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Application of concrete to the treatment and disposal of radioactive waste in Japan
Nuclear Engineering and Design 138 (1992) 179-188 North-Holland
179
Application of concrete to the treatment and disposal of radioactive waste in Japan Y a s u r o Maki a a n d Hiroshi O h n u m a b a Department of Civil Engineering, Hosei University, Tokyo, Japan b Central Research Institute of Electric Power Industry, Tokyo, Japan
Received 20 July 1992
Low level radioactive waste will be disposed at the Rokkasho site in Aomori Prefecture in Japan. During the treatment, storage and disposal of the radioactive waste, concrete is used and will be used for various purposes. For this reason a lot of research & development has been carried out on concrete at various institutes, universities and manufactures in Japan. This paper presents the present state of application of concrete to treatment, storage and disposal of low level radioactive waste in Japan. In the 2nd section, the electric power supply and the kinds and volumes of radioactive waste from nuclear power plants in Japan are described. In the 3rd section, the applications of concrete to the treatment of radioactive waste are described. These are solidification with cement and containers made by various mortars and concretes. The application of concrete to disposal structures are presented in the 4th section; these are research on the durabity of concrete under disposal site condition, research on the filling the concrete pit with 200 1 drum packed cement solidified wastes by prepacked concreting methods, and so on. And this section describes also the outlines of the low level radioactive disposal system at the Rokkasho site.
1. Introduction Nuclear power generation now accounts for about 30 percent of the total power generated in Japan. With this development, it has become a critical task to safely and rationally treat and dispose of radioactive wastes arising from nuclear power generation. To this end, the Japanese Government has been developing laws and regulations concerning treatment, storage and disposal of radioactive wastes. The electric utilities, which have been generating electricity from nuclear power, have been in charge of construction and operation of facilities for the treatment and disposal of the wastes. Japan Nuclear Fuel Industry Co., Ltd. (JNFI) has been constructing facilities for low-level radioactive wastes. Japan Atomic Energy Research Institute (JAERI) is in charge of research on, for instance, safety of treatment and disposal of radioactive wastes. The manufacturers have been energetically developing technologies for Correspondence to: Dr. Yasuro Maki, Dept. of Civil Engineering, Housei University, 3-7-2 Kajino-Cho, Koganei-shi, Tokyo 184, Japan.
treatment and disposal. Furthermore, Power Reactor and Nuclear Fuel Development Corporation (PNC) has been conducting research and construction of facilities for treatment and disposal of radioactive wastes arising from spent fuel reprocessing and plutonium fuel fabrication. Of the activities for treatment and disposal of such radioactive wastes in Japan, this report outlines the present state of application of concrete materials to the treatment and disposal of radioactive wastes, in particular, low-level wastes.
2. Electric power supply and low-level waste arising 2.1. Electric power supply
According to the interim report of the Advisory Committee for Energy (October 1990), the nuclear power now has established itself as a critical energy source in Japan. Ever since the introduction of the first commercial nuclear reactor in 1966, the development of nuclear power has been promoted by the efforts of
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Y. Maki, H. Ohnuma /Application o[ concrete
come mainly from nuclear power plants. Radioactive wastes originally arise in liquid forms but are solidified with cement, asphalt or plastics, and are regarded as solid wastes. Solid wastes coming from nuclear power plants include: concentrated liquid waste of liquid waste arising from regeneration of ion-exchange resins used in water treatment and various drains; spent ion-exchange resins and spent filters for purification of water systems; miscellaneous solid wastes such as paper, clothes, etc. used in reactor facilities, metallic and concrete pieces arising from maintenance; .astir,"from incineration of combustible miscellaneous solid wastes; and reactor internals such as spent control rods that will be treated as radioactive waste in future. Of such radioactive wastes, the concentrated liquid waste is solidified, for example, by mixing with cement, pouring into heated asphalt to evaporate moisture and mix only the solids within the asphalt, or at first evaporating moisture to turn the waste into powder and then solidifying the powder with cement or plastics. These wastes are normally encapsulated in 200-liter steel drums, and the resulting packaged wastes are called homogeneous solidified wastes in Japan. The wastes that the electric utilities are planning to dispose of at Rokkasho-mura, Aomori Prefecture are mainly of such types. Spent ion-exchange resins, filter sludge, inciner-
those concerned while constantly assuring safety. As of the end of May 1990, the number of commercial nuclear power plants in operation was 38 units with a total output of more than 30,000 MW, supplying about 30% of the electricity. Moreover, even if the increase in energy consumption could be minimized by energy saving efforts, the mean annual growth rate of energy used in the form of electricity is expected to be 2.7% for the period from 1988 to 2000, and 1.5% from 2000 to 2010. If the dependency on oil were to be reduced further, and the emission of carbon dioxide were to be kept at a certain level, the expected increase would require the utmost efforts to introduce non-fossil energy. Nuclear power is, at present, the sole non-fossil energy that can meet the needs quantitatively. The report indicated the power supply targets at the ends of the respective years, as shown in fig. 1. These figures indicate that we must rely on nuclear power for about 40 percent of the power supply; although the figures for nuclear power were modified downwards by 5 to 6 percent in comparison with the previous report of the Committee (July 1986). 2.2. Kinds and quantities o f radioactive wastes
The radioactive wastes to be covered by this report are solid wastes of low radioactivity level. These wastes
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Y. Maki, 1t. Ohnuma /Application of concrete ation ash, all in powdery form, will be mixed with cement or plastics and solidified in 200-liter steel drums as homogeneous solidified wastes. Incombustible miscellaneous solid wastes arise in bulk. Such wastes are put into plastic bags and packed in 200-liter steel drums to store in most of the power plants. In future they will be reduced in volume by, for example, high temperature combustion, plasma melting, or high pressure volume reduction with a high pressure press, then solidified in containers with some solidifying materials. The containers for this purpose may include 200-liter steel drums (including those lined with concrete), large-sized metal boxes, and concrete containers. The most useful method for solidification will be cement solidification by the prepacked concreting method. The reactor internals are small in quantity although their radioactivity levels are high. They are stored in spent fuel pools and similar facilities called site bunkers. They have not been treated yet since the final disposal method is to be decided in future. Such reactor internals might be reduced in volume by chopping and compression and then packed in steel or concrete containers having shielding functions. At present, concentrated liquid wastes are solidified in steel drums with cement, etc. Combustible miscellaneous solid wastes are incinerated, and the resulting ash is put into steel drums without any treatment. Most of the spent ion-exchange resins are stored in tanks, and the incombustible miscellaneous solid wastes are stored in steel drums. The quantities of such arisings are as shown in fig. 2 according to the tentative calculation of 1984. With various efforts to reduce the
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arisings of various wastes, the number of such steel drums are predicted to be 400 and some thousands in 1990, and more than 700,000 in 2000. If presently untreated wastes could be treated by a method having a considerable volume reduction effect, the number of the steel drums could be reduced to 300 and some thousands in 1990, and more than 500,000 in 2000. The latter figures will be the actual number of steel drums to be disposed of eventually.
