Patent Number: 048636738
Section: summary

BACKGROUND OF THE INVENTION This invention relates to control rods and control rod drives in nuclear reactor, and more particular to the testing of check valves that prevent control rod ejection responsive to ambient reactor pressure. STATEMENT OF THE PROBLEM Control rods in nuclear reactors are inserted to the core of the reactor for control of the reactor reaction. Absent any other force, the ambient pressure interior of the reactor would cause control rod ejection. Insertion of control rods usually occurs responsive to a fine movement system, either hydraulic or ball screw driven and a rapid movement system, which system is always hydraulic. In the case of a ball screw driven drive, slow and controlled movement of the control rod is responsive to rotation of a screw and change of a following screw ball in elevation for lifting the control rod to and from a control position of penetration within the reactor core. In this ball screw control rod drive, the second system for control rod insertion is hydraulic. This system causes the piston on top of the ball screw drive to leave the position of normal support on the ball screw drive. The control rod rapidly moves upwardly responsive to hydraulic force from a position of withdrawal to a position of complete penetration of the reactor core for control of the reaction. Some drives include hydraulics for both fine motion control and rapid insertion. Fine motion is achieved by a combination of controlled hydraulic power applied to the drive piston and a hydraulically controlled latch. SCRAM or rapid insertion occurs upon substantial hydraulic power being applied to the drive piston just as with the ball screw drive. It is sufficient to say that as long as the system is hydraulic, the disclosure herein is applicable. Such hydraulic systems must be fail safe. These systems must be designed with safe guards against conceivable casualty scenarios. In order to understand the test apparatus and test process of this invention, it is required to understand the hypothesized casualties. Hydraulic systems have been known to fail. In in a nuclear reactor, the high pressure hydraulic system for the immediate insertion of a control rod were to fail, other causalities could conceivably follow. Control rod drives upon insertion are equipped with ratchet mechanisms. These mechanisms maintain the rods in their inserted position unless deliberate control rod withdrawal is undertaken. However, as a fail safe procedure, it is assumed for purposes of analysis that such systems do not work. In other words, the hydraulic system itself must be designed to prevent inadvertent control rod withdrawal. Assuming failure of the hydraulic system, the control rod will be under an ejection force from the full pressure interior of the boiling water nuclear reactor. In an attempt to guard against this scenario, a check valve has been placed in the drive at the inlet of the hydraulic system. Upon inadvertent rupture of the system, the check valve is designed to close, stopping further control rod ejection. Unfortunately, the checking of the functionality of this ejection prevention check valve has heretofore not been practically possible because of its remote location in a hostile environment under the vessel without system disassembly and inspection. One possibility for checking the ejection prevention valve has been to relieve the hydraulic pressure on the control rod insertion system to see if the check valve will seat without seepage to prevent control rod ejection. Unfortunately, in such a test, should the check valve be defective at least a partial ejection of the control rod would result. In such an ejection the very casualty that the system was designed to prevent could conceivably be precipitated. Consequently, there remains a need to design a system for the inspection of the integrity of the control valve ejection prevention check valve. Such a test must not otherwise interfere with the safe operating state of the reactor. SUMMARY OF THE INVENTION In a hydraulic system of a control rod drive for insertion of control rods, a test apparatus and test process is disclosed for determining the integrity of the hydraulic system check valve for preventing inadvertent control rod ejection responsive to the reactor pressure. The check valve is located at the drive between a hydraulic valve for causing rod insertion and the drive hydraulic cylinder. In this interval downstream of the check valve, alteration is made to the hydraulic path by the installation of a conduit with a quick disconnect connected through an isolation valve. Test apparatus for temporary connection at the disconnect is disclosed consisting of a small positive displacement piston and cylinder. The small positive displacement piston and cylinder connects to the quick disconnect through a complementary quick disconnect fitting and a rapidly opening toggle valve. Provision is made for timing the excursion of piston in the piston and cylinder preferably by end of stroke microswitches. A process for the testing of the integrity of the check valve is disclosed in which quick opening valve is opened with resultant backflow from the hydraulic system through the check valve into the volume defined by the small displacement. Presuming proper check valve operation and closure, the time of excursion of the piston between the two microswitches is long, indicating that the check valve seats and remains sealed to prevent control rod ejection. Alternately, if the time of excursion of the piston between the two microswitches is short, there is an indication that the check valve has not properly seated and that control rod ejection is possible. It is important to note that control rod ejection is not permitted due to the small volume of the interrogating piston and cylinder. Provision is made for the discharging of the contents of the piston back to the hydraulic circuit before removal of the test apparatus for sequential tests at adjacent control rod drives. OTHER OBJECTS, FEATURES AND ADVANTAGES An object to this invention is to establish a test protocol for an ejection prevention check valve on the hydraulics of a control rod drive. Accordingly, a positive displacement piston and cylinder is utilized, the piston having a stroke of limited volume. Piston movement is timed from beginning of stroke to end of stroke over the small volume therebetween. The length of the piston stroke, preferably being beyond a predetermined test standard time, establishes check valve integrity. Simply stated, the volume of the piston and cylinder is sufficient to permit immediate check valve closure and testing of the seal of the check valve. An advantage of this test apparatus and process is that it in no way interferes with the integrity of the hydraulic systems. For example, SCRAM of a drive can be undertaken even at a drive under test without interfering with the operation of the control rod drive in any fashion. Yet another advantage of this procedure is that for the first time checking of the integrity of the check valve is permitted without perceptible ejection of the control rod. Yet another advantage is that the disclosed test protocol is applicable to all types of control rod drives, including those drives which are completely hydraulic as well as those drives which are partially ball screw actuated and partially hydraulic actuated. A further object to this invention is to set forth a test protocol that requires minimal modification of extant or future control rod drives for tests of their respective backflow check valves. According to this aspect of the invention, the drives are modified by the attachment of a conduit between the valve closing pressure source and the reverse flow check valve from the control rod drive hydraulics. This conduit is preferably provided with a quick disconnect fitting communicated through the isolation valve. An advantage of this aspect of the invention is that with the above simple modification, all hydraulic drives are easily and expeditiously altered for the test apparatus and protocol of this invention. A further object to this invention is to disclose a simplified apparatus for tests. According to this aspect, a positive displacement cylinder of small volume is provided. The cylinder is designed for communication to the hydraulic circuit being tested through a mating quick disconnect and a quick opening valve. Microswitches monitor the beginning of stroke and the end of stroke. By the expedient of timing the interval between microswitch actuation, a definitive test of the function of the ejection prevent check valve can be made. An advantage of this aspect of the invention is the simplicity of the test apparatus.