ML20154A917

Provides Info for Preparation of Correspondence Re Failure of Instrument Air Sys Check Valves in Supply Lines to Plant Safety Injection & Refueling Water Tank Level Detection

ML20154A917
Person / Time
Site:	Fort Calhoun
Issue date:	05/05/1988
From:	Calvo J Office of Nuclear Reactor Regulation
To:	Rossi C NRC
References
NUDOCS 8805160243
Download: ML20154A917 (5)

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;E WASHINGTON, D. C. 20555 g...../ May 5,1988 MEMORANDUM FOR

Charles E. Rossi, Director Division of Operating Events Analysis TFPU: Lester Rubenstein, Assistant Director for Region IV Reactors and Special Projects Division of Reactor Projects - III, IV, V and Special Projects FR0tu Jose A. Calvo, Project Director Project Directorate IV Division of Reactor Projects - III, IV, V and Special Projects

SUBJECT:

FORT CALHOUN SAFETY INECTION AND REFUELING PATER TANK (SIRWT) LEVEL CETECTION During the recent meeting to discuss the status of Region IV plants, the failure of instrument air system check valves in the supply lines to the subject instrurentation was determined to need further reviw. Since your division was given the lead for the preparation of corre ience on the problen, the following information is being provided to a h -his effort.

This situation appeared when the licensee found that the check valves leaked excessively during testing. In order to conduct the test, the instrument air supply to the SIRUT bubblers was secured, and a nitrogen bottle was utilized to supply the ces pressure. When the licensee realized that the valve:were failing the test, the test setup with the N,, bottle was left in place to assure a reliable source of gas pressure remained 8vailable. Subsequently, the licensee found that the check valves that were installed were designed for high terperature water or steam service. Therefore, the valves were replaced with acceptable a newfor design employing air service the N,,(soft seats bottles haveand a spring-loaded been rewoved). If thedisctesting that is had not been perfonned and the vatves not replaced, it is possible that the Recirculation Actuation Signal (RAS) during a LOCA event would have prematurely transferred the suction of the High Pressure Safety Injection Pumps to the containment sump. Since an as-found leak rate test was not done on the original check valves, it is not possible to state just how long the air .

acevrulators would have rainteined the bubblers operable. Additionally, there I are other factors which could affect the air system availability such as the  !

return of the air compressor after the load shedding sequence or the occurrence i of a coincident loss of off-site power. In these cases, the air supply source may be returned before a sufficient demand had been placed on the system to cause premature RAS actuation. In a worst case scenario assuming irr'ediate loss of air pressure, the high pressure pumps may not obtain suction from the sump. The pumps in this situation may fail in about two minutes without operator action. Since SIRWT remote level would also be lost, the operator must rely on containment surp level and injection duration to determine if an ,

1 override to return to SIRWT suction is appropriate.

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Charles E. Rossi The SIRWT level instrumentation is provided by an air bubbler system that determines tank level by sensing air backpressure. Two channels of tank level are provided with the air pressure being converted to an analog level signal by a local transmitter. Separate from this level detection is a set of four bubblers which have Static-0-Ring pressure switches that are used to provide the low tank level input into the Pecirculation Actuation Signal set-up circuitry. The 31RWT low level (tank level of 16 in.) signal combines with either the Pressurizer Pressure Low signal (PPLS) or the Containment Pressure High signal (CPHS) to set-up the RAS. The RAS performs the function of securing the low pressure safety injection pumps and transferring suction for the High Pressure Safety Injection and the Containment Spray Pumps from the SIRWT to the containment sump. Although the initiation of RAS is annunciated in the control room, no direct level readings are available from this portien of the circuit. The analog level transmitters have both high and low level alarm functions also annuciated in the control room.

Since both the tank level instrumentation art the low tank level input device for RAS set-up require instrument air, events which affect the status of the air supply have a possible consequence on their function. In addition to the obvious loss of the instrument air compressors during a loss of off-site power event, the compressors are deenergired during a load shedding secuence in a loss of coolant event. Because of this possibility, the SIRWT low level switches for RAS were designed with a supply of air that is guaranteed by the incorporation of instrument air supply check valves tc isolate the bubblers upon loss of air pressure and the use of air accumulators. The problem noted by the licensee, however, was that during leak testing, each of these valves leaked excessively. This would have resulted in a bleeding down of the air pressure in the accumulators at a rate determined by the residual pressure in ,

the instrunent air system. The prenature bleeding down of the bubbler air I supply could cause the actuation of the low level input into the RAS set-up circuit at a premature time. Sirce the analog level instrurents do not utilize air accumulators, the function of these instruments is not maintained when the instrument air pressure is lost. Depending on the leakage and demand on the instrument air system, the length of time that air will be available will vary.

The PAS input will occur when the air pressure drops to approxirately 0.5 psig, and the level indication will trend down as pressure decreases. The low level alarm will be received as sensed level reaches 186 inches.

