05000285/LER-1981-006-03, /03L-0:on 810624,reactor Protection Sys Nuclear Power Recorder Channel B Trip Setpoints Determined to Be Nonconservative.Caused by Faulty Temp Change Power Calculation Due to Grounded Hot Leg Temp Loop
| ML20010C227 | |
| Person / Time | |
|---|---|
| Site: | Fort Calhoun  |
| Issue date: | 07/07/1981 |
| From: | Mueller R OMAHA PUBLIC POWER DISTRICT |
| To: | NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION IV) |
| Shared Package | |
| ML20010C224 | List: |
| References | |
| LER-81-006-03L, LER-81-6-3L, NUDOCS 8108190264 | |
| Download: ML20010C227 (4) | |
| Event date: | |
|---|---|
| Report date: | |
| 2851981006R03 - NRC Website | |
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[Ti--] LPo i n t s in the current loop _B/122-Il has caused alternate current paths to the Subqngled1 j,l,l l Margin Monitor and RPS indication and the resultant erroneous "B" channel readings.
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LER No.81-006 Omaha Public ?ower District Fort Calhoun Station Unit No. 1 Docket No. 05000285 Attachment No. 1
Safety Analysis
The trip setpoints of "B" channel RPS units corresponding to High Power Level, Thermal Margin / Low Pressure, and a::ial power distribution were determined to be non-cor.cervative due to a faulty "B" channel 4T power calculation. This AT power calculation was in error due to a grounded "B" channel llot leg temperature loop. Thus, the "B" channel of the RPS was bypassed. During the time "B" channel was bypassed, the remaining channels "A", "C" and "D" - were operable and fully capable of performing their intended purpose, that is, to safely shutdown the reactor, should the need have arisen.
During the time the "B" subccoled Margin Monitor was in alarm (corresponding to the temperature side only) the "B" Subcooled Margin Monitor pressure loop remained operable as did the "A" Subcooled Margin Monitor temperature and pressure loop, a
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LER No.81-006 Omaha Public Power District Fort Calhoun Station Unit No. 1 Docket NO. 05000285 Attachment No. 2
Corrective Action
Upon inspection of "B" Chant.ul (RPS) abnormalities it was deter-mined that the "B" hot leg temperature channel had been grounded at the transmitter TT/B-122H.
In addition, the "B" channel Nuclear Power Recorder (which is fed via a signal from the "B" hot leg temperature loop and the "B" channci Linear Power Drawer) also had a ground at or near the Nuclear Power recorder. As a result of the dual ground, an alternate current path between the "B" hot leg temperature loop, the linear power drawer and the Nuclear Power Recorder was created. As a result of this alternate current path, the normal signal which feeds into the RPS (and more specifically, the dT power calculator) was interrupted, thus causing the liigh Power Level Thermal Margin / Low Pressure and Axial Power Distribution trip units to fail non-conser-vatively.
The grounds were isolated and repaired / alleviated via work per-formed on Maintenance Order #10647. The affected "B" channels were recalibrated and applicable sections of Surveillance Test ST-RPS-4 were performed. The loop B-122/H was thus returned to oberability and the "B" channel RPS taken out of bypass.
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LER No.81-006 Omaha Public Power Di.attict Fort Calhoun Static.: Unit No. 1 Docket No. 05000285 Attechment Nn, 3 Failure Data This is the first occurrence of this type at the Fort Calhoun Station Unit No. 1.
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