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Information Notice No. 91-53: Failure of Remote Shutdown System Instrumentation Because of Incorrectly Installed Components
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 September 4, 1991 NRC INFORMATION NOTICE 91-53: FAILURE OF REMOTE SHUTDOWN SYSTEM INSTRUMENTATION BECAUSE OF INCORRECTLY INSTALLED COMPONENTS Addressees All holders of operating licenses or construction permits for nuclear power reactors. Purpose The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alert addressees to an event where the remote shutdown system at a nuclear power plant could have failed to perform certain functions because associated components were installed incorrectly. It is expected that licensees will review the information for applicability to their facilities and consider actions, as appropriate, to avoid similar problems. However, suggestions contained in this information notice are not NRC requirements; therefore, no specific action or written response is required. Description of Circumstances On May 18, 1991, Entergy Operations, Inc., the licensee for the Waterford Steam Electric Station, performed post-modification testing on a main steam isolation valve (MSIV) at the Waterford 3 reactor with the reactor in Mode 5. The testing included closing the subject valve using fire isolation switch FR-4 in the auxiliary relay room to ensure that the control wiring changes on the MSIV did not affect the function of the switch. If the control room is evacuated because of a fire, the switch is to be used to isolate control circuits for the MSIV and several other components from the control room and to ensure that power is supplied to the components to maintain them in the desired position. When the switch was operated, it travelled past the isolation position, allowing two of the adjacent make-before-break contacts to connect, which shorted the positive contact of one circuit to the negative contact of another circuit. The short circuit caused the power supply breaker for the switch to trip, but the switch was severely damaged by smoke and fire. The licensee quickly extinguished the fire. While replacing the switch, the licensee determined that stop screws that should have been installed when the switch was installed were missing. This allowed the switch to rotate past the intended stopping position. The licensee replaced the failed switch and successfully tested the replacement. The switch is a multideck rotary switch manufactured by Electroswitch. The licensee determined that the stop screws 9108280089 . IN 91-53 September 4, 1991 Page 2 of 2 were supplied with the switch, but installation instructions did not specify that the stop screws be installed in this application. The licensee inspected several other isolation switches in adjacent panels and found another switch with missing stop screws. Further information may be found in NRC Inspection Report 50-382/91-18. Discussion Fire in certain plant areas such as the control room or the cable spreading room can affect the ability to achieve safe shutdown of the plant from the control room. In addition, the control room can become inaccessible for reasons other than a fire. These situations warrant the use of a remote shutdown system to achieve safe shutdown of the plant from a remote shutdown panel or emergency control stations that are independent of the control room. The remote shutdown system includes switches to isolate the control circuits of required safe shutdown equipment from the control room and to transfer controls to either the remote shutdown panel or emergency control stations. Therefore, licensees provide required instrumentation and controls for the remote shutdown system in accordance with the requirements of General Design Criterion (GDC) 19 of Appendix A and Appendix R to Part 50 of Title 10 of the Code of Federal Regulations (10 CFR Part 50). The significance of the problem described above is that the remote shutdown capability for the plant may be compromised if the control circuits and transfer switches (for shutdown equipment) fail because they are installed incorrectly. If these switches fail, certain instrumentation and control functions for the remote shutdown system may be lost. Licensees may not recognize that stop screws have not been installed in the fire isolation (transfer) switches unless the operator challenges the mechanical stops of these switches. Although the switches were tested for MSIV operability during pre-operational testing at Waterford, the effective- ness of the mechanical stops of these switches was apparently never tested. This information notice requires no specific action or written response. If you have any questions about the information in this notice, please contact one of the technical contacts listed below or the appropriate Office of Nuclear Reactor Regulation project manager. Charles E. Rossi, Director Division of Operational Events Assessment Office of Nuclear Reactor Regulation Technical contacts: T. Chandrasekaran, NRR 301-492-0859 C. Doutt, NRR 301-492-0847 Attachment: List of Recently Issued NRC Information Notices .
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