3. Application of concrete materials to treatment of radioactive wastes
Treatment of radioactive waste normally means changing physical or chemical characteristics of waste arose and packing it into a container for ease in transport, storage and disposal. The treatment thus includes volume reduction, solidification, and encapsulation in a container. The major application of concrete to treatment of radioactive wastes is for the treatment of low- and intermediate-level wastes and transuranic (TRU) wastes; solidification with cement, and containers made of concrete. 3.1. Cement solidification Solidification with cement can take in two forms. The first one is to mix and solidify liquid or powdery waste, such as concentrated liquid waste, spent ion-exchange resin, and incineration ash, with cement. Mixing is effected by a large-sized mortar mixer such as indrum mixer or a forced mixing type mixer. As for
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)t, Maki, H. Ohnuma / Application o f concrete
solidifying materials, ordinary portland cement and portlment blast-furnace slag cement are normally used. Such cements are required to have an appropriate strength. Moreover, such cements must be compatible with the waste to be solidified since the waste may include various substances, e.g., acidic substances and sulfates. Efforts are also being made to develop new cements including low-moisture-content cement. Attempts have also been reported to prevent adverse effects on cement hydration by, prior to cement solidification, pretreating the contents of the concentrated liquid to make them insoluble [1,2]. The second method is applied to almond-like grains called pellets that are produced from liquid or powdery wastes. It is also applied to vitrified granular ash that arises from high temperature incineration and is called granule. Incombustible miscellaneous solid wastes compressed by a high pressure press to reduce the volume are also treated by the second method. Such wastes are put into 200-liter steel drums or 200-liter steel drums with concrete lining and solidified by the prepacked concreting method. Mortar to be poured should be highly fluid and be able to fill every corner without the use of a vibrator. Moreover, its bleeding should be negligible. To this end, various methods have been tried and satisfactory results have been reported. Such methods include the use of a fluidizing agent, a self-levelling material, and the use of a special cement comprising water glass to eliminate redundant water. Figure 3 shows the outline of a process which pelletizes concentrated liquid waste and solidifies the pellets with water glass cement. It is reported that the solidified waste has satisfactory strength, resistance to leaching,
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and adsorption of radioactive nuclides. The process has been adopted in PWR and BWR power plants in Japan. Photographs 1 and 2 show cement solidification
Photo 1. Cement solidified incombustible miscellaneous waste by prepacked method.
Y. Maki, H. Ohnuma /Application of concrete
Photo 2. Incombustible miscellaneous solidified product by prepacked method.
of miscellaneous solid wastes. Photograph 1 shows the use of a self-levelling material, It will be found that the filled and solidified agent has filled every corner. Photogrpah 2 is a scene of a full-scale filling test. It shows the practicability of solidification with the prepacked concreting method. To achieve smooth filling, various additive agents as well as self-levelling agents are reportedly used [3-6]. Although not for the wastes arising from normal operation, research is also in progress at JAERI, CRIEPI, etc. on cement solidification, by the prepacked concreting method, of decommissioning concrete of relatively high radioactivity arising from decommissioning of a reactor. It should be noted that in an ordinary civil engineering work the prepacked concreting method is practiced under water. The cement solidification of radioactive wastes by the prepacked concreting method is naturally made in a dry condition. A process of solidifying waste with some natural rock materials for solidifying, such as silica and China clay, has been developed, although the process may not be classified into cement solidification. Solidification is effected by hydro-thermal reaction under high temperature and high pressure (about 300°C and 300 kgf/cmZ). Materials to be treated by this method are dried powder from concentrated liquid waste and incineration ash. The process is capable of producing solidified wastes that excel in strength and resistance to leaching. 3.2. Containers
There are many applications of concrete to containers for encapsulation in and out of Japan. The applications are varied: use of concrete for the container
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proper, for the overpacking containers, and as the lining material for metal containers. Containers for disposal must meet the following requirements: (1) The container must be compatible with the waste to be encapsulated. (2) The container must be durable under the disposal conditions. (3) The container must withstand handling for disposal. (4) The container must bear the stacking load after disposal. (5) The container must contain radioactive waste after disposal as long as possible (for example, 300 years, 500 years, or 1000 years depending on radioactivity level, etc.) The containers must have radiation shielding functions in some cases. Ordinary concrete that is normally used in civil and construction works adequately meets such requirements. On the other hand, efforts are in progress to develop special concretes or mortars for higher performance [4,8,9,10]. 3.2.1. Fiber-reinforced concrete One example is a container developed in France, which is made of mortar reinforced with asbestos fiber. The cylindrical barrel is produced by a centrifugal method, and is 15 to 20 cm in thickness. The bottom plate and the cover are joined to the cylindrical barrel by means of both asbestos concrete plate and ordinary concrete. The use of asbestos concrete is presumably intended to give shock resistance to the container so that the container can withstand a drop accident during handling. The performance of the container has been reportedly verified by experiment. Containers made of carbon-fiber reinforced concrete may have properties comparable to those of asbestos cement. Carbon fiber is expected to be investigated further as a substitute of asbestos fiber since unlike glass fiber, carbon fiber is free of degradation by alkali. In addition to asbestos fiber, steel fiber, which is commonly used, is also used for reinforcement. When concrete is used to make a disposal container, it is desirable to make the wall of the container as thin as possible to encapsulate a greater amount of waste and to reduce the weight of the waste package (waste encapsulated in a container and ready for disposal), except a thicker concrete wall is required for radiation shielding. To meet such requirements, steel fiber reinforced mortar was used in some cases to line the interior of a steel drum. The steel fiber reinforced mortar has a high shock resistance, and is also effective in coping with cracks. Naturally, containers were also experimentally produced by using thick wall concrete container reinforced with steel fiber.
