ML20085L691

LER 75/005A:on 750323,returning Backup Bearing Water to Svc Caused buffer-mid-buffer Upset,Tripping Two Circulators. Caused by Response of Pressure & Level Controllers Associated W/Backup Bearing Water.Sys Revised

ML20085L691
Person / Time
Site:	Fort Saint Vrain
Issue date:	06/21/1976
From:	Hillyard H, Swart F PUBLIC SERVICE CO. OF COLORADO
To:	Howard E NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION IV)
Shared Package
ML20085L694	List: ML20085L691 ML20085L698 ML20085L701
References
LER-75-005A, LER-75-5A, P-76140, NUDOCS 8311020344
Download: ML20085L691 (6)

Text

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80 32 I n I-NAME: W ' 30 M 7 7 7- 2*L.C2, y

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pubne service company ce cdlada P. O. Box 361, Platteville. Colorado 80651

e. .. J 2 3 SECE 7 h y,1,I'g?ygg'7g June 21, 1976 Fort St. Vrain

• --. Unit No.1

1. D' P-76140 Of b G 8 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 4

Docket No. 50-267 Dear Mr. Howard.

Enclosed please find a copy of Unusual Event Report No. 50-267/75/05A, Final, submitted per the requirements of the Technical Specifications.

Also, please find enclosed one copy of the Licensee Event Report for Unusual Event Report No. 50-267/75/05A.

Very truly yours,

& L ad[

Frederic E. Swart Superintendent, Nuclear Production Fort St. Vrain Nuclear Generating Station

• FES/alk cc: Mr. Roger S. Boyd gg ggEIOR d384

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' REPORT DATE: June 21, 1976 UNUSUAL EVENT 75/05A Page 1 of 4 OCCURRENCE DATE: March 23,1975 FORT ST. VRAIN NUCLEAR CENERATING STATION PUBLIC SERVICE COMPANY OF COLORADO P. O. BOX 361 -

PLATTEVILLE, COLORADO 80651 REPORT No. 50-267/75/05A Final IDENTIFICATION OF OCCURRENCE:

A situation occurred while testing the helium circulator backup bearing water system which resulted in "A" and "B" circulators automatically tripping off and the buffer indication on "C" and "D" circulators to drop resulting in operator action to set the brake and seal.

This is identified as an Unusual Event, per Technical Specification definition AC 7.6c, paragraph 3.

7 CONDITIONS PRIOR TO OCCURRENCE: Steady State Power Routine Shutdown Hot Shutdown Routine Load Change X Cold Shutdown Other (speckfy)

Refueling Shutdown Routine Startup The major plant parameters at the time of the event were as follows:

i Power RTR 0 MWth ELECT 0 MWe Secondary Coolant Pressurb N/A psig Temperature N/A 'F Flow N/A f/hr.

Primary Coolant Pressure 85 psig Temperature N200 *F Core Inlet

%200 *F Core Outlet

- . Flow Helium circulator self- f/hr.

turbining

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UNUSUAL EVENT 75/05A Page 2 of 4 DESCRIPTION OF

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OCCURRENCE:

The emergency feedwater header had been isolated via V-21453 in order to repair the pressure control for valve' PV-21105. Upon completion of this repair, pressurization of the emergency feedwater header was initiated by slowly opening the isolation valve V-21453. The following events then occurred:

1. Helium circulators lA and IB tripped on negative buf fer-mid-buffer differential pressure.

2. HV-2366-1 and HV-2366-2 changed from supplying makeup buffer gas to the helium circulators via the purified helium header, to the helium storage system. At a PCRV pressure of 85 psig helium density is barely suf ficient to place the purified helium com-pressors in service. Loss of a major helium user such as circu-lator lA and IB buffer caused the purified helium compressor to trip and the automatic controls to switch to the high pressure helium storage system.

y 3. The bearing water surge tank pressure in both loops increased to 140 psi, causing the buffer return flows to drop to 0 on the

~ remaining circulators. Circulator 1C buffer dif ferential pressure decreased to 10" negative but did not trip. Circulator ID buffer differential pressure dropped but did not go negative. The operator set the brake and seal on both of these circulators be-cause the return flow was so low. The Rix recovery compressors were then set up to vent to atmosphere in order to get the bearing water surge tank pressure down. When surge tank pressure equalled reactor vessel pressure, 1C and ID helium circulators were restarted.

. APPARENT CAUSE

,' 0F OCCURRENCE: X Design Unusual Service Cond.

Including Environment Manufacture

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. - - Component Failure '

, Installation /Censt. Other (specify)

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Operator ~ "

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Procedure. ,

The increased. pressure in the surge tanks due to the water inflow displaced j- -

helium from these tanks thereby increasing the buffer helium recirculstion suction pressure. This reduced the buffer return flow from the helium circu-l~ 1ators causing them to trip on buffer-mid-buffer. ,

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U C UNUSUAI. EVENT 75/05A Page 3 of 4 ANALYSIS OF ~ ~' ~ '~~~ ~ ~ '

OCCURRENCE: ~ "~

It appears that PC-21105 failed to control as did PCV-2191 and PCV-2192. With these valves not operating while V-21453 was being opened, there was no con-trol of the backup bearing water flow. The large amount of flow into the bearing water surge tanks caused a displacement of the helium from the surge tanks (due to a level increase). This helium flowed backward through the eq'u alization line 'to the sucEion' of the buffer helium recirculators, which

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caused reverse flow of the buffer return from the helium circulators. The reverse flow of buffer return caused a buffer-mid-buffer dif ferential suffici-ently nega:ive to initiate the automatic circulator trips.

The level increase would not have occurred under normal circumstances because with the normal bearing water pumps tripped and bearing water supplied from the backup bearing water system, a feedforward signal from the control system would have opened the surge tank drain valves. Under these conditions, the possibility of high rate of change in the surge tank level and the reversal of return flow from the circulators, is eliminated.

CORRECTIVE -

ACTION:

A test of the backup bearing water system was completed. This test, RT-356, was.

performed to verify that backup bearing water will function correctly and re-laced transients are repeatable. This test was performed at (1) pressures simulating the most adverse requirements for backup bearing water, and (2) at the pressure which also causes the most adverse helium buffer-mid-buffer inter-

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actions.

The test of (1) was performed five (5) times, by simultaneously transferring all four circulators to backup bearing water. The results of the test results show that on the most severe' transient there was a margin of 25 psid between the

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transient and the circulator trip point. All other test results show a larger margin.

The test of (2) was performed and the results show that the cost sev'ere differ- __.

ential pressure transient on the buffer-mid-buffer decreased but remained

, positive. The trip point is -9" water dif ferential pressure. The results of 2 test RT-356 verify the operability of the backup bearing water system and con-cludes_this report.

l No further corrective action is planned or required.

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j FAILURE DATA /

SIMILAR REPORTED OCCURRENCES: - - - - - - -

Unusual Occurrence No. 50-267/75/02 reports a failure of PV-21105-1 in the backup bearing water system which occurred on April 10, 1975.

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UNUSUAL EVENT 75/05A Page 4 of 4 PROGRAMMATIC UfPACT:

None CODE D! PACT:

None Submitted by: N c t u (L % U I. M .. .

H. W.' Hillyard , Jr.

• Technical Services Supervisor -

Reviewed by: ) M ..

K.LarryBQy

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