Patent Number: 048044989
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention where a concentrated radioactive waste liquid generated in a BWR power plant (wherein the main component is sodium sulfate (Na.sub.2 SO.sub.4)) is treated will now be described with reference to FIG. 1. A soluble barium compound is added to the radioactive waste liquid to insolubilize sodium sulfate (Na.sub.2 SO.sub.4) in the radioactive waste liquid and precipitate it as barium sulfate (BaSO.sub.4). Then or simultaneously, an adsorbent is added to the radioactive waste liquid to adsorb radioactive substances such as Cs and Sr and precipitate them. By this reaction, caustic soda is formed through the following reaction course: EQU Na.sub.2 SO.sub.4 +Ba(OH).sub.2 .fwdarw.BaSO.sub.4 +2NaOH After this caustic soda has been separated, the radioactive waste liquid slurry containing the precipitate of barium sulfate and the adsorbent is concentrated by evaporation or powdered, and a hydraulic solidifying material (such as cement, water glass or silica) is added after concentration or powdering or in the slurry state without concentration or powdering, whereby solidification is effected and a solidified body is obtained. The separated caustic soda is free of the radioactive substances and can be conveniently utilized again. As the adsorbent, copper ferrocyanide is used for adsorbing and fixing Cs and titanium chloride is used for adsorbing and fixing Sr. The effects of fixing the radioactive substances by these adsorbents are shown in FIGS. 2 and 3, where amounts of Cs and Sr exuded from the final solidified body obtained by adding these adsorbents are plotted. As is apparent from FIGS. 2 and 3, with an increase in the amounts of added copper ferrocyanide and titanium chloride, the effect of controlling exudation of Cs and Sr is enhanced. Generally, if copper ferrocyanide or titanium chloride is added to the radioactive waste liquid in an amount of about 5 moles per mole of Cs or Sr in the waste liquid, the majority of Cs or Sr can be adsorbed and fixed. The present invention will now be described in detail with reference to the following examples. EXAMPLE 1 In this example, a radioactive waste liquid containing sodium sulfate (Na.sub.2 SO.sub.4) as the main component was treated, with barium hydroxide (Ba(OH).sub.2) used as the soluble barium compound, copper ferrocyanide and titanium chloride (TiCl.sub.2) added as the adsorbent, and a solidifying material comprising an alkali metal silicate as the main component and a phosphate as the hardening agent. This example will now be described with reference to the flow-chart of FIG. 4. From a radioactive waste liquid tank 1, a radioactive waste liquid containing sodium sulfate (Na.sub.2 SO.sub.4) at a solid concentration of 20% by weight (the amount of solid sodium sulfate left when water was removed) was supplied to a reaction tank 4 so that the amount of the solid sodium sulfate was 25 tons, and 56 tons of barium hydroxide was supplied to the reaction tank 4 from an additive tank 2. Furthermore, copper ferrocyanide and titanium chloride were supplied as the adsorbent to the reaction tank 4 from an additive tank 3 so that the amount of each additive was 5 moles per mole of Cs or Sr in the radioactive waste liquid. The reaction tank 4 was heated to 80.degree. C. by a heater 5 and the radioactive waste liquid was mixed and stirred for 1 hour by a stirrer, whereby the following reaction and adsorption of Cs and Sr were completed: EQU Na.sub.2 SO.sub.4 +Ba(OH).sub.2 .fwdarw.BaSO.sub.4 +2NaOH After the above-mentioned solubilizing step and the step of adsorbing and fixing Cs and Sr had been completed, separation of the adsorbent and barium sulfate by sedimentation was carried out in the reaction tank 4, whereby caustic soda (NaOH) was separated. The separated caustic soda could be utilized again for other uses. For example, as shown in FIG. 4, the separated caustic soda could be utilized for regeneration of an ion exchange resin 14 contained in a desalting device 13 used for desalting in a condensation system 15 of a reactor. Then, the radioactive waste liquid slurry containing the precipitate of barium sulfate and the adsorbent was dried and powdered by a drier 6 and the formed powder was received in a powder hopper 7. A drum 8 was charged with 120 kg of a solidifying material comprising sodium silicate as the main component, supplied from a tank 9, and 60 kg of makeup water supplied from a tank 10, and they were kneaded to form a paste. Then, 300 kg of the above-mentioned