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Information Notice No. 80-43: Failures of the Continuous Water Level Monitor for the Scram Discharge Volume at Dresden Unit No. 2
SSINS NO.: 6835 Accession No. 8008220279 IN 80-43 UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF INSPECTION AND ENFORCEMENT WASHINGTON, D.C. 20555 December 5, 1980 Information Notice No. 80-43: FAILURES OF THE CONTINUOUS WATER LEVEL MONITOR FOR THE SCRAM DISCHARGE VOLUME AT DRESDEN UNIT NO. 2 Description of Circumstances: On December 2, 1980, following a reactor scram at Dresden Nuclear Power Station Unit 2, the licensee discovered that the recently installed scram discharge volume (SDV) continuous water level monitoring system had failed to respond as expected. The continuous water level monitoring system was designed and supplied by Nortec Corporation, and utilizes a single transmit/ receive transducer for each of the four level monitoring channels. Following installation of the continuous level monitoring system, the licensee verified its operability by tripping each alarm channel while its sensor was connected to a calibration standard consisting of a section of 4 inch pipe identical to the SDV piping. No in situ test of the equipment was accomplished. The expected high level annunciator alarms were not received in the control room following scram when the SDV filled with water. An investigation to determine the cause was initiated and once per shift checks of the SDV level on both Units 2 and 3 were re-instituted. The reactor scram system functioned normally and post scram manual ultrasonic tests on the SDV verified proper system drain. Investigations by the licensee determined that the ultrasonic detectors were inadequately coupled to the SDV piping. The coupling was improved and testing performed to show that the system could detect water flow into the SDV during single control rod scram tests. The unit was taken critical and was heating up on December 4, 1980, at approximately 400 psig when a reactor scram occurred. The expected SDV high level annunciator alarms were not received in the control room immediately following the scram, but were received about 10-15 minutes later when the SDV was draining. Following an additional scram at approximately 20 psig with control rods inserted these alarms were received properly. Investigation identified level detection system operability problems as evidenced by a low signal to noise ratio. Arrangements are being made for replacement components (including transducers) to improve the signal characteristics and reduce system noise. Until the improvements are installed and demonstrated adequate by in situ testing, use of the ultrasonic detection system is being augmented with manual ultrasonic checks of level in the SDV, using the previously required once per shift check. A manual check will be performed after each reactor scram. At Dresden Unit 2 the licensee will . IN 80-43 December 5, 1980 Page 2 of 2 complete the system improvements and required testing within 7 days in accordance with the Immediate Action Letter issued by Region III on December 4, 1980. The above operating experience indicates the need for a demonstration of the operability of ultrasonic systems recently installed to continuously monitor for water in the SDV. Until demonstrated operable by in situ testing, these systems should not be solely relied on for detection of water in the SDV. Use of new ultrasonic detection systems should be augmented by manual checks for water in the SDV each shift and after reactor scram until in situ testing is performed. It is important that a high degree of reliability be established in operability of these continuous water level monitoring systems in order to support plant operations in the interim until final scram discharge system modifications are performed. This Information Notice No. is provided as an early notification of a possibly significant matter that is still under review by the NRC staff. Recipients should review the information for possible applicability to their facilities. No specific response is requested at this time. Pending further NRC evaluations, we expect licensee actions will be requested or required. No written response to this Information Notice No. is required. If you have any questions regarding this matter, please contact the Director of the appropriate NRC Regional Office.
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