원자력발전소 연계논리계통 자기진단시험 유해성 및 개선에 관한 연구

Abstract

Interposing Logic System(ILS) controls miscellaneous plant equipment that does not fall within the scope of major plant control systems. Multiple microprocessor-based, single-board computers in the ILS control various fans, breakers, compressors, pumps and valves. The ILS receives plant operator command inputs from control modules mounted on control panels, plant process and equipment operating status inputs, and inputs from other control systems. The ILS performs On-off logic operations. The ILS provides a suitable output signal to control plant equipment and control module indicating lamps, and to operate the plant annunciator, computer, and other control systems. The ILS has using the Auto Diagnostic Test(ADT) which calls IOCTST(Input/output board auto test) and INTEST(Control board auto test) in order to protect Digital contol system. The INTEST is to check the all of the memories and board functions which are controlled properly. IOCTST is to check the all of the Input channels and Output channels on IO boards. IOCTST is generated On-Off pulse by the control board to the IO board every 5 minutes. The pulse is very short time duration such as 16µsec to the IO board in order to check the IO board performance. However, recently the failed IO board issued an unexpected output trip signal triggering the reactor coolant pump(RCP) circuit breaker to close and therefore shutting down RCP operations. Eventually the power plant output was stopped. In detail, whenever the control board sends a square wave to IO board, the actual output is turnning from Off to On in an instant. That indicates that a component may be actuated by an unexpected output signal in case of abnormal wave. The abnormal wave lasts for 500msec. That means that the abnormal wave is about 31 times as compared with a normal wave. Also ADT cannot detect IO board troubles. The Opto-coupler chip from IO board has been removed. But control board recognized that the IO board was working well. The same wave forms are monitored when Opto-coupler is installed or removed on the IO board. Thus, the control board recognizes the normal status for IO board. For solving the root cause, A decision was made to delete the IOCTST from the patch cord. Program patch is loaded in EPROM on the control board. Once the patch is applied, control board indicates what kinds of IO boards being in the system. On the other hand, control board doesn't recognize the IO board when the numbers from the program patch are deleted. Eventually bthe control board doesn't send the IOCTST signal in order to test the IO board anymore. Checking the IO board by the control board is performed by the complement error detector every 100msec instead of IOCTST. The system has to be varified to see if it works properly. The performance test was done by using the ILS simulator after deleting the program patch inside EPROM. The contents of performance tests are function test and stability test. The performance test are continued for 72days and completed successfully. The manufacturer of ILS agreed to this design change since no impacts has arisen in the system. The modified ADT has been applied for 45loops such as RCP, 13.8KV Feed PCB and other 6 months ago. The ILS system has been working well so far. This study is conducted to solve the chronic trouble in the ILS. A lot of troubles of the ILS have been issued after commercial operation in Havit nuclear power plant 3,4. But most of them have disappeared without any corrective actions. Unneccessary ADT seems to cause the troubles. This study is important as the manufacturer admitted their design mistake. It has improved the system performance and reliability for the ILS. Accordingly nuclear power plant will become safer.