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3.2.2. Resin mortar Resin mortar is mostly used for lining the steel drums. Polymer-impregnated mortar and resin mortar have been investigated, and both have reached to the stage of practical application. Polymer-impregnated mortar is applied by lining mortar of 2 to 5 cm thick over the interior of a steel drum, impregnating the lining with a resin monomer, then polymerizing the monomer by thermal polymerization. A container developed by Chichibu Cement Co. Ltd. was verified by the U.S. NRC to meet the standards of the high integrity container (HIC) prescribed in the regulations of 10CFR61 (Photograph 3). Resin mortar, on the other hand, is a lining of about several millimeters thick on the interior of a steel drum. It is close to the form of coating. The developed container is free of pores and has an excellent waterstopping capability. The resin mortar lining is quite effective in preventing corrosion of the steel drum from the interior. The container also has a special feature that the effective volume for packing is almost comparable to that of an unlined steel drum. 3.2.3. Ceramics In addition to concrete containers, ceramic containers have been developed. It is desirable to isolate radioactive wastes from the human environment as long as possible. When a geological formation is used for this purpose in view of the possible instability of the geological formation, it is desirable to contain the radioactive substances in an engineered structure as long as possible. It is quite desirable to achieve a period of the effective containment of an order of 1000
Photo 4. Cylindrical ceramic container (200 1). years. To this end, ceramics have been attracting much attention and have been investigated as highly durable materials. Damages due to freezing or salts are almost negligible in the disposal environments at several meters below the ground surface for low-level waste, and at several hundred meters or more below the ground surface for high-level waste. Hence concrete will adequately meet the requirements. Concrete, however, has not been demonstrated of the integrity over an extended period. Moreover, it has become possible to produce large-sized structures of ceramics in recent years. Such backgrounds might explain why the research efforts were initiated. Ceramic container prototypes range from an inner container which is used in a steel drum to a large-sized container which is for over-packing high-level vitrified waste. Intensive research efforts are in progress to put such containers to practical use. In particular, ceramic containers will be indispensable as containers for melting low-level waste at high temperature and solidifying the waste in the containers (Photograph 4) [4]. 3.2.4. Other efforts In addition to those efforts mentioned above, a variety of attempts have been made or are in progress, including application of concrete of high shock resistance and long-term durability.
4. Application of concrete materials to disposal of radioactive wastes Photo 3. Polymer impregnated concrete lining drum (exampie).
Disposal means a state out of human control. Irrespective of the substance to be disposed of, may it be
Y. Maki, H. Ohnuma /Application of concrete
municipal waste, industrial waste, or radioactive waste of our concern be made in a manner which assures safety of the people against the disposed waste. The period for assuring the safety is 60 to 70 years for ordinary structures; the safety period is actually the durable life of the structure. In contrast, the safety assurance period for radioactive wastes is much longer. It is about 300 years for low-level wastes of which major safety-determining radioactive materials are strontium-90 and cesium-137 with half lives of about 30 years. In disposal of radioactive wastes, its safety is assured by a combination of the performances of the engineered structures in containing and preventing migration of radioactive substances, and the performances of the surrounding soil or the like in retarding migration of and diluting the radioactive substances. The engineered structures include the waste to be disposed of, the overpacking container for encapsulating the waste, the facility, an additional artificial clay layer, etc., and are called engineered barriers. The latter is called a natural barrier. The migration of radioactive substances disposed of is effected by ground water. The roles of the engineered barriers, therefore, are to isolate the radioactive substances from the ground water, to retard the contacts between them as long as possible,, to retard the migration of radioactive substances as long as possible even if they contact with ground water, and to adsorb the radioactive substances and minimize their outflow to the surrounding soil. Hence the disposal facility must be free of through cracks and maintain a low water permeability, while ordinary reinforced concrete structures tolerate cracking provided the sectional stresses are below the allowable stress. Degradation of concrete into clay may be acceptable provided such a change call be predicted and the degradation is with a range for which the containment performance can be assured. This is a key difference from ordinary structures. The land disposal method being planned in Japan is a mixture of the monolith method of France and the silo method of Sweden. The geology of a site consists of bedrocks of tuff, sandstone, etc. except the surface layers of about three meters thick. A pit is excavated in such a bedrock and a reinforced concrete structure is built in the pit. The pit is divided into cells with partition wails similar to those of the Swedish system. Wastes are placed in these cells and mortar is filled into the ceils to form an integral structure. When completed, the structure is close to the monolith of the French system. Like the Swedish system, clay layers are formed around the pit to reduce water penetration into
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Fig. 4. Outline of the burial facilities.
the pit and to prevent the penetrating water from contacting the wastes. It is also planned to provide the pit with a porous concrete lining inside the pit and with water collecting and discharging channels. The concrete will have a compressive strength of 250 k g f / c m 2 or over (fig. 4) [11]. To contribute to the construction of the disposal facility for the low-level wastes, a variety of research programs are in progress at some research institutes and companies. Some programs relating concrete will be outlined in the following [12-14]. Filling the concrete pit with mortar is designed to integrate the pit, filling material and waste packages so that they behave as an integral structure like a bedrock against external forces of earthquake, water penetration, etc. Accordingly the mortar to be filled is required to fill in every corner of the pit. The mortar must not leave any pores due to bleeding around the steel drums. Moreover, it must have a low water permeability. To assure the execution of such a mortar, a full-scale concrete pit was constructed in an experiment. Solid wastes in 200-liter steel drums were placed in the pit, and mixed proportion determined by a preliminary experiment was filled into the pit. After the mortar cured, the filling property and water impermeability of the mortar were checked by taking boring cores of 30 cm in diameter. As a result, both the filling property and water impermeability of the mortar were found to be satisfactory (Photographs 5 and 6) [12]. With regard to the durability of the concrete pit, the present approach to the safety assessment requires a period of about 10 years at the most for assurance of
186
Y. Maki. H. Ohnuma / Application of concrete
Photo 5. Checking grouting ability by demolishing the storage pit.
containment by the concrete structure. It, therefore, is sufficient to use the ordinary structural concrete for building the concrete pit. From the viewpoint of safety, however, it is desirable that the concrete retains its integrity as long as possible. To predict the life of the concrete, accelerated tests were made under the following conditions: (1) Freezing and thawing test in air wherein temperature cycles from - 2 0 ° C to 40°C were given to the concrete twice a day. (2) Freezing and thawing test in water wherein temperature cycles from - 1 8 ° C to 5°C were given to the concrete six times a day. (3) Neutralization test wherein the concrete was left to stand in all air of which carbon dioxide gas concentration was about 500 times of that of ordinary air (15%).
