ML20195B635

Submits Response to RAI Re GL 95-07, Pressure Locking & Thermal Binding of Safety Related Power Operated Gate Valves

ML20195B635
Person / Time
Site:	Comanche Peak
Issue date:	05/25/1999
From:	Terry R, Walker R TEXAS UTILITIES ELECTRIC CO. (TU ELECTRIC)
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
GL-95-07, GL-95-7, TXX-99120, NUDOCS 9906020107
Download: ML20195B635 (8)

Text

I )

i

. . 1 mammmmmmmmes l

== '

~ ~

7 Log # TXX-99120 File # 10035 '

Ref. # 10CFR50.54(f) 7UELECTRIC GL 95-07 May 25,1999 Senior Mce President

& PrincipalNuclear Oplcer U. S. Nuclear Regulatory Commission Attn.: Document Control Desk Washington, D.C. 20555-0001

SUBJECT:

COMANCHE PEAK STEAM ELECTRIC STATION (CPSES) UNITS 1 AND 2 DOCKET NUMBERS 50445 AND 50446 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION REGARDING GENERIC LETTER 95-07," PRESSURE LOCKING AND THERMAL BINDING OF SAFETY RELATED POWER OPERATED GATE VALVES" REC: 1. Generic Letter 95-07, " Pressure Locking and Thermal Binding of Safety Related Power Operated Gate Valves," dated August 17,1995

2. TU Electric letter logged TXX-95263 from Mr. C. L. Terry to NRC dated October 16,1995

3. TU Electric letter logged TXX-95278 from Mr. C. L. Terry to NRC dated November 6,1995

4. TU Electric letter logged TXX-96046 from Mr. C. L. Terry to NRC dated February 13,1996

5. TU Electric letter logged TXX-97056 from Mr. C. L. Terry to NRC dated March 14,1997 On August 17,1995, the NRC issued Generic Letter 95-07, " Pressure Locking and Thermal Binding of Safety Related Power Operated Gate Valves"(Reference 1).

Pursuant to Section 182a of the Atomic Energy Act of 1954, as amended, and 10 CFR 50.54(f), TU Electric committed to implement the requested actions in Generic Letter 95-07.

Via Reference 3, TU Electric provided clarification to Reference 2 as requested by the NRC.

l_ ' in Reference 4, TU Electric provided a proposed schedule for modification and conclusion of L TXU Electric's (previously known as TU Electric) analysis. Reference 5 stated that based on industry test verification results, TXU Electric had concluded that Commonwealth Edison's j 9906020107 DR 990525 Y p ADOCK 05000445 PDR d$

ii

' 6

'dh CoMANCilE PEAK SILAM ELECTRIC STATION P.O. Box 1002 Glen Rose, Texas 76043-1002 L

TXX-99120 Page 2 of 8 analytical method used to calculate the additional stem thrust requirement above the pull out thrust anticipated by the typical design calculations is conservative. TXU Electric had applied this aforementioned method to predict stem thrust to overcome presstre locking on several susceptible valves. This additional thrust combined with the pullout thrust, when compared with the available motor capability, both from the GL 89-10 program, demonstrated adequate margin and valve operability. Therefore, TXU Electric concluded that valve modification plans stated in Reference 4, were not warranted.

Subsequent to submittal of Reference 5 and review of TXU Electric's submittal, NRR Staff (during a teleconference) verbally requested additional information on certain valves in the form of eight (8) questions. Two (2) questions were answered satisfactorily during the teleconference, however, additional verification / evaluation was needed for the remaining six j (6) questions, which included approximately thirty two (32) valves.  !

As a result of the teleconference, TXU Electric initiated a SMART Form (a document used to l identify issues at CPSES) SMF-1999-000241-00 and appropriately classified the issue as a potential adverse condition. To determine if the 32 motor operated valves (MOVs) were operable under the postulated pressure locking scenarios, TXU Electric implemented a  !

methodology developed by the Commonwealth Edison Company' that used the actual MOV l characteristics to calculate the required thrust to pull the valve out of its seat under the j postulated pressure-locking situation. After the thrust required to pull the valve disc out of its j seat is calculated, this value is compared to the available thrust provided to the valve by the actuator and a margin is determined. The Commonwealth Edison Company methodology allows the user to conclude that for the short-term, the power-operated valve is operable as i long as the margin is greater than zero for stiff valve bodies and greater than twenty (20)  !

percent for flexible valve bodies. To ascertain how long the valve would remain operable, an )

estimate of valve degradation over time was considered, and the valve operability period was  ;

determined. Long-term operability requires a 20 percent margin for stiff valve bodies and a forty (40) percent margin for flexible valve bodies.

The six (6) questions as we understand them, and a response to each question is as follows.  !

