United States Nuclear Regulatory Commission - Protecting People and the Environment

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 



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                                                              IN 83-43     
                                                              June 24, 1983 
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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 
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                                                              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

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