ML110420609
| ML110420609 | |
| Person / Time | |
|---|---|
| Site: | Watts Bar  |
| Issue date: | 01/31/2011 |
| From: | - No Known Affiliation |
| To: | Division of Operating Reactor Licensing |
| References | |
| Download: ML110420609 (14) | |
Text
1 WBN2Public Resource From:
Boyd, Desiree L [dlboyd@tva.gov]
Sent:
Monday, January 31, 2011 4:52 PM To:
Epperson, Dan; Poole, Justin; Raghavan, Rags; Milano, Patrick; Campbell, Stephen Cc:
Crouch, William D; Hamill, Carol L; Boyd, Desiree L; Elton, Thomas L
Subject:
TVA letter to NRC_1-31-11_ IPE COMMITMENT RESPONSE Attachments:
1-31-11_ IPE COMMITMENT RESPONSE_Final.pdf PleaseseeattachedTVAlettersenttotheNRCtoday.
- ThankYou,
~*~*~*~*~*~*~*~*~*~*~*~*~*~*~
Désireé L. Boyd WBN2LicensingSupport SunTechnicalServices dlboyd@tva.gov 4233658764
~*~*~*~*~*~*~*~*~*~*~*~*~*~*~
Hearing Identifier:
Watts_Bar_2_Operating_LA_Public Email Number:
261 Mail Envelope Properties (7AB41F650F76BD44B5BCAB7C0CCABFAF1AC0C52A)
Subject:
TVA letter to NRC_1-31-11_ IPE COMMITMENT RESPONSE Sent Date:
1/31/2011 4:52:13 PM Received Date:
1/31/2011 4:52:20 PM From:
Boyd, Desiree L Created By:
dlboyd@tva.gov Recipients:
"Crouch, William D" <wdcrouch@tva.gov>
Tracking Status: None "Hamill, Carol L" <clhamill@tva.gov>
Tracking Status: None "Boyd, Desiree L" <dlboyd@tva.gov>
Tracking Status: None "Elton, Thomas L" <tlelton@tva.gov>
Tracking Status: None "Epperson, Dan" <Dan.Epperson@nrc.gov>
Tracking Status: None "Poole, Justin" <Justin.Poole@nrc.gov>
Tracking Status: None "Raghavan, Rags" <Rags.Raghavan@nrc.gov>
Tracking Status: None "Milano, Patrick" <Patrick.Milano@nrc.gov>
Tracking Status: None "Campbell, Stephen" <Stephen.Campbell@nrc.gov>
Tracking Status: None Post Office:
TVANUCXVS2.main.tva.gov Files Size Date & Time MESSAGE 328 1/31/2011 4:52:20 PM 1-31-11_ IPE COMMITMENT RESPONSE_Final.pdf 297713 Options Priority:
Standard Return Notification:
No Reply Requested:
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Normal Expiration Date:
Recipients Received:
U.S. Nuclear Regulatory Commission Page 2 January 31, 2011 cc (Enclosure):
U. S. Nuclear Regulatory Commission Region II Marquis One Tower 245 Peachtree Center Ave., NE Suite 1200 Atlanta, Georgia 30303-1257 NRC Resident Inspector Unit 2 Watts Bar Nuclear Plant 1260 Nuclear Plant Road Spring City, Tennessee 37381
U.S. Nuclear Regulatory Commission Page 3 January 31, 2011 bcc (Enclosure):
Stephen Campbell U.S. Nuclear Regulatory Commission MS 08H4A One White Flint North 11555 Rockville Pike Rockville, Maryland 20852-2738 Charles Casto, Deputy Regional Administrator for Construction U. S. Nuclear Regulatory Commission Region II Marquis One Tower 245 Peachtree Center Ave., NE Suite 1200 Atlanta, Georgia 30303-1257
ENCLOSURE Response to IPE RAI 14 Tennessee Valley Authority - Watts Bar Nuclear Plant - Unit 2, Docket No. 50-391 E1 - 1 IPE RAI 14 Inappropriate truncation can result in significant accident sequences being erroneously eliminated; SRs QU-B3/LE-E4 provide the requirements for acceptable truncation. Provide the technical bases for using the same truncation limit for LERF; that is, provide the change in LERF if a lower truncation limit was issued. If the change in LERF is greater than 5 percent, identify what sequences were eliminated (Reference F&O 3-1).
