ML17241A275
| ML17241A275 | |
| Person / Time | |
|---|---|
| Site: | Nine Mile Point  |
| Issue date: | 08/29/2017 |
| From: | Distel D Niagara Mohawk Power Corp |
| To: | Lauren Gibson Japan Lessons-Learned Division |
| Gibson L, NRR/JLD, 415-1056 | |
| Shared Package | |
| ML17241A270 | List: |
| References | |
| CAC MF7946, CAC MF7947 | |
| Download: ML17241A275 (1) | |
Text
Request for Additional Information Nine Mile Point Nuclear Station, Units 1 and 2 Mitigating Strategies Assessment and Focused Evaluation NTTF Recommendation 2.1 - Flooding August 2017 Request:
As discussed during the audit call on August 15, 2017, the NRC notes that there are differences in the way that the warning time is addressed between the Mitigating Strategies Assessment (ML16349A029; dated December 14, 2016) and the Focused Evaluation (ML17069A005, dated March 10, 2017) for Nine Mile Point Nuclear Station. In particular, the FE contains commitments to further evaluate the consequential rainfall estimate and meteorological assessments in order to determine if the monitoring threshold should be adjusted or if the flood protection strategies should be modified. Please explain the evolution of the approach and how it impacts the Mitigating Strategies Assessment. Given that the FE mentions the possibility of warning times that may be less than the 6.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> previously determined for installing the flood barriers, please provide a justification for the plant being protected during such time as the analyses and related commitments are being completed.
Response
- a. Please explain the evolution of the [warning time] approach and how it impacts the Mitigating Strategies Assessment.
Between preparation of the Mitigating Strategies Assessment (MSA) and Focused Evaluation (FE), there was a change in an understanding of the available warning time from the 3rd party rainfall forecasting vendor associated with the installation of temporary barriers for the local intense precipitation (LIP) flood. The forecasting vendor was Accuweather at the time the MSA was written but recently changed to Murry and Trettel, LLC.
In preparing the MSA, Operations contacted Accuweather and it was confirmed that Accuweather could provide 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> warning for the rainfall action trigger criteria in the Accuweather contract (1 inch/hour or greater than 6 inches in 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />). The 24-hour warning time was written into the MSA as acceptable because the site (conservatively) needs 6.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> to install the barriers.
After the MSA was submitted and the actions to install barriers were dispositioned further for the FE (per NEI 16-05), Exelon concluded that the 24-hour warning from Accuweather, although achievable for an intense rainfall burst nested within a longer-duration storm, was not sufficiently proactive and did not provide an acceptable level of assurance that the minimum 6.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> of warning time could be achieved; particularly for storms not associated with longer-duration and organized meteorological systems. Therefore, the FE included planned updates to the sites procedure with enhanced warning time, which resulted in 4 commitments.
The commitments in the FE to perform additional analysis of consequential rainfall and enhance procedures to install barriers are also applicable to MSA. As discussed in Part b of this response (below), ingress and consequential rainfall analyses (still under development) are
Request for Additional Information Nine Mile Point Nuclear Station, Units 1 and 2 Mitigating Strategies Assessment and Focused Evaluation Page 2 showing that the MSA conclusion (the FLEX strategy can be successfully implemented as designed) remain valid. It should be noted that installation of cables and hoses are required for the FLEX strategy at 2 door locations that receive temporary barriers during a LIP-type flood.
While these barriers are needed at these two door locations for successful implementation of the FLEX strategy, the barriers can be installed after the onset of the rainfall with limited or no warning time. See Part b for additional discussion.
- b. Given that the FE mentions the possibility of warning times that may be less than the 6.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> previously determined for installing the flood barriers, please provide a justification for the plant being protected during such time as the analyses and related commitments are being completed.
As the four commitments in Enclosure 2 of the FE submittal are being addressed, Exelon maintains that NMPs key safety functions can be maintained during the LIP flood with installed equipment even without the temporary barriers in place or barriers installed with limited or no warning time. The bases for this position is the progress made to date on the four commitments and ingress volume estimates developed in NMP Calculations S0FLOODF002 and WH-C-002.
Additional details are provided below. It should be noted that Commitments 2 and 4 are optional, contingent upon the outcome of Commitments 1 and 3.
Commitment 1.
