ML20209C395
| ML20209C395 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  |
| Issue date: | 07/01/1999 |
| From: | Schuelke D NORTHERN STATES POWER CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| GL-95-07, GL-95-7, NUDOCS 9907090281 | |
| Download: ML20209C395 (10) | |
Text
,
1, i
W Northern States Power Company j
Prairie Island Nuclear Generating Plant f
1717 Wakonade Dr. East Welch, Minnesota 55089 j
i i
)
i July 1,1999 Generic Letter 95-07 l
U S Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555 PRAIRIE ISLAND NUCLEAR GENERATING PLANT Docket Nos. 50-282 License Nos. DPR-42 50-306 DPR-60 Supplemental Response to Generic Letter 95-07, " Pressure Locking and Thermal Binding of Safety-Related Power-Operated Gate Valves" Generic Letter 95-07 (dated August 17,1995) was issued by the NRC requesting licensees to provide information concerning (1) the evaluation of operational configurations of safety-related, power-operated gate valves for susceptibility to pressure locking or thermal binding; and (2) analyses, and needed corrective actions, to ensure that safety-related power-operated gate valves that are susceptible to pressure locking or thermal binding are capable of performing the required safety function.
By letters dated October 16,1995, and February 12,1996, with subject, " Response to Generic Letter 95-07: Pressure Locking and Thermal Binding of Safety-Related Power-Operated Gate Valves," Prairie Island responded to the Generic Letter.
a By letter dated July 8,1996, the NRC staff requested additional information in order to complete its review of the Prairie Island responses to Generic Letter 95-07. The Prairie
/
l Island response to the Request for Additional Information was included in a letter dated
)
August 6,1996.
Based on further review of the issue and additional discussion with NRC staff, the attachment to this letter supplements the Prairie Island response to Generic Letter 95-C;b j
07.
i in this letter we have made one new Nuclear Regulatory Commission commitment:
27 S M*J*
e P
1 i
o
r-l USNRC NORTHERN STATES POWER COMPANY July 1,1999 Page 2 Long term remedial action will consist of either permanent procedure changes, orplant design changes, to ensure the Containment Spray Pump discharge MOVs are not pressure locked when they are required to be operable.
l Please contact Jeff Kivi (651-388-1121) if you have any questions related to this letter.
/ es Y
f Donald A. Schuelke Plant Manager Prairie Island Nuclear Generating Plant c: Regional Administrator - Region Ill, NRC Senior Resident Inspector, NRC NRR Project Manager, NRC J E Silberg Attachments:
- 1. Supplemental Response to Generic Letter 95-07 GL95-07eupp. DOC
UNITED STATES NUCLEAR REGULATORY COMMISSION NORTHERN STATES POWER COMPANY PRAIRIE ISLAND NUCLEAR GENERATING PLANT DOCKET NO. 50-282 DOCKET NO. 50-306 GENERIC LETTER 95-07, PRESSURE LOCKING AND THERMAL BINDING OF SAFETY-RELATED, POWER-OPERATED GATE VALVES Northern States Power Company, a Minnesota corporation, by this letter dated July 1, 1999, hereby submits information required by Generic Letter 95-07 for the Prairie Island Nuclear Generating Plant.
This letter contains no restricted or other defense information.
NORTHERN STATES POWER COMPANY By se M)
Donald A. Schuelke Plant Manager Prairie Island Nuclear Generating Plant i
On this
/
day of OIIu
/f77 before me a notary public in and for said County, pMsonaVy appeared, Donald A. Schuelke, Plant Manager, Prairie Island Nuclear Generating Plant, and being first duly sworn l
acknowledged that he is authorized to execute this document on behalf of Northern States Power Company, that he knows the contents thereof, and that to the best of his knowledge, information, and belief the statements made in it are true and that it is not interposed for delay.
/
MARLYS E. DAVIS NOTARYPUBUC-MNNESoTA h
WyCommisson EqwwJet at feel w1*:.... : ' *::::: ' .':: '." ' ' '."" ":,' ',":a
l
\\
ATTACHMENT 1 Supplemental Response to Generic Letter 95-07, " Pressure Locking and Thermal Binding of Safety-Related Power-Operated Gate Valves" Discussions between NRC staff and Prairie Island engineering personnelidentified a number of questions related to the Prairie Island response to Generic Letter 95-07. The response to some of these questions relied on information not previously submitted.