(4) Erosion test wherein the concrete was exposed to a sulfuric acid solution of which concentration was about eight times of that contained in river water or ground water (440 ppm). As a result of these accelerated tests, the test specimens of the concrete were found to maintain the integrity except the very shallow surfaces. On the basis of such findings, a concrete pit of a sufficient thickness is estimated to have a durability over a long period of at least about 100 years [3]. A porous concrete layer will be used to line the interior of the concrete pit. Even if water penetrates into the concrete pit, the water will swiftly move through the porous concrete layer and flow out of the concrete pit through the drain channels; thus water will be prevented from contacting the waste packages. The porous concrete is a kind of "no fines concrete." It has been used as a material of sewer pipe for draining water which comes from seepage through the wall. Experiments are being made to verify the performance of the porous concrete as the concrete pit material. Like the porous concrete, as an attempt to utilize a conventional material, application of polymer-impregnated concrete (PIC) forms was investigated. The attempt of the use of PIC forms is attributed to the low water permeability and high durability of the PIC. It might be used on the exterior of the concrete pit to reduce contact between the ground water and the concrete pit. Various efforts have been made and are in progress to improve the water stopping property of the concrete and to decrease the migration of radionuclides. For instance, an inorganic cement type material is applied to the concrete surface to form crystals in the pores of
Imitative
Cement tyl grout drum
Photo 6. Grouting condition (enlarged photograph) and overall photograph of the boring core.
187
Y. Maki, tl. Ohnurna /Application of concrete /.Ground Sarface
/ Cover Soil :'!'~i~-~ ,.~'!':~i~'~i'.'~~', !,~~'.'-"..~i'~-]".~ !i:!i.':i~.\i' ~! ~-".!'~i.'~'. ..i....... i:'.~"~'~i"i ~!"."'-!'.~'if//~ MadeRocr ~ Ma/n Concrete
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Fig. 5. Outline of the rock integration burial facilities.
the concrete to make the concrete impermeable. This technique was applied to the tunnels of the JR Shinkansen lines. Mortar test specimens treated by this technique were subjected to a test to measure the diffusion coefficient of Cs-137. It was found that the coating with this impermeability treatment material is effective in preventing radionuclides from migrating by making the cement mortar impermeable. With regard to the structural design, an integral structure disposal system has been investigated. This disposal system was examined to provide for a case when the bedrock is relatively close to the ground surface. According to this system, a pit is excavated in a bedrock, and a base concrete is placed in the pit. Then solidified wastes encapsulated in large-sized concrete containers are placed in the pit, and concrete is poured into the pit to fill the gaps to make a so-called man-made rock. In this way, the solidified wastes are integrated with the rock formation. It eliminates the use of the concrete pit of which construction is under way at Rokkasho-mura, Aomori Prefecture. The system has a feature that the integration with the rock formation can produce a more structurally or hydraulically stable structure. It should be noted that, according to this system, it is ~lesirable to use large-sized concrete containers. This, in turn, will make it desirable to integrate both the processes of treatment and disposal (fig. 5) [15].
5. Concluding remarks To cope with the increasing demand for electricity, united efforts of the people are required in energy
saving, utilization of natural energy, reduction in energy loss through redistribution of demands. For the time being, however, nuclear power can not be neglected as one measure to cope with the increasing demand for energy. Hence the treatment and disposal of radioactive waste are tasks that can not be avoided. The radioactivity level of wastes range from those that must be isolated from the human environment over a very long period to those that arise from demolition of power plants and should be recycled as resources. Irrespective of whether they may be disposed of or recycled, concrete plays the main role. On the other hand, in the treatment and disposal of radioactive wastes we have encountered problems that were rather rare in the fields of civil engineering and construction. For instance, in the treatment of solidification, the chemical properties of a waste may cause degradation of cement hydrate. In the disposal we are required to predict whether the functions will be maintained over a very long period, for instance, 300 years, and if degradation occurs, when it will occur. These are our tasks of the future. It should be added that, in Japan, to achieve disposal of low-level wastes of which radioactivity exceeds the upper limit of wastes that can be disposed of at Rokkasho-mura, research and development efforts concerning land disposal are in progress; for instance, research on concrete materials and water stopping materials that are more durable, less permeable to water, and more resistant to cracking. We would like to express our deep gratitude to Japan Nuclear Fuel Industry Co., Ltd., Radioactive Waste Management Center and others for the provision of valuable data.
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Y. Maki, H. Ohnuma / Application of concrete
References [1] M. Toyohara et al., Study on the leaching ratio and long term stability of water reduced cement solidified product, Joint International Symposium on Environmental Consequences of Hazardous Waste Disposal, May 1991. [2] Teruo Iji et al., Advanced cement solidification process, Waste Management '85, April 1985. [3] Sumitomo Chemical Co., Ltd., Solidification of Radioactive Wastes (1988). [4] NGK, NGK Radwaste Treatment System (1989). [5] M. Kikuchi et al., Advanced solidification technique using cement-glass, 3rd International Conference on Nuclear Fuel Reprocessing and Waste Management, April 1991. [6] K. Yokoyama et al., Development of cement-conditioning technique for dry active waste, ASME, JASM, JAEC Waste Conference, October 1990. [7] Mitsubishi Heavy Industry, Radioactive Waste Solidification System Utilizing Hydrothermal Reaction. [8] Chichibu Cement, Ozawa Cement, SFPIC container.
[9] Chichibu Cement, Polymer-cement-mortar lining container. [10] Chichibu Cement, Properties and applications of polymer-impregnated concrete. [11] Japan Nuclear Fuel Industries, Outline of burial facility (1990). [12] Radioactive Waste Management Center, Field Test for Safe/Reliable Inspection and Final Storage of Low Level Radioactive Waste. [13] STA (Central Research Institute of Electric Power Industry), Durability of Concrete Structure (1991). [14] S. Takebe, Studies on diffusion of 137Cs in cement mortar, JAERI-M89211 (1989). [15] Y. Maki, Application of concrete to treatment and disposal of radioactive waste, Concrete Journal 29, No. 2 (1991). [16] CRIEPI Research& Investigation data, Incombustible miscellaneous solid waste treatment techniques (1989). [17] CRIEPI Research & Investigation Data, Incombustible miscellaneous solid waste solidification techniques (1990).
