Patent Number: 048636738
Section: description

Referring to FIG. 1, a reactor vessel V is shown. A control rod drive housing H depends from the reactor vessel V and holds a control rod drive D. The drive includes at its upper end a coupling S. Coupling S has mounted there upon, a control rod C. In order to understand the problem created by control rod ejection, it must be remembered that vessel V is a pressure vessel. In the case of a boiling water reactor, the pressure interior of vessel V can be 1,000 lbs. per square inch of pressure. In the absence of drive D and presuming that control rod C is fully inserted, it would be expected that the control rod C would be ejected by the pressure interior of the vessel V from the reactor. As is common, the control rod C moves to and from a core U interior of the reactor between fuel bundles F for control of the reaction. As illustrated in FIG. 1, drive D can be of two discrete types. First, drive D can be a hydraulic drive. In such hydraulic drives, incremental motion is achieved by applying controlled fluid inflow 14 through conduit 19 to piston P. Rapid insertion (SCRAM) is achieved by opening valve 18 and applying high pressure fluid from accumulator A through conduit 19 through a check valve 22. Check valve 22 is the valve whose integrity is to be tested by this invention. After passing through check valve 22, fluid--typically water--acts upon a piston P interior of the control rod drive. When acting upon piston P, the control rod is advanced and retracted interior of the core C between the fuel bundles F. During control rod insertion, it is common to latach the rod in the inserted position. Specifically, and in the embodiment here illustrated, a hydraulically actuated latch mechanism L is illustrated. Latch mechanism L acts upon the side of piston P at indentations 24. The control rod is thus maintained in its position of insertion. As has been previously emphasized, it is assumed, for the casualty control scenarios, that if the latches L and their engagement at 24 fails for some reason, the check valve 22 is then the safety system under which the hydraulic system P operates. Assuming a failure in the hydraulic system, high pressure will no longer be maintained at conduit 19. Ball 26 interior of check valve 22 will move from its lower seated position to an upper seated position blocking conduit 19. Such movement will occur as piston P drives fluid towards conduit 19 and in the case of the particular hydraulics herein illustrated, conduit 28 unseats ball 26 from its lower position to an upward seated position at 30. It will also be understood that FIG. 1 can be just as well illustrative of a prior art ball screw assisted hydraulic drive. In such drives, a motor M turns a shaft T. Shaft T operates on a ball screw B. Ball screw B raises and lowers couplings S connected to the bottom of control rod C. (Inflow 14 is used only as a purge flow to minimize contamination of the drive). Such drives typically include again a piston such as piston P. Again SCRAM is achieved by the opening of valve 18, piston P moves rapidly upward. In such movement it unseats the piston from the ball screw B with the result that the control rod C is rapidly inserted. For this particular design the latch L' is on the piston and the notch 24' is on the stationary part of the drive D. In either system, a failure of pressure at conduit 19 can cause piston P to reverse its path. The basic SCRAM hydraulic circuit is essentially the same for both types of drives. A SCRAM valve (18) connects a high pressure accumulator (A) through the insert pipe (19) to the underside of the piston (P). The piston (P), fores the control rod (C) into the core. In the event of a failure of this long thin insert pipe (19), and the required assumption that the designed method of position retention also fails, reactor pressure will force the piston (P) out of the core unless the check valve (22) closes. Hence the need to test this check valve (22). Since the check valve (22) is located in the drive, under the nuclear reactor, (in some cases in an inerted environment) the test must be run from a remote location (i.e., back near the accumulator (A)). The two drives differ in the designed method of control rod position retention. The ball screw drive has two methods of position retention. The main method is resting on the ball nut, with an electromagnetic brake preventing rotation of the ball screw. Following SCRAM, when the piston leaves the ball nut, the secondary position retention method of a latch (L') takes effect. The latch is attached to the piston and latches into an indentation (24') in the stationary part of