It should be noted if it was concluded that stiff body valves had a 20 percent margin and flexible body valves had a 40 percent margin using the Commonwealth Edison Company methodology, the valve was considered not to be susceptible to pressure locking.

Question 1 The TXU Electric submittal states that the charging pump discharge to the cold leg valves, 1/ 2 - 8801 A/B, are not susceptible to pressure locking because the charging pump is

' Commonwealth Edison Company letter dated May 29,1998 from R. M. Krich to the USNRC,

" Commonwealth Edison Company (Comed) Response to NRC Generic Letter (GL) 95-07, Pressure Locking and Thermal Binding of Safety-Related Power Operated Gate Valves"

TXX-99120 Page 3 of 8 operating when the valves open. On a loss of off-site power do the valves open when the charging pumps are not operating or are the valves sequenced to open after the charging pumps are operating? Are there any pressure locking scenarios where the valves will operate at locked rott conditions until the charging pump develops full discharge pressure?

Response

1/ 2 8801 A/B (stiff bcdy valve) are safety injection header isolation valves, which are in a closed position pre design basis accident (DBA). These valves open automatically when a safety injection signal is received. Considering the question, it was deemed that the valves 1/

2 - 8801 A/B were operable. Margins were determined to be 18,30,48, and 35 percent respectively, under actual operating conditions. Since Valve 1-8801 A had an operability margin of 18 percent, TXU Electric is evaluating long-term modification of this valve.

Question 2 Explain why the RHR to charging pump suction isolation valves,1/ 2 - 8804A/B, and RHR to SI pump suction isolation valves,1/ 2 - 8807A/B, are not susceptible to pressure locking. The bonnets of these valves could be pressurized during shutdown cooling, RHR pump surveillances, and ECCS injection evolutions and these valves would be opened later at a lower RHR pump discharge pressure.

Response

1/ 2 - 8804A/B (stiff body valve) are Residual Heat Removal (RHR) heat exchanger outlet injection header to charging pump suction and RHR to safety injection pump recirculation header valves. These valves are in a closed position pre DBA, and are opened by plant operators to complete the switchover from emergency core cooling injection mode to cold leg recirculation mode. The subject valves were deemed operable. Margins were determined to be 2,5,116 and 8 percent respectively, under actual operating conditions. The Unit 1 Valve 1- 8804A , Train A RHR heat exchanger outlet injection header to charging pump suction header isolation valve was determined to have a 2 percent margin for operability. The evaluation for this valve includes the actual valve factor determined through static testing plus a 10 percent margin. The 10 percent margin added to the valve factor was equivalent to about an additional 7 percent overall margin. The stem factor used for the operability review was the actual stem factor measured after Valve 1- 8804A was refurbished in October 1996.

No additional margin was added for possible stem factor degradation between October 1996 and October 1999, it is TXU Electric engineering's judgment that, folhwing a valve refurbishment, stem factor typically decreases notably for some months as the actuator grease became distributed around the actuator gears. This judgment is based on review of available cata for Valve 1- 8804A, where the stem factor decreased from 0.00922 to 0.007305 from October 1991 to October 1993 following a valve refurbishment in October 1991.

I 1/ 2 - 8807A/B (stiff body valve) are charging pump to safety injection pump isolation valves, l

O TXX-99120 Page 4 of 8 which are in a closed position pre DBA. These valves are opened by plant operators to complete switchover from emergency core cooling injection mode to cold leg recirculation mode. Margins were determined to be 90,40,50, and 21 percent. The subject valves were deemed operable.

Question 3 Explain why containment spray pump discharge valves,1/2HV4776 and 1/2HV4777, are not susceptible to pressure locking following operation of the containment spray pumps with the discharge valves shut.

Response

1/ 2 - HV4776 and 1/ 2 - HV4777 (flexible body valve) are containment spray discharge valves. These valves are in a closed position pre DBA, and are opened automatically by a containment spray actuation signal. These valves have greater than 40 percent margin using the Commonwealth Edison Company methodology. These valve 3 were deemed operable.

No hardware modification is required for these valves.

Question 4 Technical Specifications require an operable emergency core cooling system (ECCS) injection i flow path from the refueling water storage tank (RWST) to the reactor coolant system (RCS) via the RHR pump when RCS temperature is less that 350 F. During the shutdown cooling mode of operation, the RWST suction valve to the RHR pump, 1/ 2 - 8812A or B, is shut. Is the valve required to be opened to realign the RHR pump to inject into the RCS. If so, why isn't the valve susceptible to pressure locking?

