Abstract:
A method of packaging a nuclear reactor vessel for decommissioning and removal, wherein closure plates are installed onto the vessel, concrete is injected into the vessel, shielding material is installed around the exterior of the vessel and the main nozzles of the vessel, the installed shielding materials are welded to themselves, the vessel is placed on shipping cradles and attached to longitudinal restraint mechanisms for transport.

Description:
FIELD OF THE INVENTION 
     This invention relates to a method of packaging a nuclear reactor vessel for decommissioning and removal, and more particularly, to a method wherein low density concrete (in a wet mixture) is injected into the vessel and external radiation shielding of different thicknesses is mechanically attached to the vessel and then welded to itself to reduce the cost of removal of a reactor vessel. 
     BACKGROUND OF THE INVENTION 
     Various methods for disposing of nuclear reactor pressure vessels (“RPV&#39;s”) exist. As described in  American Nuclear Society Transactions , (November 1999), RPV&#39;s may be disposed of by segmenting the RPV into small pieces and placing the pieces into liners and shielded casks for transport to disposal sites or by placing an entire RPV inside a shielded transport cask. 
     These methods are extremely costly and are not always suited for the disposal of large full-size (&gt;900 MW(electric)) RPV&#39;s. For example, the known methods may result in high fabrication and transportation costs, high grouting, shielding and burial site disposal volumes, and often high worker radiation doses. 
     It is therefore an object of the present invention to provide a method of packaging a nuclear reactor vessel for decommissioning and removal which reduces cost and which can be implemented on large full-size RPV&#39;s without incurring the foregoing disadvantages. 
     SUMMARY OF THE INVENTION 
     The object of the invention can be attained and the disadvantages of the prior methods can be overcome by providing a method of packaging a nuclear reactor vessel for decommissioning and removal, including the steps of: installing reactor vessel permanent closure plates onto the vessel; injecting concrete into the vessel; installing a first ring of shielding material around the main nozzles of the vessel; enclosing the vessel core area with a second shielding ring; welding longitudinal seams of the first shielding ring; welding longitudinal seams of the second shielding ring; welding the second shielding ring to the first shielding ring; placing the vessel on shipping cradles; and tightening a longitudinal restraint mechanism to the vessel. The method can also include the step of installing impact limiters on each end of the vessel. 
     The concrete injected into the vessel can be wet, low density cellular concrete or the like with a density between 0.721 g/cm 3  to 1.041 g/cm 3  and can be prepared with foaming agents and curing additives on the site where the decommissioning is to take place. The concrete is allowed to harden prior to final closure and sealing of all the reactor openings. 
     The method of packaging the nuclear reactor vessel also includes the step of circulating air into the vessel to remove heat from inside the vessel, which is performed prior to the step of injecting concrete into the vessel. In addition, prior to installing the first shielding ring, the method of packaging a nuclear reactor vessel for decommissioning and removal can include the steps of: allowing the vessel to vent and cool; removing temporary closure plates; verifying that the vessel includes a requisite amount of the concrete; verifying that there are no empty spaces in the vessel; and confirming that no free standing water is in the vessel. 
     The first and second shielding rings can preferably be of steel, or the like, and in a preferred embodiment have respective thicknesses sized to provide the requisite amount of radiation shielding. The second shielding ring can be applied to the vessel by lowering the vessel into the second shielding ring and mechanically fastening the second shielding ring to the vessel. The closure plates are preferably made of steel and can be welded to the vessel. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the invention will become apparent upon review of the following detailed description of preferred embodiments, taken in conjunction with the following drawings, in which: 
     FIG. 1 is a perspective view of an example of a reactor pressure vessel which is to be dismantled and removed according to an embodiment of the present invention; 
     FIG. 2 is a cross-sectional view of a reactor pressure vessel taken along line A—A of FIG. 1; 
     FIG. 3 is a cross-sectional view of a reactor pressure vessel taken along line A—A of FIG.  1  and prepared according to an embodiment of the present invention; 
     FIG. 4 is a schematic view of reactor pressure vessel shielding details according to an embodiment of the present invention; 
     FIG. 5 is a cross-sectional view of a reactor pressure vessel taken along line A—A of FIG. 1 containing low density cellular concrete according to an embodiment of the present invention 
     FIG. 6 is a schematic view of an incore closure plate; and 
     FIG. 7 is a schematic view of the reactor pressure vessel reactor pressure vessel of FIG. 1 on its side and resting in its shipping cradles. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in FIGS. 1 and 2, an exemplary RPV  100  is ellipsoid in shape and includes a vessel shell  101 , an upper head  102 , an upper instrumentation and support assembly  103 , a pair of lifting lugs  104 , a pair of head studs  105 , O-ring seals  106 , an upper support plate  107 , control rod guide tubes  108 , an upper core plate  109 , a lower core plate  110 , a lower core support plate  111 , a lower core support column  112 , a core barrel  113 , neutron shield pads  114 , former plates  115 , core baffle plates  116  and an interior surface  117  which is usually clad with a thin stainless steel liner. 
