ML051450522
| ML051450522 | |
| Person / Time | |
|---|---|
| Site: | Millstone  |
| Issue date: | 12/22/2004 |
| From: | Kevin Mangan Division of Reactor Safety I |
| To: | Jenison K, Paul Krohn NRC Region 1 |
| References | |
| FOIA/PA-2005-0208 M2-EV-04-0024, Rev 1 | |
| Download: ML051450522 (6) | |
Text
2 , 9 %R REG 0
MISTONE ESIODENTS' OFFICE FAX NO:860-443-5893 OFFICE NO.:860-447-3170 TO: APaul Krohn
-zZKenneth Jenison O Keith Heater o Other FROM: a Max Schneider, Sr. Resident Inspector O Silas Kennedy, Resident Inspector 4-Kevin Mangan, Resident Inspector 0 Ellen Bartels, Secretary o Other DATE:
PAGES: (including this form)
MESSAGE:
. . . ......... f)
TO'd DEGS EPP 09es BNOI15 IW dZ:ST60 SO0W-9E-NUf
QA Non-QA al DB or LB document change required? yes 0 no Z TECHNICAL EVALUATION for Evaluation of Pressure Entering the Enclosure Building Filtration Region On Main Steam Safety Valve Lift Millstone Unit 2 M2-EV-04-0024 Revision I 12/22/04 5 pages
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.J..P -Prep xery Date I, Lcst~ '- t P /ad / -0 Independent Reviewer Datc P. Enginreen A rovI Engineering Approver Date BNOIS11 I W St:60 S00Z-9z-Nhr 0*d £685.2Ev 096
1.0 Purpose This evaluation is written to discuss the impact of the pressure transient created by actuation of the Main Steam Safety Valves (MSSV) on the ability of the Enclosure Building Filtration System (EBFS) to draw and maintain a vacuum in the enclosure building filtration region. This evaluation fulfills AR 04001502-04.
Revision 1 to this evaluation is written to address NRC Unresolved Item (URI)05000336/2004007-01.
2.0 Background
As reported in reference 1, the MSSVs (one or more) lift on a plant trip and dislodge the fabric boots which have been installed on the discharges of these safety valves as described in reference 2, Dislodging of the fabric boots is probably caused by movement of the safety valve discharge tee and discharge vent piping due to reaction forces created on actuation of the safety valves. In addition to pipe movement, actuation of the safety valves will cause a momentary pressure transient and movement of air through the small gaps which exist around the safety valve discharge pipe. A concern was raised concerning the impact that this pressure transient had on the ability of the EBFS to fulfill its Technical Specification requirement to draw a vacuum of 0.25" wg within 60 seconds after EBFS actuation signal (reference 3).
References:
- 1. CR-04-02193
- 2. DCN DM2-01-0177-02 MSSV Exhaust Line Boots Basis Documentation
- 4. Safety Functional Requirements Manual (SFRM) Sections 2.3 and 2.10, Rev 6
- 5. Drawing 25203-20097, Rev 2
- 6. Calculation 2D10-3 "Main Steam Piping Relief Valves-Reaction Force", Rev 0
- 7. Calculation 2D 10-2 "Vent Piping for Main Steam Relief Valves", Rev 0
- 9. SP 2609D "Enclosure Building Filtration System Filter Test Refuel" Rev 013-02
- 10. Calculation 99EBFS-02845M2 "MP2 Enclosure Building Post LOCA Relative Humidity" Rev 0 3.0 Discussion Design of the Safety Valves and Discharge Piping Millstone 2 has two steam generators, each of which has a 34" steam header. There are eight safety valves on each of these two headers located immediately outside containment and upstream of the main steam isolation valves. The design function of these valves is to open on high Steam Generator (SG) pressure to prevent over pressurization of the SG.
Decay heat is credited to be removed from the RCS via the MSSVs following a reactor trip when the steam dump to the main condensers does not operate.
Each safety valve has a minimum capacity of 794,060 Ibm/hr and an expected capacity of 882,289 ibrn/hr at a valve inlet pressure of 1098.2 psia. The opening time for each of the MSSVs is less than 30 milliseconds (reference 4).
Technical Evaluation M2-EV-04-0024 Page 2 O14015IW s :60 s0OF-9F~-Pr
£0 d £66S.EtVt 098
Each safety valve discharges to a 10 inch tee and discharge pipe (about 22" in length) which discharges into an 18" vent pipe which extends through the roof of the enclosure building. The 18" piping sits on a steel plate inside an 18 1/8" diameter bushing. This gap allows the vent pipe to move to absorb the reaction force created when the safety valve lifts. In addition to the gap between the vent pipe and the bushing, the bushing itself has a triangular wedge cut out of the bottom to prevent the collection of water in the bushing. These gaps are installed by design and are shown on reference 5.
