Abstract:
An apparatus, system, and method of storing and transferring a canister of spent nuclear fuel. In one aspect, the apparatus is a lid for a ventilated vertical overpack having a chamber for receiving spent nuclear fuel, the lid having ventilation ducts. In one aspect, the system comprises: a lid for a ventilated vertical overpack having a chamber for receiving spent nuclear fuel, the lid having ventilation ducts; and a ventilated vertical overpack having a cylindrical body including lower ventilation ducts, a bottom, and a chamber formed by the body and the bottom adapted for receiving a canister of spent nuclear fuel.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to the field of storing spent nuclear fuel and specifically to a ventilated vertical overpack for storing spent nuclear fuel. 
     In the operation of nuclear reactors, it is customary to remove fuel assemblies after their energy has been depleted down to a predetermined level. Upon removal, this spent nuclear fuel is still highly radioactive and produces considerable heat, requiring that great care be taken in its packaging, transporting, and storing. In order to protect the environment from radiation exposure, spent nuclear fuel is first placed in a canister. The loaded canister is then transported and stored in large cylindrical containers called casks. A transfer cask is used to transport spent nuclear fuel from location to location while a storage cask is used to store spent nuclear fuel for a determined period of time. 
     One type of storage cask is a ventilated vertical overpack (“VVO”). A VVO is a massive structure made principally from steel and concrete and is used to store a canister loaded with spent nuclear fuel. Typically, VVOs are cylindrical in shape and are extremely heavy, weighing over 150 tons and often having a height greater than 15 feet. VVOs have a flat bottom, a cylindrical body having a chamber adapted to receive a canister of spent nuclear fuel, and a removable top lid. 
     In using a VVO to store spent nuclear fuel, a canister loaded with spent nuclear fuel is placed in the chamber of the cylindrical body of the VVO. Because the spent nuclear fuel is still producing a considerable amount of heat when it is placed in the VVO for storage, it is necessary that this heat energy have a means to escape from the VVO chamber. This heat energy is removed from the outside surface of the canister by ventilating the VVO chamber. In ventilating the VVO chamber, cool air enters the VVO chamber through bottom ventilation ducts, flows upward past the loaded canister, and exits the VVO at an elevated temperature through top ventilation ducts. The bottom and top ventilation ducts are located circumferrentially near the bottom and top of the VVO&#39;s cylindrical body respectively. 
     Because it is imperative that the canister of spent nuclear fuel not be directly exposed to the external environment, the chamber has a pedestal situated at its bottom. When the canister is placed in the chamber for storage, the canister rests on the pedestal, ensuring that the canister is located at an elevation well above the openings of the bottom ventilation ducts. Additionally, the top ventilation ducts are positioned on the cylindrical body so that the openings are located well above the canister when the canister is resting on the pedestal inside the VVO chamber. Because the canister is not directly exposed to the external environment, the extent of radiation emanating through the ducts to the external environment is negligible. However, positioning the bottom ventilation ducts on the VVO body so that they are below the canister and positioning the top ventilation ducts on the VVO body so that they are above the canister results in an increased length of the VVO body. This increased length can result in the VVO being too tall to complete canister transfer operations inside a nuclear power plant building because the VVO will not fit through the door. As such, extra money must be spent either to construct an autonomous external canister transfer facility, or to enlarge the door of the power plant. 
     In most nuclear power plants, a canister loaded with spent nuclear fuel is transferred from a radiation pool to a VVO by a transfer cask. In transferring the loaded canister from the transfer cask to the VVO, the transfer cask is stacked atop a VVO with its lid removed so that the canister can be lowered into the VVO&#39;s chamber. During the lowering operation, the canister must pass through the elevation where the top ventilation ducts are located, creating a direct path for radiation escape. Because it is undesirable to directly expose a loaded canister of spent nuclear to the environment at any time, the openings of the top ventilation ducts on the VVO body must be closed during transfer. This is done by installing temporary shield plugs in the openings. Because these temporary shield plugs must provide ample radiation blockage, they are made of concrete and are often massive, posing logistical problems in the handling effort needed to install and remove them in the top ventilation ducts that are located at least 15 feet above the floor. For example, installing and removing four shield blocks entails eight heavy load-handling evolutions which increases the potential of a load drop mishap (and operator injury). Additionally, removal of the shield blocks following canister transfer operations increases the radiation dose exposure to the operations personnel. 
