Patent Publication Number: US-6660972-B1

Title: Container for storing hazardous material and a method of enclosing hazardous material in a concrete container body

Description:
REFERENCE TO RELATED APPLICATIONS 
     The present application is the national stage under 35 U.S.C. 371 of international application PCT/SE00/00783, filed Apr. 26, 2000 which designated the United States, and which international application was published under PCT Article 21(2) in the English language. 
     This invention relates to storage of hazardous materials, especially low-level radioactive materials and chemical or biological hazardous materials. More particularly, the invention relates to a container for storing such materials, whether waste or useful materials, in a sealed storage space and a method for fluid-tight enclosure of the hazardous materials in a container body of concrete. 
     Prior art techniques for containing radioactive materials, such as fuel elements for nuclear reactors, include enclosing the materials in a shipping or storage container of reinforced concrete (DE-A-35 15 871). The radioactive material is introduced in a generally cylindrical monolithic container body with a vertically elongate storage space and an access opening connecting the storage space with the exterior surface of the container body at one end thereof. Apart from this opening the storage space is jointless. 
     After the radioactive material has been introduced in the storage space through the access opening, a pre-cast concrete closure body is placed in the access opening and bolted to the container body. A sealing member positioned in the gap between the wall of the access opening and the closure body ensures that the containment of the radioactive material will be fluid-tight. 
     In this prior art shipping and storage container, the sealing member is a factor of uncertainty. Although the sealing member may initially provide an adequate sealing, it may in the course of time lose some or all of its sealing ability, e.g. under the influence of the stored material. 
     An object of the invention is to ensure in a concrete container for storing a hazardous material a satisfactory containment of the material for a very long time, such as several decades. 
     In the container and the method according to the invention, the features of which are set forth in the claims, this object is achieved by casting concrete in an access opening of a container body after the introduction of the hazardous material in a storage space formed by the container body through the access opening and allowing the concrete to harden while supplying heat to the portion of the container body which surrounds the access opening. The supply of heat will cause the access opening to be expanded as a result of the thermal expansion of the heated portion of the container body. After a suitable heating period, the heated portion is allowed to assume the ambient temperature and thereby provide a shrink fit with the closure body formed by the hardened concrete cast in the access opening. 
     The shrink fit will be particularly effective if the container body comprises a metal reinforcement, preferably a prestressed reinforcement, extending about the access opening and this reinforcement is heated together with the concrete. 
     Preferably, the heating is accomplished by a heater, e.g. an electric heater, embedded in the concrete and extending about the access opening. 
    
    
     The container and the method of the invention will be described in more detail with reference to the illustrative exemplary embodiment of a container for storing hazardous materials shown in the drawings. 
     FIG. 1 is a vertical sectional view of the container; and 
     FIG. 2 is an enlarged sectional view of the container shown in FIG.  1 . 
    
    
     In the illustrated exemplary embodiment, the container according to the invention comprises a circular cylindrical container body  11  shown in its normal upright position in the drawings. The container body  11  is a monolithic body of concrete and forms a central, likewise circular cylindrical storage space  12 . A tubular mouth portion  11 A at the upper end of the container body  12  defines an access opening  13 . Through this opening  13  material to be enclosed in the container body  11  and held in it for a shorter or longer time can be introduced in the storage space  12  and removed therefrom if required. The access opening  13  forms an upward extension of the storage space  12 . 
     As shown in the drawings, the container body  11  has been sealed in accordance with the method of the invention after a number of inner vessels C containing hazardous material have been stacked in the storage space  12 . The hazardous material may be radioactive material, particularly low-level radioactive material, chemical or biological material or any other material that has to be stored such that it is reliably prevented from escaping from the container body. 
     The container body  11  is provided with both an axial steel reinforcement  14  embedded in the wall  15  of the container body and a further reinforcement  16  formed by a steel wire wound about the cylindrical outer surface of the wall  15 . A steel reinforcement (not shown) is also embedded in the bottom wall  17  of the container body  11 . All reinforcements may be prestressed. 
     A heater  18  is embedded in the upper portion of the container body  11 , including the mouth portion  11 A. Preferably, the heater  18  is an electrical heater, but the heating energy supplied to the heater for heating the upper portion of the container body  11 , and especially the mouth portion  11 A, may also be non-electric energy. Associated with the heater  18 , but not shown in the drawings, are means for connecting the heater to an energy source and means for controlling and monitoring the heating. 
     In the illustrated embodiment, the heater  18  is in the shape of a cylindrical helix which is coaxial with the wall  15  of the container body  11 , the storage space  12  and the access opening. The heater axially subtends the portion of the container body wall  15  which defines the access opening  13 , that is, the mouth portion  11 A, and an adjoining axial section of the portion of the wall which defines the storage space  12 . Over that section the pitch of the helical heater may increase gradually in the direction away from the mouth portion  11 A so that the heating power per unit volume of the concrete will be lower than in the mouth portion. During the heating a temperature gradient will thus develop in the concrete beneath the mouth portion  11 A. 
     After the vessels C have been inserted in the storage space  12 , a fluid-tight sealing of the container body  11  is accomplished by casting a closure body  19  of concrete in the access opening  13 . A plate  20  inserted in the access opening  13  prior to the casting of the concrete forming the closure body  19  limits the space occupied by the concrete. This plate may be omitted, however, so that the concrete poured into the access opening  13  to form the closure body  19  can reach the vessels C and also fill the gap between the vessels and the container body wall  15 , thereby immobilising the vessels in the storage space  12 . 
     To seal the container body  11 , concrete is poured into the access opening  13  and at the same time, or some time before the pouring, the heater is energised to heat the upper portion of the container body  11  and thereby expand it, including the reinforcement  16 , so that the access opening  13  is widened. The cast and still wet concrete is vibrated intensely and then revibrated after a few hours so that a very intimate contact between the concrete of the container body  11  and the concrete of the closure body  19  is brought about. 
     During the first phase of the hardening of the concrete of the closure body  19  it may be advantageous to cool the wet concrete. This can be done by sticking cooling rods or other cooling members into the still viscous concrete. 
     When the concrete in the access opening  13  has hardened sufficiently, the heating is discontinued so that the temperature of the upper portion of the container body  11  will be reduced and the mouth portion  13  will thereby be somewhat constricted and subject the closure body  19  formed in the access opening to an omnidirectional radial pressure. This pressure will enhance the bond between the closure body  19  and the mouth portion  19  surrounding it so that a perfectly fluid-tight and permanent sealing results. If desired, the heating can be controlled such that the temperature of the concrete of the mouth portion  11 A varies in dependence on the progress of the hardening of the concrete forming the closure body  19 . Preferably, the mouth portion  11 A is heated to a temperature within a predetermined temperature range while the adjoining section of the container body  11  is heated to a temperature that drops from the first-mentioned temperature adjacent the mouth portion  11 A to the ambient temperature adjacent the lower end of the heater  18 . 
     The container body  11  can be manufactured centrally and stored in suitable numbers for future use. All that is required to contain the hazardous material after it has been introduced in the storage space  12  of the container body  11  is to cast concrete in the access opening  13  to form the closure body  19  and control the heating of the upper portion of the container body. These sealing operations can readily be carried out at any suitable location, e.g. where the hazardous material to be contained is kept. 
     As is apparent from the foregoing description, the sealed closure body  19  cannot be easily removed. However, should it be necessary to open the sealed container body  11  to gain access to the contained hazardous material, opening can be accomplished with a reasonable effort by cutting away the closure body  19  using chiselling or other fragmenting tools. A container body opened in this way may be reused.