ML18025C043: Difference between revisions
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| number = ML18025C043 | | number = ML18025C043 | ||
| issue date = 01/31/2018 | | issue date = 01/31/2018 | ||
| title = Request for Additional Information Regarding License Amendment Request to Resolve Nonconformances Relating to Containment Dome Truss | | title = Request for Additional Information Regarding License Amendment Request to Resolve Nonconformances Relating to Containment Dome Truss | ||
| author name = Chawla M | | author name = Chawla M | ||
| author affiliation = NRC/NRR/DORL/LPLIII | | author affiliation = NRC/NRR/DORL/LPLIII | ||
| addressee name = Coffey R | | addressee name = Coffey R | ||
Line 18: | Line 18: | ||
=Text= | =Text= | ||
{{#Wiki_filter:UNITED STATES NUCLEAR REGULATORY COMMISSION Mr. Robert Coffey Site Vice President NextEra Energy Point Beach, LLC 6610 Nuclear Road Two Rivers, WI 54241 | {{#Wiki_filter:UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, O.C. 20555-0001 January 31, 2018 Mr. Robert Coffey Site Vice President NextEra Energy Point Beach, LLC 6610 Nuclear Road Two Rivers, WI 54241 | ||
==SUBJECT:== | ==SUBJECT:== | ||
REQUEST FOR ADDITIONAL INFORMATION FOR POINT BEACH NUCLEAR PLANT, UNITS 1 AND 2, REGARDING LICENSE AMENDMENT REQUEST TO RESOLVE NONCONFORMANCES RELATING TO CONTAINMENT DOME TRUSS (EPID L-2017-LLA-0209) | REQUEST FOR ADDITIONAL INFORMATION FOR POINT BEACH NUCLEAR PLANT, UNITS 1 AND 2, REGARDING LICENSE AMENDMENT REQUEST TO RESOLVE NONCONFORMANCES RELATING TO CONTAINMENT DOME TRUSS (EPID L-2017-LLA-0209) | ||
==Dear Mr. Coffey:== | ==Dear Mr. Coffey:== | ||
By letter dated March 31, 2017 (Agencywide Documents Access and Management System (ADAMS) Accession No. | |||
The additional information requested within the enclosure to this letter is needed by the NRC staff to complete its review. Please provide a response to the request within 60 days of the date of this letter. If you have any questions, please contact me, at (301) 415-8371. | By letter dated March 31, 2017 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML17090A511), NextEra Energy Point Beach, LLC (NextEra, the licensee) submitted a license amendment request (LAR) for the Point Beach Nuclear Plant PBNP), Units 1 and 2. The proposed LAR uses a risk-informed resolution strategy to resolve legacy design code non-conformances associated with construction trusses in the containment buildings of PBNP, Units 1 and 2. | ||
The U.S. Nuclear Regulatory Commission (NRC) staff conducted the audit on November 29 and 30, 2017, at the NextEra office in Jupiter, Florida, to support the review of the license amendment. The additional information requested within the enclosure to this letter is needed by the NRC staff to complete its review. Please provide a response to the request within 60 days of the date of this letter. | |||
If you have any questions, please contact me, at (301) 415-8371. | |||
Sincerely, Mahesh Chawla, Project Manager Plant Licensing Branch Ill Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket Nos. 50-266 and 50-301 | |||
==Enclosure:== | ==Enclosure:== | ||
Request tor Additional Information cc: ListServ | |||
REQUEST FOR ADDITIONAL INFORAMTION FOR LICENSE AMENDMENT REQUEST 278 TO FACILITY OPERATING LICENSE NOS. DPR-24 AND DPR-27 RISK-INFORMED APPROACH TO RESOLVE CONSTRUCTION TRUSS DESIGN CODE NONCONFORMANCES NEXTERA ENERGY POINT BEACH, LLC POINT BEACH NUCLEAR PLANT, UNITS 1 AND 2 DOCKET NOS. 50-266 AND 50-301 | |||
However, Section 3.4 of Enclosure 1 of the submittal states that "[t]he alternative evaluation methods and acceptance criteria are not proposed as part of the license basis revision" which appears to be contradictory in that it seeks acceptance of the final proposed configurations of the trusses and associated equipment without any change to the current licensing basis. a. Clarify the change(s) to the licensing basis being sought by the submittal and provide a tabular comparison of the current licensing basis and the proposed change, including any specific changes to the current COR. b. Clarify whether the intent of reference to the submittal in the proposed text of the new Section A.5.1 O of the UFSAR, as shown in Enclosure 3 of the submittal, is to include the alternative evaluation methods and acceptance criteria used in the submittal as part of the proposed new licensing basis or explain which sections of the submittal are intended to