Patent Number: 048308145
Section: description

DETAILED DESCRIPTION The present invention is an improved integrated head package for a nuclear reactor which includes a lifting rig and missile shield assembly that transfers the momentum of postulated head missiles into a lifting motion of the lifting rig and missile shield assembly and thus prevents missile generated loads from being transferred to the lift rods, head lugs, closure studs and reactor vessel supports. Referring now to FIGS. 2 to 8, the integrated head package 41 for an advanced pressurized water reactor system is adapted for sealing of a reactor vessel and has a pressure vessel closure head 43, sealable to the reactor vessel by use of closure studs 44, with control rod drive mechanisms 45 and displacer rod drive mechanisms 47 which pass through and extend above the hemispherically shaped portion 49 thereof. A shroud 51 surrounds the drive mechanisms 45 and 47 of the control rods and displacer rods and rests on shroud support pads 53 on the closure head 43. A plurality, preferably three, equiangularly, circumferentially spaced vertical lift rods 55 are provided which are attached at their bottom ends 57 by means of a clevis 59 and pin assemblies 61 to head lift lugs 63 which are permanently secured to the closure head 43. The lift rods 55 form an integral part of the closure head lifting assembly such that the closure head 43 and all associated operative components can be removed from the reactor pressure vessel to a storage position, not shown, during a refueling operation, as well as for re-installation on the pressure vessel following refueling. Adjacent the top ends 65 of the lift rods 55 is a vertically slidably retained missile shield assembly 67 which includes a flat, solid, preferably circular, missile shield plate 69 having apertures 71 adjacent only to the periphery thereof through which the lift rods 55 extend. The missile shield plate 69 extends across the area between the lift rods 55 and is vertically slidably retained at the upper portion 65 of the lift rods 55 between spaced stop members 73, 73', illustrated as a lower flanged member 75 fixed to the lift rod 55, and an upper nut 77 threadedly secured to a threaded section 78 of the lift rod. On the missile shield plate 69 (FIGS. 4 and 7) are equiangularly, circumferentially spaced support blocks 81 having a wide section 83 and outwardly extending arm 85, the support blocks 81 being permanently secured, such as by welds 87 to the upper surface 79 of the missile shield plate 69. The arms 85 have apertures 89 therethrough which are aligned with apertures 71 in missile shield plate 69 and through which the upper portions 65 of the lift rods 55 also extend. The stop members 73, 73' are spaced vertically from each other on the lift rod 55 such that a gap 91 is present between the upper surface 93 of the arm 85 and the upper stop member 73'. A recess 95 may be provided in the lower surface 97 of the missile shield plate 69 for seating of the lower stop member 73 therein. The support block 81 has thereon a clevis 99 to which there is pin connected, by means of pins 101, a lift rig comprising a support system 103 preferably a tripod support system, that has a lift ring 105 affixed at the top thereof. Spreader support means, such as cylindrical members 107 are secured to adjacent support blocks 81. The cylindrical members 107 distribute forces acting on the missile shield plate 69 and reinforce the same. Depending downwardly from the outer periphery of the missile shield plate 69 is a skirt 109 which will deflect downwardly any missile that strikes the underside 97 of the plate and is turned outwardly towards the plate periphery. The lifting gap 91 is of a size sufficient that the missile shield plate 69, upon impact by a missile against the lower surface 75 thereof, will slide upwardly on the lift rods 55 but will not contact the upper stop member 73'. By so sizing the lifting gap 91, all of the missile momentum is transferred to a lifting force and is not transferred to the lifting rods, head lugs, closure studs and pressure vessel supports. The missile shield plate 69 with the associated weight of the support blocks 81, support system 103 and spreader support means 107 thus absorbs missile impact forces and permits the use of smaller size lift rods than would be required if the lift rods had to absorb such an impact through a rigid missile shield plate fixed to the lift rods. The present integrated head package 41 uses a separate rod position indicator 111, such as collars 113, on each of the control rod drive mechanisms 45 and the displacer rod drive mechanisms 47, the collars 113, in the form of horizontally extending adjacent plate-like members, aligned in a common horizontal plane (FIGS. 2 and 3) to act as a seismic plate. A seismic sleeve on those mechanisms is thus not required. The integrated head package 41 is also stabilized by the use of a plurality, usually six to eight, spaced adjustable stabilizing members 115 which are secured to the walls 117 of the containment well, and to a seismic ring 119 on the shroud 51. The present improved integrated head package, where a rod position indicator is used for each rod drive mechanism eliminates the need for seismic sleeves on such mechanisms. Current seismic sleeves are approaching the structural limit of the sleeves and such may not be adequate for use with advanced pressurized water reactor plants. Also, the perforation of a missile shield, as in prior art integrated head packages, for interface with the seismic sleeves, requires an increase in the plate thickness since the perforated plate is structurally weaker than a solid plate. Since seismic sleeves, if used in an advanced pressurized water reactor would increase the amount of congestion already present in upper head area designs and increase the difficulty associated with assembling the lifting rig/spreader/missile shield assembly on the head due to the need for engagement of all of the seismic sleeves, the present system, eliminating the need for seismic sleeves is beneficial.