The consequences of an event which requires the initiation of safety injection will change fron that analy7ed if the time of inititation of the RAS signal is premature. Transient analysis for the Loss of Coolant Accidents assume that a specified cuantity of SIRWT water of a specified boron concentration and temperature is injected. A prenature RAS will secure the low pressure injection pumps and shift the high pressure safety injection pumps to a recirc-ulation mode. This may occur at a time which is insufficient to allow proper suction to be established from the sump. Since the RAS also cause the mini-flow ,

recirc lines back to the SIRWT to shut, the high pressure pumps may fail within I two minutes due to insufficient flow for cooling if suction cannot be estab-  !

lished. Although the RAS circuit design has a means to override the signal and realign the valves, the loss of instrunent air fails the SIRWT control roem l I

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Charles E. Rossi level instruments low such that the operator will not know true level. His only information is that the time to drawdown the tank should be about 20 ninutes.

An early RAS would be indicative of an instrument problem or a major loss of water inventory.

In the case of Fort Calhoun, the instrument air system is not classified as  !

safety-related. Also, the air isolation check valves are not required to be tested during surveillance or inservice testing or periodically inspected.

This fact was first addressed by a Safety System Outage Modification Inspection (SSOMI) in September 1986. The need to verify the condition and functionally of this valves was further highlighted during the water ingress I into the instrument air system event on July 6, 1987. Subsequently, the licensee has agreed to periodically leak test various air supply check valves.

If you should need further assistance in the preparation of the Information  !

Notice on this subject, please contact Patrick Milano, on ext. 21347. )

1 Jose A. Calvo, Project Director Project Directorate IV Divisicn of Reactor Projects - III, IV, V and Special Projects cc: T. Murley J. Sniezek F. Miraglia D. Crutchfield G. Holahan W. Jensen DIJTRIBUTION

\ POCKET FILE NRC & LOCAL PDRs PD4 READING FILE i DCRUTCHFIELD l GHOLAHAN PN00 NAN ABOURNIA OGC JCALVO EJORDAN l JPARTLOW -

ACRS (10) .

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• See previous concurrences: 7_

F04/BFM* PD4/PM* PD4/D / @ DR4A PMilano ABournia JCalvo LRubenstein 4/29/88 4/29/88 r // v/88 4/ f-/88

1 level instruments low such that the opeator will not know true level. His only I information is that the time to drawdown the tank should be about 20 minutes.

An early RAS would be indicative of an instrument problem or a major loss of water inventory. '

In the case of Fort Calhoun, the instrument air system is not classified as safety-related. Also, the air isolation check valves are not reqfuired to be tested during surveillance or inservice testing or periodically inspected.

This fact was first addressed by a Safety System Outage Modii ation l Inspection (SSOMI) in September 1986. The need to verify the condition and .

functionally of this valves was further highlighted during );he water ingrees into the instrument air system event on July 6, 1987. Sub equently, the licensee has agreed to periodically leak test various air supply check valves.

If you should need further assistance in the preparati of the Infonnation Notice on this instrumentation, please feel free to c tact Patrick Milano.

Jose A. Cy vo, Project Director I Project irectorate IV l Divisio of Reactor Projects - III, IV, and Special Projects cc: T. Murley ,

J. Sniezek i F. Miraglia l

0. Crutchfield G. Holahan W. Jensen l P. Milano 1 I

DISTRIBUTION DOCKET FILE NRC & LOCAL PDRs PD4 READING FILE I DCRUTCHFIELD GHOLAHAN PN00 NAN ABOURNIA OGC_ _ j ggy , j E0RDAN ./ I JPARTLOW /

ACRS (10) /

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PD4/ BPM PD4/PM PD4/D PMilano

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/ ABournia JCalvo 4/8/88 4/11/88 4/ /88 (4[omm.J f / pp i

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Charles E. Rossi level instruments low such that the operator will not know true level. His only information is that the time to drawdown the tank should be about 20 minutes.

An early RAS would be indicative cf an instrument problem or a major loss of water inventory.

In the case of Fort Calhoun, the instrurent air system is not classified as safety-related. Also, the air isolation check valves are not reouired to be tested during surveillance er inservice testing or periodically inspected.

This fact was first addressed by a Safety System Outage Modification Inspection (SS0MI) in September 1986. The need to verify the condition and functionally of this valves was further highlighted during the water ingress into the instrurent air system event on July 6,1987 Subsequently, the licensee has acreed to periodically leak test various air supply check valves.

If ycu should need further assistarcs in the preparation of the Information Notice on this subject, please contact Patrick Milano, on ext. 21347.

kc Lt, b. [ W

/ ose J A. Calvo, Project Director Project Directorate IV Division of Reactor Projects - III, IV, V and Special Projects cc: T. Murley J. Sniezek F. Miraglia D. Crutchfield G. Holahan W. Jensen j l

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