be incorporated by reference. | : 1. Section 2.4 of Enclosure 1 of the submittal proposes "[a]cceptance of the final modified configuration of the Unit 1 construction truss and associated equipment, and the current configuration of the Unit 2 construction truss and associated equipment, using a risk-informed approach for resolution." The current licensing basis for the trusses is the code of record (COR) against which nonconformance is noted for certain truss members. It appears that the licensing basis change includes the use of a different analysis method and acceptance criteria, such as those listed in Tables 4-1 and 4-2 of Enclosure 5 of the submittal, for certain truss members. Enclosure 3 of the submittal includes the text for the new section (Section A.5.1 O) of the updated final safety analysis report (UFSAR), | ||
which also appears to change the licensing basis for certain truss members to a different code and/or acceptance criteria by incorporating the submittal by reference. However, Section 3.4 of Enclosure 1 of the submittal states that "[t]he alternative evaluation methods and acceptance criteria are not proposed as part of the license basis revision" which appears to be contradictory in that it seeks acceptance of the final proposed configurations of the trusses and associated equipment without any change to the current licensing basis. | |||
: a. Clarify the change(s) to the licensing basis being sought by the submittal and provide a tabular comparison of the current licensing basis and the proposed change, including any specific changes to the current COR. | |||
: b. Clarify whether the intent of reference to the submittal in the proposed text of the new Section A.5.1 O of the UFSAR, as shown in Enclosure 3 of the submittal, is to include the alternative evaluation methods and acceptance criteria used in the submittal as part of the proposed new licensing basis or explain which sections of the submittal are intended to be incorporated by reference. | |||
: c. Explain how the cited nonconformances can be reconciled if "{t]he alternative evaluation methods and acceptance criteria are not proposed as part of the license basis revision" as stated in Section 3.4 of Enclosure 1 of the submittal. | : c. Explain how the cited nonconformances can be reconciled if "{t]he alternative evaluation methods and acceptance criteria are not proposed as part of the license basis revision" as stated in Section 3.4 of Enclosure 1 of the submittal. | ||
Enclosure | Enclosure | ||
One of the commitments states that the licensee will "implement new seismic operating limits applicable to both Units ... Site procedures will be revised .... " Section 3.2 of Enclosure 1 of the submittal provides the proposed "new seismic operating limits" and Section 2.2.2 of the same enclosure provides the maximum ground accelerations in the horizontal and vertical directions for the operational basis earthquake (OBE). It appears that the "new seismic operating limit" for the vertical direction exceeds the corresponding value for the OBE. a. Provide the basis and justification for the selection of the "new seismic operating limits." b. Clarify the purpose of the "new seismic operating limits" as compared to the QBE limits when the "new seismic operating limit" for the vertical direction appears to exceed the corresponding value for the QBE. 3. According to Section 3.1.1 of Enclosure 1 and Section 1.3 of Enclosure 4 of the submittal, the initiating events considered are seismic events and thermal loading arising from postulated accidents. | : 2. Enclosure 2 of the submittal identifies the regulatory commitments made by the licensee in conjunction with the submittal. One of the commitments states that the licensee will "implement new seismic operating limits applicable to both Units ... Site procedures will be revised .... " Section 3.2 of Enclosure 1 of the submittal provides the proposed "new seismic operating limits" and Section 2.2.2 of the same enclosure provides the maximum ground accelerations in the horizontal and vertical directions for the operational basis earthquake (OBE). It appears that the "new seismic operating limit" for the vertical direction exceeds the corresponding value for the OBE. | ||
These initiators are used in the risk assessments for both the "bounding" and the "demonstrably conservative" analyses. | : a. Provide the basis and justification for the selection of the "new seismic operating limits." | ||