Response

l 1/ 2 - 8812A/B (stiff valve body ) are RHR pump suction valves. They are in the open position pre DBA. These valves are closed after switchover from emergency core cooling injection mode to cold leg recirculation mode to provide redundant isolation of the RWST from the RHR emergency sumps and are not required to reopen. During shutdown cooling one of these valves may be closed. Margin for these valves were determined to be 28,64,31 and 29 i percent respectively. These valves were deemed operable. No additional actions are  !

warranted for these valves.

l l

l i

E -

TXX-99120 Page 5 of 8 Question 5 The TXU Electric submittal states that the containment spray containment sump suction valves,1/ 2 - HV4782 and 1/ 2 - HV4783, and RHR pump containment sump suction valves, 1/ 2 - 8811NB were modified by installation of pressure relief valve in the bonnet to prevent

. the valves from pressure locking. What is the setpoint of the relief valve and how do you

- demonstrate that the valves will not become pressure locked at pressure less than the relief valve setpoint?

L Response

' 1/ 2 - HV4782 and 1/ 2 - HV4783 (flex valve body) are containment spray pump containment.

sump isolation valves. These valves are in closed position pre DBA. They are opened by the plant operators when the RWST level reaches the containment spray pump switchover level

. setpoint. These valves have a bonnet relief valve set at 76 psid, the minimum margin of 49 percent was derived when Commonwealth Edison Company methodology was utilized.

Therefore, these valves were considered to be operable, and no additional analysis was

. warranted.

1/ 2 - 8811NB (stiff valve body) are RHR pump containment recirculation sump isolation

. valves. These valves'are in closed position pre DBA, and open when the RWST level reaches 4 the lo-lo'setpoint. - Valves 1-8811NB and 2-8811NB are deemed operable for the following reasons:

a) , using the worst possible pressure locking conditions (489 psid), the Commonwealth Edison Company methodology did show an equal to or greater -

than 5 percent positive margin; b) ' the actual maximum pressure locking pressure these valves have been 1 subjected to prior to power ascension was 350 psid which yields an equal to or j greater than 24 percent margin; i

^ c) these valves are stroked with conditions worse than DBA conditions (days after shutdown cooling with the piping drained) and have not pressure locked; i

d) .- these valves are mounted upside down and are not closed under water solid I conditions; and . <

Le)' an air bubble would be trapped even if water leaked into the bonnet during

shutdown cooling.

The presence of an air bubble woiald allow the contained water to expand under constant  !

. pressure conditions. ' The maximum theoretical expansion for a DBA would be for the 2000 F  ;

difference between the max sump temperature and normal ambient conditions at the valves or  !

a'7 percent increase in volume. The actual temperature increase in the valve would be l a

I

B W

. vi . .

TXX-99120.

Page 6 of 8l[ f l

- significantly less due to heat transfer to the concrete and the surrounding environment. )

3.5 percent would be the approximate maximum expected expansion, and an air bubble of this size is a reasonable expectation.

During the CPSES Unit 2 fourth refueling outage (2RF04) a repair to replace the Unit 2 relief valve intemals to allow a 76 psid set point has been completed. A design change has been initiated and will be completed during the upcoming CPSES Unit i seventh refueling outage (1RF07). This design change will repair or replace the Unit i relief valve intemals to allow a 76 psid set point; Question 6 The July 18,1996,~ submittal states that the RHR pump crosstie valves,1/ 2 - 8716A/B, are not susceptible to thermal induced pressure locking because of small temperature changes at the valve. Attachment 3 of the submittalindicated that temperatures could increase as much as 40' F. Explain why this increase in temperature will not create a thermal induced pressure locking condition. These valves have pressure locked at Vogtie and Clinton.

Response

1/ 2 - 8716A/B (stiff valve body) are RHR crosstie isolation valves. These valves are in the 1 open position pre DBA, and are closed by operators after 1/ 2 - 8811 A valves are opened to prevent damage to an RHR pump if only one pump is running. These valves are also

. reopened later to initiate hot leg recirculation mode to remove any crystalized boron in the

- core. The subject valves are considered operable because they are open and are required to i be open for ECCS injection mode, and closed for the cold leg recirculation mode. These l valves are operable fur the depressurization pressure mode, because a positive margin was l

indicated when Commonwealth Edison Company methodology was applied. However, these i valves may not be operable post DBA for hot leg recirculation as the Commonwealth Edison -

Company methodology application did not result in a positive margin for thermally induced j pressure locking.- j lt was also concluded that CPSES Technical Specification operability is not affected for ECCS operation, the RHR crosstie valves are required to be open to meet the requirements of Tech  ;

Spec 3.5.2 (Modes 1,2, and 3) and of Technical Specification 3.5.3.1 (Mode 4).. These operability requirements are predicated on the requirement to provide ECCS injection to all four cold legs.: Additionally, the RHR crosstie valves do not affect the required Tech Spec j

, operability of the RHR loops required by Technical Specification 3.4.1.3,3.4.1.4.1 and '

t 13.4.1.4.2.

