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Information Notice No. 93-78: Inoperable Safety Systems at a Non-Power Reactor
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 October 4, 1993 NRC INFORMATION NOTICE 93-78: INOPERABLE SAFETY SYSTEMS AT A NON-POWER REACTOR Addressees All holders of operating licenses or construction permits for test and research reactors. Purpose The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alert addressees to a problem which resulted in inoperable safety systems at a non-power reactor. It is expected that recipients 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 April 28, 1993, after a number of spurious scrams had occurred, the licensee for the University of Virginia Research Reactor shut down the reactor and began troubleshooting activities to determine the cause of the problem. The reactor had been experiencing spurious scrams for some time and the licensee had concluded that the scrams were not caused by electrical transients or line noise. The licensee suspected the reactor scram logic to be the cause of the scrams. The reactor scram logic consists of two trains that cause power to different magnets to be cut off on receipt of a scram signal. This in turn releases control rods and shuts down the reactor. While investigating the problem, the senior reactor operator (SRO) interchanged components of the scram logic system between the two trains. Among these components were solid-state relays and mixer-drivers (M/Ds) which act as a 28-channel "OR" gate in the scram logic. After consulting the facility safety analysis report and visually inspecting the exterior of the components, the SRO assumed that the interchanged components were identical. However, the M/Ds had been internally modified in the 1970s to tie unused inputs together and were no longer identical. After the M/Ds had been exchanged and no spurious scram signals were received for 30 minutes, the reactor administrator and the SRO started the reactor. Neither the reactor administrator nor the SRO recognized that the troubleshooting activities were actually a maintenance or modification function and that testing to verify the operability of the reactor scram system was required before the reactor was restarted. 9309290007. IN 93-78 October 4, 1993 Page 2 of 3 After operating for approximately 5� hours, the reactor was shut down by driving the rods into the core. With three of the four control rods seated, licensee personnel then introduced a scram signal into the reactor electronics using the intermediate range channel switch. However, the electronics did not generate a scram signal as expected. The licensee investigated and found that the M/Ds were not internally identical as described above. Further investigation showed that the following scram signals were inoperable: the two power level scrams, intermediate range period scram, primary coolant low flow scram, loss of power to the primary pump scram, intermediate channel range switch scram, and key switch scram. The licensee returned the M/Ds to their original positions and tested the reactor to ensure that the reactor electronics had not been damaged. To prevent a recurrence of this problem, the licensee made the following changes: (1) revised the facility standard operating procedures (SOPs) to clearly define maintenance and troubleshooting activities, (2) added a checklist to the SOPs to specifically control maintenance activities, and (3) added a checklist to the SOPs to verify the operability of the reactor safety systems after an unplanned reactor scram. Other changes were also made to the SOPs to ensure management control over maintenance. The licensee checked the reactor electronics against the schematics and found that changes had been made to console modules that were not reflected in the schematics. The licensee found two instances where externally identical components were internally different and therefore not interchangeable. After completing this check, the licensee labeled the modules that had been modified to clearly indicate that they were unique and not interchangeable. Discussion The circumstances described above demonstrate the importance of recognizing and controlling maintenance and modification actions. Clearly defining maintenance activities in facility procedures and providing training to personnel can be effective methods for controlling such activities. Also important to reactor safety is updating schematics of reactor electronics to reflect modifications to safety systems. Performing testing of the affected safety systems after activities of the type described above may prevent the occurrence of similar events. . IN 93-78 October 4, 1993 Page 3 of 3 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 (NRR) project manager. /s/'d by AEChaffee/for Brian K. Grimes, Director Division of Operating Reactor Support Office of Nuclear Reactor Regulation Technical contacts: A. Adams, Jr., NRR (301) 504-1127 C. Bassett, RII (404) 331-5570 Attachment: List of Recently Issued NRC Information Notices .
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