Patent Number: 041585992
Section: description

DETAILED DESCRIPTION OF THE PRACTICE OF THIS INVENTION The apparatus shown in the drawings includes a nuclear reactor 11 which is mounted in a pit 13. This apparatus also includes a lifting device 15, typically a crane, for lifting and moving the upper package of the reactor 11. The crane 15 includes the usual drives (not shown) necessary for operation and is preferably movable on a track (not shown). There is also a pit 17 for replacement apparatus. The cart 19 containing the replacement fuel before refueling and the spent fuel after refueling is temporarily retained in pit 17. There is a spent fuel pit 18 connected to the pit 17 through a gate 20. During refueling the pit 17 and the pit 13 are filled with water. In the event that there is a necessity to effect repairs the gate 20 is closed and the water is pumped from pit 17. A crane 23 is provided for carrying out the fuel replacement operation. This crane is movable on a track 25 and has the necessity movable mast 27 to hold and move the fuel assemblies. The reactor 11 includes a lower package 31 and an upper package 33. For the details of the structure the reactor 11 and the above-listed U.S. patents (particularly patent No. 3,607,629) are referred to. For the purpose of the invention it is necessary only to describe briefly the features shown in FIGS. 1 and 2. The lower package 31 includes the body 35 of a pressure vessel. Within the body 35 there is a fuel core 37 composed of fuel assemblies (FIG. 2). Conductors 39 and 41 are connected peripherally to the body 35 for transmitting fluid between steam generators 43 and the pressure vessel and between pumps 45 and the pressure vessel. The upper package 33 includes the head 51 of the pressure vessel. The housings (not shown) for the control rid drive mechanism (not shown) are joined pressure-tight to the head 51. The driving pistons (not shown) the drive shafts (not shown), and the control rods (not shown) extend from the housings (not shown) through the head into the core 37. These drives, shafts, and control rods are held magnetically in the retracted position during refueling (see U.S. Pat. No. 3,766,006) and are removable as part of the upper package 33. The upper package also includes the missile shield 53. The shield 53 has a lifting lug 55 to which the lifting device 15 is connected for removal of the upper package 33. The casing 57 includng the control rod drive mechanisms (not shown) has a ring 59 on which a cable tray 61 is pivoted. The cables 65 are connected to the usual cabinets 63 in a compartment 67. Because the tray 61 is pivoted and the cables 65 hang loosely from the tray, the upper package may be moved without disconnecting the cables, (U.S. Pat. No. 3,836,430). The head 51 is secured pressure-tight to the body 35 of the pressure vessel by studs 71. Each stud 71 is provided with a hydraulically operable detensioner 73 for rapid movement of the studs. Typically, there are 26 large studs rather than 52 smaller studs. The studs 71 and detensioners 73 and head 51 are enclosed in a thermal insulating housing 75. The pit 13 is surrounded by an outer concrete building 81 which serves as a radioactive shield. Within building 81 there is a containment pressure shell 83, typically of steel, which has a base 85 that lines the base 87 of the building 81. The crane 23 and its tracks 25 are disposed within the shell 83. Within the shell 83 there is a concrete inner structure 91 in which the reactor 11 and its associated apparatus such as the steam generator 43, the pumps 45 and fluid conductors 39 and 41 are mounted. The reactor 11 is disposed in the pit 13 which is part of this structure 91. The vessel 35 has linings 95 of thermally insulating material separated from the wall 93 of the pit by an air space (not shown). The conductors 39 and 41 pass through openings in this structure, and have linings 97 of thermally insulating material also separated from the walls 97 by air spaces. The stem generators 43 and the pumps 45 are supported from the base 100 of the structure. The components 39, 41, 43 and 45 and other like components are accessible or may be viewed through inspection hatches or openings 101 and 103 (see FIG. 1). The operating deck above the reactor is shielded from the generators 43 by arcuate concrete shields 105 which span walls 107 secured to an arcuate concrete structure 109 extending from the shell 83. Openings 104 are provided for venting the pit 13. The base 99 is provided with a circular receptacle 111 to which the upper package 33 may be removed during refueling. There may also be receptacles 113 and 115 for storing the head 51 and the upper or lower internals (not shown) where necesssary. There is also a pressurizer 116 for maintaining the pressure during operation of the reactor 11. The inner structure 91 is provided with a hatch cover 117 of steel and concrete shown raised in FIG. 2. This cover 117 is mounted on hinges 119 and when raised affords access to the reactor 11 and its associated apparatus. When lowered, during normal operation of the reactor 11, the cover 117 affords shielding protection. Within the shell 83 there is a cavity 121 where the cart 19 with replacement fuel assemblies (or, after refueling, with the spent assemblies) is disposed during refueling. This cavity 121 is enclosed by concrete walls 123 and is separated from the inside of the inner structure 91 by a removable gate 125. The refueling pit 17 is a cavity in the wall 131 of the spent fuel storage pit 18. This wall 131 on one side abuts the adjacent wall 133 of the concrete shield 81. There is a passage 135 extending through the walls 131, 133 and 123. The passage 135 has a cylindrical metal wall 136. It is closed on the side of the pit 17 by a gate 137 which is slidable between open and closed position. On the side of the cavity 121 this passage is closed by a cover 139 which is bolted to a flange 141 of the wall defining the passage 135. OPERATION The refueling is carried out under water to protect against radioactivity. It is essential that the upper internals be maintained under water as they are raised as part of the upper package. The water level is then raised as the upper package is raised. But, the water level must be maintained below the detensioners 73, the cable tray 61 and the cable 65. Initially, the inside of the pit 13 is dry. At the start of the refueling the fuel cart 19 is disposed under water in pit 17 with replacement assemblies 151. The boron concentration is now appropriately raised to the refueling magnitude (2500) p.p.m. for annual cycle and 1440 p.p.m. or less for shorter cycles. The control rod drive mechanisms (not shown) are operated to remove the rods (not shown) to the retracted position (U.S. Pat. No. 3,766,006). The detensioners 73 are now operated and the studs 71 tilted out of locking position. The upper package is now lifted to position A (FIG. 1) and moved to the receptacle 111 in position B. At this point, or before, the level of the water in the pit 13 is raised. The level of the rising water during this operation is always maintained above the upper internals in the rising package but below the stud detensioners, cable tray 61 and cables 65. Gate 137 is then raised and cart 19 moved into cavity 121. Gate 125 is now raised and refueling carried out by crane 23. The replacement assemblies 151 one-by-one, replace spent assemblies; the spent assemblies being placed in the cart 19. After the replacement of fuel assemblies has been completed the cart 19 of spent assemblies are removed to the pit 17 whence the assemblies may be transferred under water into pit 18. The upper package is replaced as the water level is gradually lowered. The studs are secured sealing the head 51 to the body 35. The shield 53 is lowered to operating position and the control rods reinserted in core 37. The boron concentration of the water in the reactor vessel is now reduced to operating magnitude (1200 p.p.m. for annular refueling cycle, 650 for semiannual and 350 for three-months). The reactor 11 may now be set into normal operation. In lieu of refueling from a cart 19 of assemblies, the assemblies may be brought one-by-one into the containment 13 as they are at present. FIG. 3 shows the relationship between refueling cycle in months, plotted horizontally, and fuel cost saving in dollars per kilowatt, plotted vertically. Curves are plotted for three parameters of refueling time. FIG. 3 shows that the fuel savings is a maximum for a three-months cycle and refueling time of three days. While a method of practicing this invention has been disclosed herein, many modifications thereof are feasible. This invention then is not to be restricted except insofar as is necessitated by the spirit of the prior art.