2CAN089010, Voluntary Rept 50-368/90-V02-00 Re Result of Postulating Failure of Check Valve in LPSI Sys Not Previously Postulated.Caused by Higher Differential Pressure Not Considered.Valve Mfg Consulted

Voluntary Rept 50-368/90-V02-00 Re Result of Postulating Failure of Check Valve in LPSI Sys Not Previously Postulated.Caused by Higher Differential Pressure Not Considered.Valve Mfg Consulted

ML20056A565
Person / Time
Site:	Arkansas Nuclear
Issue date:	08/03/1990
From:	Ewing E ENTERGY OPERATIONS, INC.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
2CAN089010, 2CAN89010, NUDOCS 9008080234
Download: ML20056A565 (5)

Text

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, August 3, 1990' ml . 2CAN089010

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U. S. Nuclear Regulatory Commission -l Document-Control Desk Mail Station P1-137.

Washington, D. C.. 20555

SUBJECT:

Arkansas Nuclear One - Unit 2 Li Docket No._ 50-368 '

License No. NPF-6 Voluntary Report No. 50-368/90-V02-00 -  !

Gentlemen: -

This . report is being-submitted as a voluntary report to provide information ,

.obtained as a result of postulating a failure of a check valve in the Low ,~

Pressure Safety Injection System which had not previously been postulated.- >

Very truly yours, -

/'E!C.-Ewing. '

General Manager,

• Assessment ECE/DM/sgv Attachment cc:- Regional Administrator Region IV U. S. Nuclear Regulatory Commission 611 Ryan Plaza-Drive, Suite 1000  !

Arlington, TX 76011 INP0 Records Center .

Suite 1500 l 1100 Circle 75 Parkway .l Atlanta, GA 30339-3064 l l

4008080234 900803 ff

-hO POCK 05000368 4 PDC , l l

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'A. Plant' Status At the time of discovery of this condition Arkansas Nuclear One, Unit <

Two (ANO-2) was operating at 100 percent of rated thermal power in Mode 1 (Power Operation). Reactor Coolant System (RCS) [AB] temperature was-approximately 580 degrees Fahrenheit and RCS pressure about 2250 psia.

B. Event Description As part of an ongoing program implemented to ensure that switch settings on_c_ertain motor operated valves (MOVs) are. set and maintained ,

correctly to accommodate the maximum differential pressures expected on these valves, the. calculations used in establishing the design and functional _ requirements (e.g., torque switch settings, valve actuator sizing, etc.), for MOVs in various plant systems are being reviewed and revised,.if~necessary. The_ calculation revisions are sometimes necessary due to the development of new differential pressures (APs) that:are postulated to exist across the valve (s). The new APs are being derived by utilizing different assumptions for plant operating conditions (usually worst case conditions) than those used to establish the current design requirements for the valves. Postulating various equipment failures (act_ive and passive component failures), that were not assumed to occur when the original calculations were performed, is a major factor in establishing new APs and thus revised functional requirements for the valves. This approach essentially creates new

' operability' requirements for MOVs and established a new and enhanced design basis for individual MOVs. In some cases, the new postulated AP -

that a valve is required to operate against (open and close) is higher than that used in the original design calculations, thus requiring modifications to the valve and/or valve actuator to provide the identified functional capability. Additionally, if modification of the-valve or actuator is necessary to ensure the valve will operate properly under the_new postulated conditions, a question of whether the valve (s) was operable during previous plant operations is generated.

The Emergency Core Cooling System (ECCS) is comprised of the High -

Pressure Safety Injection (HPSI) [BQ], Low Pressure Safety Injection (LPSI) [BP] pumps and piping, and the Safety Injection Tanks (SITS).and associated piping. There are four injection headers from the ECCS to provide a cooling water flowpath to each of the RCS cold legs. A typical. injection header is supplied by a LPSI header and HPSI header which are joined together. (See Figure 1.) Design pressures for the different portions of system piping are as indicated on Figure 1.

In April 1989, revisions to calculation k-2054-00 for the LPSI system header isolation M0V's (2CV-5017, 2CV-5037, 2CV-5057 and 2CV-5077) were completed. This calculation indicated that based on assumed worst case conditions the maximum AP these valves should be required to operate against was approximately 1260 psid. This AP was significa M y larger that the original maximum AP (approximately 480 psid), wb k n was used to establish the existing design requirements for these MOVs.

Additionally, the new calculated AP for these valves was somewhat

unique in that contrary to the original calculations, the highest L system pressure was now postulated to occur on the downstream side of the valve disc. The original calculation had assumed that the valve

, would be required to operate against the maximum discharge pressure

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1 produced by an operating LPSI pump. The LPSI pump pressure is exerted i

-on the upstream side of the valve disc.

i The revised calculation utilized new assumptions for plant conditions and considered that during certain accident conditions (a limited range - ,

of small break LOCAs) both a HPSI and LPSI pump would be operating and with the current system design all the safety injection MOVs would be open. Under the postulated conditions RCS pressure could remain elevated above LPSI pump pressure and since HPSI discharge pressure would be significantly higher than LPSI discharge pressure, the LPSI v check valves (2SI-14s) would be in a closed position essentially acting as passive components under these conditions.- Following a postulated i failure (passive failure) of one_of the 251-14 check valves, the LPSI system could be subjected to the pressure of the operating HPSI pump (approximately 1450 psig). With a design pressure of approximately 500 psi, the LPSI-system could be overpressurized. The AP across the LPSI MOVs would be approximately 1260 psid (1450 psig minus the LPSI pump ',

discharge pressure of approximately 190 psig). Additionally, utilizing these assumptions the highest pressure exerted across the valve would

- become the_ operating pressure of the HPSI pump; the highest pressure would exist on what is'normally considered to be the downstream side of ,

the valve. Due to the physical design of the LPSI valves,.the high pressure would b exerted under the valve disc and would significantly increase the torque required to close the valve if open (i.e., the valve would have'to close against a AP of 1260 psid). The previous t LPSI MOV calculations had assumed a maximum AP of 480 psid across the valve with the highest pressure on the LPSI pump side of the valve.

