PLA-6473, Response to Request for Additional Information on Amendment Request No. 299 PRA Questions
| ML090420496 | |
| Person / Time | |
|---|---|
| Site: | Susquehanna  |
| Issue date: | 01/23/2009 |
| From: | Spence W PPL Corp, Susquehanna |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| PLA-6473 | |
| Download: ML090420496 (9) | |
Text
A William H. Spence Executive Vice President and Chief Operating Officer/Chief Nuclear Officer Tel. 610.774.3683 Fax 610.774.5019 Whspence@pplweb.com PPL Corporation Two North Ninth Street Allentown, PA 18101-1179 Tel. 610.774.5151 www:pplweb.com Pa JAN 2 3 2009 U. S. Nuclear Regulatory Commission Attn: Document Control Desk Mail Stop OP1-17 Washington, DC 20555 SUSQUEHANNA STEAM ELECTRIC STATION RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ON AMENDMENT REQUEST NO. 299 FOR OPERATING LICENSE NO. NPF-14 AND AMENDMENT REQUEST FOR OPERATING LICENSE NO. NPF-22: PRA QUESTIONS PLA-6473 Docket Nos. 50-387 and 50-388
Reference:
- 1) PPL Letter (PLA-6328) from B. T. McKinney (PPL) to NRC Document Control Desk titled "Proposed License Amendment No. 299for Unit 1 and Proposed License Amendment No. 269for Unit 2for Changes to Technical Specifications 3.6.4.1 Secondary Containment and 3.6.4.3 Standby Gas Treatment System, "dated March 28, 2008.
- 2) PPL Letter (PLA-6154) from B. T McKinney (PPL) to NRC Document Control Desk titled "Application for Renewed Operating Licenses Numbers NPF-14 and NPF-22 Response to SAMA RAIs, "dated April 12, 2007.
The purpose of this letter is to respond to questions asked during a telephone conversation held on December 22, 2008 between PPL and NRC with regard to Reference 1. contains the responses to the questions.
This supplement does not affect the no significant hazards consideration evaluation provided in the original request (Reference 1).
Any questions regarding this letter should be directed to Mr. Cornelius T. Coddington at (610) 774-4019.
Document Control Desk PLA-6473 I declare, under penalty o e
that the foregoing is true and correct.
Executed on:
W. H. Spence Copy:
NRC Region I Mr. R. R. Janati, DEP/BRP Mr. F. W. Jaxheimer, NRC Sr. Resident Inspector Mr. B. K. Vaidya, NRC Project Manager
ATTACHMENT 1 TO PLA-6473 RESPONSES TO REQUESTS FOR ADDITIONAL INFORMATION PLA-6473 Page 1 of 6 Ouestion 1 It is stated in Section 4.4.2 that the majority of postulated containment failures involve catastrophic containment over-pressure failures, which are also predicted to fail secondary containment, thereby rendering SGTS ineffective'. Explain why use of the containment vent in accordance with SSES's plant-specific implementation of the BWROG Emergency Procedure and Severe Accident Guidelines (EP/SAGs) is not effective in reducing the overall contribution from containment over-pressure failures.
The explanation may take the form of discussing the top LERF sequences and why venting does not reduce the containment failures. (The generic EP/SAGs direct containment venting irrespective of dose in order to prevent containment failure.) If this statement is the result of the SSES PRA treating containment venting in a different manner than directed in the plant-specific EP/SAGs, provide a re-characterization of: (1) severe accident containment response/failure assuming appropriate credit for containment venting (i.e., operator actions consistent with the plant-specific EP/SAGs), and (2) the effectiveness of SGTS in reducing severe accident releases/risk assuming appropriate credit for containment venting.
Response 1 The use of the primary containment vent in accordance with SSES's plant specific procedures is effective in reducing primary containment over-pressure failures. Venting of the primary containment will result in the failure of secondary containment due to the large amount of gas released. Failing secondary containment renders SGTS ineffective.
Therefore, the end result is the same; when the primary containment is either failed or vented, secondary containment will fail which renders SGTS ineffective.
Comparison of PPL Emergency Procedures and BWROG EP/SAGs for Venting with Core Damage The BWROG EP/SAGS (PC/P-4) states, "Before suppression chamber pressure reaches Primary Containment Pressure Limit A, vent the primary containment, defeating isolation interlocks and exceeding offsite radioactivity release rate limits if necessary, to control pressure below Primary Containment Pressure Limit A."
The PPL procedure states, "Before primary containment pressure reaches 65 psig, if determined appropriate, vent the primary containment, defeating isolation interlocks if necessary and using vent lineups not needed for adequate core cooling, to control pressure below 65 psig."
PLA-6473 Page 2 of 6 The difference between the two documents is that the PPL procedures deleted, "and exceeding offsite radioactivity release rate limits if necessary" and added "if determined appropriate." SSES venting strategy concludes that the decision to vent must be made with an awareness of the off-site consequences realized against the consequences of the venting action.
Venting Containment Prior to Core Damage Venting primary containment can be effective in preventing core damage for certain scenarios. Core damage can be prevented by depressurizing the reactor and using low pressure makeup water to keep the core covered. The continued use of low pressure makeup depends on keeping the Automatic Depressurization System (ADS) valves open.