     FIGS. 3,  4  and  7  show an RPV  100  that has been prepared for decommissioning and removal according to the method of the present invention by attachment of shielding plates. The RPV  100  is clad with a two inch thick shield plate  201  surrounding the upper portion of the vessel shell  101  around the outlet and inlet nozzles  204 ,  205 , a five inch thick shield plate  202  surrounding the middle portion or core area of the vessel shell  101 , a one inch thick shield plate  203  around the lower portion of the shell  101  (beneath the lower core support plate  111 ), outlet nozzles  204 , inlet nozzles  205 , a drain  206 , incore penetrations  207 , a head vent  208 , Control Rod Drive Mechanism (“CRDM”) penetrations  209 , flange monitoring tubes  210 , a transport cradle  212  and longitudinal restraint jaws  213  which are tightened to hold the RPV  100  in place during transport. Impact limiters  211  can be installed on each end of the RPV  100  to prevent knocking or bumping of the RPV  100  during transport. 
     As shown in FIGS. 3 and 6, outlet and inlet nozzles  204 ,  205  are covered by respective closure plates  404  and  405 . Similarly, a drain opening is covered by a closure plate  406 , an incore opening  207  is covered by a closure plate  407 , a head vent opening  208  is covered by a closure plate  408 , a CRDM opening  209  is covered by a closure plate  409  and a flange monitoring tube  210  is covered by a closure plate  410 . The closure plate material can be ASME SA-240, Type 304L, ASME SA-516 GR. 70, or any suitable alloy providing requisite radiation shielding and welding characteristics. The closure plates  404 - 410  are welded to the RPV  100  so that the closure plates  404 - 410  cover their respective openings  204 - 210 . 
     FIG. 4 provides details of shielding on an RPV  100  and illustrates typical longitudinal seam welds  501  between each respective two inch thick shield plate  201  or between each respective five inch thick shield plate  202  and circumferential seam welds  502  between a two inch thick shield plate  201  and a five inch thick shield plate  202 . FIG. 4 also illustrates typical longitudinal seam closure plates  503  and typical circumferential seam closure plates  504 . Longitudinal seam welds that do not use the closure plates  503  are also used on some of the seams. 
     FIG. 5 shows a cross section of the RPV  100  housing Low-Density Cellular Concrete (LDCC)  600 . The void space inside the RPV  100  should be limited to &lt;15% of the total volume of the RPV  100 . LDCC  600  is used to fill the RPV  100 . LDCC  600  is a heterogeneous mixture of organic surfactants/admixtures, portland cement, water and air and is sensitive to overpressurization. For an approximately 42 foot RPV 100,  130  megagrams (Mg) of 10° C. liquid LDCC  600  can be injected into the drained RPV  100  (weighing ˜907 Mg) still positioned in the vertical position. 
     Due to the high internal metal temperature, caused by component decay heat of the reactor vessel, compensating action should be taken to cool the inside of the RPV  100  prior to injection of the LDCC  600  into the vessel. A grout chiller system circulates air from outside the containment region into the RPV  100  to remove heat from the inside of the RPV  100 . Before exiting the containment region, exhaust air passes through a cooling coil, high efficiency particulate air (HEPA) filters, and an exhaust fan. The grout chiller system is put into operation prior to the injection of LDCC  600  into the RPV  100  and can lower RPV  100  internal metal temperatures to &lt;75° C. The LDCC  600  is prepared in batches on-site using special foaming agents and curing additives, pumped into the containment building and routed to proper RPV  100  injection ports. The density of the LDCC is within a safety range of 0.721 to 1.041 g/cm 3 . 
     Prior to installing the shield plates  201 ,  202 , the RPV  100  is allowed to vent and cool. In addition, closure plates, for example the drain closure plate  406 , can be removed to verify that the RPV  100  includes a requisite amount of the LDCC  600 , that there are no empty spaces in the RPV  100 , and that no free standing water is in the RPV  100 . 
     The two inch thick shield plate  201  is mechanically fastened around the main nozzles of the RPV  100 . The  5  inch thick shield plate  202  is mechanically closed around the RPV  100 . The 5 inch thick shield plate  202  can be mechanically closed around the RPV  100  by lowering the RPV  100  into a void enclosed around its circumference/perimeter by the 5 inch thick shield plate  202  and subsequently mechanically fastening the 5 inch thick shield plate  202  to the RPV  100 . The mechanical fasteners are removed after the shield plates are welded as described below. 
     The shield plates  201 ,  202  are not welded to the RPV  100  so as to avoid exposing workers to high radiation doses and so that the integrity of the RPV  100  is not compromised. Two inch and five inch shield plate  201 ,  202  vertical seams are welded and then the five inch shield plate  202  is welded to the two inch shield plate  201  on the circumference as shown in FIG.  11 . The shield plate material can be steel or the like. 
     As shown in FIG. 7, the RPV  100  may now be removed and placed on its side in preparation for removal and disposal. The final step is to transport the RPV  100  to a burial site by means of a barge or the like. The RPV  100  package can be buried in a trench or the like. 
     While this invention has been described in terms of specific embodiments, this invention, including this disclosure and appended claims, is not so limited and is to be construed in accordance with the full spirit and scope of the invention including alternatives and modifications made apparent to those of skill in the art.