Perform an analysis to more accurately define the consequential rainfall estimate using the existing FLO-2D model. (Commitment date 12/31/2017)
The purpose of the consequential rainfall analysis is to evaluate the consequential rainfall depth at selected exterior door locations that are protected with portable flood barriers, as described in operating procedures N1-OP-64 and N2-OP-102. The consequential rainfall is defined in NEI 15-05 as the point at which flooding from rainfall (e.g., as determined by hydraulic analysis) rises above the permanent and passive flooding barriers (e.g., walls, door sills, dikes, berms, administratively closed openings, etc.) such that Structures, Systems, and Components (SSCs) important to safety are impacted. The results of the hydraulic analysis will be reported in the form of water surface elevation (WSEL)-rainfall depth curves at the selected exterior door locations, which will be used to determine rainfall depth(s) that can produce consequential flooding at each door. The consequential flood level at door openings typically defaults to the sill elevation (the level at which floodwater begins to enter the building) but could be higher if ingress can occur before impacting SSCs important to safety. The hydraulic analysis is complete.
The plant is currently performing additional assessments to define consequential flood levels based, in part, on ingress volume estimates. These estimates, discussed further below, appear to justify setting consequential flood levels above the plants finish floor and consequential 1-hour rainfall values generally greater than approximately 4 inches. Final determination of door-specific consequential flood levels and associated 1-hour rainfall depths is ongoing and expected to be completed before the commitment date.
NMPs exterior flooding ingress calculations for Units 1 and 2 (S0FLOODF002 and WH-C-002, respectively) evaluate the potential for flooding to impact safe shutdown equipment. For NMP Unit 1, S0FLOODF002 concluded that significant water buildup will not occur on the ground elevation of 261 feet [location of Safe Shutdown Equipment in Unit 1] as a result of the total
Request for Additional Information Nine Mile Point Nuclear Station, Units 1 and 2 Mitigating Strategies Assessment and Focused Evaluation Page 3 flow from the 72-hour PMP event. Assuming no water flows lower than floor elevation 250 feet, the depth from the flood across all areas on floor elevation 250 feet will average 31 inches.
For NMP Unit 2, WH-C-002 concluded that, as a result of the 72-hour PMP event, flooding through the Control Building doors C261-24, C261-29, and C261-31 and Electrical Tunnel doors ET261-1, ET261-2, ET262-3, and ET262-6 will flood the Control Building and the Electrical Tunnels to an approximate elevation of 225 feet. No Safe Shutdown Equipment is located in the Control Building and Electrical Tunnel below elevation 261.0 feet (plants finish floor). The Electrical Tunnel does contain insulated cables that are designed to function while inundated.
From WH-C-002, no flooding is expected in the Reactor Building and minimal flooding, if any, is expected in the Diesel Generator Building.
Ingress calculations S0FLOODF002 and WH-C-002 are based on the original 2013 FHRR LIP results (before changes to the more conservative roof runoff modeling approach were made as part of the 2014 sensitivity analysis). As discussed in Section 4 of the MSA Report, the 2014 sensitivity analysis showed minimal changes to the maximum flood depths (+/- 0.1 foot) with one notable exception; the maximum depths increased between 0.2 and 0.3 foot in an area located northeast of the Radwaste Building. Comparing stage hydrographs (Figure 1 through Figure 8) at select locations of the same duration and location in the power block area shows that the duration of flooding above the plants finish floor elevation (261.0 feet USLS35) approximately doubled. (See also Figure 9 for location of grid identification numbers.) Nevertheless, Exelon maintains that conclusions drawn from the ingress volume calculations (Safe Shutdown Equipment will not be impacted by the LIP flood if the barriers are not in place prior to the onset of rainfall) remain valid with the following justifications:
The water depth above the plants finish floor for the extended period of flooding resulting from the 2014 sensitivity analysis, shown by comparing stage hydrographs in Figure 1 through Figure 8 of the same duration and location, is relatively small (approximately 0.1 foot). Therefore, any additional ingress volume would also be small.
Even with the longer duration flooding above the plants finish floor elevation, as shown by the 2014 sensitivity analysis, the temporary barriers would be installed (per procedures N1-OP-64 and N2-OP-102 for NMP Units 1 and 2, respectively), even with little or no advanced warning, to cut off ingress before reaching the extended period. Therefore, the calculated ingress volume would likely never be reached or exceeded. Timed installation exercises performed in July 2013, that form the basis for the 6.5-hour installation time, include environmental adjustment factors between 1.5 and 3 to account for high winds, rain, and poor lighting. Therefore, it is reasonable to conclude that the barriers can be installed during localized rain and flood conditions and stop or limit ingress.
Both the original LIP analysis (supporting the 2013 FHRR) and the 2014 LIP sensitivity analysis include conservative assumptions. In addition to the conservative treatment of roof runoff in the 2014 analysis, Section 7.1.2 of the FE describes the conservatisms as follows:
- The use of HMR-51 and HMR-52 PMP values is generally considered conservative compared to site-specific PMP studies.
