ML20216J916

Forwards Response to NRC 990723 RAI Re GL 95-07, Pressure Locking & Thermal Binding of SR Power-Operated Gate Valves

ML20216J916
Person / Time
Site:	Vogtle
Issue date:	10/01/1999
From:	Beasley J SOUTHERN NUCLEAR OPERATING CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
GL-95-07, LCV-681-H, NUDOCS 9910060337
Download: ML20216J916 (6)

Text

o J. Bernie Beesley.Jr P.E. Southern Nuclear

. Vice President Operating Company. Inc.

Vogtle Project 40 inverness Center Parkway

+ I" R0. Box 1295 Birmingharn, Alabama 35201 Tel 205 992.7110 Fax 205 992.0403 SOUTHERN COMPANY Energy to Sern YourWorld" October 1,.1999 Docket Nos.: 50-424 LCV-681-H 50-425 U.S. Nuclear Regulatory Commission

. ATTN: Document Control Desk Washington, D.C. 20555-0001

'L,adies and Gentlemen:

Vogtle Electric Generating Plant Response to Reauest for Additional Information Renardinn GL 95-07

' By letter dated July 23,1999, NRC requested additional information regarding responses to Generic Letter 95-07, " Pressure Locking and Thermal Binding of Safety-Related Power-Operated Gate Valves." Attached are specific responses to the Request for Additional Information.

Please contact this office if you have any questions.

Sincerely, b

.B. Beasle , Jr.

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U. S. Nuclear Regulatory Commission

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cc: Southem Nuclear Operating Company Mr. J. T. Gasser -

Mr. M. Sheibani-SNC Document Management U. S. Nuclear Regulatory Commission .

Mr. L. A. Reyes, Regional Administrator Mr. Ramin R.' Assa, Vogtle Project Manager, NRR Mr. J. Zeiler, Senior Resident Inspector, Vogtle l.

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Attachment l

7 Vogtle Electric Generating Plant Response to Request for' Additional Information Regarding GL 95-07 NRC Question 1:

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Your February 9,1998, submittal states that (1) the temperaure of the residual heat l'

removal (RHR) loop suction valves,1/2HV-8701 A/B and 1/2HV-8702A/B, trends with the reactor coolant system (RCS) temperature, (2) the temperature of the valves located adjacent to the RCS (1/2HV-8701B and 1/2HV-8702B) increases from approximately -

350 F to approximately 620*F during a plant heatup, and (3) bonnet thermal pressurization rates of up to 100 psi / F are theoretically possible. During a telephone call

. conducted on June 30,1999, you stated that (1) the weak link in the analysis for the i pn:ssure retaining capability of these valves is the gasketed joint between the valve body and bonnet, (2) in cases of excessive pressure in a valve bonnet, the pressure would cause  !

the botting at the gasketed joint to stretch and the pressure to relieve through the gasketed

' joint,' (3) no valve structural degradation resulting from the relief will occur, and (4) there

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j is evidence that there has been some minor leakage from the gasketed joints on these valves. Discuss if the American Society of Mechanical Engineers Code design stress limits for the body to bonnet fasteners on valves 1/2HV-8701 A/B and 1/2HV-8702A/B l are exceeded in instances where exce'ssive bonnet pressure could be relieved through the gasketedjoint.

SNC Response to Question 1:

l l r i A detailed analysis was performed for Vogtle Electric Generating Plant by Westinghouse j t i assess pressure in the bonnet for the RHR loop suction valves,1/2HV-8701 A/B and i 1/2HV-8702A/B, and to assess if the pressure resulted in stresses in the body to bonnet fasteners for the valves w; thin American Society of Mechanical Engineers Code design stress limits. This analysis addressed thermally induced pressure locking due to valve i eatup during reactor coolant system heatup and hydraulically induced pressure locking

~ due to reactor coolant system pressure, both in conjunction with seismic and operating loads.

l The analysis was based on methods developed by the Westinghouse Owners Group and

the following assumptions

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. Pressurization rate of 23 psi / F , i e Depressurization rate dependent on the valve differential pressure, e Average heatup rate of 40 F / hour, and

.. Seismic loads, i 4

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The analysis demonstrated that maximum bonnet pressure resulted in body to bonnet joiiit stresses within the ASME code allowable limits. Additionally, the analysis demonstrated that the preload on the bolting, not assuming a concurrent seismic event, is not exceeded. With a seismic event concurrent with the maximum bonnet pressure, the body to bonnet fastener preload is' exceeded by.4R With preload exceeded by 4%, any pressure relief through the gasketed joint would be limited to weepage.

This conservative analysis demonstrates that ASME code design stress limits for the VEGP RHR loop suction valve body to bonnet fasteners are not exceeded.

With the maximum bonnet pressure determined by this analysis, the results of previous engineering reviews only regarding the opening of the v' alve following cooldown of the -

' RCS from rated thermal power conditions to RHR service conditions (approximately 350 F and 350 psig), remain valid. These previous engineering reviews are discussed in letter LCV-%81-F, dated February 9,1998.- These results described in this letter were confirmed as valid if a pressurization rate of 23 psi / F is assumed to also apply for this 1 cooldown.

~ Based on this new analysis, for the case of plant heat up with a need to place the isolated l train of RHR in service, the analyzed pressure c'ould exceed the capability of the actuator to open the valve. However, based on the analysis, RHR could be placed in service after a l_ _ ' duration of 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> to allow bonnet pressure to decay to the point where the upstream W RCS loop suction valve could be opened.

