Patent Number: 047073250
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a gauge plate for determining and measuring the actual locations of the guide pins for the lower internal structure in a nuclear reactor vessel for use in customizing a replacement upper internals structural package, and to a method of using this gauge plate. More particularly, the present invention relates to a gauge plate, and a method of using same, whereby the actual position and dimensions of the existing guide pins in the barrel of a nuclear reactor vessel for the lower internal structure can be determined so that the conventional inserts for the upper core plate which mate with these guide pins can be customized to the required close tolerances when the upper internals structural package for an existing nuclear reactor is replaced. In a liquid cooled nuclear reactor, the internal structure of the core barrel generally includes, in addition to the core itself, an upper internals structural package, which is installed and can be removed as a unit, and a lower internals structure which is axially aligned with the upper internals structural package. To ensure that the upper and lower internals structures are correctly oriented and aligned with one another, the inner surface of the core barrel is provided with a plurality, usually four, symmetrically disposed guide pins which are fixedly aligned with the lower internal structure and which extend radially inwardly from the inner wall of the core barrel and engage in respective slots formed in the peripheral surface of the upper core plate, which, in turn, forms the lower most portion of the upper internals structural package. In view of the relatively close tolerances required for the orientation of the upper and lower internal structures, it is customary to machine the surfaces of the peripheral slots in the upper core plate to general relatively large tolerances relative to the guide pins, and then to customize the upper core plate to the actual position and size of the guide pins by providing each of the perpheral slots of the upper core plate with an insert which has been finely machined to provide the desired small clearances, e.g. 0.013 cm (0.005 in.), based on actual clearance measurements. In order to make these measurements when the reactor is initially being built at the factory, the upper internals structural package or unit, or at least the portion of same necessary to properly position the upper core plate, is placed in the core barrel and properly aligned with the lower internal structure including the baffle plate arrangement which surrounds the area in which the core is located. Thereafter, the various clearances between the surfaces of the peripheral slots in the upper core plate and the adjacent surfaces of the respective guide pins are actually measured by an individual, and then the upper internals structural package is removed from the core barrel. Thereafter, the actual measurements taken are used to machine the relatively small inserts which are then positioned and fastened in the respective perpheral slots of the upper core plate so as to customize the core plate to the actual positions of the lower internal structure. Although the above technique is satisfactory when a new reactor is being built, a number of significant problems are presented when it is necessary to replace the upper internals structural package of a reactor which has been in use for some time. Initially, since the reactor vessel, including the core barrel with its guide pins and the fixed portions of lower internal structure, e.g., the baffle plate arrangement, are no longer located at the situs of the factory, it would be extremely difficult, time consuming and expensive, particularly in view of the large size and weight of even the upper core plate, which for example is in the order of 13-1400 kg (3000 lbs.) for a typical reactor, to ship the upper core plate, or a sufficient portion of the upper internals structural package, to the situs of the reactor, to take the necessary measurements, return the upper core plate to the factory for customizing, and then return the completed package to the reactor for ultimate installation. Moreover, since the reactor vessel of a previously operating reactor is somewhat radioactive and accordingly is conventionally flooded with water during the replacement or refitting period, it would be very difficult and extremely dangerous for an individual to take the necessary measurements in order to customize the new upper core plate to the existing guide pins in the core barrel. SUMMARY OF THE INVENTION It is therefore the object of the present invention to provide an apparatus for taking the measurements necessary for the customization of a replacement upper core plate, and in particular the upper core plate inserts for the peripheral guide slots, without requiring the use of the actual upper core plate for such measurements, as well as to provide a method of using the apparatus to take the measurements which overcomes the above problems. The above object is initially achieved according to the invention by a gauge plate for use in customizing replacement upper core plate inserts of a nuclear reactor of the type including a pressure vessel, a core barrel disposed within the pressure vessel, a lower internal structure disposed in the core barrel and including a baffle plate arrangement, and an upper internals structural package having an upper core plate at its lower end, with the upper core plate being provided with a plurality of peripheral grooves, each including an insert machined to close tolerances, for engaging respective approximately rectangular shaped guide pins extending radially inwardly from the inner surface of the core barrel for aligning the upper internal structural package relative to the lower internal structure; wherein