powder was added to the drum 8 from the powder hopper 7, and the mixture was kneaded and solidified. According to this example, since adsorption and 5ixation of the radioactive substances could be performed simultaneously with the insolubilization, separated caustic soda could be directly used for regeneration of the ion exchange resin and other purposes. Moreover, by the synergistic effect attained by insolubilization and fixation of the radioactive waste liquid, it was found that in the solidified body obtained in this example, the exudation ratio of Cs was reduced to 1/100 of the level attained in the conventional solidified body. EXAMPLE 2 The radioactive waste liquid slurry containing the precipitate of barium sulfate and the adsorbent, prepared in Example 1, was concentrated so that water in an amount corresponding to the amount of water added together with the solidifying material in Example 1 was left in the radioactive waste liquid slurry, and then, only the powdery solidifying material was added. An effect similar to the effect attained in Example 1 was obtained. However, in this example, it was impossible to adjust the water to solidifying material ratio completely in the concentration step. In this point, the process of Example 1 is preferable. EXAMPLE 3 In the foregoing two examples, the adsorbent for Cs and Sr was added to the radioactive waste liquid. In the present invention, the intended effects of the present invention can be similarly attained even if the adsorbent is not added to the radioactive waste liquid. In order to demonstrate this fact, the procedures of the foregoing two examples were repeated in the same manner except that the adsorbent was not added to the reaction tank 4. When caustic soda formed in the insolubilization step was separated, a solidified body having a sufficient strength was obtained. FIG. 5 illustrates increase of the strength of the solidified body by separation of caustic soda. In FIG. 5, the ordinate represents the relative strength of the solidified body calculated based on the supposition that the strength for the solidified body obtained by powdering the radioactive waste liquid without insolubilization of sodium sulfate and solidifying the powder with sodium silicate is 1. The abscissa represents the ratio of separation of caustic soda from the radioactive waste liquid. As is apparent from FIG. 5, if caustic soda formed in the insolubilization step is not separated and the radioactive waste liquid is directly powdered and solidified, a solidified body cannot be obtained. If the separation ratio of caustic soda exceeds a certain level, a solidified body can be obtained and the strength of the solidified body can be improved. In this example, it was confirmed that if the separation ratio of caustic soda exceeded about 97%, a solidified body could be obtained and the strength of the solidified body was increased with increase of the separation ratio of caustic soda. In this example, the radioactive waste liquid slurry containing the precipitate of sodium sulfate (BaSO.sub.4) is powdered and solidified. Similarly to the foregoing Example 2, when the radioactive waste liquid slurry is concentrated by evaporation and solidified, the strength of the solidified body is improved. In this example, the radioactive substances contained in caustic soda, such as Cs and Sr, where not removed. However, a radioactive substance filter member packed with copper ferrocyanide and titanium chloride may be disposed in the midway of piping from the reaction tank 4, if necessary. In this case, caustic soda separated from the radioactive waste liquid can be conveniently utilized again. Furthermore, after the filter member is used, the filter member may be supplied to the drier 6 together with the slurry for powdering and solidification. According to the embodiments of the present invention, since the main component of the concentrated radioactive waste liquid, that is, sodium sulfate or sodium borate (Na.sub.2 SO.sub.4 or Na.sub.2 B.sub.4 O.sub.7), is insolubilized, the final solidified body is very good. Furthermore, since the radioactive substances are adsorbed in the adsorbent and precipitated, the radioactive substances are not substantially contained in caustic soda formed in the insolubilizing treatment. Accordingly, this caustic soda can be separated and utilized for regeneration of an ion exchange resin or other purposes. Moreover, since the radioactive substances are fixed in the solidified body, a very stable solidified body can be obtained.