Patent Number: 047073250
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, and in particular FIG. 1, there is shown an elongated, generally cylindrically shaped nuclear reactor vessel 1 of conventional design for use in a pressurized water nuclear reactor system. Vessel 1 has the usual hemispherical bottom 3, at least one cooling water inlet nozzle 5, and at least one cooling water outlet nozzle 7. As is conventional, vessel 1 forms a pressurized container when sealed at its open and 9 by a head assembly (not shown). Disposed within the vessel 1 is a cylindrical core barrel 11 which is suspended from an inwardly extending flange 13 of vessel 1. Core barrel 11 includes a bottom forging 15 having a plurality of projections 17 disposed about its circumference for engaging a corresponding number of key members 19 on the vessel 1 for stablizing the position of the core barrel 11 in the circumferencial and radial directions. Mounted within the core barrel 11 near its lower end, and connected to the inner wall of the core barrel 11 by connecting elements 21, is a lower core plate 23 on which the nuclear reactor core (not shown) and the lower internals structure of the reactor normally rest. Of the lower internals structure, the only element shown in Figure 1 is the baffle plate arrangement or baffle structure 25 formed of a plurality of individual interconnected baffle plates 27. The baffle plate structure 25 is connected to the core barrel 11 by a plurality of separater plates or spacers 29, and normally contains and surrounds the nuclear reactor core. As is more clearly shown in FIG. 2, the baffle plate structure 25 has a shape corresponding to the arrangement of the generally rectangular configurations of the fuel rod assemblies which make up the nuclear reactor core. Normally, the space within the core barrel 11 above the baffle plate arrangement 25 contains the upper internals structural unit which, in a conventional manner, is simply suspended within the core barrel 11 in that its upper end, which is constituted by an upper support plate (not shown) is supported on the upper flange 31 of the core barrel 11. To accurately align the upper internals structural unit with the lower internals in both the circumferential and radial directions, the upper core plate (not shown in FIG. 1), which forms the lower end or bottom of the upper internals structural unit, is provided with a plurality of symmetrically disposed peripheral grooves, which engage with a like plurality of radially inwardly directed guide pins 33 fastened to the inner wall of the core barrel 11 at the normal elevation of the upper core plate. The guide pins 33 are fixed to the inner wall of the core barrel 11 so that they have a defined relationship to the lower internals structure of the reactor vessel, and in particular a defined relationship to the baffle structure 25. As indicated above, in order to provide compensation for the various possible machining and positioning tolerance values and in order to provide accurate alignment between the upper internals structural unit and the lower internals structure, the peripheral guide or alignment grooves in the large upper core plate are machined to relatively large tolerance values, and a respective relatively small insert, which is machined to the actual required tolerance values on the basis of actual measurements, is provided and fastened in each of the peripheral guide slots or grooves and forms the actual bearing or guide surfaces between the guide pins 33 and the upper core plate. Such an insert is shown, for example, in FIG. 3. As shown in FIG. 3, the upper core plate 35 has a peripheral groove 37 which extends between the two major surfaces of the core plate 35. The dimensions of this groove 37 are substantially larger than the dimensions of the corresponding guide pin 33 (FIG. 1) in both the circumferential and the radial directions. To provide the close tolerances required, a core plate insert 39 is secured within each groove 37 by means of a flange portion 41 which extends laterally beyond the dimensions of the groove 37 along the bottom surface of the upper core plate 35 and is fastened to same by means of four screws 43. The remaining portion of the core plate insert 39, which is actually within the groove 37, extends upwardly from the flange portion 41 between the two major surfaces of the upper core plate 35 and is provided with a U-shaped groove formed by two accurately machined guide surfaces 45 and 49 whose dimensions are determined on the basis of actual measurements of a respective guide pin 33 relative to a respective slot 37. The two opposed surfaces 45 and 49 are dimensioned to bear against the side surfaces of generally rectangular shaped guide pin 33 so as to accurately position the core plate 35 in the circumferential