The focus on these initiators is presumably due to the source of the nonconformances. | : b. Clarify the purpose of the "new seismic operating limits" as compared to the QBE limits when the "new seismic operating limit" for the vertical direction appears to exceed the corresponding value for the QBE. | ||
However, a risk assessment needs to consider events and hazards that can credibly impact the structural integrity of the truss and the equipment supported from it. Provide quantitative or qualitative technical justification for the exclusion of internal and external initiating events other than those already considered in the analysis that could impact the application. | : 3. According to Section 3.1.1 of Enclosure 1 and Section 1.3 of Enclosure 4 of the submittal, the initiating events considered are seismic events and thermal loading arising from postulated accidents. These initiators are used in the risk assessments for both the "bounding" and the "demonstrably conservative" analyses. The focus on these initiators is presumably due to the source of the nonconformances. However, a risk assessment needs to consider events and hazards that can credibly impact the structural integrity of the truss and the equipment supported from it. Provide quantitative or qualitative technical justification for the exclusion of internal and external initiating events other than those already considered in the analysis that could impact the application. | ||
: 4. According to Enclosure 2 and Section 1.2 of Enclosure 4 of the submittal, no modifications are to be performed for the trusses in Unit 2. However, Section 6.4.2.2 of Enclosure 5 of the submittal states that "[t]rimming the first panel point at 14 locations for Unit 1 and 11 locations for Unit 2" while Section 6.5.1 states that "[t]he Unit 1 truss was analyzed in the configuration with limited modifications (six first panel point locations require trimming)." a. Clarify, with technical justification for the selection, the modifications that are proposed for the Unit 1 trusses and provide the rationale for not proposing all the modifications for Unit 1 trusses mentioned in Section 6.4.2.2 of Enclosure 5 of the submittal. | : 4. According to Enclosure 2 and Section 1.2 of Enclosure 4 of the submittal, no modifications are to be performed for the trusses in Unit 2. However, Section 6.4.2.2 of Enclosure 5 of the submittal states that "[t]rimming the first panel point at 14 locations for Unit 1 and 11 locations for Unit 2" while Section 6.5.1 states that "[t]he Unit 1 truss was analyzed in the configuration with limited modifications (six first panel point locations require trimming)." | ||
: b. Clarify whether any modifications are proposed for the Unit 2 trusses. If no modifications are proposed provide the rationale for not performing the modifications for Unit 2 trusses mentioned in Section 6.4.2.2 of Enclosure 5 of the submittal. | : a. Clarify, with technical justification for the selection, the modifications that are proposed for the Unit 1 trusses and provide the rationale for not proposing all the modifications for Unit 1 trusses mentioned in Section 6.4.2.2 of Enclosure 5 of the submittal. | ||
: c. Confirm that the configuration of the Unit 2 trusses that were analyzed to determine the seismic and thermal fragility values, which are used in the submittal are consistent with the configuration of the Unit 2 trusses as proposed in the submittal. | : b. Clarify whether any modifications are proposed for the Unit 2 trusses. If no modifications are proposed provide the rationale for not performing the modifications for Unit 2 trusses mentioned in Section 6.4.2.2 of Enclosure 5 of the submittal. | ||
If differences are found between the two configurations, provide and propagate the updated values or justify the continued use of the values in Section 6.4.2.2 of Enclosure 5 (and throughout Enclosure 4 ). 5. Section 6.4.1 of Enclosure 5 of the submittal discusses the determination of the probability of truss failure as a function of temperature, which is used for the risk calculations in Sections 2 and 5 of Enclosure 4 of the submittal. | : c. Confirm that the configuration of the Unit 2 trusses that were analyzed to determine the seismic and thermal fragility values, which are used in the submittal are consistent with the configuration of the Unit 2 trusses as proposed in the submittal. If differences are found between the two configurations, provide and propagate the updated values or justify the continued use of the values in Section 6.4.2.2 of Enclosure 5 (and throughout Enclosure 4 ). | ||
