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Information Notice No. 83-65: Surveillance of Flow in RTD Bypass Loops Used in Westinghouse Plants
SSINS No.: 6835 IN 83-65 UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF INSPECTION AND ENFORCEMENT WASHINGTON, D.C. 20555 October 7, 1983 Information Notice No. 83-65: SURVEILLANCE OF FLOW IN RTD BYPASS LOOPS USED IN WESTINGHOUSE PLANTS Addressees: All Westinghouse nuclear power reactor facilities holding an operating license (OL) or a construction permit (CP). Purpose: The intent of this Information Notice is to alert Westinghouse licensees to the possibility of low flow through a reactor coolant loop resistance temperature detector (RTD) bypass line. Such low flow can degrade the response of the temperature measurements utilized in the reactor protection system. The presence of low flow in this line, should be revealed by a low flow alarm; however, no regulatory requirements or vendor recommendations are established regarding periodic surveillance of the flow sensor or alarm. Description of Circumstances: On January 29, 1983, during routine shutdown operation at Salem Generating Station, Unit 1, a low flow alarm was received at the control room from the No. 14 reactor coolant loop RTD bypass line. Investigation of the problem, by alternately isolating the hot- and cold-leg sides of the flowpath, indicated that the hot-leg flowpath was obstructed. The No. 14 reactor coolant average temperature channel was therefore declared inoperable and a limiting condition for operation was entered, retroactive to the time of discovery of the problem. All bistables associated with the channel were immediately placed in the tripped condition. The redundant loop T avg channels were operable throughout the occurrence. In an attempt to dislodge the obstruction from the hot-leg bypass loop, its isolation valve was cycled, and then bypass flow was restored to normal. No other problems with the channels were evident, and the occurrence was assumed to involve an isolated instance of corrosion products restricting flow in the bypass line. Discussion: The reactor coolant loop temperature instruments are utilized in the reactor protection system. Operability of the temperature instruments is required to provide the overall reliability, redundancy, and diversity available in the facility design for the mitigation of accidents. The reactor coolant system hot- and cold-leg RTDs are located in reactor coolant bypass loops. A bypass 8308300378 . IN 83-65 October 7, 1983 Page 2 of 2 loop from upstream of the steam generator to the reactor coolant pump inlet is used for the hot-leg RTD, and a bypass loop from downstream of the reactor coolant pump to the pump inlet is used for the cold-leg RTD. The bypass loop flowrate affects the overall time response of the temperature signals provided for reactor protection. These response times are considered in performing FSAR accident analyses and are included in plant technical specification surveillance requirements. The RTD bypass loops have low flow alarms whose setpoint is 90% of the flow for clean piping. Bypass flow at or above this alarm setpoint is sufficient to maintain the proper RTD response time. Flow at or below the setpoint results in a control room alarm, which indicates to the operator that the overall RTD response time may have been degraded to a level inconsistent with plant technical specifications. As a result of the Salem experience the significance of the RTD bypass low flow alarm has been demonstrated. No regulatory requirements or vendor recommendations are established regarding periodic surveillance of the RTD bypass flow sensors; however, the Salem procedures require calibration of the flow detectors every three years. No action or written response to this notice is required; however, Westinghouse licensees may find it prudent to calibrate the flow sensors on a refueling outage basis and to verify the alarm setpoint on a monthly basis to assure the operability of this monitoring function. If you have any questions regarding this notice, please contact the Regional Administrator of the appropriate NRC Regional Office, or a Technical Contact listed below. Edward L. Jordan, Director Division of Emergency Preparedness and Engineering Response Office of Inspection & Enforcement Technical Contact: I. Villalva, IE M. Wigdor, NRR (301) 492-9635 (301) 492-7592 Attachment: List of Recently Issued IE Information Notices .
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