TVA Response:
As noted in the peer review report documentation, a WBN LERF truncation evaluation was not provided to the Peer Review Team. Subsequent to the peer review, a truncation study of the WBN LERF model was performed by quantifying at different truncation levels, and the results are shown in Table 1. The quantification was performed for a range of truncation values to demonstrate the impact on the LERF value with decreasing truncation levels. Demonstration of Level 2 model convergence to meet QU-B3 of RA-Sa-2009 (final change is less than 5%) could not be achieved. The percent difference between truncation levels 1E-12 /r-yr and 1.0E -13/r-yr was 37%.
Table 1: WBN U2 Results of Truncation Evaluation for LERF - Base Model Truncation Limit U2-LERF (r-yr) difference 1.00E-08 3.27E-07 1.00E-09 4.88E-07 49%
1.00E-10 1.30E-06 166%
1.00E-11 1.93E-06 48%
1.00E-12 2.62E-06 36%
1.00E-13 3.58E-06 37%
When comparing the differences between the 1.0E-12, 1.0E-13, and 1.0E-14 cutsets, several HRA dependency factors were identified as driving the increase in results. Since the original LERF truncation was performed at 1.0E-12, these HRA dependency combinations were not adequately reviewed, and the default values from the HRA calculator were used in the quantification. Several of these values were overly conservative. As a result, several LERF HRA dependency combinations were reviewed and updated as noted in Table 3. Table 2 displays the truncation study performed on the WBN model with the updated LERF HRA dependency values. Quantification with a 1E-14 truncation value had to be performed in parts by IE group and then the cutset files were merged to generate the total LERF frequency.
Quantification at 1.0E-15 was not possible due to computing limitations.
ENCLOSURE Response to IPE RAI 14 Tennessee Valley Authority - Watts Bar Nuclear Plant - Unit 2, Docket No. 50-391 E1 - 2 Table 2: WBN U2 Results of Truncation Evaluation for LERF - Updated HRA Dependency Truncation Limit U2-LERF (r-yr) difference 1.00E-08 3.27E-07 1.00E-09 4.88E-07 49%
1.00E-10 1.30E-06 166%
1.00E-11 1.94E-06 49%
1.00E-12 2.24E-06 15%
1.00E-13 2.46E-06 10%
1.00E-14 2.64E-06 7%
ENCLOSURE Response to IPE RAI 14 Tennessee Valley Authority - Watts Bar Nuclear Plant - Unit 2, Docket No. 50-391 E1 - 3 Figure 1: WBN U2 Results of Truncation Evaluation for LERF - Updated HRA Dependency 1.00E07 6.00E07 1.10E06 1.60E06 2.10E06 2.60E06 1.00E14 1.00E13 1.00E12 1.00E11 1.00E10 1.00E09 1.00E08 LERF(ryr)
TruncationLevel
ENCLOSURE Response to IPE RAI 14 Tennessee Valley Authority - Watts Bar Nuclear Plant - Unit 2, Docket No. 50-391 E1 - 4 Figure 2: Percent Change in LERF vs. Change in Truncation Level -
Updated HRA Dependency 0%
20%
40%
60%
80%
100%
120%
140%
160%
180%
1.0E08to1.0E09 1.0E09to1.0E10 1.0E10to1.0E11 1.0E011to1.0E
12 1.0E12to1.0E13 1.0E13to1.0E14 PercentChangeinLERF ChangeinTruncationLevel HRA Dependency Impact on Convergence HRA dependencies are addressed in a recovery file. The LERF recovery file (RecruleLERF.caf) is run after the fault tree file is quantified using the defined truncation level set in the PRAQuant file. The recovery file reviews each cutset for combinations of independent HEP basic events that are replaced with the applicable HRA dependency combination event and its associated probability. This method of applying HRA dependency can impact convergence.