Because of the high level of conservatism in the analysis of these valves, further l evaluations are planned to look at plant specific factors affecting potential pressure locking to allow additional operational flexibility. The SNC plan for further evaluations to more accurately model conditions for these valves will take into account:

• The low probability that the isolated train of RHR would need to be placed in service in a plant heat up due to emergent demands, e The in-depth capability to control RCS tempesature via diverse means for the assumed 12-hour ~ duration until the loop suction valve could be opened, e "Ihe high level of conservatism in the analysis of these valves.

. This evaluation will be completed and a follow up response will be provided by June 1, 2000,: i l

NRC Question 2:

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' Your submittal dated July 26,1996, states that the pressurizer power operated relief valve l(PORV) block valves,1/2HV-8000A/B, are not susceptible to pressure locking because the temperature of the valves trends with the pressurizer steam temperature and as the temperature of the steam decreases, pressure in the bonnets of the valves will decrease.

c Also stated was that during a steam generator tube rupture (SGTR) event, the RCS pressure will decrease and the temperature of the steam space in the pressurizer will rapidly deuease. However, the temperature of the piping and of the pressurizer (metal) will decrease at a slower rate than the steam in the pressurizer. The NRC staff considers that the pressure of the' steam in the bonnets of 1/2HV-8000A/B will be higher than the pressurizer pressure until the temperature of the steam in the pressurizer is the same as the pressurizer (metal) and piping. Describe how long it will take for the temperature of-

'the valves to equalie with the temperature of the steam in the pressurizer following a

. SGTR and if the temperatures will equalize prior to opening the valves during the event.

SNC Response to Question 2:

As an additional conservative measure to demonstrate the capability of the PORV block valves to open when pressure locking is assumed, VEGP applied the computer software program PRESLOK to calculate the estimated force required to open the PORV block valves in the pressure locking scenano. The PRESLOK program was prepared by the Westinghouse Owner's Group based 'upon the calculational methods developed by Commonwealth Edison.' This e' ulation is performed using the following conservative assumptions:

e. No pressure decay or heat transfer to depressurize the valve bonnet, e Maximum pressurizer pressure equal to the safety valve setpoint trapped in the bonnet, e Upstream pressure equal to the secondary side relief valve setpoint, j e Almospheric pressure in containment, and l e Other design basis assumptions, such as friction coefficients and degraded motor capability, consistent with the Vogtle Generic Letter 89-10 program.

Based on the results of this calculation, the PORV block valves have sufficient opening margin, including the 20% additional margin recommended by PRESLOK to be applied to the calculated required thrust to operate the valve at pressure locked conditions. 1 Therefore, no further action is required. I n

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. NRC Question 3:

During a telephone call conducted on June 30,1999, you stated that the RHR to charging ,

and safety _ injection pump suction valves,1/2HV-8804A/B, are not susceptible to pressure locking following operation of the RHR system in the shutdown cooling mode j because low temperature water from the reactor water storage tank will cool the valves ,

and decrease the temperature and pressure in the bonnets of the valves during the l

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injection phase of an accident. Discuss if the bonnets of valves 1/2HV-8804A/B are

. susceptible to thermal induced pressure increase and the maximum potential pressure that could be in the bonnets of the valves during heatup of the plant when ti.e RHR system is aligned for shutdown cooling. Describe how long it will take for the temperature of the

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valves to decrease, following operation of the RHR in the shutdown cooling mode during l a plant heatup, to the point where pressure in the bonnets will be less than the RHR pump discharge pressure during the injection phase of an accident.

SNC Response to Question 3: -

. A detailed engineering evaluation was performed for VEGP by Westinghouse to assess the susceptibility to thermally induced pressure locking of RHR to charging and safety

- injection pump suction valves,1/2HV-8804A/B. The evaluation concluded these valves are not susceptible to thermally induced pressure locking.

The evaluation takes into account the piping configuration, an assumed maximum RHR temperature of 350 F, and insulated piping. The evaluation concluded that the

. temperature increase at the valve would be less than 1 F. The evaluation was 1

determined using the approach outlined in EPRI report TR-103581, " Thermal  !

. Stratification, Cycling and Striping (TASCS)." Based on the engineering evaluation, the

. maximum potential pressure that could be reached in the bonnet when RHR system is aligned for shutdown cooling is 583 psig.

Previous analysis has assessed these valves as not susceptible to pressure locking, however, a theoretical maximum pressure of 583 psig in the bonnet due to hydraulic pressure locking considerations has led to reassessment of these valves as susceptible to pressure locking.

VEGP applied the computer software program PRESLOK to calculate the estimated I force required to open the 1/2HV-8804A/B valves in the pressure locking scenario. This calculation is performed using the following conservative assumptions:

• - No pressure decay or heat transfer to depressurize the valve bonnet, o Conservative RCS pressure, containment sump elevation, and RWST elevation, e Other design basis assumptions, such as friction coefficients and degraded motor capability, consistent with the Vogtle Generic Letter 89-10 program.

Based on the results of this calculation, the 1/2HV-8804A/B valves have the capability to

, be opened, including the 20% additional margin recommended by PRESLOK to be

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applied to the calculated required thrust to operate the valve at pressure locked conditions.

This engineering analysis shows that in cases of maximum hydraulic pressure locking

- these valves have the capability to be opened although the torque rating of the actuators is exceeded. The amount by which Ge torque rating is exceeded is less than the valve vendor's stated limit for one time use of the actuator of two times the torque rating.

. Therefore, no further action is required.

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