direction, while similar surfaces oriented at 90.degree. to the surfaces 45 and 49 serve to accurately position the upper core plate 35 in the radial direction. Of course, although only one such peripheral guide groove 37 with its insert 39 is shown, it is to be understood that the upper core plate 35 has a plurality of such grooves 37 which are symmetrically disposed about its circumference, and in particular contains four such guide grooves 37 and inserts 39 disposed on the orthogonal or cardinal axes of the upper core plate 35, i.e. each groove 37 is displaced from the two adjacent grooves by 90.degree. relative the center of the plate 35. To take the measurements necessary to customize the inserts 39 for a replacement upper core plate, according to the invention, a gauge plate 51, as shown in section in FIG. 1 and in plan view in FIG. 2, is provided, with the gauge plate 51 being formed of metal, for example, of 304 stainless steel. The gauge plate 51 has an outer diameter which corresponds to that of the upper core plate which is to be replaced, but has a substantially reduced thickness in order to reduce the weight of the gauge plate 51 during use. For example, whereas the actual upper core plate may have a thickness of 7.62 cm (3 in.), the gauge plate 51 may have a thickness in the order of 1.9 cm (3/4 in.). Moreover, in order to further reduce the weight of the gauge plate 51 and so as to permit it to be more easily lowered through the body of water in which the reactor vessel of FIG. 1 is normally submerged during the replacement of refitting period, the interior portion of the circular engaged plate 51 is provided with a number of cut out sections 53. As shown in FIG. 2, the gauge plate 51 is provided with four U-shaped peripheral gauging slots 55 which are located on the cardinal axes of the gauge plate 51 and are positioned to coincide with the locations of the guide pins 33 for the lower internal structure. The gauging slots 55 are of a known size and location relative to one another and sufficiently wide so that the guide pins 33 can enter these gauging slots 55 with sufficient clearance. For example, the slots 55 may be approximately 7.16 cm (2.82 in) wide which, in a typical installation, would allow for an approximately 0.15 cm (0.06 in) radial gap on each side of the respective guide pin 33. Since, as indicated above, the gauge plate 51 is substantially thinner than the upper core plate, it is necessary to provide some arrangement for postioning the gauge plate 51 at the elevation of the guide pins 33 within the core barrel 11. According to the preferred illustrated embodiment of the invention, this is achieved by providing the lower surface of the gauge plate 51 with a plurality, and preferably at least three, of pads 57 which are formed of metal, for example, stainless steel, and which are positioned so that they will rest on the top ends of the baffle plates 27 when the gauge plate 51 is inserted into the core barrel 11. The pads 57 are of a thickness, for example, 2.86 cm (11/8 in), sufficient to cause the gauge plate 51, and in particular the gauging slots 55, to be at the elevation of the guide pins 33 when the pads 57 are resting on the top surface of the baffle plate structure 25 as shown in FIG. 1. In addition to generally positioning the gauge plate 51 relative to the guide pins 33, it is likewise necessary to accurately position the gauge plate 51 relative to the baffle plate arrangement 25. For this purpose, the gauge plate 51 is provided with a plurality of positioning pins 59 which extend downwardly perpendicular to the lower major surface of the gauge plate 51, i.e. the same major surface containing the pads 57. The positioning pins 59 are located on the plate 51 at positions corresponding to the outboard or outer most positions of the fuel assembly top nozzle locations in the upper core plate so as to simulate such positions, and are of sufficient length, for example 10.16 cm (4 in) so that they can extend into the area delimited by the baffle plate arrangement 25 when the gauge plate 51 is resting on the top or end surfaces of the baffle plates 25. As shown, eight such positioning pins 59 are provided in four pairs, with two pairs of positioning pins 59 being diametrically oppositely disposed along each of the cardinal or orthogonal axes of the gauge plate 51, and with the positioning pins 59 of each pair being symmetrically disposed with respect to its associated cardinal axis. As shown in FIG. 2, the eight positioning pins 59 are in fact disposed in the respective outer most corners 61 formed between two adjacent baffle plates 27, with the individual positioning pins 59 being located on the gauge plate 51 so that they will, based on the original drawings of the particular nuclear reactor being gauged, provided an expected very small