Request for Additional Information Nine Mile Point Nuclear Station, Units 1 and 2 Mitigating Strategies Assessment and Focused Evaluation Page 4
- Rainfall loss rates were conservatively not considered in the analysis and the entire model area was assumed to be impervious.
- The use of conservative Mannings n-values.
- Concrete security barriers that restrict flood water from flowing onto the site were conservatively not included in the analysis.
- Culverts that convey flow into the site were included in the 2-dimensional analysis, while culverts that convey flow away from the site were considered blocked.
In summary, the consequential rainfall analysis and ingress volume calculations currently show that the sites Safe Shutdown Equipment are protected for extreme rainfalls without the temporary portable barriers or barriers being installed with little or no warning.
Commitment 2.
As an optional task, if the consequential rainfall is determined to be low, a site-specific evaluation of storm types and seasonality will be conducted to determine the types of storms that could produce consequential rainfall and the meteorological parameters that could produce such events. This step 2017 may not be required if the consequential rainfall is sufficiently large to use available NWS and/or meteorological vendor tools and provide the necessary 6.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> of warning time. (Commitment date 12/31/2017)
As discussed above, the hydraulic analysis developed under Commitment 1 appear to indicate that the consequential 1-hour rainfall depth is generally greater than approximately 4 inches. The plant is currently performing additional assessments to define consequential flood levels based, in part, on ingress volume estimates. These estimates appear to justify setting consequential flood levels above the plants finish floor. Final determination of door-specific consequential flood levels and associated 1-hour rainfall depths is ongoing. Once a final determination is made, the site will assess the need to perform a site-specific evaluation of storm types and seasonality to support enhancements to site procedures (rather than following the standard procedure in NEI 15-05). The final determination and site-specific evaluation (if necessary) are expected to be completed by the commitment date.
Commitment 3.
Enhance site procedures to better define a monitoring threshold for longer forecasting periods (3 to 7 days) and the action trigger (per NEI 15-05). Based on the results of the above actions, existing site severe weather procedures will be updated and/or the meteorological vendor contract will be modified to incorporate the monitoring threshold and action trigger. (Commitment date 06/30/2018)
Enhancements to site procedures have not yet begun, pending the outcome of Commitments 1 and/or 2, but expected to be completed by the commitment date.
Request for Additional Information Nine Mile Point Nuclear Station, Units 1 and 2 Mitigating Strategies Assessment and Focused Evaluation Page 5 Commitment 4.
Modify the flood protection strategy if severe weather procedure enhancements and/or NWS/meteorological vendor contract forecast periods are determined not to be viable once the consequential rainfall and meteorological assessments described in Commitment Nos. 1 and 2 above are complete. (Commitment date 06/30/2018)
The need for modifications to the flood protection strategy has not yet been determined, pending completion of Commitment 3.
Figure 1 - Stage Hydrograph at the Grid Element 8156 (DG Rollup Door) from 2013 FHRR (6-hour)
Figure 2 - Stage Hydrograph at the Grid Element 7978 (Admin) from 2014 Sensitivity (6-hour)
Request for Additional Information Nine Mile Point Nuclear Station, Units 1 and 2 Mitigating Strategies Assessment and Focused Evaluation Page 6 Figure 3 - Stage Hydrograph at the Grid Element 8156 (DG Rollup Door) from 2013 FHRR (72-hour)
Figure 4 - Stage Hydrograph at the Grid Element 7978 (Admin) from 2014 Sensitivity (72-hour)
Request for Additional Information Nine Mile Point Nuclear Station, Units 1 and 2 Mitigating Strategies Assessment and Focused Evaluation Page 7 Figure 5 - Stage Hydrograph at the Grid Element 10043 (SE Corner of DGB) from 2013 FHRR (6-hour)
Figure 6 - Stage Hydrograph at the Grid Element 10043 (SE Corner of DGB) from 2014 Sensitivity (6-hour)
Request for Additional Information Nine Mile Point Nuclear Station, Units 1 and 2 Mitigating Strategies Assessment and Focused Evaluation Page 8 Figure 7 - Stage Hydrograph at the Grid Element 10043 (SE Corner of DGB) from 2013 FHRR (72-hour)
Figure 8 - Stage Hydrograph at the Grid Element 10043 (SE Corner of DGB) from 2014 Sensitivity (72-hour)
Request for Additional Information Nine Mile Point Nuclear Station, Units 1 and 2 Mitigating Strategies Assessment and Focused Evaluation Page 9 Figure 9 - Grid Identification (from 2013 FHRR)