179
Application of concrete to the treatment and disposal of radioactive waste in Japan Y a s u r o Maki a a n d Hiroshi O h n u m a b a Department of Civil Engineering, Hosei University, Tokyo, Japan b Central Research Institute of Electric Power Industry, Tokyo, Japan
Received 20 July 1992
Low level radioactive waste will be disposed at the Rokkasho site in Aomori Prefecture in Japan. During the treatment, storage and disposal of the radioactive waste, concrete is used and will be used for various purposes. For this reason a lot of research & development has been carried out on concrete at various institutes, universities and manufactures in Japan. This paper presents the present state of application of concrete to treatment, storage and disposal of low level radioactive waste in Japan. In the 2nd section, the electric power supply and the kinds and volumes of radioactive waste from nuclear power plants in Japan are described. In the 3rd section, the applications of concrete to the treatment of radioactive waste are described. These are solidification with cement and containers made by various mortars and concretes. The application of concrete to disposal structures are presented in the 4th section; these are research on the durabity of concrete under disposal site condition, research on the filling the concrete pit with 200 1 drum packed cement solidified wastes by prepacked concreting methods, and so on. And this section describes also the outlines of the low level radioactive disposal system at the Rokkasho site.
1. Introduction Nuclear power generation now accounts for about 30 percent of the total power generated in Japan. With this development, it has become a critical task to safely and rationally treat and dispose of radioactive wastes arising from nuclear power generation. To this end, the Japanese Government has been developing laws and regulations concerning treatment, storage and disposal of radioactive wastes. The electric utilities, which have been generating electricity from nuclear power, have been in charge of construction and operation of facilities for the treatment and disposal of the wastes. Japan Nuclear Fuel Industry Co., Ltd. (JNFI) has been constructing facilities for low-level radioactive wastes. Japan Atomic Energy Research Institute (JAERI) is in charge of research on, for instance, safety of treatment and disposal of radioactive wastes. The manufacturers have been energetically developing technologies for Correspondence to: Dr. Yasuro Maki, Dept. of Civil Engineering, Housei University, 3-7-2 Kajino-Cho, Koganei-shi, Tokyo 184, Japan.
treatment and disposal. Furthermore, Power Reactor and Nuclear Fuel Development Corporation (PNC) has been conducting research and construction of facilities for treatment and disposal of radioactive wastes arising from spent fuel reprocessing and plutonium fuel fabrication. Of the activities for treatment and disposal of such radioactive wastes in Japan, this report outlines the present state of application of concrete materials to the treatment and disposal of radioactive wastes, in particular, low-level wastes.
2. Electric power supply and low-level waste arising 2.1. Electric power supply
According to the interim report of the Advisory Committee for Energy (October 1990), the nuclear power now has established itself as a critical energy source in Japan. Ever since the introduction of the first commercial nuclear reactor in 1966, the development of nuclear power has been promoted by the efforts of
0 0 2 9 - 5 4 9 3 / 9 2 / $ 0 5 . 0 0 © 1992 - Elsevier Science P u b l i s h e r s B.V. All rights reserved
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Y. Maki, H. Ohnuma /Application o[ concrete
come mainly from nuclear power plants. Radioactive wastes originally arise in liquid forms but are solidified with cement, asphalt or plastics, and are regarded as solid wastes. Solid wastes coming from nuclear power plants include: concentrated liquid waste of liquid waste arising from regeneration of ion-exchange resins used in water treatment and various drains; spent ion-exchange resins and spent filters for purification of water systems; miscellaneous solid wastes such as paper, clothes, etc. used in reactor facilities, metallic and concrete pieces arising from maintenance; .astir,"from incineration of combustible miscellaneous solid wastes; and reactor internals such as spent control rods that will be treated as radioactive waste in future. Of such radioactive wastes, the concentrated liquid waste is solidified, for example, by mixing with cement, pouring into heated asphalt to evaporate moisture and mix only the solids within the asphalt, or at first evaporating moisture to turn the waste into powder and then solidifying the powder with cement or plastics. These wastes are normally encapsulated in 200-liter steel drums, and the resulting packaged wastes are called homogeneous solidified wastes in Japan. The wastes that the electric utilities are planning to dispose of at Rokkasho-mura, Aomori Prefecture are mainly of such types. Spent ion-exchange resins, filter sludge, inciner-
those concerned while constantly assuring safety. As of the end of May 1990, the number of commercial nuclear power plants in operation was 38 units with a total output of more than 30,000 MW, supplying about 30% of the electricity. Moreover, even if the increase in energy consumption could be minimized by energy saving efforts, the mean annual growth rate of energy used in the form of electricity is expected to be 2.7% for the period from 1988 to 2000, and 1.5% from 2000 to 2010. If the dependency on oil were to be reduced further, and the emission of carbon dioxide were to be kept at a certain level, the expected increase would require the utmost efforts to introduce non-fossil energy. Nuclear power is, at present, the sole non-fossil energy that can meet the needs quantitatively. The report indicated the power supply targets at the ends of the respective years, as shown in fig. 1. These figures indicate that we must rely on nuclear power for about 40 percent of the power supply; although the figures for nuclear power were modified downwards by 5 to 6 percent in comparison with the previous report of the Committee (July 1986). 2.2. Kinds and quantities o f radioactive wastes
The radioactive wastes to be covered by this report are solid wastes of low radioactivity level. These wastes
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Y. Maki, 1t. Ohnuma /Application of concrete ation ash, all in powdery form, will be mixed with cement or plastics and solidified in 200-liter steel drums as homogeneous solidified wastes. Incombustible miscellaneous solid wastes arise in bulk. Such wastes are put into plastic bags and packed in 200-liter steel drums to store in most of the power plants. In future they will be reduced in volume by, for example, high temperature combustion, plasma melting, or high pressure volume reduction with a high pressure press, then solidified in containers with some solidifying materials. The containers for this purpose may include 200-liter steel drums (including those lined with concrete), large-sized metal boxes, and concrete containers. The most useful method for solidification will be cement solidification by the prepacked concreting method. The reactor internals are small in quantity although their radioactivity levels are high. They are stored in spent fuel pools and similar facilities called site bunkers. They have not been treated yet since the final disposal method is to be decided in future. Such reactor internals might be reduced in volume by chopping and compression and then packed in steel or concrete containers having shielding functions. At present, concentrated liquid wastes are solidified in steel drums with cement, etc. Combustible miscellaneous solid wastes are incinerated, and the resulting ash is put into steel drums without any treatment. Most of the spent ion-exchange resins are stored in tanks, and the incombustible miscellaneous solid wastes are stored in steel drums. The quantities of such arisings are as shown in fig. 2 according to the tentative calculation of 1984. With various efforts to reduce the
181
arisings of various wastes, the number of such steel drums are predicted to be 400 and some thousands in 1990, and more than 700,000 in 2000. If presently untreated wastes could be treated by a method having a considerable volume reduction effect, the number of the steel drums could be reduced to 300 and some thousands in 1990, and more than 500,000 in 2000. The latter figures will be the actual number of steel drums to be disposed of eventually.
