05000315/LER-1985-069, :on 851117,intermediate-range Neutron Flux Instrument N-35 Inoperable.Caused by Instrument Drift. Detector Replaced & Instrument Will Be Trended in Effort to Identify Future Drift
| ML20202F654 | |
| Person / Time | |
|---|---|
| Site: |  |
| Issue date: | 07/07/1986 |
| From: | Blind A, Will Smith INDIANA MICHIGAN POWER CO. (FORMERLY INDIANA & MICHIG |
| To: | NRC OFFICE OF ADMINISTRATION (ADM) |
| References | |
| LER-85-069, LER-85-69, NUDOCS 8607150209 | |
| Download: ML20202F654 (4) | |
| Event date: | |
|---|---|
| Report date: | |
| 3151985069R00 - NRC Website | |
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e NRC Form 30g U S. NUCLEM KE1ULATOJV COMMtSSION
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FACILITY NAME (11 DOCKET NUMSER (2)
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C. Cook Nuclear Plant, Unit 1 0l5l0l0l0131115 1lOFl0l3
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Operation With Inoperable Intermediate Range Neutron Flux Detector, Caused by Instrument Drift EVENT DATE (Si LER NUMSER (6)
REPORT DATE (7)
OTHER FACILITIES INVOLVED (8)
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,,r,no This revision is subnitted to update the cause for the inoperable instrumenti.
On Novanber 17,1985, at 0043 hours4.976852e-4 days <br />0.0119 hours <br />7.109788e-5 weeks <br />1.63615e-5 months <br />, Intermediate Range (IR) Neutron Flux Instrument N-35 (EIIS/IG) was determined to be inoperable. The reactor trip setpoint for N-35 was discovered to be above the allowable limit of less than or equal to 30 percent Reactor Thermal Power (RTP). The detennination was made while RTP was at 29 percent. With 1 inoperable IR instrument and power above 5 percent RTP, power escalation continued as allowed by Technical Specifications. It was not evident frcm the data available below 5 percent RTP that the setpoint was above allowable limits. The cause has been attributed to instrument drift which is presumed to be the result of a faulty detector tube (EIIS/DET).
The N-35 detector has been replaced and the instrument will be trended in an effort to identify any future drift.
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US NUCLEAR REGULATORY Con.netessON NRC Fe,m 3084 UCENSEE EVENT REPORT (LER) TEXT CONTINUATION AreRovao ome No. siso oio.
EXPIRES: S/31/3 PACILITY NAME (1)
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Conditions Prior to Event
Unit 1 was operating at 29 percent reactor thermal power (RTP) during the initial power escalation for Fuel Cycle 9.
Description of Event
On November 17,1985, at 0043 hours4.976852e-4 days <br />0.0119 hours <br />7.109788e-5 weeks <br />1.63615e-5 months <br />, intermediate range (IR) nuclear instrument N-35 (EIIS/IG) was discovered to be inoperable. We reactor trip setpoint was in excess of Technical Specification 2.2.1 Table 2.1-1 which requires the setpoint to be less than or equal to 30 percent RTP. We reactor trip setpoint for N-35 was determined to be above the allowable limit while plotting detector current versus reactor thermal power (while at approximately 29 percent RTP) during startup. Technical Specification 3.3.1.1 Table 3.3-1, Item 5, Action 3, allowed the unit to continue power escalation with one inoperable IR nuclear instrument as KrP was above 5 percent. It was not evident frcm the data available below 5 percent power that the setpoint.was above allowable limits.
We reactor trip setpoint for N-35 was lowered to less than 25 percent RTP at 1051 hours0.0122 days <br />0.292 hours <br />0.00174 weeks <br />3.999055e-4 months <br /> on Nov mber 17, 1985, fulfilling Technical Specification requirements. Se detector (EIIS/DET) current output for N-35 continued to be tremled and was later found to be decreasing. On February 13, 1986, the setpoint was found to have drifted above 30 percent RTP. W e Unit during this tine period was operating at 90 percent RTP. The IR detector high voltage and empensating voltage were checked and found to be set correctly.
Based on vendor reccamendations, voltage was increased to 1200 volts to place the detector tube at what appeared to be its saturation point. We detector output current was trended with the indication of continued instrumeht drift evident.
Following an unrelated Unit 1 reactor trip on May 28, 1986, the unit was placed in cold shutdown to replace the N-35 detector. h ere were no other cmponents or systms related to this event that were inoperable at the start of the event.
Cause of Event
The intermediate cause for IR instrument N-35 reactor trip setpoint being in excess of Technical Specifications was instrument drift. W e root cause is not known, but it is, presumed to be the result of a faulty detector tube, g....
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Analysis of Event
This supplanental report is subnitted to report on actions taken since the original LER. 'Ihe additional actions taken, including replacment of the N-35 detector, were based on trending of the instrument outpit. The original LER reported that the setpoint was erroneous based on reduced neutron leakage fra" the core, it has since been determined that the instrument setpoint had drifted.
The IR instrument (N-35) provides redundant protection for high startup rates (SUR). 'Ihis trip is blocked above 10 percent RTP, The second of two IR instruments (N-36) remained operable and would have tripped the reactor if required as the coincidence is one out of two instruments.
In addition, the four power range instruments provided high positive rate trip (5 percent RfP per 2 seconds) and high flux low setpoint trip (25 percent RTP) protection with a two out of four coincidence. Based on the redundant IR instnment (N-36) and the four power range instruments it is concluded that adequate core protection existed and that this condition did not constitute an unreviewed safety question nor did it create a substantial hazard to the health and safety of the public.
Corrective Actions
The irmediate corrective action was to reduce the setpoint below 25 percent RTP. 'Ihis action did not correct the problan because the instrument continued to drift.
The instrument (N-35) detector has been replaced and the Plant will base its future setpoint predictions on detector current data and average powbr distribution fran the end of the previous fuel cycle. 'Ihis will give a more conservative prediction as it will account for any instrument drift.
Previous predictions were based on data frun the beginning of the last fuel cycle. In addition, the detector currents will be trended in order to identify any further instnment drift that may occur during a fuel cycle.
Failed Canponent Identification Westinghouse intennediate range nuclear instrunent detector nodel number ht-23707, EIIS:DLT.
Previous Similar Events
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$ hUI INDMMR & MICHIGAN ELECTRIC COMPANY Donald C. Cook Nuclear Plant P.O. Box 458, Brusgman, Mctugan 49106 i
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l July 7, 1986 l
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United States Nuclear Regulatory Commission Document Control Desk j
Washington, D.C.
20555 l
Operating License DPR-58 i
Docket No. 50-315 4
j Document Control Manager:
j In accordance with the criteria established by 10CFR50.73 entitled Licensee Event Reporting System, the following
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report /s are being submitted:
j 85-069-01 Sincerely, l
1
/
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p 1
.G.
- Smith, j
Plant Manager l
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I Attachment I
cc:
John E. Dolan J.G. Keppler, RO:III M.P. Alexich R.F. Kroeger H.B. Brugger j
R.W. Jurgensen i
NRC Resident Inspector l
R.C. Callen, MPSC G. Charnoff, Esq.
j D. Hahn l
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PNSRC j
A.A. Blind i
i Dottie Sherman, ANI Library l
File i
l 4
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