Patent Number: 050376047
Section: summary

CROSS REFERENCES TO RELATED APPLICATIONS This application and the following co-assigned, co-pending U.S. patent applications relate to reactor vessel annealing: U.S. Pat. application Ser. Nos. 07/368,454 entitled "Modular Annealing Apparatus For In Situ Reactor Vessel Annealing And Related Method of Assembly," Bauer and Mavretish; 07/368,433 entitled "Water Filled Tanks For Temporary Shielding of Reactor Vessel Internals and Method of Assembly," Grimm and Sejvar; and 07/368,503 entitled "Annealing Unit Insertion and Removal System," Reiner and Kordaliski. BACKGROUND OF THE INVENTION This invention relates to shielding radioactivity emanating from reactor vessel stored internals and, more particularly, to temporarily shielding personnel from the stored internals during operations such as in situ reactor vessel annealing. DESCRIPTION OF THE PRIOR ART At various times during the operational life of a nuclear reactor, there is a need to remove the fuel and internals from the reactor vessel. Such instances include inspections, reactor vessel annealing, etc. In particular regard to annealing, as a result of constant bombardment by neutron radiation during a nuclear reactor's operation over time, the metal in certain portions of a nuclear reactor vessel tends to lose its ductility and becomes brittle. This decreases the safety margin when cold water must suddenly be introduced into the vessel cooling system as a result of a malfunction. Brittleness, consequently, is a major concern impacting the safe operating life of the reactor vessel. It is known that annealing can restore the ductility of the metal so that the useful life of the reactor can be safely prolonged. Annealing is a process of heating and cooling a substance such as steel to make the steel less brittle. In order to avoid the creation of additional stresses, a major portion of the reactor vessel must be annealed. To perform annealing, the nuclear reactor is shut down, the reactor vessel is opened, the fuel is stored in a fuel storage pit and the reactor upper and lower internals are stored. Due to the extremely high radiation levels of the stored internals and the reactor vessel, they are stored underwater in a refueling canal to significantly reduce the personnel radiation exposure during the operations involved in the installation and use of the annealing apparatus. In order to place the annealing apparatus in the reactor vessel and to allow workers to make the necessary heater connections and actuate thermocouples used for temperature sensing, the region directly above the reactor vessel should be dry. Further, water must eventually be removed from the reactor vessel to reach the required annealing temperature. Removal of this water would remove the radiation shielding from the stored reactor internals in fluid communication with the reactor vessel, thereby endangering the personnel. It is known that the radiation emitted from the stored internals can be blocked using temporary shielding in the form of a coffer dam, as described in EPRI, Jan. 1983, P-2712, entitled "Feasibility of Methodology for Thermal annealing an Embrittled Reactor Vessel," and NUREG/CR 42/2, EGG-125--6708, Apr. 1985, entitled "In Place Thermal Annealing of Nuclear Reactor Pressure Vessels." More particularly, a unitary cylindrical coffer dam is seated directly on the bottom of the refueling canal adjacent the vessel and extends to the top of the refueling canal. By flooding the canal, the internals are shielded by the water and the annealing apparatus can be introduced into the reactor vessel through the top of the coffer dam. However, with this method, water leakage is known to occur into the reactor vessel at the interface of the coffer dam and the bottom of the refueling canal. This seal is weak because of the uneven surface of the bottom of the refueling canal and because this type of shielding merely relies on the weight of the coffer dam to create the seal clamp force. Further, the conventional coffer dam, which is fabricated in a factory and intended for a large commercial reactor vessel, is very difficult to transport due to its size since containers suitable for truck or train transport are limited in size. An additional feasibility concern of the conventional coffer dam includes the ability to get a coffer dam as big in diameter as the reactor vessel flange through a containment building equipment hatch. The equipment hatch opening is significantly smaller than the diameter of the reactor vessel, making entry of a one-piece coffer dam impossible without major modification to the containment structure. There is a strong desire to keep the containment structure as intact as possible. Further, any modification to the containment building would require rigorous requalification and verification of the containment's integrity, which is a very costly and time-consuming activity and would, most likely, require Nuclear Regulatory Commission approval. Because of the problems inherent in the abovedescribed conventional coffer dam, some efforts at temporary shielding have been directed away from coffer dams. For example, in the above-referenced application entitled "Water Filled Tanks ..." a set of vertical, water-filled tanks interposed between the stored internals and the reactor vessel, and a set of horizontal tanks placed above the stored internals and the vertical tanks, are described. However, such tanks are not necessarily suitable for all reactor designs, such as where very limited space exists between the reactor vessel and the stored internals in the refueling canal. In light of the above a need exists for reliable and leak-proof temporary shielding of stored internals, which allows the reactor vessel to be drained of water, is applicable to various reactor designs, allows a safe work place for personnel, and which lends itself to a relatively easy method of assembly. SUMMARY OF THE INVENTION Accordingly, it is a purpose of the present invention to provide a coffer dam pre-fabricated in segments in a factory, which segments can be easily transported to a nuclear reactor, introduced individually into the hatch of the containment building, assembled as a complete coffer dam and installed on the reactor vessel flange. It is another purpose of the present invention to provide a coffer dam which reliably seals the interior of the reactor vessel from the water-filled refueling canal. It is another purpose of the present invention to provide a coffer dam which can be easily disassembled and removed from the containment building, and a related method for disassembly and removal. It is another purpose of the present invention to provide a coffer dam and a related method of assembly which allow the coffer dam to be reused at a plurality of reactors. It is another purpose of the present invention to provide a method of assembling a coffer dam requiring a minimum of personnel involvement in the containment building. To achieve the foregoing and other purposes of the present invention, there is provided a coffer dam assembly made up of a plurality of vertical and/or horizontal cylindrical segments small enough to pass through the equipment hatch of the containment building. The segments are manufactured at a factory, transported to the containment building and introduced individually into the containment building via the containment hatch. The segments are then assembled at a low radiation area of the operating floor into a total coffer dam using a minimum of effort and personnel intervention. Seals are placed between the edges of each of the segments and along a bottom flange of the coffer dam. The edges of the segments are bolted together and the completed coffer dam is positioned on and bolted directly to the reactor vessel flange in sealing relation. The bolting can be accomplished, either before or after the annealing apparatus is inserted into the reactor vessel, using long handled tools while the refueling canal is flooded. A cover is connected in sealing relation to the top of the coffer dam to control airborne radiation during annealing and is hinged to allow personnel entry when needed. The method invention includes the steps of: pre-fabricating the segments at a factory; transporting a plurality of the segments to the reactor; introducing each segment through the hatch of the containment building; assembling the plurality of segments in sealing relation into a coffer dam; and installing the coffer dam in sealing relation to the reactor vessel flange. Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.