the drive. When the ball nut finally catches up to the piston, a boss on the piston nut unlatches the piston and the ball nut takes over in retaining the control rod position. The hydraulic drive has only one method of position retention. It consists of a spring loaded, hydraulically operated latch (L) located on the stationary part of the drive. This latch engages indentations (24) located along the length of the piston (P). This same latch is used to retain the piston position following SCRAM. The modification necessary for the utilization of the test apparatus of this invention can be easily understood. Typically, conduit 19 from between the exit to the check valve and the insert or SCRAM valve 18 is provided with conduit 40. Conduit 40 includes an isolation valve 42 and a quick disconnect 44. It is to the quick disconnect that connection occurs. Having set forth the simple modification that changes the prior art drive shown in FIG. 1, attention may now be devoted to the test apparatus of this invention. Referring to FIG. 1, SCRAM valve 18 is provided with a high pressure fluid source from accumulator A. The valve discharges through conduit 19 to the piston. Check valve 22 with ball 26 is in a position permitting the fluid flow from accumulated through conduit 19 to the piston. Assuming that backflow occurs through conduit 19 responsive to control rod ejection, reverse flow will occur. Such flow will occur through the check valve 22 and out the now failed high pressure inlet 19. Conduit 40 has been tapped to conduit 19. An isolation valve 42 is present, which isolation valve includes a quick disconnect 44. The test apparatus includes a piston and cylinder 60, a hydraulic cylinder 62 has a free floating piston 64 placed therein. in the embodiment here shown, a microswitch 66 is placed at the bottom of stroke. As will hereinafter be more fully explained, a threaded rod 68 and handle 70 is utilized for returning piston 64 from end of stroke at microswitch 66 to the beginning of stroke at the opposite end of the cylinder 62 after each test. A quick opening valve 71 is provided. This quick opening valve includes a quick opening toggle 72. Toggle 72 is moved at right angles to open the valve. When handle 72 opens valve 71, a microswitch 74 is actuated. Microswitch switch 74 starts a timer 76. Timer 76 continues to run until it receives a signal through conduit 78 from microswitch 66. Quick opening toggle valve 72 is connected through a conduit 80 and a mating quick disconnect 84 to quick disconnect 44. It will be understood that a reactor vessel V includes a multiplicity of hydraulic drives. Since a multiplicity of hydraulic drives is utilized, it will be understood that the simple test apparatus consisting of the quick opening valve 71 and the piston and cylinder 60 can be repetitively used one drive after another drive for the required test. The test procedure is easily understood. Typically, connection is made at the quick disconnect across the mating fittings 44, 84. Thereafter, isolation valve 42 is opened communicating the high pressure circuit to the quick opening valve 71. At this juncture, piston 64 is at the end of the chamber remote from microswitch 66. Quick opening toggle 72 is opened. This toggle triggers microswitch 74 to start timer 76. The timer commences to run. At the same time, the volume of piston 64 can be seen to be small. This small volume begins to fill. As the volume begins to fill, fluid flow occurs from conduit 19 through the isolation valve 42, the quick opening valve 71 into the piston and cylinder 60. Assuming safe operation of check valve 22, such reverse flow should cause check ball 26 to move from a lower position to the upper position 30. In upper position 30, flow from piston at conduit 19 should be stopped. This piston, assuming an operable check valve 22, should occur before the full stroke of piston 64 to microswitch 66. It will be understood that if check 22 is not given proper seating or when seated lacks a complete seal, piston 64 will rapidly traverse cylinder 62. It will impact microswitch 66 stopping timer 76 in a short interval. By knowing the volume of the piston stroke of piston 64 interior of cylinder 62, a standard can rapidly be set for check valve 22 to pass. Returning briefly to FIG. 1, it will be realized that the volume interior of piston and cylinder 60 is extremely small when compared to the fluid volume required to be displaced by piston P for complete ejection of control rod C. This being the case, it will be understood that the test here disclosed does not require appreciable ejection of the control rod C to test the integrity of the check valve 22.