual carbonaceous chars will be in relatively small amounts and may be oxidized to carbon dioxide. This and other gases may be collected and concentrated in several ways, e.g., (i) with cooling at lowered temperatures and at pressures higher than atmospheric, (ii) by low-temperature sorption, (iii) by collection on chemical scrubbers, or (iv) or by combinations of ways. Carbon dioxide is collected and held in a concentrated form. Therefore, simple analyses can be given enough time and sufficient concentration of decaying atoms to assure accurate measurements. The environmentally benign collected gas can be released to the atmosphere. Experimental Case 1 A typical case with the preferred embodiment using barium hydroxide anchor material proceeded as follows: First, solid UF.sub.4 was contacted for 15 minutes with fresh, sulfonated polystyrene cation-exchange resin in water, thereby adding a distinct U.sup.++++ color to the resin. Next, the wet resin was mixed with enough Ba(OH).sub.2 anchor material in slurry form to allow ultimate formation of BaSO.sub.4 from all the sulfonyl groups in the resin present. This mixture was stirred occasionally for a half hour, allowed to settle, and freed of much of the water by decantation. The wet mixture of anchor material, radioactive resin, and some solid UF.sub.4 was put into a sealed borosilicate-glass system with provision for displacing air, evacuating, heating, and vaporizing and condensing both water and volatile organic materials. The water was largely dried away, either by partial evacuation or by flow of carrier gas, with vapor collection in either case. Consideration of the experimental behavior and theoretical objectives leads the inventor to conclude that anchoring ions had attached to sulfonyl groups and anchored them. At this point one Ba.sup.++ attached to two sulfonyl groups, and hydrated barium hydroxide was also present. Later analyses showed the water to be substantially free of decaying atoms. The system was further heated toward 300.degree. C., again with partial evacuation or use of argon carrier gas to sweep organic vapors to condensation sites. Fog from vaporization of large organic molecules became increasingly evident as heating proceeded. It is interpreted that heating in the presence of water and additional anchor ions allows breakage of the anchored sulfonated groups away from the organic portion of the cation-exchange resin: A water molecule replaces the water molecule which was removed during manufacture of the sulfonated resin, giving back a sulfate; also the hydrogen which had been lost in manufacture returns to the resin. These actions leave BaSO.sub.4 and, locally, the original polystyrene resin. The material that vaporized was near totally condensible at room temperature--very little noncondensible material collected in a ballast vacuum chamber. The condensed vapors were liquid at room temperature, but, after weeks of standing, sometimes show some solid formation due to limited repolymerization. Unlike ion-exchange resin decompositions with NaOH-KOH anchor (bonding) material, which formed some charry residue, as discussed in Case 2, the Ba(OH).sub.2 anchor material did not yield clear evidence of any carbonaceous residue. Apparently the barium hydroxide provides catalysis for depolymerization of ion-exchange resin that NaOH-KOH does not give. The resin depolymerization gives the vapor, and the organic material is largely decomposed. Apparently, even the cross-linked material is decomposed more effectively than in Case 2. The radioactive barium sulfate synthetic barite has not appeared to be wet when the reaction zone is viewed in a borosilicate glass container. Apparently, vaporization largely keeps up with depolymerization. The barite is as crystals which are ghosts of the original ion-exchange resin beads; they are not dusty as they were prepared. The uranium turned black, coloring the barite, but, as noted, there was no obvious carbonaceous deposit. The final location of the decaying atoms was all with the unvaporized residue, as well as was detectable with the Eberline beta-gamma counter used. Experimental Case 2 A typical case with the earlier preferred embodiment using NaOH-KOH bonding material proceeded as follows: Wet cation-exchange resin, U.sup.++++, and solid NaOH-KOH mixtures were heated in a sealable borosilicate glass operated either with vapor boiling or with sweeping argon gas. Heating drove off much of the water as vapor, leaving melted hydroxide mixtures with some extra water in solution. A second liquid phase formed from the solid radioactive ion-exchange resin, and vaporization started. Unlike the condensate with barium hydroxide, as temperatures rose, the heated organic solutions changed both boiling temperatures and colors. Finally, at temperatures approaching 500.degree. C., charry residues remained with the inorganic material, but most of the resin had vaporized. The separation of radioactive from nonradioactive material was again good, with the radioactive material being with the hydroxides, sulfates, and sulfites, which are not as advantageous for permanent radioactive disposal as barite.