ML20085L598
| ML20085L598 | |
| Person / Time | |
|---|---|
| Site: | Fort Saint Vrain  |
| Issue date: | 06/18/1975 |
| From: | Brey H PUBLIC SERVICE CO. OF COLORADO |
| To: | Howard E NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION IV) |
| Shared Package | |
| ML20085L596 | List: |
| References | |
| RO-50-267-75-10, NUDOCS 8311020109 | |
| Download: ML20085L598 (5) | |
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P. O. Box 361, Platteville. Colorado 80651 June 18. 1975 Mr. E. Morris Howard, Director s'.-
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Nuclear Regulatory Commission Region IV
.e Office'of Inspection and Enforcement
-Suite 1000 Arlington, Texas 76012 Ref:
Facility Operating
- teense No. DPR-3a t
Docket No. 50-267
Dear Mr. Howard:
Enclosed please find a copy of Unusual E-ent Report No. 50-267/75
'O, Preliminary, submitted per.the requirements of tne Technical Specifications.
Very truly yours.
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H. Larry Brey Superintendent-0perationc
. Fort St. Vrain;uclear Generating'St:rion llLE:11 cc:
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- A Unitual DISTRIBlTf0M Events and Abn'otm41 t'nu sual herating Monthly occurrence Occurrence Cost s port.
f.etter Reports Reporte e
Addreesee San Francisco Operations Office EPhA 1333 Broadway Oakland, California 494612 2
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Atens Manager 4
Finance & Budget v.
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Calif. Patent Group Division of Reactor R*sestch sad 11evelopment ERDA Washington, D. C.
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Attns Director 1
Asst. Dir. for INBR Programs 2
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Aest. Dir. for Cas cooled Reactor Project 1
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Asst. Dir. for Engrg. sad Technology
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1 Asst. Dir. for Reactor Safety e
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ERDA -SCRP0-SD P. O. Box 81325 San Diego, Californis 92138 1
1 W. Soule'. Project Engr.
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ITdUSUAL EVEhT REPORT <DATE:
June 11, 1975 OCCURRENCE DATE: April 28, 1975 PORT ST. VRAIN NUCLEAR CENERQTING STATION PUBLIC SERVICE COMPANY OF COLORADO P. O. BOX 361 PLATTEVILLE, COLORADO 80651 REPORT NO. 50-267/75/10 Preliminary IDENTIFICATION OF EVENT:
During routine analysis of PCRV liner cooling tube outlet temperatures, as required by SUT B-3, four individual cooling tubes in the top head region of the PCRV were observed to be indicating a temperature rise of 2'*F.
The expected temperature rise should be on the order of~3 to 5*F.
CONDITIONS PRIOR TO EVENT:
X Steady State Power Routine Shutdown Hot Shutdown
' Routine Lead Chan;;e Cold Shutdown Other (specify)
Refueling Shutdown Routine Startup The major parameters at the time of the event were as fellows:
Power Rtr.
13
>Tch Elcet.
O ice Secondary Coolant Pressure 1212 psig Temperature 316
- F 240,000-
- /hr Flow Primary Coolant Pressure 411 psig Temperatute 274
- F Core ::tlet 370
- F Core 0utlet Flow 312,000
_ #/hr.
m-m
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e Page two DESCRIPTION OF EVENT:
The four tubes which showed larger-than-expected temperature rises are associated uith the lower end of each of the 39 top head penetrations which communicate with the PCRV cavity upper plenum.
It was thought that hot gas from the upper plenum was being drawn into the annulus between the control rod drive housing and the refueling penetration.
In normal operation, such a flow of gas could result from the lower static pressure which exists inside the control rod drive housing relative to the static pressure associated with the core inlet plenum.
APPARENT CAUSE OF EVENT:
X Design Unusual Service Cond.
Including Environ.
Manufacture Component Failure Installation /Const.