3. Application of concrete materials to treatment of radioactive wastes
Treatment of radioactive waste normally means changing physical or chemical characteristics of waste arose and packing it into a container for ease in transport, storage and disposal. The treatment thus includes volume reduction, solidification, and encapsulation in a container. The major application of concrete to treatment of radioactive wastes is for the treatment of low- and intermediate-level wastes and transuranic (TRU) wastes; solidification with cement, and containers made of concrete. 3.1. Cement solidification Solidification with cement can take in two forms. The first one is to mix and solidify liquid or powdery waste, such as concentrated liquid waste, spent ion-exchange resin, and incineration ash, with cement. Mixing is effected by a large-sized mortar mixer such as indrum mixer or a forced mixing type mixer. As for
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182
)t, Maki, H. Ohnuma / Application o f concrete
solidifying materials, ordinary portland cement and portlment blast-furnace slag cement are normally used. Such cements are required to have an appropriate strength. Moreover, such cements must be compatible with the waste to be solidified since the waste may include various substances, e.g., acidic substances and sulfates. Efforts are also being made to develop new cements including low-moisture-content cement. Attempts have also been reported to prevent adverse effects on cement hydration by, prior to cement solidification, pretreating the contents of the concentrated liquid to make them insoluble [1,2]. The second method is applied to almond-like grains called pellets that are produced from liquid or powdery wastes. It is also applied to vitrified granular ash that arises from high temperature incineration and is called granule. Incombustible miscellaneous solid wastes compressed by a high pressure press to reduce the volume are also treated by the second method. Such wastes are put into 200-liter steel drums or 200-liter steel drums with concrete lining and solidified by the prepacked concreting method. Mortar to be poured should be highly fluid and be able to fill every corner without the use of a vibrator. Moreover, its bleeding should be negligible. To this end, various methods have been tried and satisfactory results have been reported. Such methods include the use of a fluidizing agent, a self-levelling material, and the use of a special cement comprising water glass to eliminate redundant water. Figure 3 shows the outline of a process which pelletizes concentrated liquid waste and solidifies the pellets with water glass cement. It is reported that the solidified waste has satisfactory strength, resistance to leaching,
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and adsorption of radioactive nuclides. The process has been adopted in PWR and BWR power plants in Japan. Photographs 1 and 2 show cement solidification
Photo 1. Cement solidified incombustible miscellaneous waste by prepacked method.
Y. Maki, H. Ohnuma /Application of concrete
Photo 2. Incombustible miscellaneous solidified product by prepacked method.
of miscellaneous solid wastes. Photograph 1 shows the use of a self-levelling material, It will be found that the filled and solidified agent has filled every corner. Photogrpah 2 is a scene of a full-scale filling test. It shows the practicability of solidification with the prepacked concreting method. To achieve smooth filling, various additive agents as well as self-levelling agents are reportedly used [3-6]. Although not for the wastes arising from normal operation, research is also in progress at JAERI, CRIEPI, etc. on cement solidification, by the prepacked concreting method, of decommissioning concrete of relatively high radioactivity arising from decommissioning of a reactor. It should be noted that in an ordinary civil engineering work the prepacked concreting method is practiced under water. The cement solidification of radioactive wastes by the prepacked concreting method is naturally made in a dry condition. A process of solidifying waste with some natural rock materials for solidifying, such as silica and China clay, has been developed, although the process may not be classified into cement solidification. Solidification is effected by hydro-thermal reaction under high temperature and high pressure (about 300°C and 300 kgf/cmZ). Materials to be treated by this method are dried powder from concentrated liquid waste and incineration ash. The process is capable of producing solidified wastes that excel in strength and resistance to leaching. 3.2. Containers
There are many applications of concrete to containers for encapsulation in and out of Japan. The applications are varied: use of concrete for the container
183
proper, for the overpacking containers, and as the lining material for metal containers. Containers for disposal must meet the following requirements: (1) The container must be compatible with the waste to be encapsulated. (2) The container must be durable under the disposal conditions. (3) The container must withstand handling for disposal. (4) The container must bear the stacking load after disposal. (5) The container must contain radioactive waste after disposal as long as possible (for example, 300 years, 500 years, or 1000 years depending on radioactivity level, etc.) The containers must have radiation shielding functions in some cases. Ordinary concrete that is normally used in civil and construction works adequately meets such requirements. On the other hand, efforts are in progress to develop special concretes or mortars for higher performance [4,8,9,10]. 3.2.1. Fiber-reinforced concrete One example is a container developed in France, which is made of mortar reinforced with asbestos fiber. The cylindrical barrel is produced by a centrifugal method, and is 15 to 20 cm in thickness. The bottom plate and the cover are joined to the cylindrical barrel by means of both asbestos concrete plate and ordinary concrete. The use of asbestos concrete is presumably intended to give shock resistance to the container so that the container can withstand a drop accident during handling. The performance of the container has been reportedly verified by experiment. Containers made of carbon-fiber reinforced concrete may have properties comparable to those of asbestos cement. Carbon fiber is expected to be investigated further as a substitute of asbestos fiber since unlike glass fiber, carbon fiber is free of degradation by alkali. In addition to asbestos fiber, steel fiber, which is commonly used, is also used for reinforcement. When concrete is used to make a disposal container, it is desirable to make the wall of the container as thin as possible to encapsulate a greater amount of waste and to reduce the weight of the waste package (waste encapsulated in a container and ready for disposal), except a thicker concrete wall is required for radiation shielding. To meet such requirements, steel fiber reinforced mortar was used in some cases to line the interior of a steel drum. The steel fiber reinforced mortar has a high shock resistance, and is also effective in coping with cracks. Naturally, containers were also experimentally produced by using thick wall concrete container reinforced with steel fiber.