     Moreover, the potential for leakage of radiation through the top ventilation ducts during the lowering of the loaded canister into the VVO chamber is quite real, even with the shield plugs installed in the top ventilation ducts, because of the narrow crevice that must exist between the shield plugs and the top ventilation duct openings for tolerance reasons. 
     SUMMARY OF THE INVENTION 
     These problems and other are solved by the present invention which in one aspect is a lid for a ventilated vertical overpack having a chamber for receiving spent nuclear fuel, the lid having ventilation means. The lid can comprise a lid cap and a lid body wherein the ventilation means are located on the lid body. 
     Preferably, the ventilation means are one or more lid ventilation ducts. Also preferably, the lid comprises a lid shear ring and has means to secure the lid to the ventilated vertical overpack body. 
     In another aspect, the invention is a system for storing spent nuclear fuel comprising the lid as described above and a ventilated vertical overpack having a cylindrical body including lower ventilation ducts, a bottom, and a chamber formed by the body and the bottom adapted for receiving a canister of spent nuclear fuel. 
     Preferably, the lower ventilation ducts are located on the cylindrical body near the bottom and there are no upper ventilation ducts included on the cylindrical body of the overpack. Also preferably, the lid is secured to the overpack by bolts that extend through the lid and threadily engage the overpack body. 
     The system&#39;s lid can comprise a lid cap which substantially encloses the chamber and a lid body in which the upper ventilation means are located. The ventilation means in the lid of the system are preferably one or more lid ventilation ducts. Preferably, the system&#39;s lid has a lid shear ring and the cylindrical body has a top surface having a body shear ring, wherein the lid shear ring engages the body shear ring when the lid is placed on the cylindrical body, restricting lateral movement of the lid with respect to the cylindrical body. 
     Also preferably, when the canister of spent nuclear fuel is received in the overpack and the lid is secured, air within the chamber is warmed by heat from the spent nuclear fuel, cold air entering through the lower ventilation ducts and warmed air exiting through the ventilation means of the lid. 
     In yet another aspect, the invention is a method of storing spent nuclear fuel comprising placing a canister of spent nuclear fuel in the chamber of the overpack of the system described above; and securing the lid so that air within the chamber is warmed by heat from the spent nuclear fuel, cold air entering through the lower ventilation ducts and warmed air exiting through the ventilation means of the lid. Preferably, the lid is secured to the body of the overpack by bolting it thereto. Also preferably in the method, the lid has a lid shear ring and the cylindrical body has a top surface having a body shear ring, wherein lateral movement of the lid with respect to the cylindrical body is restricted when the lid is placed on the cylindrical body. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a prior art ventilated vertical overpack (“VVO”). 
     FIG. 2 is a perspective view of the prior art VVO with its lid removed. 
     FIG. 3 is a perspective view partially in section of the prior art VVO with a canister of spent nuclear fuel fully inserted therein and showing ventilation of the canister. 
     FIG. 4 is perspective view of a transfer cask being placed atop a partially shown prior art VVO with shield plugs in place. 
     FIG. 5 is a cut-away view of a transfer cask secured atop a prior art VVO showing a canister being lowered into the partially shown prior art VVO with shield plugs in place. 
     FIG. 6 is a perspective view of the top portion of a prior art VVO with temporary shield plugs being installed in top ventilation duct openings. 
     FIG. 7 is a perspective view of a transfer cask being removed from a partially shown prior art VVO after a canister has been transferred from the transfer cask to the prior art VVO. 
     FIG. 8 is a perspective view of the ventilated vertical overpack of the present invention having upper ventilation ducts in its lid. 
     FIG. 9 is a perspective view of the ventilated vertical overpack of the present invention having upper ventilation ducts in its lid and having a canister fully inserted in the chamber of the ventilated vertical overpack body. 