These questions re!ated to pressure locking concerns for two sets of motor-operated I
valves (MOVs):
- 1. Containment Spray (CS) Pump discharge MOVs (MV-32103, MV-32105, MV-32114, and MV-32116), and
- 2. Residual Heat Removal (RHR) Pump suction (on sump side) MOVs (MV-32075, MV-32076, MV-32178, and MV-32179).
Because the response to these questions relies on new information, NSP is submitting the responses to these questions for the information of the NRC staff.
- 1. With respect to the Containment Spray (CS) Pump discharge valves (MV-32103, MV-32105, MV-32114, and MV-32116): The NSP submittals did not indicate that these valves are susceptible to pressure locking.
The NRC questions and associated NSP responses are as fo!.'c w NRC Question:
Explain why the Containment Spray (CS) Pump discharge valves (MV-32103, MV-32105, MV-32114, and MV-32116) are not susceptible to pressure locking following pump surveillance testing.
NSP Response:
Upon further review of these valves, NSP agrees that these valves are potentially susceptible to pressure locking following pump surveillance testing.
I NRC Question:
l Are these valves sequenced to automatically open after the CS Pump is operating and developed full discharge pressure?
I NSP Response:
l l
f.
)
Attachment i
July 1,1999 Page 2 The associated pump discharge valve receives a signal to open upon containment spray actuation and therefore begins and continues to open prior to and during the starting of the containment spray pump. Pump discharge pressure is likely to be low i
i when the valve begins to open.
NRC Question:
l Are there any pressure locking scenarios where the valves would operate at locked rotor conditions until CS Pump develops full discharge pressure?
NSP Response:
If the valve were pressure locked, it is likely that motor would be at locked rotor for a short period of time until pump discharge pressure is built up sufficiently to equalize upstream pressure to the bonnet and relieve the pressure locked situation.
NRC Question:
l If applicable, why is it acceptable to operate at locked rotor conditions?
NSP Response:
The valves were determined to be operable. However, because it is neither desirable nor acceptable to operate at locked rotor conditions, NSP has instituted short-term remedial actions and will institute long term remedial actions to further i
assure that the valves will remain operable.
l The short term remedial action has been to stroke each valve to ensure its j
operability (i.e., not pressure locked). Also, temporary changes have been made to the quarterly pump surveillance procedures to ensure that each valve is operable
)
I (i.e., not pressure locked) after operating the pump and prior to exiting the Technical i
Specification Limiting Condition for Operation for that train of containment spray.
l Other containment spray surveillance procedures were reviewed and found to not
.cause any potential pressure locking scenano.
I Long term remedial action will consist of either permanent procedure changes, or plant design changes, to ensure these valves are not pressure locked when they are required to be operable.
l In addition, Prairie Island engineering staff is conducting an independent re-l verification of MOVs susceptible to pressure locking and thermal binding.
- 2. With respect to the containment sump to RHR suction (on sump side) MOVs (MV-32075, MV-32076, MV-32178, and MV-32179): The August 6.1996, NSP submittal GL95-07eupp. DOC l
Attachment July 1.1999 Page 3 indicates that these valves are not susceptible to pressure locking because the piping segment is drained.
The attached drawings (based on Figures 6.2-3 and 6.2-4 from the USAR) show the relationship of the two valves and the containment sump.
j The two NRC questions and associated NSP responses are as follows:
NRC Question:
Could the bonnets be filled during normal operations (due to sump or RHR leakage) and pressure lock due to increase in sump temperature and containment pressure during the injection phase of a design basis accident?
NSP Response:
The containment sump is empty during normal operation. After cycling the sump j
side valve to drain the bonnet and the section of pipe between the two valves, the sump is pumped out. Periodically containment is entered for at-power inspection.