184
E Maki, H. Ohnuma /Application of concrete
3.2.2. Resin mortar Resin mortar is mostly used for lining the steel drums. Polymer-impregnated mortar and resin mortar have been investigated, and both have reached to the stage of practical application. Polymer-impregnated mortar is applied by lining mortar of 2 to 5 cm thick over the interior of a steel drum, impregnating the lining with a resin monomer, then polymerizing the monomer by thermal polymerization. A container developed by Chichibu Cement Co. Ltd. was verified by the U.S. NRC to meet the standards of the high integrity container (HIC) prescribed in the regulations of 10CFR61 (Photograph 3). Resin mortar, on the other hand, is a lining of about several millimeters thick on the interior of a steel drum. It is close to the form of coating. The developed container is free of pores and has an excellent waterstopping capability. The resin mortar lining is quite effective in preventing corrosion of the steel drum from the interior. The container also has a special feature that the effective volume for packing is almost comparable to that of an unlined steel drum. 3.2.3. Ceramics In addition to concrete containers, ceramic containers have been developed. It is desirable to isolate radioactive wastes from the human environment as long as possible. When a geological formation is used for this purpose in view of the possible instability of the geological formation, it is desirable to contain the radioactive substances in an engineered structure as long as possible. It is quite desirable to achieve a period of the effective containment of an order of 1000
Photo 4. Cylindrical ceramic container (200 1). years. To this end, ceramics have been attracting much attention and have been investigated as highly durable materials. Damages due to freezing or salts are almost negligible in the disposal environments at several meters below the ground surface for low-level waste, and at several hundred meters or more below the ground surface for high-level waste. Hence concrete will adequately meet the requirements. Concrete, however, has not been demonstrated of the integrity over an extended period. Moreover, it has become possible to produce large-sized structures of ceramics in recent years. Such backgrounds might explain why the research efforts were initiated. Ceramic container prototypes range from an inner container which is used in a steel drum to a large-sized container which is for over-packing high-level vitrified waste. Intensive research efforts are in progress to put such containers to practical use. In particular, ceramic containers will be indispensable as containers for melting low-level waste at high temperature and solidifying the waste in the containers (Photograph 4) [4]. 3.2.4. Other efforts In addition to those efforts mentioned above, a variety of attempts have been made or are in progress, including application of concrete of high shock resistance and long-term durability.
4. Application of concrete materials to disposal of radioactive wastes Photo 3. Polymer impregnated concrete lining drum (exampie).
Disposal means a state out of human control. Irrespective of the substance to be disposed of, may it be
Y. Maki, H. Ohnuma /Application of concrete
municipal waste, industrial waste, or radioactive waste of our concern be made in a manner which assures safety of the people against the disposed waste. The period for assuring the safety is 60 to 70 years for ordinary structures; the safety period is actually the durable life of the structure. In contrast, the safety assurance period for radioactive wastes is much longer. It is about 300 years for low-level wastes of which major safety-determining radioactive materials are strontium-90 and cesium-137 with half lives of about 30 years. In disposal of radioactive wastes, its safety is assured by a combination of the performances of the engineered structures in containing and preventing migration of radioactive substances, and the performances of the surrounding soil or the like in retarding migration of and diluting the radioactive substances. The engineered structures include the waste to be disposed of, the overpacking container for encapsulating the waste, the facility, an additional artificial clay layer, etc., and are called engineered barriers. The latter is called a natural barrier. The migration of radioactive substances disposed of is effected by ground water. The roles of the engineered barriers, therefore, are to isolate the radioactive substances from the ground water, to retard the contacts between them as long as possible,, to retard the migration of radioactive substances as long as possible even if they contact with ground water, and to adsorb the radioactive substances and minimize their outflow to the surrounding soil. Hence the disposal facility must be free of through cracks and maintain a low water permeability, while ordinary reinforced concrete structures tolerate cracking provided the sectional stresses are below the allowable stress. Degradation of concrete into clay may be acceptable provided such a change call be predicted and the degradation is with a range for which the containment performance can be assured. This is a key difference from ordinary structures. The land disposal method being planned in Japan is a mixture of the monolith method of France and the silo method of Sweden. The geology of a site consists of bedrocks of tuff, sandstone, etc. except the surface layers of about three meters thick. A pit is excavated in such a bedrock and a reinforced concrete structure is built in the pit. The pit is divided into cells with partition wails similar to those of the Swedish system. Wastes are placed in these cells and mortar is filled into the ceils to form an integral structure. When completed, the structure is close to the monolith of the French system. Like the Swedish system, clay layers are formed around the pit to reduce water penetration into
185
Fig. 4. Outline of the burial facilities.
the pit and to prevent the penetrating water from contacting the wastes. It is also planned to provide the pit with a porous concrete lining inside the pit and with water collecting and discharging channels. The concrete will have a compressive strength of 250 k g f / c m 2 or over (fig. 4) [11]. To contribute to the construction of the disposal facility for the low-level wastes, a variety of research programs are in progress at some research institutes and companies. Some programs relating concrete will be outlined in the following [12-14]. Filling the concrete pit with mortar is designed to integrate the pit, filling material and waste packages so that they behave as an integral structure like a bedrock against external forces of earthquake, water penetration, etc. Accordingly the mortar to be filled is required to fill in every corner of the pit. The mortar must not leave any pores due to bleeding around the steel drums. Moreover, it must have a low water permeability. To assure the execution of such a mortar, a full-scale concrete pit was constructed in an experiment. Solid wastes in 200-liter steel drums were placed in the pit, and mixed proportion determined by a preliminary experiment was filled into the pit. After the mortar cured, the filling property and water impermeability of the mortar were checked by taking boring cores of 30 cm in diameter. As a result, both the filling property and water impermeability of the mortar were found to be satisfactory (Photographs 5 and 6) [12]. With regard to the durability of the concrete pit, the present approach to the safety assessment requires a period of about 10 years at the most for assurance of
186
Y. Maki. H. Ohnuma / Application of concrete
Photo 5. Checking grouting ability by demolishing the storage pit.
containment by the concrete structure. It, therefore, is sufficient to use the ordinary structural concrete for building the concrete pit. From the viewpoint of safety, however, it is desirable that the concrete retains its integrity as long as possible. To predict the life of the concrete, accelerated tests were made under the following conditions: (1) Freezing and thawing test in air wherein temperature cycles from - 2 0 ° C to 40°C were given to the concrete twice a day. (2) Freezing and thawing test in water wherein temperature cycles from - 1 8 ° C to 5°C were given to the concrete six times a day. (3) Neutralization test wherein the concrete was left to stand in all air of which carbon dioxide gas concentration was about 500 times of that of ordinary air (15%).
