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Information Notice No. 92-16, Supplement 2: Loss of Flow from the Residual Heat Removal Pump During Refueling Cavity Draindown
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 August 23, 1993 NRC INFORMATION NOTICE 92-16, SUPPLEMENT 2: LOSS OF FLOW FROM THE RESIDUAL HEAT REMOVAL PUMP DURING REFUELING CAVITY DRAINDOWN Addressees All holders of operating licenses or construction permits for nuclear power reactors. Purpose The U.S. Nuclear Regulatory Commission (NRC) is issuing this supplement to Information Notice 92-16 to alert addressees to several events that could have resulted in loss of shutdown cooling. This supplement describes three events where the licensees approached or entered reduced inventory operation without reliable reactor coolant system (RCS) level monitoring instrumentation. 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 September 11, 1992, Carolina Power and Light Company, the licensee for H.B. Robinson, was lowering the RCS level to 63.5 cm (25 inches) below the vessel flange so that a reactor coolant pump seal could be replaced. The licensee procedure erroneously directed the use of 34.5 kPa (5.0 psig) nitrogen overpressure instead of 3.5 kPa (0.5 psig) overpressure while the RCS is being drained. During the draindown, repeated inquiries from the NRC resident inspector about the validity of the RCS level indication led to securing the draindown and venting the RCS overpressure. The tygon tube level indication dropped 304 cm (10 feet) when the RCS pressure was fully vented. On September 13, 1992, the licensee lowered the RCS level to 91.4 cm (3 feet) below the vessel flange, the entry point for mid-loop operation, while the level indicators continued to show an error of 5 cm (2 inches). On March 19, 1993, the New York Power Authority, licensee for Indian Point Unit 3, began RCS draindown so that the ultrasonic level monitoring system could be functionally tested. In response to Generic Letter (GL) 88-17, "Loss of Decay Heat Removal," the licensee committed to have two independent means of level indication when RCS level was in the reduced 9308170095. IN 92-16, Supplement 2 August 23, 1993 Page 2 of 3 inventory range. One system was a standpipe system and the other was the ultrasonic level monitoring system. The ultrasonic transducer, which had been recently replaced, was bench-tested prior to installation; however, a functional test with RCS level in the mid-loop range was needed to consider the monitoring system operable. In the early part of the draindown, the indicated level differed by as much as 17.5 cm (7 inches) between the permanent standpipe system and a temporary standpipe that was in service but settled to within the acceptance value, 15 cm (6 inches). With RCS level reduced to the mid-loop area to functionally test the ultrasonic level monitoring system, the only operable level indication was the permanent standpipe system. While lowering the RCS level further to a second data point for the level monitoring system functional test, the indicated level differed by 7.5 cm (3 inches) between the standpipe and the monitoring system. The acceptable difference in level indication in the mid-loop range was 2.5 cm (1 inch). Therefore, the licensee began troubleshooting the standpipe system and, in the process, isolated the only operable means of level indication with RCS level at mid-loop. The resident inspectors, who were in the control room during portions of the draindown, questioned the reliability of the level indicators and notified licensee management. As a result, RCS level was raised above the mid-loop range and further troubleshooting revealed that a loop seal, which had been overlooked, existed in the original standpipe vent. Shutdown cooling was not lost during this event. On March 9, 1993, at Wolf Creek Nuclear Generating Station, divergence between the two types of level sensing instrumentation occurred during RCS draindown. The licensee determined that the divergences were due to problems involving both the tygon tube and permanently installed level transmitters. Nitrogen added to the reactor coolant system during the draindown in order to assist the removal of fission gases, was determined to have formed a loop seal in the tygon tubing. The problem with the level transmitters was water droplets which adhered to the instrument tube walls during draining of the reference legs and, subsequently, collected in the reference legs when the RCS was drained. Discussion Generic Letter 88-17, "Loss of Decay Heat Removal," recommended specific actions to ensure reliable RCS level indication because it is very important in a PWR when approaching a mid-loop condition. The high potential for the loss of shutdown cooling in mid-loop operation due to inadequate net positive suction head in the shutdown cooling pumps necessitates precise RCS level control. Level measurement of a closed RCS can be sensitive to pressure differences between the RCS and the instrumentation vent tubes. Diversity in level measurement methods can reveal some of the potential errors. In the events addressed above, the licensee procedures lacked specific guidance to ensure sufficient accuracy for the level monitoring system before entering risk-significant RCS level manipulations. As discrepancies were noted between level indications they were not successfully resolved before. IN 92-16, Supplement 2 August 23, 1993 Page 3 of 3 continuing with additional RCS draindown. When level readings were subsequently found to be outside the acceptable range, the suitability of remaining in a reduced inventory condition during the instrumentation repair was not considered. Entering or remaining in a reduced inventory condition without reliable instrumentation increases the potential for a loss of shutdown cooling. This information notice requires no specific action or written response. If you have any questions about the information in this notice, please contact the technical contact listed below or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager. /s/'d by CIGrimes/for Brian K. Grimes, Director Division of Operating Reactor Support Office of Nuclear Reactor Regulation Technical contact: Thomas Koshy, NRR (301) 504-1176 Attachment: List of Recently Issued NRC Information Notices .
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