Further evaluation of the condition was completed in August 1989, and-included a detailed review of the design capability of the LPS1 MOVs in their existing configuration. Based on this evaluation'it was determined that if the LPSI valves were open the valve actuators would not be capable of closing the valves under these new postulated conditions..

C. Root Cause L The criteria that existed during initial plant construction for l establishing the design and functional requirements for the LPSI header l- isolation MOVs did not consider the assumptions aiscussed above which were used to derive a new AP across the valves. Therefore,'this higher

'AP was not considered and the valve actuator was not originally l

designed to accommodate the effects of the higher AP.

D. Corrective Actions l The valve manufacturer, Target Rock Corporation (TRC) [T020], was consulted and concurred that if the LPSI MOV was open, the Limitorque actuator presently used on the valve would not be able to fully close against the.AP created by the postulated conditions. TRC stated that the Limitorque actuator could be modified by changing the gearing, and torque switch setting which would allow the actuator to close the valve against the postulated AP. .

Modifications to the LPSI valve actuators were developed and implemented during the 2R7 refueling outage (September 1989 to November 1989) to increase the capability of the valve actuator to close against a AP of 1260 psid with the highest pressure on the downstream side of the valve.

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Additionally, the ANO-2 Emargency Operating Procedure was revised to specify closing the LPSI MOVs after a Recirculation (Actuatior Signal (RAS) was initiated. This should ensure that the LPSI piping would not' be' exposed to pressures greater _ than its design pres ore if one of the 2SI-14s failed and RCS pressure or HPSI pressure were still above,the design pressure for the LPSI piping.

As afresult'of the MOVATS program established by Engineering in 1986' to evaluate the design and functional requirements for safety related ,

MOVs, deficiencies related to MOVs installed in.the plant are being identified. The valves or actuators'are being modified (as necessary)_

to; ensure design requirements are satisfied. "

E. Safety Significance

-The function of the LFSI MOVs in an accident condition (SBLOCA) is to open to provide a flovpath for injection of water to the RCS. The  :

ability of the valve (:) to open was not affected prior to or after the  :

, calculations were revised and the valve actuators modified. One of the [

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primary concerns related to the as found condition of the LPSI NVs  !

considering the postulated scenario was the potential effect of the condition on the functioral capability of the ECCS, while operating in i a post LOCA containment sump recirculation mode. If a failure of one 1 of the 2SI-14 check vaives were to occur while in this configuration and the size of the iwitiating LOCA was such that RCS pressure and HPSI system pressure were still above the design pressure of the LPSI system at the. time of an RA'i, then the' LPSI piping might be exposed to pressure greater than its design pressure and could possibly rupture.-

This could result in diversion of HPSI flow through the ruptured LPSI piping thus reducing' flow delivered to the core. Additionally, any water wst through such a postulated break would not be recoverable  !

(not returned to the containment sump for continued recirculation).

However, adequate Control Room indications were available to detect such a condition and procedural guidance existed to mcnitor HPSI flow during an accident situation. Therefore, should such a rupture have-occurred, the resultant decrease in cooling water injection' flow to the ,

RCS could have been detected and the appropriate Operator, actions taken

, (i.e., close HPSI injection MOVs associated with leakpath or secure

'HPSI pump if appropriate). Therefore, the safety concerns associated 1 with this condition are considered to be minimized by these factors.

F. Additional Information

• This report is being submitted as a voluntary report to provide information obtained as a result of postulating a failure of a single component which had not previously been postulated. The condition was reviewed considering the 10CFR50.72 and 10CFR50.73 reporting criteria and not considered to be reportable to the NRC based on original plant design criteria.

Energy Industry Identification System (EIIS) codes are identified in the text as [XX].

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SIT '2 -

_HPSI HEADER 1 7'

(TYP0 CAL)

( u2

_ HPSI HEADER 2 7'

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(TYPICAL) ncs M '

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/ - 2 8-18 i j(o i

asi-is ass-1s @

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_ , _ LP. EA.E.  ;

F OP (H L)@' 2Ss-14 f '@' (TYPICAL) -

(H L) i  ;

i DRAW U EGEND I

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Q = E:ZGIE PRESSURE / I4tf PREShBE.

i h=DESIGNPRESSUREPORNPSENEADERIUPSTREANOF80095IST.,485PSIG.

h = DESIGN NOURE FOR 1sti PERESSURE SIDE IS IMO PSIG.

h = DESIGN / RESSUGE FOR IAlf PERESSUttE SIDE IS SM PSIG.

h=DESIGIEPa'?3URE IS 2368 PSIG.

h=DESIGNP9tESCUmaIS700PSIG.

NPSI = NI2 PR2SSURE SAPETY IHJBCTION.

LPSI = I4W' P9tESSURE SAFETY IIEJECTIces.

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