The plant capability for hot leg recirculation is still available as described in the TXU Electric's

• Hot Leg Recirculation". discussion below.

l- ,

l l

I' m

r; ,

L -.TXX-99120 -

Page.7 of 8 Hot Leg Recirculation Thermally-induced' pressure locking scenario for valves 8716A & 8716B:

- The premise for this scenario is that the valve bonnet is completely full of fluid. As the valve L heats up,' the fluid expands, creating a.large pressure in the valve bonnet that cannot be l overcome by the valve operator (while maintaining the integrity of the valve).

The 8716A/B valves are potentially susceptible to this phenomenon during a loss of coolant accident (LOCA) event, where relatively coid ECCS water from the RWST (minimum ~

temperature of 40*F) initially flows through the injection lines from the RHR pumps to the cold legs through the 8809NB. valves. Although the RHR pumps are " flow balanced," there may .

. be some flow through the open 8716NB valves. Altematively, if one RHR pump fails, there will be flow through the 87_16NB valves as the operating RHR pump provides flow to all four s RCS cold legs. It is assumed that during the ECCS injection phase, the temperatures of the

valves are at the same temperature as the RWST water (equal to or greater than 40*F) and the bonnet completely fills with fluid.

)

.Upon transfer to the cold leg injection phase, the RHR crosstie valves (8716NB) are closed.

l L The sump recirculation fluid will not impinge directly on the 8716 valves, but will flow through l the RHR-RCS cold leg injection lines through the 8809 valves. Near the time of switchover to

. sump recirculation, the maximum RHR heat exchanger outlet temperature is predicted to be approximately 192*F. Based on " preliminary calculations," the steady-state temperatures of

, , the 8716 valves are expected to rise by approximately.40*F. The expansion of the 40*F fluid in the valve bonnets is expected to result in significant bonnet pressures that result in the failure of the valves to open upon demand when transferring to the ECCS hot leg injection-

' mode of operation. An evaluation of the effect of the valve failure to open is provided below, l

During hot leg recirculation:

' During the transfer from cold leg recirculation to hot leg recirculation per Emergency

["

. Operating Procedures, only one train of RHR is transferred at a time. Should hot leg flow not be verified, the train is retumed to the cold leg recirculation mode of operation.

An attempt is made to transfer the second train to hot leg recirculation. As before, if m

' hot leg recirculation flow is not verified, the train is returned to the cold leg recirculation mode of operation. In addition, the system can provide hot leg recirculation via the

. safety injection pumps using the RHR pumps to supply water to the Safety injection

. Pump suctions.

The requirement to transfer to hot leg recirculation is based on the perceived need to prevent boron precipitation in the core. During the recovery phase of a large break LOCA, boiling in the core is expected to result in the concentration of boron in the core regions. The precipitation of boron has been postulated to hinder the heat transfer from the core to the .

.7

I TXX-99120 Page 8 of 8 recirculating ECCS fluid. Localized depositions could result in flow blockages and " hot spots" with additional clad failures. This issue is primarily driven by the assumption that the core boil-off rate is equal to the ECCS mass injection rate. Lower core boil off rates and higher ECCS injection rates will result in increased core recovery and reduced potential for boron precipitation. In a 1992 " Reasonable Assurance of Safe Operation" report generated by Westinghouse to support continued operations of their plants while an issue associated with the failure of valve 8840 was investigated. It was concluded the ECCS injection rates at Westinghouse plants were sufficiently high as to minimize the potential for significant boron precipitation in the core regions. This conclusion was later confirmed for CPSES (WPT-15131,1/13/93, & NSAL 92-010) and is maintained in current calculations. Given the current flow capabilities of the CPSES ECCS, boron precipitation is not expected to occur following a LOCA.

The safety function of the ECCS is to provide sufficient fluid such that the 10CFR50.46 LOCA criteria are satisfied. Valves 8716A/B can only affect the long term cooling requirement of the 10CFR50.46, and only then if the delivered ECCS flow is not sufficient to preclude or minimize boron precipitation. Current calculations have concluded that sufficient flow is available at CPSES; therefore, it is concluded that the ECCS will meet its intended long term cooling safety function, even with the assumed failures of valves 8716A/B in the closed positions.

If you have any questions, please contact Obaid Bhatty at (254) 897-5839.

Sincerely,

8. W 9 C. L. Terry By: JM -

ffoger CfWalker Regulatory Affairs Manager OBlob cc: Mr. E. W. Merschoff, Region IV Mr. J. l. Tapia, Region IV Mr. D. H. Jaffe, NRR Resident inspectors, CPSES L_