Patent Number: 041815726
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT In designs of nuclear reactors that utilize rotatable plugs in the closure head of a reactor vessel, there exist annular passageways defined between the rotatable plugs and associated apparatus which allow the rotation of the plugs. In order to meet design requirements, seals must be provided that are capable of sealing these passageways under a variety of reactor conditions. The invention described herein serves to seal those kinds of passageways. Referring to FIG. 1, a core 10 comprising fuel assemblies 12 that produce heat is contained within a reactor vessel 14. The reactor vessel 14 has an inlet 16 and an outlet 18 that permit a coolant 20 to circulate in a heat transfer relationship with the fuel assemblies 12. The coolant 20, which in a fast breeder reactor may be liquid sodium, fills the reactor vessel 14 to a coolant level 22. The reactor vessel 14 is closed at its top end by a closure head comprising a stationary outer ring 24, a first rotatable plug 26, a second rotatable plug 28, and a third rotatable plug 30. The stationary outer ring 24 may be integral with reactor vessel 14 or it may be attached to reactor vessel 14 by suitable means known in the art. In addition, a gear mechanism 32 may be mounted on each rotatable plug such that gear mechanism 32 may be driven by a drive mechanism (not shown) which in turn will rotate the particular rotatable plug. The first rotatable plug 26 is supported from stationary outer ring 24 by a first bearing assembly 34. The outer peripheral surface of the first rotatable plug 26 together with the inner peripheral surface of stationary outer ring 24 define a first annulus 36 therebetween. The first bearing assembly 34 enables the first rotatable plug 26 to move relative to stationary outer ring 24 while maintaining a fluid-tight boundary between the outside and the inside of reactor vessel 14. Again referring to FIG. 1, second rotatable plug 28 is dispsosed eccentrically within first rotatable plug 26 and supported by second bearing assembly 38 defining a second annulus 40 therebetween in a manner similar to that of first bearing assembly 34. Likewise, third rotatable plug 30 is similarly eccentrically disposed within second rotatable plug 28 and supported by a third bearing assembly 42 defining a third annulus 44 therebetween. In addition, third rotatable plug 30 has disposed therein an in-vessel transfer collar 46 which provides access for an in-vessel transfer machine (not shown). During refueling, an in-vessel transfer machine which may be chosen from those wellknown in the art, is placed in the bore of the in-vessel transfer collar 46. When the in-vessel transfer machine is in place in the in-vessel transfer collar 46, a selected combination of rotations of the three rotatable plugs 26, 28, and 30 will align the in-vessel transfer machine in appropriate relationship with a chosen fuel assembly 12 of the core 10. As is well known in the art, the in-vessel transfer machine may then remove the chosen fuel assembly from the core and replace it with a fresh fuel assembly. When the reactor coolant 20 is liquid sodium, as in the case of liquid metal fast breeder reactors, it is necessary to avoid contact of the liquid sodium with oxygen because this interaction will result in the formation of impurities in the liquid sodium. To thus avoid this interaction, the space between the bottom of the closure head and the coolant level 22 is filled with a cover gas 48 such as argon. The cover gas 48 not only fills the cover gas space between the bottom of the closure head and the top of the coolant level 22, but it also fills the annuli 35, 40, and 44. While the cover gas 48 prevents oxygen from contacting the coolant 20, the cover gas 48 itself is subjected to radiation exposure from the core and thus becomes contaminated with radioactive particles. It is, therefore, necessary to have the cover gas 48 circulated between the reactor vessel and the cleaning process to remove most of the radioactive particles in a manner well known in the art. As previously indicated, it is, nevertheless, necessary to prevent this cover gas 48 from escaping up the annuli 36, 40, and 44 through the seals in the closure head, and out of the reactor vessel. Referring now to FIGS. 2 and 3, while FIG. 2 is a partial view of the closure head of FIG. 1 illustrating the three bearing assemblies 34, 38, and 42, FIG. 3 is an enlargement of the first bearing assembly 34 which shows the elements of a typical bearing assembly. The bearing assembly comprises a bearing support 48 which rests upon and is sealed to the stationary outer ring 24 by two O-rings 50 which may be chosen from those well known in the art. The bearing inner race 52 is supported by the bearing support 48 and is bolted thereto by bolt 54. Bearing ball 56 is disposed in inner race 52 in a manner such that an additional clearance 58 is provided on the inner diameter of the inner race 52. Clearance 58 is provided to accommodate differential thermal expansion among the components of the closure head which allows the closure head to be manufactured of materials having various coefficients of thermal expansion. An outer race 60 rests on the bearing ball 56 and is bolted to the first rotatable plug flange 62 by bolt 64. Flange 62 may be attached to the first rotatable plug 26 by common means such as bolts or flange 62 may be an integral part of first rotatable plug 26. The arrangement of the first bearing assembly 34 and flange 62 of first rotatable plug 26 is such that the weight of first rotatable plug 26 is transmitted through the flange 62 and through bearing assembly 34 to the stationary outer ring 24 thereby providing a mechanism for allowing rotation of the first rotatable