Other (specify)
Operator Procedure The static pressure inside a control rod drive housing is slightly greater than that at the core outlet, but less than that of the core inlet plenum.
These pressure differences are intended to cause the normal purge flew into the drive housing to flow downward to exit the control rod guide tubes, flow over the control rods for cooling, and discharge into the core eutlet plenum.
Due to the equipment maintenance access holes in the control rod
' drive housing, plus the articulated lower guide tube and orifice valve ass.embly, no positive seal exists between the annulus surrounding the drive unit and its encased internals. Therefore, a pressure difference sufficient to draw hot gas from the upper plenum into the annulus is present whenever the primary coolant is being circulated. The temperatures of the het gas is equal to the core inlet primary coolant temperature, which varies from 210*F at zero power, up to 760*F at full power.
The four het cooling tubes are physicc:ly welded to the refueling penetratien nozzles at the upper headliner, and therefore they responded promptly to the presence of hot hellun flowing into the penetration between the CRD assembly and the penetration liner.
ANALYSIS OF EVFNT:
The Icakage of hot gas into the annulus between the centrol rod drive housing and the refueling penetration liner has been deternined to be the cause of the unusual temperature. rise associat ed with 't op head t ubes T-39, T-40, - T-41, and T-42.
This gas flow is caused by pressure differentials which exist
-between the inside and outside of a control rod drive unit.
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Page three Analysis of Event (continued)
To determine the specific flow paths that exist in the CED assembly, a series of diagnostic tests were performed on a spare control rod drive unit. The results indicated that the flow entered the drive housing through numerous access and maintenance ports, and exited from the control rod guide tubes and from the duct which directs the reserve shutdown material from the drive unit into the receiver channel in the core.
Subsequent to these mechanical flow tests, a computer simulation of the various flow processes was developed and used to further analyze the physical situation and to evaluate the effects of proposed modifications designed to alleviate the situation.
CORRECTIVE ACTION:
The design of corrective seals at specific locations both inside' and outside of the control rod drive unit is currently in progress.
Basically, the drive honsis.gs are expected to be modified as follows:
- 1) A modified flexible seal will be installed at the lower outside end of the housing in place of the existing anti-convection ring currently in use.
- 2) A spring-loaded piston ring / cable busaing assembly will be positioned at the upper end of each' control rod guide tube.
- 3) A seal welded piston ring seal will be positioned between the inner and outer feed pipes-below the rupture disc assembly of the reserve shutdown hopper contained in the control red drive unit.
This seal is so designed that it in no way impedes the flow of reserve shut'down material which would flow through this tube if released by the rupture disc above.
These three seals are. expected to reduce the leakage flow to an acceptab'e 1cvel.
FAILURE DATA /
SIMil. TAR REPORTED EVENTS:
Nonc pROGRABDIATIC IMPACT:
Not known - modiff. cations to be performed in parallel with other prograr obj ec t ives.
b'e do not know at this time what iten will be controlling.
CODE IUPACT:
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Recommended:
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Approved:. 2
- 11. Larry Bre *
'/Frederic I'.
Swart Superintendent-Operat i mn Supt. Nucicar Prodnet ton Fort St.- Vrain Nuclear Public Service Comp.'ny Cencrating Station of Coloindo n
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P-77063 Mr. E. Morris Howard, Director Nuclear Regulatory Commission Region IV Office of Inspection and Enforcement Suite 1000 Arlington, Texas 76012 REF: Facility Operating License No. DPR-34 Docket No. 50-267
Dear Mr. Howard:
Enclosed please find a copy of Unusual Event Report No. 50-267/75-10B, Final Supplement, submitted per the requirements of the Technical Speci-fications.
Also, please find enclosed one copy of the Licensee Event Report for Unusual Event Report No. 50-267/75-10B.
Very truly yours, f
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Ac' Ju <,
Frederic E. Swart Nuclear Production Manager FES/alk l
cc: Director, Office of Management Information and Program Control l
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