(4) Erosion test wherein the concrete was exposed to a sulfuric acid solution of which concentration was about eight times of that contained in river water or ground water (440 ppm). As a result of these accelerated tests, the test specimens of the concrete were found to maintain the integrity except the very shallow surfaces. On the basis of such findings, a concrete pit of a sufficient thickness is estimated to have a durability over a long period of at least about 100 years [3]. A porous concrete layer will be used to line the interior of the concrete pit. Even if water penetrates into the concrete pit, the water will swiftly move through the porous concrete layer and flow out of the concrete pit through the drain channels; thus water will be prevented from contacting the waste packages. The porous concrete is a kind of "no fines concrete." It has been used as a material of sewer pipe for draining water which comes from seepage through the wall. Experiments are being made to verify the performance of the porous concrete as the concrete pit material. Like the porous concrete, as an attempt to utilize a conventional material, application of polymer-impregnated concrete (PIC) forms was investigated. The attempt of the use of PIC forms is attributed to the low water permeability and high durability of the PIC. It might be used on the exterior of the concrete pit to reduce contact between the ground water and the concrete pit. Various efforts have been made and are in progress to improve the water stopping property of the concrete and to decrease the migration of radionuclides. For instance, an inorganic cement type material is applied to the concrete surface to form crystals in the pores of
Imitative
Cement tyl grout drum
Photo 6. Grouting condition (enlarged photograph) and overall photograph of the boring core.
187
Y. Maki, tl. Ohnurna /Application of concrete /.Ground Sarface
/ Cover Soil :'!'~i~-~ ,.~'!':~i~'~i'.'~~', !,~~'.'-"..~i'~-]".~ !i:!i.':i~.\i' ~! ~-".!'~i.'~'. ..i....... i:'.~"~'~i"i ~!"."'-!'.~'if//~ MadeRocr ~ Ma/n Concrete
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Fig. 5. Outline of the rock integration burial facilities.
the concrete to make the concrete impermeable. This technique was applied to the tunnels of the JR Shinkansen lines. Mortar test specimens treated by this technique were subjected to a test to measure the diffusion coefficient of Cs-137. It was found that the coating with this impermeability treatment material is effective in preventing radionuclides from migrating by making the cement mortar impermeable. With regard to the structural design, an integral structure disposal system has been investigated. This disposal system was examined to provide for a case when the bedrock is relatively close to the ground surface. According to this system, a pit is excavated in a bedrock, and a base concrete is placed in the pit. Then solidified wastes encapsulated in large-sized concrete containers are placed in the pit, and concrete is poured into the pit to fill the gaps to make a so-called man-made rock. In this way, the solidified wastes are integrated with the rock formation. It eliminates the use of the concrete pit of which construction is under way at Rokkasho-mura, Aomori Prefecture. The system has a feature that the integration with the rock formation can produce a more structurally or hydraulically stable structure. It should be noted that, according to this system, it is ~lesirable to use large-sized concrete containers. This, in turn, will make it desirable to integrate both the processes of treatment and disposal (fig. 5) [15].
5. Concluding remarks To cope with the increasing demand for electricity, united efforts of the people are required in energy
saving, utilization of natural energy, reduction in energy loss through redistribution of demands. For the time being, however, nuclear power can not be neglected as one measure to cope with the increasing demand for energy. Hence the treatment and disposal of radioactive waste are tasks that can not be avoided. The radioactivity level of wastes range from those that must be isolated from the human environment over a very long period to those that arise from demolition of power plants and should be recycled as resources. Irrespective of whether they may be disposed of or recycled, concrete plays the main role. On the other hand, in the treatment and disposal of radioactive wastes we have encountered problems that were rather rare in the fields of civil engineering and construction. For instance, in the treatment of solidification, the chemical properties of a waste may cause degradation of cement hydrate. In the disposal we are required to predict whether the functions will be maintained over a very long period, for instance, 300 years, and if degradation occurs, when it will occur. These are our tasks of the future. It should be added that, in Japan, to achieve disposal of low-level wastes of which radioactivity exceeds the upper limit of wastes that can be disposed of at Rokkasho-mura, research and development efforts concerning land disposal are in progress; for instance, research on concrete materials and water stopping materials that are more durable, less permeable to water, and more resistant to cracking. We would like to express our deep gratitude to Japan Nuclear Fuel Industry Co., Ltd., Radioactive Waste Management Center and others for the provision of valuable data.
188
Y. Maki, H. Ohnuma / Application of concrete
References [1] M. Toyohara et al., Study on the leaching ratio and long term stability of water reduced cement solidified product, Joint International Symposium on Environmental Consequences of Hazardous Waste Disposal, May 1991. [2] Teruo Iji et al., Advanced cement solidification process, Waste Management '85, April 1985. [3] Sumitomo Chemical Co., Ltd., Solidification of Radioactive Wastes (1988). [4] NGK, NGK Radwaste Treatment System (1989). [5] M. Kikuchi et al., Advanced solidification technique using cement-glass, 3rd International Conference on Nuclear Fuel Reprocessing and Waste Management, April 1991. [6] K. Yokoyama et al., Development of cement-conditioning technique for dry active waste, ASME, JASM, JAEC Waste Conference, October 1990. [7] Mitsubishi Heavy Industry, Radioactive Waste Solidification System Utilizing Hydrothermal Reaction. [8] Chichibu Cement, Ozawa Cement, SFPIC container.
[9] Chichibu Cement, Polymer-cement-mortar lining container. [10] Chichibu Cement, Properties and applications of polymer-impregnated concrete. [11] Japan Nuclear Fuel Industries, Outline of burial facility (1990). [12] Radioactive Waste Management Center, Field Test for Safe/Reliable Inspection and Final Storage of Low Level Radioactive Waste. [13] STA (Central Research Institute of Electric Power Industry), Durability of Concrete Structure (1991). [14] S. Takebe, Studies on diffusion of 137Cs in cement mortar, JAERI-M89211 (1989). [15] Y. Maki, Application of concrete to treatment and disposal of radioactive waste, Concrete Journal 29, No. 2 (1991). [16] CRIEPI Research& Investigation data, Incombustible miscellaneous solid waste treatment techniques (1989). [17] CRIEPI Research & Investigation Data, Incombustible miscellaneous solid waste solidification techniques (1990).