     FIG. 10 is a bottom perspective view of the ventilated lid apparatus of the present invention. 
     FIG. 11 is perspective view of the ventilated vertical overpack of the present invention wherein the ventilated lid comprises a lid body and a lid cap. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates prior art ventilated vertical overpack (“VVO”)  2 . Prior art VVO  2  comprises flat bottom  17 , cylindrical body  12 , and lid  14 . Lid  14  is secured to cylindrical body  12  by bolts  18 . Bolts  18  also serve to restrain lateral sliding of lid  14  with respect to cylindrical body  12  if prior art VVO  2  were to tip over. Cylindrical body  12  has top ventilation ducts  15  and bottom ventilation ducts  16 . Top ventilation ducts  15  are located at or near the top of cylindrical body  12  while bottom ventilation ducts  16  are located at or near the bottom of cylindrical body  12 . Both bottom ventilation ducts  16  and top ventilation ducts  15  are located around the circumference of the cylindrical body  12 . 
     Referring to FIG. 2, cylindrical body  12  of prior art VVO  2  forms chamber  25 . Chamber  25  is adapted so as to be capable of receiving a canister loaded with spent nuclear fuel when lid  14  is removed. 
     Referring to FIG. 3, prior art VVO  2  is illustrated partially in section with canister  13  loaded in chamber  25  for storage. Prior art VVO  2  has pedestal  19  located at the bottom of chamber  25 . When canister  13  is placed in chamber  25  of prior art VVO  2  for storage, canister  13  rests on pedestal  19 . Bottom ventilation ducts  16  are positioned on cylindrical body  12  so that canister  13  is above the elevation of bottom ventilation ducts  16  when canister  13  is placed in chamber  25 . Top ventilation ducts  15  are positioned on cylindrical body  12  so that canister  13  is below the elevation of top ventilation ducts  15  when canister  13  is placed in chamber  25 . This positioning mitigates the extent of radiation emanating through both bottom ventilation ducts  16  and top ventilation ducts  15  to the external environment. Cylindrical body  12  and portions of lid  14  are made of gamma absorbing material such a concrete  22 . 
     When lid  14  is secured to cylindrical body  12  with canister  13  resting in chamber  25 , ventilation of chamber  25  (and thus cooling of the spent nuclear fuel) must occur. In prior art VVO  2 , cold air  30  enters bottom ventilation ducts  16 , flows upward passed canister  13  removing heat energy form canister  13  by convention, and exits prior art VVO  2  as warm air  31  through top ventilation ducts  15 . 
     Referring to FIG. 4, in order to transfer loaded canister  13  from transfer cask  27  to prior art VVO  2 , transfer cask  27  is positioned above and stacked atop prior art VVO  2  by overhead crane  26 . 
     Referring to FIG. 5, transfer cask  27  has a retractable bottom  28  capable of opening so that loaded canister  13  can be lowered directly into chamber  25  of prior art VVO  2  while transfer cask  27  is stacked atop and secured to prior art VVO  2 . During this lowering operation, canister  13  will pass by top ventilation ducts  15  (FIG.  1 ), creating a direct path for radiation to escape. 
     Referring to FIG. 6, in order to block this radiation from escaping through top ventilation ducts  15  during lowering, temporary shield plugs  29  are installed in the openings of top ventilation ducts  15  before transfer cask  27  is positioned above prior art VVO  2 . Temporary shield plugs  29  are made of gamma absorbing material such as concrete. 
     Referring to FIG. 7, upon fully lowering canister  13  into prior art VVO  2 , transfer cask  27  is removed. Temporary shield plugs  29  must then be removed before lid  14  is again secured to cylindrical body  12  so that chamber  25  can be properly ventilated. Once temporary shield plugs  29  are removed, lid  14  is secured to cylindrical body  12  by bolts  18  (FIG.  1 ). 
