Patent Number: 047298693
Section: description

DESCRIPTION OF THE DISCLOSED EMBODIMENT FIG. 1 depicts a typical steam generator 10 for a nuclear power plant. The steam generator has tubes 12 which are typically 3/4 or 7/8 of an inch outside diameter with a 0.05 inch wall thickness. Each of the thousands of tubes is inserted into a hole in the tubesheet face or lower surface 16. Experience has shown that the tubes 12 are prone to deterioration. Accordingly, they must be inspected and when necessary taken out of service by plugging or repaired by sleeving, through their open ends at the tubesheet face 16. Servicing personnel gain access to this space by crawling through the manway 18 and standing in the primary head 20 which may typically prove an approximate five foot clearance. Since the primary head 20 is highly radioactive, time on this task in this area is very limited but can be extended if proper radiation shielding can be provided. FIG. 2 shows a modular radiation shielding panel 22 for use on the lower side 16 of the nuclear steam generator tubesheet 14. The modular lead shielding panel 22 is typically of rectangular or other simple plain geometric shape for both ease of manufacture and ease of use. Although steam generator tubesheets and tubes supported thereby vary in dimension, a typical modular tile might be 85/8" in length and 73/8" in width. Such a lead plate conveniently is 1" thick and has a 1" diameter hole located on an axis of symmetry therewith which bisects its longest dimension. This axis is labeled 4--4 in the various figures. An asymmetric axis 6--6 perpendicular to the axis of symmetry 4--4 and also passing through the center of the transverse hole 24, may typically be located 4 5/16" from the longitudinal edge of the imaginary rectangle of lead tile 22 which makes it equidistant from both the longitudinal edge 26 and the longitudinal edge 28 of the rectangle which defines the geometric shape of the lead plate 22. The location of the hole 24 is determined by the particular size and location of tubes 12 in the tubesheet and whether they are arranged in a square pitch or a triangular pitch. In the typical pattern illustrated, for example, the 73/8" width of the lead tile 22 will shield six openings width-wise, and with its 85/8" of length will shield seven holes length-wise on the tubesheet 14. The position of tubes along the symmetrical axis 4--4 is schematically shown in FIG. 4 in the form of broken-line half circles 12' and they are separated by the tubesheet material 16 ' of tubesheet 16 lying between them. The lead tiles are arranged with the hole 24 in the location as shown such that if a previously plugged tube is encountered where the hole 24 is intended to be aligned, a quick 180.degree. rotation permits the hole to be lined up with another tube 12. Because of the softness of the lead and its propensity to be dented or otherwise misshapen by external forces and impacts, the lead tile 22 is provided with a means for protecting it. The means for protecting the modular panels 22 of lead sheathing, is a sheath of material such as stainless steel, which is harder than lead, and which is easily decontaminated. In FIGS. 5 and 6, a sheath is illustrated in the form of a box-like structure 30 slightly larger than the lead panel 22. For example, in the dimensions given, the sheath may have an outer dimension of 71/2" width, by 83/4" length, with a hole center 34 located at the corner of a square 43/8" from the outside edge of the sheath 30 to the center of the hole. The inside height of the sheath would be approximately 5/8". The hole 34 of the sheath 30, and 36 of the top 32, will thus be in register with a transverse hole 24 of the lead tile 22 when placed in the sheath or box 30 with the top 32 thereon welded or otherwise secured in place. To complete the sheathing, the box 30, after having the lead insert placed therein and the top 32 welded to its sides 38 is provided with a tube 50 having a washer 52 welded at either end. Tube 50 is inserted through the holes 34 and 36, and hole 24 in register therewith, thus completing the sheathed tile or modular radiation shielding panel assembly. The modular radiation shielding system for use on the lower side of a nuclear steam generator tubesheet 14 to protect inspection and repair workers in the primary head 20 from radiation of the tubesheet 14 and tubes 12 that it secures preferably utilizes a particular fastening means 40. The fastening means 40 extends through the transverse openings 24, 34 and 36 in register, and a tube 12 aligned therewith, thereby releasably fastening the modular panel to the tube sheet face 16. The "Rapid Installation Tube Gripper" 40 can be rapidly attached inside the end of the tube 12 which is held by means of weld 42 in tubesheet 14. The tube engaging balls 44 are manipulated by means of a flange 46 secured to an internal sleeve in a manner fully disclosed in the previously referenced U.S. patent application Ser. No. 686,114 filed Dec. 24, 1984. It is sufficient for this disclosure to state that the locked and released positions of the balls 44 and tapered shaft 48 are maintained by the internal sleeve to which flange 46 is attached until the device 40 is inserted into or removed from the tube. It will be understood by those skilled in the art that upon the installation of a plurality of the modular radiation shielding panels according to the system described herein, there will be areas within particular tubes 12 which the rapid installation tube grippers 40 are filling which will not include any shielding from radiation. This is because there will be no lead between the workers under the lead shielding panel assemblies and the inside surface of the tube. To alleviate this condition and to further insure maximum shielding, the projecting tapered end 48 of the rapid installation tube gripper 40 can be lead filled, thus creating a lead finger portion which prevents radiation from the tube 12 "to shine" into the primary head 20, to any appreciable degree, from the otherwise unshielded tube containing the gripper 40. While the foregoing description is of a system to be installed on the lower surface of a tubesheet in a nuclear reactor steam generator, it is obvious that the invention can be used in many other applications that require lead shielding. The adaptability of the modular concept, both from a standpoint of ease of manual handling of the shielding assemblies, and from the standpoint of being able to be quickly installed and removed in various confined areas makes the modular radiation shielding system of the instant invention particularly useful whether provided in the disclosured embodiment or in equivalent structures.