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Information Notice No. 83-43: Improper Settings of Intermediate Range (IR) High Flux Trip Setpoints
SSINS No.: 6835 IN 83-43 UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF INSPECTION AND ENFORCEMENT WASHINGTON, D.C. 20555 June 24, 1983 Information Notice No. 83-43: IMPROPER SETTINGS OF INTERMEDIATE RANGE (IR) HIGH FLUX TRIP SETPOINTS Addressees: All nuclear power facilities holding an operating license (OL) or a construction permit (CP). Purpose: This notice is to inform holders of OLs and CPs of events that occurred at the North Anna nuclear power facility that could be of safety significance at their facilities. Namely, the improper settings of the high flux trip setpoints of the IR nuclear instruments subsequent to reloading the core with a low neutron leakage fuel configuration. Description of Circumstances: On June 6, 1983, the North Anna Unit 2 plant was being returned to full power operation following its second refueling outage, during which time the core was reloaded with a low neutron leakage fuel configuration to reduce the neutron fluence on the reactor pressure vessel. The plant was at about 50% full power when the shift supervisor noticed that there was no indication on the trip status annunciator panel that the IR high flux bistables had tripped. These IR trips had been properly blocked when reactor power reached 10% of rated power; however, the IR bistables on channels N35 and N36 should have tripped and illuminated the associated annunciator windows when reactor power reached 25%. In addition, the plant's technical specifications require the IR high flux bistables to trip at 30% power or less. The station's reactor engineer was promptly notified of the above trip failures. His evaluation was that the trip values being used (4xlO-4 amperes) were equivalent to 25% power for the previous core pattern, but that they were improper for the new low neutron leakage fuel load pattern. Subsequent to this determination, both IR high flux trip channels were reset to trip at 2.5xlO-4 amperes, the value computed by the licensee as being approximately equivalent to 25% power. Since the North Anna Unit 1 plant had also been previously refueled with a low leakage core, the licensee reviewed the settings of the Unit 1 IR high flux trip settings. During this review, it was discovered that the Unit 1 IR high flux 8305110483 . IN 83-43 June 24, 1983 Page 2 of 3 trip settings were also improper. As a result the trip setpoint for Channel N36 was changed to 2.5XlO-4 amperes, the value equated by the licensee as being equivalent to 25% power. Channel N35, however was believed to be properly calibrated and was left at its original setting of 4xlO-4 amperes. On a subsequent startup, however, Channel N35 failed to trip prior to reaching the 30% power limit of the technical specifications; therefore, a new setpoint adjustment was made. The value of 4xlO-4 amperes was determined by the licensee as being equivaient to 25% of rated power during the initial startup of Unit 2. Subsequently, the term "current equivalent to 25% power" was dropped from the startup procedure and the term "4xlO-4 ampere" was substituted as the standard setpoint for the IR high flux trip. The licensee now recognizes that the required setpoint may vary with care age, rod pattern and core design, especially new core designs using a low neutron leakage fuel pattern. Evaluation of these factors is underway for North Anna Units 1 and 2 to find an accurate method of determining the IR current equivalent to 25% power for a given core at anytime during its life. Discussion: The IR high flux trips provide protection against nuclear excursions during plant startup. Such protection is afforded by tripping the reactor if nuclear power exceeds 25% of rated power provided that the IR high flux trip had not been manually blocked prior to the excursion. (The technical specification for the trip setpoint is 30% of rated power.) Thus, improper IR high flux trip setpoints degrade the reactor protection system during plant startup. Reactor cores are being reloaded with low neutron leakage fuel patterns to decrease the rate of neutron embrittlement of the reactor pressure vessel. However, the effects of a low neutron leakage fuel pattern on ex-core detector calibration had not been fully accounted for by certain licensees. As a result, the high flux IR trips are effectively at higher power levels than assumed in the safety analyses. Such a condition existed at Zion Unit 1 subsequent to changing the core pattern from a conventional pattern to a low neutron leakage pattern. This particular event occurred on July 7, 1982 and in addition to LER No. 82-031/03 L-0, dated October 27, 1982, it has been described by the Institute for Nuclear Power Operations (INPO) in INPO Significant Event Report (SER) 72-82. Subsequent to the event, calculations performed by the licensee revealed that the power range detectors were indicating 46% less core power than actual power until they were recalibrated. On December 9, 1982, subsequent to the Zion event and alerted by the information contained in INPO SER 72-82, Maine Yankee discovered that its IR monitors were not responding as expected during low power physics tests. As a consequence, prior to power escalation Maine Yankee conservatively calculated adjustments of the instrument gains based on design power distribution calculations. The customary calibrations, based on plant heat balance, were later performed, at . IN 83-43 June 24, 1983 Page 3 of 3 power. More recently the NRC has been informed that Surry Units 1 and 2 had conditions similar to those at North Anna subsequent to reconfiguring the cores to low leakage patterns. No written response to this notice is required; however, licensees should review the information contained in this notice for applicability at their facilities, especially if a low leakage core has been installed or is being contemplated. If you have any questions regarding this matter, please contact the Regional Administrator of the appropriate NRC Regional Office or the Technical Contacts listed in this notice. Edward L. Jordan Director Division of Emergency Preparedness and Engineering Response Office of Inspection and Enforcement Technical Contact: I. Villalva, IE (301) 492-9635 C. Julian, RII (404) 221-5535 Attachments: List of Recently Issued IE Information Notices .
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