NSD-NRC-97-5228, Forwards W Responses to Two NRC RAI on AP600 PRA
| ML20196G959 | |
| Person / Time | |
|---|---|
| Site: | 05200003 |
| Issue date: | 07/10/1997 |
| From: | Mcintyre B WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP. |
| To: | Quay T NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NSD-NRC-97-5228, NUDOCS 9707230216 | |
| Download: ML20196G959 (5) | |
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Westinghouse Energy Systems Ba 355 Pittsburgh Pennsylvania 15230 0355
- Electric Corporation l
NSD-NRC-97-5228 DCP/NRC0951 Docket No.: STN-52-003 l
July 10,1997 Document Control Desk U.S. Nuclear Regulatory Commission Washington, DC 20555
- ATTENTION: T..R. QUAY
SUBJECT:
AP600 RESPONSE TO REQUESTS FOR ADDITIONAL INFORMATION
Dear Mr. Quay:
l Enclosed are Westinghouse responses to'two NRC requests for additional information on the AP600 PRA. Specifically, responses are provided for RAI 720.383, which is a Level 1 PRA ATWS question, and RAI 720.403, which is a Level 2 PRA ATWS question. The OITS numbers associated with these RAls are 5131 and 5500, respectively.
These responses close, from the Westinghouse perspective, the RAls. The Westinghouse status t.
column in the Ol_TS will be changed to " Action N." The NRC should review these responses and inform Westinghouse of the status to be designated in the "NRC Status" column of the OITS.
Please contact Cynthia L. llaag on (412) 374-4277 if you have any questions concerning this I
transmittal.
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Brian A McIntyre, Manager 7-
- Advanced Plant Safety and Licensing jml Enclosure cc:
J. M. Sebrosky, NRC (Enclosure)
N. J. Liparuto, Westinghouse (w/o Enclosure) 3293 A wpf i
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Enclosure to Westinghouse l
Letter DCP/NRC0951 July 10,1997 4
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t NRC REQUEST FOR ADDITIONAL INFORMATION
- n= amr.nu Question
- 720.383 It is assumed that the majority of the transient initiating event categories (grouped as event IEV-ATWS-T) do not require reactor trip for about 10 minutes (see page 6-58). This assumption may be optimistic since event IEV-ATWS-T includes some relatively frequent transients which tend to produce RCS pressure transients, such as loss of RCS flow, turbine trips and loss fo main feedwater to one steam generator. Please explain.
Res9onse:
Following an event in which main feedwater remains available, there is sufficient heat removal from the RCS via the steam generators such that RCS pressure does not approach the level at which RCS integrity cot.ld be challenged (i.e., pressure does not reach the ASME Service Level C stress limit). If there is an RCS heatup resulting from the failure of reactor trip, in particular following a turbine trip with associated loss of steam flow from the steam generators or following a partial loss of main feedwater flow, core reactivity feedback effects serve to reduce reactor power which also limits the pressure rise. These combined effects result in a slower, much less severe RCS pressure rise, with additional time for the operator actions modeled in the PRA, relative to that available following a loss of main feedwater.
The PRA success criteria discussion for operator actions ATW-MANOS and ATW-MAN 06 (trip the reactor via PMS or DAS, respectively, for a transient in which main feedwater is available) indicates that it is expected that the time available for such action would be significantly greater than the time available for an event in which main feedwater is unavailable. A 10-minute action time is an engineering judgment estimate, not based on specific analysis. In the human reliability analysis, the calculations for the human error probabilities (HEP) for these actions (PRA subsections 30.6.41 and.42) assumed a 7-minute time window. Per the criteria for unproceduralized recovery in the HRA, availability at a time window of at least 5 minutes is sufficient for a simple action such as tripping the reactor to j
apply the recovery factor. Therefore, although the success criteria specify about 10 minutes for this action, the values calculated for the analysis would be valid for a time window as short as 5 minutes. A recovery factor of 0.1 was used, resulting in an HEP of 5.2E-3 for ATW MANOS, as opposed to the value of 5.2E-2 calculated for ATW-MANO3, the corresponding action for the case of loss of main feedwater. The HEP for ATW-MAN 06 is the same as for ATW-MANOS except for sequences in which ATW-MAN 06 failure follows ATW-MANOS failure,in which a dependent failure value of 0.5 is assigned to ATW-MAN 06.