plug 26 with respect to the stationary outer ring 24 along annulus 36. A spacer 66 attached to inner race 52 is provided to maintain proper alignment of bearing ball 56. The configuration of bearing support 48 in conjunction with the configuration of stationary outer ring 24 define two reservoirs between them; a first reservoir 68 and a second reservoir 70. In addition, the configuration of the components of bearing assembly 34 further define first annulus 36. Still referring to FIG. 3, an outer seal 72 is disposed on the bearing support 48 so as to seal the annulus between flange 62 and bearing support 48. Outer seal 72 comprises a tubular seal element 74 which may be a stainless steel hollow O-ring disposed in annulus 36 that extends the circumference of the stationary outer ring 24 and load spring assembly 76 attached to the bearing support 48 so as to force the tubular seal element 74 against flange 62 thereby sealing the annulus. Outer seal 72 may be disposed on the bearing support 48 in various configurations; however, the preferred angle is approximately 30 degrees from the vertical. Likewise, an inner seal 78 is similarly disposed on the bearing support 48. Inner seal 78 also comprises a second tubular seal element 80 which may also be a stainless steel O-ring and a second load spring assembly 82 attached to the bearing support 48 so as to compress the second tubular seal element 80 against flange 62. A lubricant inlet 84 which may be a conduit chosen from those well known in the art is disposed in stationary outer ring 24 and bearing support 48 such that the outlet of lubricant inlet 84 is disposed on the underside of inner race 52. Lubricant inlet 84 is connected on its outer end to a lubricant pump 86 which may be a constant volume pump which is capable of pumping a lubricant such as silicone through the lubricant inlet 84, through a channel 88 where the lubricant flow divides into two flow paths, one flowing through first annulus 36 toward outer seal 72 and the other flowing through first annulus 36 toward bearing ball 56. The lubricating fluid flowing through the second path under pressure is forced around bearing ball 56 and over tubular seal element 80 thereby compressing load springs 82 and allowing the lubricant to pass between the tubular seal element 80, and flange 62. From inner seal 78, the lubricant flows into first reservoir 68 where it fills first reservoir 68 to a level 90. At the same time, the lubricating fluid flows through the first path over outer tubular seal element 74 and into second reservoir 70. As the lubricant passes over the tubular seal elements 74 and 80 a film of lubricant is established between the tubular seal element and flange 62 such that no gases may pass therebetween. In addition, the force of the lubricant on flange 62 can reduce the bearing load by as much as 10 to 20%. A typical silicone lubricant may be Dow Corning No. 710 cracked at 482.degree. F. to remove low volatility fractions. The cracking avoids most of the off gassing at 450.degree. F., the seal operating temperature. Still referring to FIG. 3, a return conduit 92 is connected between second reservoir 70 and a first valve 94 which may be a three-way valve chosen from those well known in the art while another return conduit 96 is provided between first reservoir 68 and first valve 94. The return conduits 92 and 96 serve to direct the lubricating fluid to first valve 94 where the lubricating fluid is recirculated to lubricant pump 86. Furthermore, a recirculating conduit 98 is connected to lubricant inlet 84 and around lubricant pump 86 with a gate valve 100 and a pressure relief valve 102 disposed therein to enable lubricant pump 86 to maintain a constant volume flow even under varying operating conditions. However, during reactor refueling gate valve 100 is closed which prevents flow in recirculating conduit 98 and results in increased pressure on flange 62 which reduces the load on the bearing at a time when it is necessary to rotate the plugs. In addition, a gas inlet line 104 is connected to first annulus 36 while a gas outlet line 106 is disposed in first reservoir 68 with an opening above lubricant level 90 so that a gas such as argon may be pumped through first annulus 36 to thereby entrain contaminants in the gas flow thus purging the annulus. Also a check valve 108 may be disposed in gas inlet line 104 to prevent reverse flow in that line. Still referring to FIG. 3, it should be noted that a circumferential extension 110 of flange 62 extends into bearing support 48 thereby defining a liquid dip seal 112 in first annulus 36. While the lubricating fluid is being pumped through first annulus 36 the lubricating fluid fills liquid dip seal 112 creating a fluid seal against gas leakage through first annulus 36. Moreover, should lubricant pump 86 not be operating, the lubricating fluid will, nevertheless, remain in liquid dip seal 112 thus sealing the annulus even when the lubricating fluid is not flowing. Referring now to FIG. 4, a typical load spring assembly such as load spring assembly 82 comprises a housing 114, a biasing mechanism such as a coil spring 116 mounted in housing 114, a platform 118 mounted in housing 114 on an end of coil spring 116, and a contact surface 120 attached to platform 118 for contacting tubular seal element 80. Coil spring 116 serves to force the tubular seal element against a surface such as flange 62 to seal the annulus 36. Of course, under pressure from the lubricating fluid coil spring 116 may be compressed thereby relieving pressure on the tubular seal element. Therefore, the invention provides a closure head for a nuclear reactor having a sealing and lubricating system for allowing rotation of rotatable closure head plus while sealing the annuli defined by the rotatable plugs.