The following example cutset is a WBN LERF cutset that contains HRA dependency combination 1295. Also provided is the text from the recovery file that replaces the independent operator actions with the dependency combination value. This is a Small Loss of Coolant Accident (%2SLOCAL) followed by a common cause failure of the ESFAS (U2_ESF_SGD_CF_517_CCF_1_2) to start AFW automatically. Operator failure to start AFW manually (HAOS3) and operator failure to cooldown the RCS with MFW (HACD1) are the two actions contained in HRA dependency combination 1295 (HRADEP-LERF-POST-1295). All remaining basic events in this cutset are branch probabilities from the level 2 event tree for its corresponding level 1 plant damage state.
ENCLOSURE Response to IPE RAI 14 Tennessee Valley Authority - Watts Bar Nuclear Plant - Unit 2, Docket No. 50-391 E1 - 5 Example Text from RecruleLERF.caf
- CHANGEEVENTS** +HRADEP-LERF-POST-1295 -HACD1 -HAOS3 HACD1 HAOS3
- SET EVENT PROBS**
HRADEP-LERF-POST-1295 1.70E-03 Example Cutset 1.73E-11
%2SLOCAL U2_ESF_SGD_CF_517_CCF_1_2 U2_L2_NOTPISGTRNOSBO U2_L2_NOTRCSDEPNOSBO U2_L2_TISGTRNOSBO U2_L2FBLERF001 HRADEP-LERF-POST-1295 The joint probability of the two operator actions (HAOS3
- HACD1) prior to the dependency analysis was 6.5E-06. Using the joint probability, the cutset frequency would be 6.63E-14 r-yr.
This would only contribute to LERF when the model is quantified with a truncation value less than 1.0E-13. Using the updated joint probability from the dependency analysis of 1.7E-03 results in a cutset frequency of 1.73E-11 r-yr. This increase in LERF due to cutsets involving multiple operator actions impacts the demonstration of convergence.
A sensitivity of the WBN LERF model was performed by quantifying without accounting for any HRA dependency between human actions. The LERF recovery file was updated to remove all text associated with HRA dependency. The following results were obtained:
Table 3: WBN U2 Results of Truncation Evaluation for LERF - No HRA Dependency Truncation Unit 2 LERF(r-yr)
% Difference 1.00E-12 1.86E-06 1.00E-13 1.95E-06 4.9%
1.00E-14 1.98E-06 1.7%
As expected, the corresponding LERF values are lower than the values presented in Tables 1 and 2. Comparison of the results from Table 3 to Tables 1 and 2 clearly shows the impact that HRA dependency has on convergence.
ENCLOSURE Response to IPE RAI 14 Tennessee Valley Authority - Watts Bar Nuclear Plant - Unit 2, Docket No. 50-391 E1 - 6 Summary of the HRA Dependency Review and Update Table 4 summarizes the results of the HRA dependency combinations review and update.
Please note that other HRA dependency combinations may have also been updated since they may share a subset of HEP events contained within these combinations. The dependency levels assigned for several combinations were reviewed and updated based on several different reasons. If two actions were occurring in the same time window, the default dependency assigned was completed based on the availability of the crew. These actions were reviewed to ensure adequate resources, and the level of dependency was assigned based on the locations of the action and the stress level. Dependency levels were also adjusted to more realistically match the timing and recovery time available to support the actions. For example, the timing for RWST to sump swap over was based on a Large LOCA event for the independent HEP evaluation. For sequences going to bleed and feed, the time to cue and the time until core damage after injection are longer than the time windows based on a Large LOCA event. For these cases dependency was assigned based on the more realistic timing for that given initiating event and sequence.
