DEVELOPMENT OF HIGH VOLUME REDUCTION AND CEMENT SOLIDIFICATION TECHNIQUE FOR PWR CONCENTRATED WASTE

The hyper-Cement solidification technique was developed, which reduced the volume of concentrated liquid wastes from PWR plant by means of stable cement solidification. Drying the liquid wastes into powder using a wiped film evaporator and then solidifying the large amount of powdered wastes attained high volume reduction of the wastes due to the special cement paste with low viscosity. Using this technique, The volume reduction ratio of the wastes of was larger than in the case of using bitumen technique, which is conventionally employed to solidify the concentrated liquid wastes in PWR plant. This paper presents the outline of the hyper-cement technique and discusses the mechanical and chemical properties of the cement solidification. INTRODUCTION Cement materials are known to be attractive for solidifying radioactive wastes because of its low water permeability after hardening and the high absorbability of some radioactive species onto hardening materials. In view of the simplicity of the solidification process using these materials , cement solidification systems are operating at many nuclear facilities. However, its hardening is sometimes hindered by the presence of certain components such as boric acid and phosphate acid because they impair the hydration reactions of cement. These impairments are of particular concern in the case of the solidification of concentrated liquid wastes from pressurized water reactor (PWRs), since their main components is boric acid. To avoid these impairments, it is necessary to reduce the amount of these components in cement solidification, but this causes the increase of the volume of the products of waste solidification. F or this reason, the bitumen technique in which both salts of boric compounds and radioactive elements are mixed into molten asphalt is introduced to reduce the generation of the products of waste solidification in PWR power plants. But this solidification process is complicated and sometimes it is necessary to repair equipment because the activation of molten asphalt at high temperature results in the corrosion of metals. Thus, there is a need to develop a s olidification technique using cement materials that increases the borate content in products. We have already developed a new cement solidification technique that realizes high volume reduction of waste. This technique consists of two processes: a dryin g process to reduce the volume of liquid waste, and a cementation process to solidify a large amount of the dried product with a given cement. Using this technique, The volume reduction ratio of the wastes was larger than that in the case of using the bitumen technique. This paper presents the outline of a new cement solidification technique for concentrated liquid waste generated from PWR power plants and investigates the detailed properties of the products solidified by the technique.