During this inspection, the sump is looked at to ensure that water has not j
accumulated.
i As noted, the section of pipe between the two valves is drained. In order for the line and the bonnet to fill solid, leakage past the pump side valve from the RHR system would need to purge all of the air that was introduced during the draining. Any conceivable mechanism that would allow the air to leak out would also prevent the valve from being pressure locked.
Therefore, it is not credible for the valve bonnet filling during normal operations to result in the valve being pressure locked.
NRC Question:
Could the bonnets be filled with liquid and be pressurized to containment pressure during the injection phase of a design basis accident and, thus, be susceptible to pressure locking?
NSP Response:
NUREG/CR-6611 and Millstone Nuclear Power Station, Unit 2, LER 97-034-00 were reviewed. Two mechanisms for potentially pressure locking the valve during the injection phase of a design basis accident are evaluated:
Hydraulically induced GL95 07supp. DOC
Attachment July 1,1999 Page 4 Thermally induced Hydraulically Induced The postulated scenario is a design basis accident (in this case a large break LOCA) which pressurizes containment to the design maximum containment internal pressure. At the same time, liquid collects on the containment floor, flooding the sump. Pressure is transmitted from containment to the sump isolation valve and pushes the containment side disk off the seat. This allows the bonnet to partially fill with liquid and pressurize to maximum containment pressure plus the head difference between the water elevation in containment and the valve. As containment depressurizes, due to containment cooling
)
system operation, the bonnet could remain pressurized at this higher pressure.
When the sump isolation valve is called upon to open, the containment pressure will be reduced, developing a differential pressure across both the upstream and i
the downstream disks.
For this scenario, the bounding case would exist with the maximum pressure in the bonnet and completely depressurized on both the upstream and downstream side. This is very conservative, as containment pressure analyses show that containment would not be completely depressurized at the time of the initiation of recirculation.
An evaluation has been performed and determined that the valve actuator is capable of opening the valve under the bounding conditions.
Thermally Induced i
The scenario of concern is the bonnet partially fills due to the phenomena described above. If the liquid in the valve bonnet were heated up, the pressure could increase which could cause the valve to become pressure locked. The potential severity of this scenario is mitigated by the following mechanisms:
As described in this scenario, the liquid in the bonnet needs to be heated up to cause a potential pressure lock scenario. Due to the length of the pipe between the sump and the valve (> 17 pipe diameters) and the configuration of the line running through the concrete, and the fact that the valve itself is inside of an enclosure, a significant temperature increase of the liquid in the valve bonnet may not be possible.
Since the bonnet is initially drained, when it refills there will be air entrapped in the bonnet area. If this air volume is significant (i.e., large enough not to go into solution due to the postulated pressure increase), it can prevent a significant pressure increase.
CL95-07supp. DOC
Attachment July 1,1999 Page5 Evaluating these two factors individually:
The first factor is the temperature increase of the liquid in the bonnet. As noted above (and shown on the attached figures), there is over 20 feet of pipe between sump and the valve. The 14" pipe is routed through an 18" sleeve that is embedded in the grout between the Containment shell and the Shield Building.
The annular region between the 14" pipe and the 18" sleeve is sealed to prevent liquid in the sump from entering the valve enclosure. The 20 feet corresponds to 4
. a little more than 17 pipe diameters.
Temperature measurements of other valve bonnets; RHR to CS MOV and RHR hot leg isolation MOV, showed insignificant temperature increases during normal j
plant operations. These valves were closed with a significant temperature difference between the valve and the system they were in contact with (RHR and RCS, respectively). Due to system configuration differences (vertical vs.
horizontal piping runs, pipe elbows vs. straight pipe, etc.) it is difficult to make a direct comparison to the sump isolation vaives. However, it does provide some evidence that a substantial temperature increase wouH not be expected for the sump isolation valves during the initial hour after the accident prior to initiating sump recirculation.
The second factor (conservatively assuming a temperature increase) is quantifying the potential pressure increase. As previously discussed, the valves are cycled to ensure that the bonnet is drained. Any credible means for the bonnet to refill during normal plant operations that would allow the air to leak out would also prevent the valve from being pressure locked. That is, any path for i
air leakage would also provide a path for a small amount of water leakage; which would relieve the pressure. Therefore, it can be assumed that the bonnet starts out with 100% air void. During the accident, the bonnet will partially fill and the pressure could increase from atmospheric to maximum containment pressure.
