ML20209E977

Preliminary Rept of non-routine Occurence Re Identification of Possible Single Failure for Safety Related Temperature Sys on 990706.Low Temperature Auto Rundown Proposed & Problem Rept Initiated to Document Review of Event

ML20209E977
Person / Time
Site:	University of Michigan
Issue date:	07/08/1999
From:	Becker C MICHIGAN, UNIV. OF, ANN ARBOR, MI
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
99-024, 99-24, NUDOCS 9907150191
Download: ML20209E977 (1)

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j MICHIGAN MEMORIAL-PHOENIX PROJECT l pg PHOENIX MEMORIAL LABORATORY FORD NUCLEAR REACTOR l ANN ARBOR, MICHIGAN 48109-2100 8 July 1999 Docket 50-2 License R-28 U.S. Nuclear Regulatory Commission Document Control Room Washington, DC 20555 Re: Preliminary Report of Non-routine Occurrence -Identification of a Possible Single Failure for the Safety Related Temperature System This is a preliminary report to infonn the Commission of a condition involving a possible safety-related, single failure, could result m the loss of the capability of the Temperature System to protect Limiting Safety Settings. The condition was discovered on July 6,1999 at i130 while designing an upgrade to the system. A review of Technical Specifications 6.6, Reporting Requirements determined that the submittal of a written report is required by August j 5,1999. At the time that the single failure vulnerability was discovered, the reactor was in the second week of a two  ?

week maintenance outage.

Temperature System The Temperature System uses resistance temperature detectors (RTDs) as the sensing element. These RTDs are I connected 'o a resistance bridge to provide a 4 - 20 mAmp current loop. This 4-20 mA current loop is dropped 3 across a pru-ision 250 ohm resistor to provide the voltage signal necessary to drive the temperature recorder. The temperature recorder has pre-set alarms which provide the required technical specification action upon reaching a Limiting Safety System Setting. The resistance bridge and 4 - 20 mA current loop are driven by a +15 Vdc power supply., This 15 Vdc power supply is powered from 110 Vac Reactor Control Power and has a fuse for overload protection.

Summary ofSingle Failure -)u ],

The possible single failure is a loss of the 15 Vdc power supply to the precision bridge and current loop. A loss of the 15 Vdc power supply results in a zero mA current in the current loop, and subsequently a zero voltage is provided to the temperature recorder. The temperature recorder sees low voltage as low temperature and as such, a loss of the 15 Vdc power supply results in low temperatures and prohibits the Reactor Coolant inlet Temperature, Reactor Pool Temperature, and Reactor Coolant Exit Temperature Auto Rundowns from occurring.

SafetyImplications A failure of the 15 Vdc power supply would result in the loss of all temperature indication for the primary and secondary coolant systems. A failure of this nature would be promptly observed by the Console Operator and would result in a Manual Shutdown of the Reactor. Preliminary review mdicates that a failure of this type has not occurred in the past. The 15 Vdc power supply appears to have been installed as part of a modification in 1985.

Corrective Action As part of the ongoing modification to the Temperature System, a Low Temperatt're Auto Rundown has been proposed to eliminate this single failure possibility. A Problem Report has been mitiated to document the review of this event. This modification will be reviewed by the Safety Review Committee before actual implimentation.

Sincerely, 1

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Christopher W. Becker i Nuclear Reactor Laboratory Manager i

i hone: (734) 764-6224 Fax: (734) 936-1571 yQQf{

File: . Correspondence 99-024 NRC Corre.pondence 1999 CAMy DocumentsTWBecker\ Letter File \99 024. doc 9907150191 990708 _ _ _ . _ _ _ _ _ _ _ . _ _ _ ._ . . _ _ . .

I PDR- ADOCK 05000002 www.umich.edu/~mmpp/

l 0 PDR Fax: (734) 936-1571 l