ENCLOSURE Response to IPE RAI 14 Tennessee Valley Authority - Watts Bar Nuclear Plant - Unit 2, Docket No. 50-391 Table 4: Reviewed and Updated HRA Dependency Probabilities Event Name HEPs in Combinations Event Joint Probability (prior to dependency assessment)
(after initial dependency assessment)
(after review and update of initial dependency assessment)
HRADEP-LERF-POST-837 HACD1: Cooldown with MFW 9.20E-08 3.40E-02 1.70E-03 HAOB2: Establish RCS Bleed and Feed Cooling HACI1:
Backup Containment Isolation HRADEP-LERF-POST-853 HACD1: Cooldown with MFW 2.20E-08 3.40E-02 8.50E-04 HARR1: Align High Pressure Recirculation HACI1:
Backup Containment Isolation HRADEP-LERF-POST-840 HACD1: Cooldown with MFW 2.50E-09 3.40E-02 8.50E-04 HAOS2: Start ECCS HACI1:
Backup Containment Isolation HRADEP-LERF-POST-851 HACD1: Cooldown with MFW 1.30E-08 3.40E-02 8.50E-04 HARL1: Recover from Auto Swapover Failure HACI1:
Backup Containment Isolation HRADEP-LERF-POST-839 HACD1: Cooldown with MFW 1.80E-11 8.70E-04 3.80E-05 HAOS3: Start AFW HAOB2: Establish RCS Bleed and Feed Cooling HACI1:
Backup Containment Isolation E1 - 7
ENCLOSURE Response to IPE RAI 14 Tennessee Valley Authority - Watts Bar Nuclear Plant - Unit 2, Docket No. 50-391 E1 - 8 Table 4: Reviewed and Updated HRA Dependency Probabilities Event Name HEPs in Combinations Event Joint Probability (prior to dependency assessment)
(after initial dependency assessment)
(after review and update of initial dependency assessment)
HRADEP-LERF-POST-1389 SSIOP:
Terminate Safety Injection to Prevent PORV Water Challenge 5.90E-08 5.00E-04 1.00E-05 HARR1: Align High Pressure Recirculation HAHH1: Place Hydrogen Igniters in Service HRADEP-LERF-POST-1415 SSIOP:
Terminate Safety Injection to Prevent PORV Water Challenge 2.50E-05 1.00E-03 1.00E-03 HARR1: Align High Pressure Recirculation HRADEP-LERF-POST-838 HACD1: Cooldown with MFW 4.00E-11 1.70E-02 1.20E-04 HAOS2: Start ECCS HAOB2: Establish RCS Bleed and Feed Cooling HACI1 : Backup Containment Isolation HRADEP-LERF-POST-843 HACD1: Cooldown with MFW 2.20E-14 1.70E-02 6.00E-05 HAOS2: Start ECCS HCRL1: Inadvertently Reset SI Signal, Failure of Auto Sump Swapover HARL1: Recover from Auto Swapover Failure HACI1:
Backup Containment Isolation HRADEP-LERF-POST-1295 HAOS3: Start AFW 6.50E-06 3.40E-02 1.7E-03 HACD1: Cooldown with MFW
ENCLOSURE Response to IPE RAI 14 Tennessee Valley Authority - Watts Bar Nuclear Plant - Unit 2, Docket No. 50-391 E1 - 9 Summary of the Updated Initiating Event LERF Contributions The Initiating Event LERF contribution pie chart was updated based on the results with the updated HRA dependency combinations failure probabilities and is displayed in Figure 3. When compared to the results of the 1.0E-12 quantification using the original HRA dependency combinations probabilities (LERF = 2.62E-06/r-yr), the LERF contribution due to a Secondary Side Break Outside Containment decreased from 6% to 3%. The LERF contribution due to Grid and Plant Centered Loss of Offsite Power increased from 22% to 24% and from 19% to 21%,
respectively. All remaining IE LERF contributions were within a 1% change.
Figure 3: Updated Unit 2 LERF IE Pie Chart Conclusion Figures 1 and 2 clearly illustrate decreasing changes in LERF following successive reductions in the truncation value. These results demonstrate convergence of the model with the updated recovery file and that no significant cutsets have been inadvertently eliminated. As an independent perspective, we also received feedback from EPRI on the WBN LERF result, and they recommended quantifying the WBN LERF results at 1E-13/r-yr based on the small relative change in LERF. A representative from EPRI also noted that, at this truncation level, the Level 2 model uncertainty is likely to dominate the results.