nominal clearance, for example in the order of 0.028 to 0.03 cm (0.011 to 0.014 in) with each of the associated baffle plates 27 forming the respective corner 61. Finally, in order to determine the actual position of the existing baffle plate arrangement 25, relative to the gauge plate 51, the gauge plate is provided with a plurality of gauging holes 63, which are located at positions corresponding to the expected actual as-built locations of respective baffle plates 27. The gauging holes 63 extend completely through the gauge plate 51 and are of a sufficient diameter so that a gauging device can be inserted through each of the respective gauging holes 63 to accurately locate the actual position of the inner surface of the associated baffle plate 27. Although it is possible to provide such gauging holes 63 for a substantial plurality of the individual baffle plates 27, preferably a minimum of three such gauging holes 63 distributed as shown in FIG. 2 are provided. In particular, as shown in FIG. 2, two of the gauging holes 63' and 63" are associated with the individual baffle plates 27 forming one outer most corner 61, while third gauging hole 63 is associated with the baffle plate 27 diametrically opposed to one of the two baffle plates 27 associated with the pair of gauging holes 63' and 63". Preferably, as shown, the single gauging hole 63 is associated with a radially extending baffle plate 27. According to the preferred embodiment of the invention, the gauging device used with the gauging holes 63 in order to determine the actual position of the inner surface of the respective baffle plates 27 is a step gauge pin 65 as shown in FIG. 4, which includes a cap 67, a collar 69 of the same diameter as the gauging hole 63, and a lower pin portion 70 of a know gauging diameter. To take the measurements, a plurality of pins 65 with different known diameter portions 70 are provided. Alternatively, a single step gauge pin 65 with a plurality of successive different diameter portions 70 may be utilized. In any case, in order to utilize the step gauge pin 65 as a go, no-go type gauge, each of the gauging holes 63 is positioned relative to its associated baffle plate 27 so that the center line of the gauging hole 63 is displaced or offset by a given known amount from the expected location of the inner surface of the associated baffle plate 27 in a direction perpendicular to this inner surface. More particularly, the center line of each gauging hole 63 is offset by a distance equal to the radius of a desired size step gauge pin portion 70 so that if, for example, the center line of the gauging hole 63 is offset by 1.27 cm (0.50 in), the gauge plate 51 will be in the optimum or best position relative to the associated baffle plate 27 if a gauge pin with a portion 70 having a diameter of 2.54 cm (1 in) can be inserted into a gauging hole 63, but a gauge pin with a diameter of 2.67 cm (1.05 in) cannot be inserted. To utilize the above described gauge plate 51 for its intended purpose, the gauge plate 51 is lowered into the core barrel 11, for example, by means of an overhead crane, until its pads 57 rest on the upper end surfaces of the baffle plate arrangement 25. During the lowering procedure the gauge plate 51 is oriented so that, when the plate 51 is at rest the respective guide pins 33 extend into the respective gauging slots 55 and the plurality of positioning pins 59 extend into the respective corners 61 formed by adjacent baffle plates 27. Thereafter, the desired measurements at each of the gauging slots 55 and at each of the gauging holes 63 is carried out with the gauge plate 51 in the same position. Preferably, in order to clearly establish a reference position for the gauge plate 51 relative to the baffle plate arrangement 25, the measurements at the respective gauging holes 63 are carried out before the measurements at the respective gauging slots 55. More specifically, at each of the gauging holes 53, a step gauge pin 65 with the standard or desired diameter portion 71 is inserted into a respective gauging hole 63, and if it can be inserted, an attempt is made to insert the next larger diameter step gauge pin 65 until the step gauge pin with the largest diameter which can be inserted into the gauging hole 67 is determined and noted. This procedure is repeated for each of the gauging holes 63 with the largest diameter step gauge pin 65 which can be inserted preferably being allowed to remain in the respective gauging holes 63 so as to firmly fix the position of the gauging plate 51 relative to the baffle plate arrangement 25. Thereafter, the desired measurements are taken at each of the gauging slots 55. As illustrated in FIG. 5, four measurements are taken at each of the gauging slots 55. More specifically, using the known outer diameter of the gauge plate 51 