the gauge plate comprises a circular metal plate of a known diameter corresponding substantially to that of the upper core plate of the nuclear reactor to be gauged; a plurality of U-shaped gauging slots formed in the peripheral surface of the gauge plate and extending between its major surfaces, with the gauging slots being formed at locations corresponding to the respective locations of the guide pins of the reactor vessel to be gauged and being of a known size sufficient to receive the respective guide pins with clearance on all sides; first means for positioning the gauge plate within the core barrel of the nuclear reactor vessel to be gauged, while it contains the baffle plate arrangement but not the upper internals structural package, at the normal elevation of the upper core plate inserts and the guide pins; second means, disposed on the gauge plate, for positioning the gauge plate relative to the baffle plate arrangement of the reactor to be gauged; gauge means for the gauge plate for determining the actual position of the gauge plate relative to the baffle plate arrangement; and, remotely controlled measuring means, disposed on the gauge plate, for measuring the respective clearances between each of the U-shaped gauging slots and the adjacent surfaces of a respective guide pin and the clearance between the peripheral surface of the gauge plate and the inner surface of the core barrel adjacent each gauging slot, and for providing an indication of the measured clearances at a remote location. Preferably, the gauge plate has a thickness substantially less than that of the upper core plate for the reactor, and is provided with cutouts in its interior portion in order to reduce its weight and to permit it to be more easily lowered through the flooding water. According to the preferred embodiment of the gauge plate according to the invention, the first means comprises a plurality of support pads disposed on one major surface of the gauge plate and positioned so as to be able to rest on the upper end of the baffle plates of the baffle plate arrangement of the reactor in which said gauge plate is to be used, with the support plates being of the thickness so as to position the gauge plate at the elevation of the interface of the upper core plate inserts and the guide pins when the pads are resting on the upper ends of the baffle plates, and the second means comprises a plurality of positioning pins extending from the above mentioned major surface of the gauge plate, with the positioning pins being located on the gauge plate at respective positions corresponding to the outer most positions of the fuel assembly top nozzles of the reactor, and with each positioning pin being of a length so that it can extend into the area enclosed by the baffle plate arrangement when the gauge plate is resting on the upper ends of the baffle plates. Moreover, in the preferred embodiment of the gauge plate according to the invention, the gauging means includes a plurality of gauging holes extending through the gauge plate for receiving gauge pins, with the gauging holes being located at positions corresponding to the expected positions of respective baffle plates of the reactor in which the gauge plate is to be used. According to the method of the invention the actual dimensions of the lower internals guide pin locations of a nuclear reactor vessel are measured by positioning a gauge plate of the type described above in a nuclear reactor vessel core barrel containing a baffle plate arrangement, but with the upper internals structural package removed, so that the gauge plate rests on the upper end of the baffle plate arrangement with the lower internal structure guide pins of the reactor vessel extending into the gauging slots of the gauge plate and with the positioning pins of the gauge plate being properly positioned relative to the baffle plate arrangement; at each of the gauging slots and with the gauge plate in the same position, (a) measuring the difference between the peripheral surface of the gauging plate and the inner surface of the reactor barrel, and (b) measuring the clearance between each of the three sides of the U-shaped gauging slot and the adjacent sides of the associated guide pin; and, with said gauge plate in the same position, determining the actual position of the gauge plate relative to the existing baffle plates by inserting gauging means into each of the gauging holes. According to the preferred embodiment of the method wherein the gauging plate utilized includes at least three gauging holes with two of the gauging holes being located at positions corresponding to the positions of two adjacent baffle plates and the third gauging hole being located at a position corresponding to a baffle plate diametrically opposite one of the two adjacent baffle plates, and with the center line of each of the gauging holes being displaced by a common given dimension from the expected position of the upper edge of the respective baffle plate in a direction perpendicular to the inner surface of the respective baffle plate, the actual position of the gauge plate relative to the baffle plates is determined by inserting gauge pins of known size into the gauging holes until the inner surface of the respective baffle plate is located. Preferably, gauge pins of known different size are successively inserted into one of the gauge holes to determine the largest diameter gauge pin which can be inserted, and then, with this largest diameter gauge pin inserted in its respective gauge hole, the sequence of successively inserting gauge pins is carried out for each of the other gauge holes. Finally, according to a preferred feature of the invention, the step of determining the actual position of the gauge plate relative to the baffle plates is carried out before the measurements are taken at the respective gauging slots.