In the baseline PRA, ATWS is a sufficiently small contributor to the results that a change in the credit allowed for ATW-MANOS and ADV-MAN 06 for a subset of the events m the ATW-T initiating event category would not affect PRA conclusions or insights. In the focused PRA sensitivity, actions ATW-MANOS and ATW-MAN 06 are not modeled. ATW-MAN 06 is not included because DAS is not credited in the focused PRA, and ATW-MANOS is replaced by ADV-MAN 03, since the focused PRA assumes that all nonsafety-related systems are unavailable such that all ATWS transients involve loss of main feedwater. Thus, the focused PRA already assumes no credit for the longer amount of time available for operator action with main feedwater available, and the results are unaffected by an assumption of 5-10 minutes for this action.
PRA Revision: None.
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NRC REQUEST FOR ADDITIONAL INFORMATION
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5 Question: 720.403 Based on information in Chapter 24, each valve in the containment air filter supply line (valves V03 and V04) and exhaust line (valves V09 and VIO) appears to be assigned a failure rate of IE.6/h (for failure to reclose) based on the EPRI URD. However, the URD indicates somewhat higher failure rates (2.0E-6/h). Please justify that the valve failure rates used are: (1) applicable to the large diameter valves specified for AP600. (2) consistent with operating experience with valves of similar size and design, and (3) consistent with the stroking frequency assumptions in the URD and the stroking frequency specified in the AP600 design.
Response
The failure rate used in the PRA for air-operated valves VFS-C003, -V004, -V009, and -V010 was derived from the URD, Volume III, Revisions 5 & 6, Issued 12/93. On page A.A-20 of the URD, the suggested failure rate for air-operated valve - failure to operate to deenergized position is 1.0E-06/hr. A copy.tf the URD page is attached to this i
RAI response.
Even if the higher failure rate of 2.0E-06/hr, suggested by the NRC, is used for these AOVs, the failure probability of the affected containment isolation fault tree (CIC) changes from 1.65E-03 to 1.70E-03, an increase of 3 percent.
The effect of this 3 percent increase of CIC failure probability on the PRA results is insignificant.
(1) & (2)
The URD failure rates for AOVs are derived from operating plant experience with AOVs and from generic data sources. As is typical of generic databases, the failure rates of mechanical equipment is not broken down by valve size or specific internal type (i.e., globe, gate, etc.), but rather by failure mode (e.g., fail to open, fail to close, spuriously operate). Thus, the failure rates used are applicable regardless of the valve size.
(3)
The failure rate derived in the URD and used for AP600 is an hourly rate. The failure probability calculated for the AP600 valves from this hourly rate incorporates the AP600 valve testing frequency.
Thus, the failure probability for the AOVs is based on the AP600 testing frequency, PRA Revision: None.
720.403-1 W Westinghouse
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ANNEX A
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j RELIABILITY DATA BASE FOR PASSIVE ALWR PRAS l
Table A21 RECOMMENDED COMPONENT FAILURE RATES Survey Component Fa!!ure Mode Failure Rate Entry Note Motor operated valve Fails to operate on 4.0 x 10-3/d 1
1 demand Fails to operate on 1.0 x 10 5/hr 2
1 demand (standby rate)
Fails to remain open 1.4 x 10 7/hr 3
2 Air operated valve Fails to operate on 2.0 x 10 3/d 4
1.3 demand (to other than
'deenergized position) 4 Fails to operate on 3.0 x 10 /hr 5
1,3 demand.(to other than deenergized position; standby rate)
Fails to operate on 1.0 x 10-3/d 6
1 demand (to deenergized position) 4 Failure to operate (to 1.0 x 10 /hr 7
1 deenergized position; standby rate) 4 Spuriously transfers to 1.0 x 10 /hr 8
4 deenergized position Solenoid valve Fails to operate on 1.0 x 10 3/d 9
1 demand 4
Fails to operate on 2.0 x 10 /hr 10 1
demand (standby rate)
Spuriously c.hanges state 5.0 x 10 7/hr 11 Explosive valve Fails to operate on 3.0 x 10 3/d
-12 5
demand Operates spuriously 4.0 x 10 7/hr 13 Check vane (other than Fails to open on demand 1.0 x 10 4/d 14 Wop check)
Fails to open on demand 2.0 x 10 7/hr 15 1
(standby rate) 4 Fails to open standby 1.0 x 10 /hr 16 6
rate, adverse c(onditions)
Fails to close on demand 1.0 x 10-3/d 17 Check valve (alltypes)
Fails to remain open or 2.0 x 10 7/hr 18 plugs 4
Reverse leakage 1.0 x 10 /hr 19 Internal rupture 5.0 x 10-8/hr 2D Page A.A 20 t 20.403-2