Patent Number: 046612912
Section: summary

FIELD OF THE INVENTION AND RELATED ART STATEMENT The present invention relates to a method for fixation of incinerator ash or iodine sorbent, and more particularly to a method for fixation of incinerator ash or iodine sorbent by hydrothermal reaction. When it comes to disposal, incinerator ash needs special attention because it contains various kinds of metals. If it is disposed of as such by landfill or ocean dumping, metals will leach out from it. Incinerator ash of radioactive waste should undergo the fixation process prior to its disposal in order to suppress radioactivity and ensure safe handling. The conventional fixation method includes cement fixation method and asphalt fixation method. These methods have a disadvantage that incorporation of incinerator ash into a large amount of fixation material results in an increase of waste. The ashphalt fixation method has another disadvantage that asphalt is combustible, the fixed product needs fire protection, and the fixed product is poor in strength. For the disposal of radioactive waste, the plastics fixation method or melt fixation method are employed. Their disadvantages are that solidification matrices and containers are expensive and disposal can be accomplished only by large-scale facilities which need a high running cost. In the case of volatile radioactive iodine discharged from nuclear fuel reprocessing plants or nuclear power stations, it is usually captured in the form of solution or by adsorption on sorbent such as activated carbon and silver-loaded zeolite. Radioactive iodine in the form of solution or adsorbed on a sorbent is very poor in both physical properties (mechanical strength and heat resistance) and chemical properties (resistance to water, acid, and alkali), and it is not suitable for long-term storage. (See Report UTNL-R0160 (1982) published by the Nuclear Engineering Research Institute affiliated to the Department of Technology, Tokyo University.) As the result of recent development in the fixation method, there has been proposed a method for fixing in cement a silver-loaded sorbent which has adsorbed radioactive iodine. (See PNL-4689 (1983) by Burger and Scheele.) The cement fixation method is expected to be of practical value because of its simplicity. However, the cement fixed body is difficult to be made compact, and there is a possibility of iodine leaching out. (Note that .sup.129 I has a half life as long as 17 million years.) Thus it has to be sealed in a compact high-strength fixed body. Unfortunately, fixation requires a large amount of cement and the resulting fixed bodies are inevitably bulky and they need a large storage space. These disadvantages cause a bottleneck when the cement fixation method is put to practical use. There is disclosed in DOE/ET/41900-9 (ESG-DOE-13354) Nuclear Waste Management, US-70 a method for disposing of radioactive iodine. According to this method, an alkaline waste liquid containing radioactive iodine is mixed with a silicate compound to make sodalite, which is then sintered at 1000.degree. C. Sintering has to be accomplished completely at a high temperature, while avoiding the volatilizaton of captured iodine, which needs a complex apparatus and involves technical difficulties in practical use. As mentioned above, the conventional methods for fixation of incinerator ash have disadvantages that fixed bodies are bulky, combustible, and weak in strength, and expensive large-scale facilities are required. Also, the conventional methods for fixation of iodine sorbents involve technical difficulties in the sealing of iodine. SUMMARY OF THE INVENTION It is an object of this invention to provide a method for fixation of incinerator ash or iodine sorbent, said fixation being accomplished by forming very strong, compact fixed bodies. It is another object of this invention to provide a method for fixation of incinerator ash or iodine sorbent, said fixation being accomplished by forming fixed bodies which are superior in storage stability, with very little leaching of metal ions or iodine. It is another object of this invention to provide a method for fixation of incinerator ash or iodine sorbent, said fixation being accomplished by forming fixed bodies which are very small in volume. According to the method of this invention for fixation of incinerator ash, incinerator ash containing SiO.sub.2 and Al.sub.2 O.sub.3 is incorporated with NaOH or NaOH and an SiO.sub.2 -containing substance in such an amount that the resulting mixture contains 25 to 90 wt% of SiO.sub.2, 2 to 10 wt% of Na.sub.2 O, and more than 3 wt% of Al.sub.2 O.sub.3 which is less than the content of SiO.sub.2 in terms of mol%; 100 parts of the mixture is mixed with more than 5 parts by weight of water; and the resulting mixture is subjected to hydrothermal reaction under a pressure of 70 kg/cm.sup.2 and above and at a temperature of 150.degree. C. and above, whereby the fixation is accomplished. According to the method of this invention for fixation of iodine sorbent, iodine is adsorbed on a silver-loaded silicate sorbent; the sorbent is mixed with 1 to 30 wt% of at least one of sodium hydroxide, potassium hydroxide, and barium hydroxide; and the resulting mixture is subjected to hydrothermal reaction under a pressure of 70 kg/cm.sup.2 and above and at a temperature of 150.degree. C. and above, whereby the fixation is accomplished.