Design Function of EBFS EBFS is designed to induce a small negative pressure on the enclosure building filtration region following an actuation signal. This negative pressure (0.25 inches wg) is established within 60 seconds of an Enclosure Building Filtration Actuation Signal to collect containment leakage and filter it prior to release from the Millstone stack.
Operation of the Safety Valves When the safety valves lift, atmospheric pressure is providing all of the backpressure on the valve and in the discharge piping. On initial opening, some of the existing air in the relief valve discharge pipe and vent stack is forced through the designed gaps in the vicinity of the vent pipe inlet until full flow of steam is established up the vent pipe. The opening time of the MSSVs is less than 30 seconds and thus full flow is established within 1 second. Displaced air is the source of the pressure transient that probably cotritbutes to dislodging the fabric boots on the safety valves. Since this is a small amount of air and a short transient (less than one second), there is no effect on operation of EBFS. The steam pressure calculated at the outlet of the safety valves is approximately 109 psia (reference 6). There is some amount of additional pressure drop as a result of travel through the 10" diameter discharge pipe and expansion into the 18" vent pipe. The vent pipe is sized to avoid excessive backpressure and ensure that the steam discharge travels up the stack (reference 7).
No significant steam is likely to be released through these gaps since the steam will preferentially travel from the 10" relief valve discharge pipe straight up into the 18" discharge vent stack. The path through the gaps between the vent pipe and the bushing shown on reference 5 will not see significant steam escape due to its much higher flow resistance.
When the safety valves lift following an accident, the initial lifts will be followed during the course of the accident by several additional valve lifts to relieve pressure until there is another heat removal path for the steam via the Atmospheric Dump Valves (ADV's) which are credited for long term heat removal. For example, the MSSVs could lift periodically for the first 400-500 seconds of a SBLOCA (reference 8). However, these lifts will not hamper EBFS operation due to the small volume of air which is potentially displaced into the enclosure building filtration region.
Since no significant steam is released to the Enclosure Building due to lifting of the MSSVs, there is no significant effect on the performance of the Enclosure Building Filtration System (EBFS). The EBFS charcoal filters (which remove airborne contaminants such as iodine) are tested to operate up to a relative humidity of 96 % per reference 9. The non-QA heaters on the inlet to the filters are intended to heat the incoming air to lower the relative humidity to less than 90%, but these are not credited Technical Evaluation M2-EV-04-0024 Page 3 P *d £685 at 098 3401S11IW 9T :60 G00Z-9EZ-N'j
for post-accident operation. An analysis (reference 10) of the maximum relative humidity entering the charcoal filters without the beaters determined that the maximum relative humidity in the Enclosure Building post accident is 86%. This analysis included outside air at 100% RH mixing with steam from the containment building leakage.
Appropriately, this calculation did not consider steam from the MSSV discharge since hMSSV discharge steam is routed up the 18" diameter vent stack. Any increase in humidity associated with a minimal release of steam from the MSSV lift is insignificant and will have no effect on the operation of EBFS. The vent pipe for each MSSV is sized to ensure that the steam discharge is directed up the stack and out of the building. The proper sizing of this vent pipe and stack ensures that significant backpressure due to MSSV lifting does not hamper MSSV operation. Small amounts of debris (such as scale from the vent pipe, dirt, and occasional bird feathers) may collect in the vicinity of the MSSVs and be dislodged by the air pressure transient which occurs on the MSSV lift.
This debris simply collects on the floor of the surrounding area and does not hamper the operation of any safety related system or component.
4.0 Safety Significance This technical evaluation is written to answer a question about the operation of the MSSVs and the EBFS. Since it makes no changes to the plant and only explains the existing design, no RAC 12 review is required.
5.0 Conclusion There is no significant effect from any momentary pressure transient due to air escape from the main steam safety valve discharge pipe upon lift of a main steam safety valve.
No significant volume of gas is added to the enclosure building filtration region.
6.0 Attachments
Attachment 1: Independent Reviewer Comnent Sheet Technical Evaluation M2-EV-04-0024 Page 4 IW 9T :60 SOO-s9-_Ntr
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90 d. -1, 01 Approved 8/27/02 Effective 8/30/02 Independent Reviewer Comment and Resolution Sheet(s)
(ER/EV)No H 2 -ot+'D° tf Page -57 of 4 Independent Reviewer:; Opa)
,Vj . ? Date Comment No. ERIEV Section Comment I hla L- O° t O.S^ Cub- -171 10 g1-A %.e T(-
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__________ I ___________
MP-03-DCC-FAP 1.4-001 Rev. 000 I of I 90-d . £685 £EPP 09 N1UJ5.1 I P to ~ -n -c -n<7_C7_ -'Vel nun