as a reference, the inner diameter of the core barrel 11 at the elevation where it interfaces with the upper core plate is determined by measuring the gap or distance 71 between the periphery of the gauge plate 51 and the inner surface of the core barrel 11. Additionally, the two radial gaps 73 and 75 between the respective radially extending surfaces 77 and 79 of the generally U-shaped gauging slot 55 and the respective facing adjacent side surfaces of the guide pin 33 are measured and recorded. Finally the circumferential gap 81 between the circumferential gauging surface 83 of the gauging slot 55 and the end surface of the guide pin 33 is measured and recorded. The measurement of the gaps 71, 73, 75 and 81 are carried out at each of the four gauging slots 55 of the gauge plate 51. Since all of the dimensions of the gauge plate 51 are accurately known, the measured data, as indicated above, can be used to customize the replacement upper core plate inserts so that the replacement upper internals structural unit will be compatible with the existing lower internals structure in an operating nuclear reactor plant. Moreover, as can easily be appreciated, since all of the dimensions of the gauge plate 51 are accurately known prior to the measurements, the gauge plate 51 itself need not even be returned to the factory where the replacement parts are being manufactured. That is, all that need be sent to the factory is the results of the various measurements. As indicated above, during the replacement or refitting period of the nuclear reactor, as well as during the measuring process with the gauge plate 51, the reactor vessel 1 is normally submerged in water in order to provide protection against radioactivity. Accordingly, it is desirable for all of the measurements indicated above to be controlled and/or read out from a remote location. This can be accomplished, for example, by means of remotely controlled robot arms (not shown) as are well known in this art and/or by conventional remotely controlled linear measuring instruments disposed on the gauge plate 51 or on a remotely controlled robot arm. According to the preferred mode of carrying out the method according to the invention, the insertion of the step gauge pins 65 is carried out by a remotely controlled robot arm, while the respective measurements of the gaps 71, 73, 75 and 81 at each gauging slot 55 are carried out by respective sets of four measuring devices which are appropriately mounted at known positions on a major surface of the gauge plate 51. More particularly, as shown in FIGS. 1 and 2, respective sets of measuring devices 85, 87, 89 and 91 are mounted at known positions on the upper surface of the gauge plate 51 adjacent each of the gauging slots 55. Each of the measuring devices 85, 87 and 89 is disposed adjacent, and has its longitudinal axis perpendicular, to a respective one of the surfaces 77, 83 and 79 of the associated slot 55 so as to measure the respective gaps 73, 81 and 75 between the respective slot surfaces and the adjacent surfaces of the guide pin 33. The remaining measuring device 91 of each set is disposed adjacent the peripheral surface of the gauge plate 51 and has its longitudinal axis extending in a raidal direction so as to measure the gap 71 between the peripheral surface of the gauge plate 51 and the adjacent inner surface of the core barrel 11. The measuring devices 85, 87, 89 and 91 may, for example, each simply be a spring loaded plunger which, when released from a remote location, will move forward until it rests against a respective one of the three measuring surfaces of the associated guide pin 33 (devices 85, 87, 89) and against the inner surfaces of the core barrel 11 (device 91) and reamin locked in that position. The actual dimensions of the various gaps are then determined by manually measuring the positions of sixteen plungers, for example, by means of a micrometer, after the gauging plate 51 has been removed from the reactor vessel 1. Preferably, however, measuring devices 85, 87, 89, 91 are provided which can be remotely controlled and will additionally produce a direct remote readout of the measured clearances. For example, each of the measuring devices 85, 87, 89 and 91 can be a spring loaded plunger which is provided with an electrical position sensor which produces an electrical signal proportional to the distance moved by the respective plunger and which is individually connected to a remotely located control and indicator circuit 93 via respective wires of a multi-conductor cable 95 (shown schmatically in FIG. 1). It is of course understood, that the position of the circuit 93 in FIG. 1 is schematic only and in reality would be located outside of the reactor vessel 1 and at a safe distance from same. It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.