     FIG. 8 illustrates an embodiment of the system of the present invention, ventilated vertical overpack (“VVO”)  40 . Ventilated vertical overpack  40  comprises cylindrical body  43 , bottom  44 , and ventilated lid  41 . Cylindrical body  43  and bottom  44  form chamber  46  which is capable of receiving a canister of spent nuclear fuel. Cylindrical body  43  also has lower ventilation ducts  45  located at or near the bottom of cylindrical body  43  for ventilating chamber  46 . However, unlike cylindrical body  12  of prior art VVO  2  (FIG.  1 ), cylindrical body  43  of VVO  40  does not have upper ventilation ducts located at or near its top. Instead, ventilated lid  41  has one or more lid ventilation ducts  42  for ventilating chamber  46 . 
     Referring to FIG. 9, because cylindrical body  43  does not have upper ventilation ducts, top surface  47  of cylindrical body  43  is approximately a flat horizontal surface, providing a gamma absorbing shield all around its top. As such, when canister  13  is fully lowered into chamber  46 , cylindrical body  43  does need to extend taller than canister  13  to ensure that radiation does not emanate directly into the external environment. Moreover, this flat top surface  47  eliminates the need for installing temporary shield plugs  29  (FIG. 6) during canister  13  lowering operations being that there are no openings through which radiation can escape once a transfer cask is properly secured to top surface  47 . However, as in the prior art VVO  2  (FIG.  1 .), lower ventilation ducts  45  are positioned on cylindrical body  43  elevationally below canister  13  when canister  13  is placed in chamber  46 . Body shear ring  20  is attached to top surface  47 . Body shear ring  20  has an upper surface that is also a substantially flat horizontal surface. 
     Loaded canister  13  of spent nuclear fuel is stored in VVO  40  by placing canister  13  in chamber  46  (FIG. 8) of ventilated vertical overpack  40  and securing ventilated lid  41  to cylindrical body  43  so that the air within chamber  46  is warmed by the heat of the spent nuclear fuel, cold air enters through lower ventilation ducts  45  and exits as warmed air through the lid ventilation ducts  42 . Ventilated lid  41  can also be bolted to cylindrical body  43 . 
     Referring to FIG. 10, ventilation lid  41  has four lid ventilation ducts  42  on side wall  30 . Ventilated lid  41  has a bottom surface  51  that is a substantially flat horizontal surface. Ventilation lid  41  also comprises lid shear ring  21  attached to bottom surface  51 . Lid shear ring  21  has a lower surface that is a substantially flat horizontal surface. Referring to FIG. 9, cylindrical body  43  has body shear ring  20  secured to its top surface  47 . When ventilated lid  41  is placed atop cylindrical body  43 , the lower surface of lid shear ring  21  (FIG. 10) contacts top surface  47  to form a first substantially horizontal interface. Concurrently, the upper surface of body shear ring  20  contacts bottom surface  51  of ventilated lid  41  to form a second substantially horizontal interface. As such, lid shear ring  21  and body shear ring  20  interact to restrain lateral motion of ventilated lid  41  with respect to cylindrical body  43 , even if bolts  18  are not secured. Ventilated lid  41  is secured to cylindrical body  43  by extending bolts  18  through ventilated lid  41  and threadily engaging bolt holes  48  of cylindrical body  43 . Alternatively, shear rings do not have to be used. In this alternative embodiment, the bottom surface of the lid and the top surface of the overpack body will be in direct contact so as to form a substantially horizontal interface. 
     Referring to FIG. 11, ventilated lid  41  can be constructed so that ventilated lid  41  comprises two pieces, lid cap  49  that substantially encloses chamber  46  and lid body  50  which contains the means to ventilate chamber  46 , illustrated as lid ventilation ducts  42 . Lid ventilation ducts  42  are located within side wall  30  (FIG. 10) of lid body  50 . When constructed as lid cap  49  and lid body  50 , lid cap  49  can be removed from cylindrical body  43  without removing lid body  50 . In such an embodiment, lid body  50  is an annular ring-like structure capable of being removed and secured to cylindrical body  43  independent of lid cap  49 . Alternatively, ventilated lid  41  can be constructed so that it is one rigid piece that must be removed in its entirety. 
     The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. As will be understood by those skilled in this art, the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the disclosure of the present invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.