Patent Number: 046718974
Section: summary

BACKGROUND OF THE INVENTION This invention relates to a process and apparatus for solidification of radioactive waste occurring in a nuclear power station, and more particularly to a process and apparatus for its solidification utilizing a hydraulic solidifier. The amounts of radioactive waste occurring in nuclear power stations and related facilities have been increasing year by year, and a need for the volume reduction of such radioactive waste has consequently been increasing in order to secure a storage space within the facilities. Methods which have so far been examined for the volume reduction of radioactive waste includes the following. Concentrated liquid waste obtained by concentrating the liquid waste formed in the regeneration of spent ion-exchange resin and the slurry of powdery ion-exchange resin which occur in large amounts in a nuclear power station are dried into powder. Thus the liquid waste is freed of water which accounts for the major or part of its volume and, if necessary, the powder is further pelletized and solidified collectively by packing in a solidification vessel. (U.S. Pat. No. 4,299,271). However, such a method is still defective in that the liquid waste cannot necessarily be converted into a stable solid when using a hydraulic solidifier such as cement or alkali silicates (e.g. water glass). The concentrated liquid waste occurring in a boiling water reactor (BWR) nuclear power station is composed chiefly of a sodium salt, i.e. sodium sulfate (Na.sub.2 SO.sub.4). In a pressurized water reactor (PWR) nuclear power station, on the other hand, the concentrated liquid waste is composed chiefly of a sodium salt, i.e. sodium borate (Na.sub.2 B.sub.4 O.sub.7). These sodium salts are both water-soluble. In case the concentrated liquid waste occurring in a BWR nuclear power station is dried, powdered or, if necessary, further pelletized, and then solidified with a hydraulic solidifier, sodium sulfate which is its main ingredient will absorb free water contained in the solidifier paste and water formed by the solidification reaction, and thereby form a swollen hydrate, Na.sub.2 SO.sub.4.10H.sub.2 O, to cause cracking in the solidified body. In addition, in case of a cement solidifier, sodium sulfate will react with calcium hydroxide which is formed when cement is hydrated, and thereby form gypsum, which will prevent the cement from hardening too rapidly but will, on the other hand, accelerate the formation of ettringite (3CaO.Al.sub.2 O.sub.3.3CaSO.sub.4.32H.sub.2 O) to cause the solidified body to be swollen or broken. In case the concentrated liquid waste occurring in a PWR nuclear power station is solidified, sodium borate, which is the main ingredient, will likewise cause the solidified body to lower its strength. It will form a hydrage, Na.sub.2 B.sub.4 O.sub.7.10H.sub.2 O, to generate heat. In the case of a cement solidifier, it will inhibit the formation of a hydrate of calcium silicate (3CaO.2SiO.sub.2.3H.sub.2 O) and of a hydrate of calcium aluminate (3CaO.Al.sub.2 O.sub.3.6H.sub.2 O) by the hydration of cement. Since, in either of these cases, the powdered or pelletized waste mainly comprises the water-soluble sodium salts, the solidified body suffers from degradation of its structure, reduction in the leaching rate, and lowering in the strength and specific gravity owing to exudation during a prolonged storage. In the solidification procedure, furthermore, sodium borate reacts with the hydraulic solidifier very promptly, and the solidification proceeds so rapidly as to disturb the smooth pouring of the solidification mixture. To prevent this, the content of the liquid waste in the solidification mixture will have to be limited to at most 30 wt % and the volume reduction ratio be correspondingly lowered. SUMMARY OF THE INVENTION The object of the present invention is to provide a process for the solidification of radioactive waste wherein the solidified body is obtained having high consistency for a long time. Another object of the present invention is to provide a process for the solidification of radioactive waste wherein the solidified body is obtained having high volume reduction. Another object of the present invention is to provide a process for the solidification of radioactive waste wherein the solidified body is obtained having less degradation of its structure owing to exudation. Another object of the present invention is to provide a process for the solidification of radioactive waste wherein the solidified body is obtained having low leaching rate. The inventors have drawn their attention to the finding that the above-mentioned problems are all due to the soluble salt contained as the main ingredient in the liquid waste. Thus, they have made various studies in the belief that these problems could be solved by converting the radioactive waste into a hardly water-insoluble salt structure (including an insoluble structure) before it is submitted to a solidification process, and have finally attained the present invention. The process for solidification of radioactive waste according to this invention is characterized in that the radioactive waste is first converted into a hardly water-soluble powder (including a water-insoluble powder) and then solidified with a hydraulic solidifier in a solidification vessel. The radioactive waste may be powdered (including granulated and encapsulated) by incorporating the radioactive waste with a substance which is capable of reacting with the water-soluble salt contained in said radioactive waste to form a hardly water-soluble salt (including a water-insoluble salt) and then powdering the mixture with drying, or by powdering the radioactive waste with drying, granulating the powder with drying and then microencapsulating the granules with a hardly water-soluble substance (including water-insoluble substance). Na.sub.2 SO.sub.4 and Na.sub.2 B.sub.4 O.sub.7 which are main ingredients of liquid radioactive waste occurring in a nuclear power station have high solubilities in water. In order to see how Na.sub.2 SO.sub.4 could be converted into salts hardly soluble (including insoluble) in water and what type of salts they should be, and with attention drawn to the fact that alkaline earth metal sulfates and metal chelate salts were hardly soluble in water, in general, the present inventors selected calcium sulfate, strontium sulfate and barium sulfate for the former, and ammonium cobalt oxalate sulfate and hexaammonium chromium sulfate for the latter to examine their solubilities. The results are shown in Table 1. This table shows the values observed at 20.degree. C. TABLE 1 ______________________________________ Solubility Solubility Substance (wt %) Substance (wt %) ______________________________________ sodium sulfate 16.0 ammonium cobalt 0.8 calcium sulfate 0.205 oxalate sulfate strontium sulfate 1.3 .times. 10.sup.-2 hexaammonium 2.7 barium sulfate 2 .times. 10.sup.-4 chromium sulfate ______________________________________ It was found that all these substances had lower solubilities than sodium sulfate and that conversion into barium sulfate was more effective than into the rest for the intended purpose. In respect of cost, however, conversion into calcium sulfate was thought to be most economical and most practical. Various borates were also tested for solubilities, and conversion into calcium borate was likewise found to be appropriate in respect of cost and practical application. The radioactive waste materials which can be solidified by the procedures include not only dried granulates of concentrated liquid waste and sludge consisting of sodium sulfate, sodium borate, etc. but also a slurry waste of ion-exchange resin, and the so-called miscellaneous solid matters, such as HEPA filters, vinyl sheet clothings and wooden pieces, and their fragments. The solidifer includes not only an alkali silicate composition but also fluid solidifier, such as a thermosetting or thermo-fusible plastic, asphalt, mortar or cement. According to this invention providing a process and apparatus in which the dry powder obtained from the radio-active waste occurring in a nuclear power station is solidified with a hydraulic solidifier, the solidified body can not only be extensively protected from its deterioration and damage caused by water absorption, hydration, exothermic reaction, swelling and leaching due to the sodium sulfate and sodium borate contained in the radioactive waste to thereby retain its consistency for a long time, but also be improved outstandingly in volume reduction ratio.