Patent Number: 048448389
Section: description

PREFERRED EMBODIMENTS OF THE INVENTION FIG. 1 is a flow chart showing the method of treatment of a radioactive liquid waste according to the present invention. First, a radioactive liquid waste containing fission products and a thermally decomposable sodium compound such as sodium nitrate is transferred to a heating step to be heated therein. In this heating step, the evaporation and denitration of the liquid waste are carried out and the nitrate and water contained therein are evaporated. By further heating, the thermal decomposition proceeds and a nitrogen oxide (NO.sub.x) gas is eliminated. For example, sodium nitrite is decomposed at 320.degree. C., while sodium nitrate is decomposed at 380.degree. C. Accordingly, the heating of the radioactive liquid waste may be carried out at a suitable temperature exceeding these temperatures. It is preferred to use microwave as a heat source in the heating step, because microwave heating gives a porous calcination product. By continuing the heating, there is formed a calcination product or denitrated product essentially comprising fission products, sodium oxide and sodium peroxide. Among initial fission products, volatile nuclides are evaporated by the above heating, so that the exhaust gas must be separately subjected to necessary treatment such as condensation, adsorption or absorption. Most of the nonvolatile nuclides are converted into oxides by the above heating. Then, the denitrated product (oxides) thus formed are transferred to a reaction step. In this reaction step, water vapor is sprayed directly to the denitrated product to convert the oxides into sodium hydroxide. In order to carry out the formation of sodium hydroxide more gently, the denitrated product may be reacted with carbon dioxide gas to convert the oxides of sodium into sodium carbonate, which is then converted to sodium hydroxide by reacting sodium carbonate with water vapor. The thus obtained sodium hydroxide containing fission products is transferred to a purification step. In this step, the sodium hydroxide is dissolved in a pure alcohol such as ethyl alcohol to be converted into its ethylate (sodium ethoxide), and the thus obtained sodium ethylate is separated by solid-liquid separation from an impurity residue. The impurity residue essentially comprises fission products. The separated sodium ethylate is then transferred to a decomposition step. In this step, the ethylate is decomposed with water into ethyl alcohol and sodium hydroxide. The sodium hydroxide is recovered and reused. The impurity residue separated in the purification step may be transferred to a solidification step. In this step, the residue is melted together with a glass forming agent to be vitrified. Alternatively, it is mixed with bitumen under heating to produce a product solidified by bitumen. Since the sodium content of the impurity residue is remarkably reduced, a vitrified product having excellent properties can be formed by the vitrification or the risk of fire and explosion can be reduced in the bitumen solidification. Thus, in either case, the volume of the radioactive liquid waste to be solidified can be greatly reduced. Some of the steps constituting the method of the present invention can be applied to the treatment of a metallic sodium waste accompanied with a radioactive corrosive product from a fast breeder reactor. Such a waste containing metallic sodium is generally washed with water vapor or the like and the condensed liquid waste is subjected to vaporization by heating and concentration, and the concentrated liquid waste is melted together with a small amount of a glass forming agent to be vitrified. However, the obtained vitrified product exhibits unfavorable properties including deliquescence. In order to solve this problem, the treatment of the metallic sodium waste from a fast breeder reactor may be carried out as follows: the metallic sodium waste is directly contacted with water vapor to convert the metallic sodium into sodium hydroxide. The obtained sodium hydroxide is then transferred to the purification step of the present invention, in which the sodium hydroxide is reacted with an alcohol to form a sodium alcoholate. Then, the sodium alcoholate is separated from an impurity residue, and the separated sodium alcoholate is decomposed into sodium hydroxide. This application of the latter two steps of the method according to the present invention to the metallic sodium waste from a fast breeder reactor allows the reuse of sodium and the vitrification of the impurity residue into an excellent vitrified product, similarly to the method according to the present invention. FIG. 2 shows a preferred embodiment of an apparatus to be used in the present invention. This apparatus comprises a heating apparatus 10 and a reaction apparatus 40. The heating apparatus 10 is provided with a feeder 12 of a radioactive liquid waste and a heating chamber 18 having a bottom tilted so as to form a sink 16 in the heating chamber 18. The feeder 12 and the sink 16 are connected by a pipe 14. The heating chamber 18 is provided with a heater 20 at the bottom and sides of its outer wall and with a plurality of microwave applying apertures 22 at the top thereof. A screw 24 for continuously transferring (discharging) the denitrated product is rotatably provided at the bottom in the heating chamber 18 and can be driven by a driving motor 26 disposed outside the heating chamber 18. Further, the interior of the heating chamber 18 is partitioned into three zones A, B and C by partition plates 28 and 30. The heating chamber 18 is simultaneously subjected to irradiation with microwave and heating by the heater 20, while the radioactive liquid waste containing fission products and sodium nitrate (NaNO.sub.3) is continuously fed to the sink 16 from the feeder 12 via the pipe 14. The screw 24 is rotated by the driving motor 26. In the zone A, the heating and concentration of the radioactive liquid waste are carried out, and in the zone B, the concentration and denitration (decomposition into NO.sub.x) of the liquid waste are carried out. The oxygen required in the reaction is fed via an air feed opening 32 provided at the upper part of the heating chamber 18 and the exhaust gas is discharged via an exhaust vent 34. In the zone C, the reaction is completed to form oxides of sodium, i.e., sodium oxide and sodium peroxide. The denitrated product thus obtained is discharged via a discharge opening 36 into the reaction apparatus 40 which will be described below. The reaction apparatus 40 is provided with a screw 46 at the lower part in a reaction chamber 42 and with a water vapor sprayer 48 at the top of the chamber 42. The screw 46 can be rotated by a driving motor 44. The denitrated product formed in the heating chamber 10 is fed to the reaction chamber 42 via a feed opening 50 provided at the top thereof and transferred by the screw 46, while being sprayed with water vapor from the sprayer 48 to thereby convert the oxides of sodium in the denitrated product into sodium hydroxide. The denitrated product containing sodium hydroxide and fission products is discharged via a discharge opening 52 to be collected in a collection vessel 54, while the gaseous product generated in the formation of sodium hydroxide is discharged via an exhaust vent 56. The sodium hydroxide containing fission products thus collected is transferred to the following purification step as shown in FIG. 1. As described hereinabove, according to the present invention, it becomes possible to reuse sodium contained in the radioactive liquid waste and to remarkably reduce the volume of the radioactive waste to be solidified. In addition, since the sodium content of the radioactive waste to be solidified is greatly reduced, the solidification thereof is significantly facilitated. Accordingly, the vitrification can be carried out with a reduced amount of a glass forming agent to give a vitrified product having excellent properties. In the bitumen solidification, on the other hand, the solidification treatment can be carried out safely with reduced risk of fire or explosion. Although the present invention has been described with reference to the preferred embodiments thereof, many modifications and alterations may be made within the scope of the appended claims.