The ADS valves are inside containment and pneumatically operated. As primary containment pressure increases, due to a failure of decay heat removal, the differential pressure between primary containment and the instrument gas, which holds the ADS valves open, decreases. This will ultimately cause the ADS valves to close. As a consequence of the ADS valves closing, the reactor re-pressurizes and its pressure exceeds the head capacity of the low pressure makeup pumps. When makeup flow ceases, the reactor will boil off water, which ultimately results in the SRVs cycling, the core being uncovered, and core damage. Thus, if low pressure water is the makeup source for the reactor and there is insufficient decay heat removal, venting primary containment allows for continued low pressure makeup operation preventing core damage.
Containment Venting Impact on Large Early Release Frequency Given core damage has occurred, venting or not venting primary containment does very little to the Large Early Release Frequency (LERF). As seen from the bar graph and table below, 99.4% of the LERF is unaffected by primary containment venting.
PLA-6473 Page 3 of 6 Unit 1 LERF (HE) Contribution By Sequence 1IS-2-001 3"/
5T8(0%
1 LT-7-001 I1(0*
1IS-1-003 17 3101,
=***".*
~ ~
/~1,920%ý 1TR-1-008
.0 2 35 1 LT-2-022 0 167, 1TR-6AH-001 0,
/3082%
333'1 S
1 LT-3-060 3<-0,028"/,,
O" 1TR-7-012 0 021','1,,,*
i 1TR-6AH-009 02 1TR-1-012 0019/"
/
1TR-6AH-007 0 0 0 /o 1TR-6AH-005 0
00:
1 LT-6-015
/'"-0.002'%
4 1TR-3-075 l~3 0 002'0/
1 LT-3-038 1
0 0 1 1TR-1-010 001100 1TR-2-025 i!i :<
1 LT-6-048 0.000%
0.001%
0.010%
0.100%
1.000%
10.000%
100.000%
Percent Contribution to Unit I LERF PLA-6473 Page 4 of 6 Cumulative Top 99.4% of LERF by Sequence Cumulative %
% Contribution Cmltv Sequence Sequence Description to LERF Contribution to LERF Interfacing System LOCA for 1IS-2-001 RHR Pump Suction (F008-F009) 57.8 57.8 Break Containment Failure due to 1TR-7-013 Direct Containment Heating 15.6 73.4 following Vessel Failure due to delayed High Pressure Boil Off LOCA, with Containment 1LT-7-001 Failure due to Primary 10.1 83.5 Containment Vacuum Breakers failing to Close IIS-1-003 Interfacing System LOCA for 7.31 90.81 RHR Pump Discharge Division I Containment Failure due to 1RX-001 Excessive LOCA or Reactor 5.79 96.6 Vessel Rupture Containment Failure due to 1 TR-2-026 Energetic Containment Failure 1.92 98.52 caused by Vessel failure due to early High Pressure Boil Off 1BC-1-002 Break Outside Containment 0.926 99.446 PLA-6473 Page 5 of 6 Question 2 Explain how paragraph 4.2.2 of the submittal is consistent with License Renewal SAMA
- 12 and response to RAI 1.G in Reference 2.
Response 2 Paragraph 4.2.2 discusses the effectiveness of SGTS for large releases. SGTS is a filtration system that has the capability to remove approximately 10,000 SCFM from secondary containment. The point being made is that, for releases that involve containment failure, SGTS is ineffective since the failure of containment is likely to also fail secondary containment. The failure of secondary containment will render SGTS ineffective in reducing the quantity of radionuclides released.
An important feature is that SSES has a "soft" pipe vent. Effective primary containment venting without loss of secondary containment would require an SGTS flow rate higher than 10,000 SCFM. Hence venting primary containment would exceed the capability of SGTS and rupture the vent ductwork. Since the venting flow rate exceeds the capacity of SGTS, secondary containment would pressurize and breach. However, venting primary containment would allow for a scrubbed release via wetwell venting.
SAMA #12 recognizes that given core damage, venting primary containment may not be likely using the SSES Emergency Operating Procedures, although the procedures do not prohibit venting. The SAMA evaluates crediting venting, with the benefit of scrubbing, after core damage. This SAMA also assumes the vent can be opened. The SAMA identifies all the sequences that can benefit from venting. A review of these sequences resulted in only one sequence, 1TR-6AH-001, that was a large early release sequence.
This one sequence (if venting were credited) would go to a low early release. As can be seen from the bar graph, this sequence represents an insignificant, 0.082%, amount of the total LERF.
Venting primary containment is not credited in the SSES PRA after core damage. SSES has two methods of venting primary containment, using a hand switch from the control room or locally by manually turning a jack screw on a 24-inch butterfly valve. The 24-inch valve is a containment isolation valve between primary containment and SGTS.
This valve is installed in "pipe" which transitions to duct work after the valve. The ductwork is predicted to fail given the containment venting flow rate, allowing a release directly to the secondary containment. If manual venting is to be performed, a hatch in the duct is opened on elevation 779' of the reactor building to direct the hot gas from primary containment away from the area in which the operator is manipulating the PLA-6473 Page 6 of 6 24-inch valve. The manual venting can be performed without risk of exposing the operator to excessive radiation given the manual venting is performed prior to core damage. Since manual venting may not be possible given core damage due to dose concerns for the operator, remote venting from the control room may be the only choice.
Venting from the control room requires the availability of instrument air and AC power.
From a PRA perspective, if enough equipment or support systems have failed to cause core damage and fail containment heat removal, it is not likely that the support systems needed for remote actuation of the vent valves will be available. Therefore, although allowed by our emergency procedures, venting may not be possible given previous core damage. As a result, the PRA does not credit venting post core damage.