)
Using the gas law, the remaining air void would be approximately 22% of the original volume.
From containment pressure and temperature calculations, a conservative value for the initial sump liquid temperature entering the valve bonnet is 140 F, and the maximum sump liquid temperature is 240 F. Making the bounding assumption that the liquid in the bonnet starts at 140'F and is heated up to 240 F we can determine if the air void will be collapsed due to expansion of the liquid.
Using conservation of mass, the temperature change will cause the liquid volume to expand by 4%; which will decrease the air void to approximately 19%. The pressure in the air void is greater than the saturation pressure for 240 F.
Therefore, the liquid in the bonnet will still be subcooled.
GL9S+07supp. DOC
Attachment July 1,1999 Page 6 The pressure in the bonnet willincrease due to compressing and heating of the air void. This effect can again be determined using the gas law. The result is that these effects could increase the pressure in the bonnet by aluost 50%.
Again, a bounding case would be this pressure inside the bonnet and completely depressurized both upstream and downstream of the valve. An evaluation has shown that the valve actuator is capable of opening the valve under these conditions.
This conclusion that a significant pressure increase would not be experienced with a large air void in the bonnet is consistent with information in NUREG/CR-6611.
Therefore, in conclusion, there is no pressure lock concern for the capability to open these valves under DBA post-accident conditions.
GL95-07supp. DOC i.
,,-4 1
,-4 S
G CQ N
CONT.VE3SE~.
I SHIELD WALL j
'.b-SCREEN 2
,r VALVE ENCLCSCRE}4 Q. '.,
i
'\\
EASEMENT
\\
'N f EL E97,.
ISCLATiCN WALL s
- ),
i,
-;;; 9
- '....y F w 09 ?'g..
- a..c'.
FLCCR EL i
l yE o g...
i ees-r s.2 - a.
,1 1
, c' 0% c-
_._.r--.------=
'k x.
O
- r.. * *.
L l
's
~
',.~. [*
/
20' INLET i, C 18' P!FE SLEDE e
./
TRENC..n rND _i::
j
,,.. =,,.
- /.
CONN.
puC.
e f
egr-r m'
HEACE3
- a d vA scs j
_.s. $=-a-i ' FRCM i
r-UP Yc n D-8 To saa LCCF,s Aa.
PLMF SUCECN l
ELEVATION VIEW l
l gfg 3 4820 CWN INN Cart!O-25-97 s:cw: cant.o.
8630 200 1&2 CHECxEO CaCUP t
2 3
4 5
CL 6
P90JECf Pro.
ETNSUR CONTAINMENT SUMP B "o ' ce"-
ELEVATION VIEW FILMEO scatt NONE REV O NORTHERN STATES POWER COMPANY PRAlRIE ISLAND NUCLEAR GENERATING PLANTNL-173001-1 no m.
FIGURE 6.2-3
r 1
o Y
N l
1 W
CONTAINMENTvF333EL S
1 G
\\
I g
b 5 '-
1 r CIA.
?.
P!PE f
SHIELD WALL s
\\
SCREEN g
\\
i
. 7. -
7
[
SUMPT
'k l
\\
I l SOLATION WALL -
\\
, ", 0
/
i, A.
14* DlA.
20' INLET
.\\
P!PE
/Y
[x
/i\\
,i
\\
\\
T Y
4 s
s,
\\,
12'TO RHR PUMPS SUGON HEADER q
PLAN VIEW 9503 2wn TAM carz li-15-97 s::mr: cant ac.
8620 200 1&2 3 4820 M610 CPCt,P L
l 2
3 4
5 CLl 6 f
CbE0xE3 P90, JECT No.
ETNSUR j
+
CONTAINMENT SUMP B l
a, o. etar.
PLAN VIEW l
vtteto NORTHERN STATES POWER COMPN 4Y scat: NCNE REY @
PRAIRIE ISLAND NUCLEAR GENERATING PLANT NL-173001-2 e om -
4 FIGURE 6.2-4
,