The discussion states that a probability of failure was "assigned" and provides separate curves for the Unit 1 and Unit 2 trusses. a. Describe the process used to "assign" the probability of failure as a function of temperature for the trusses including information from any expert elicitation, any guidance, such as NUREG/CR-6372 and NUREG/CR-6771 (ADAMS Accession Nos. ML080090003 and ML022410135, respectively), used for the expert elicitation process, and a description of the qualification of the experts. b. Section 2.2.1 of Enclosure 4 of the submittal uses a single ''thermal fragility curve" for both units as opposed to the two different curves presented in Section 6.4.1 of Enclosure 5 of the submittal. | : 5. Section 6.4.1 of Enclosure 5 of the submittal discusses the determination of the probability of truss failure as a function of temperature, which is used for the risk calculations in Sections 2 and 5 of Enclosure 4 of the submittal. The discussion states that a probability of failure was "assigned" and provides separate curves for the Unit 1 and Unit 2 trusses. | ||
Provide the technical justification for using a single curve to represent the "thermal fragility" of the trusses in both units. c. Demonstrate, preferably quantitatively, the impact of changing the "assigned" probability of failure as a function of temperature for the trusses on the "bounding" and "demonstrably conservative" risk calculations. | : a. Describe the process used to "assign" the probability of failure as a function of temperature for the trusses including information from any expert elicitation, any guidance, such as NUREG/CR-6372 and NUREG/CR-6771 (ADAMS Accession Nos. ML080090003 and ML022410135, respectively), | ||
: 6. Section 3.1.2 of Enclosure 1 of the submittal states that "the peer review of the PRA [probabilistic risk analysis] | used for the expert elicitation process, and a description of the qualification of the experts. | ||
was conducted | : b. Section 2.2.1 of Enclosure 4 of the submittal uses a single ''thermal fragility curve" for both units as opposed to the two different curves presented in Section 6.4.1 of Enclosure 5 of the submittal. Provide the technical justification for using a single curve to represent the "thermal fragility" of the trusses in both units. | ||
... and the peer review of the seismic and thermal fragility analyses was conducted. " The risk calculations presented in Enclosure 4 of the submittal do not include all the technical elements of a PRA as defined in Regulatory Guide (RG) 1.200, Revision 2 (ADAMS Accession No. ML090410014). | : c. Demonstrate, preferably quantitatively, the impact of changing the "assigned" probability of failure as a function of temperature for the trusses on the "bounding" and "demonstrably conservative" risk calculations. | ||
Attachment C of Enclosure 4 of the submittal provides a "statement of compliance" against the supporting requirements (SRs) in the 2009 American Society of Mechanical Engineers (ASME)/American Nuclear Society (ANS) PRA Standard for external hazard screening (Part 6 of the Standard) and seismic PRA (Part 5 of the Standard). | : 6. Section 3.1.2 of Enclosure 1 of the submittal states that "the peer review of the PRA | ||
Further, Table 1 and Section 6 of Enclosure 4 of the submittal include core damage frequency (COF) and large early release frequency (LERF) values from different hazards. However, the sources of those values are not mentioned. | [probabilistic risk analysis] was conducted ... and the peer review of the seismic and thermal fragility analyses was conducted. " The risk calculations presented in Enclosure 4 of the submittal do not include all the technical elements of a PRA as defined in Regulatory Guide (RG) 1.200, Revision 2 (ADAMS Accession No. ML090410014). Attachment C of Enclosure 4 of the submittal provides a "statement of compliance" against the supporting requirements (SRs) in the 2009 American Society of Mechanical Engineers (ASME)/American Nuclear Society (ANS) PRA Standard for external hazard screening (Part 6 of the Standard) and seismic PRA (Part 5 of the Standard). Further, Table 1 and Section 6 of Enclosure 4 of the submittal include core damage frequency (COF) and large early release frequency (LERF) values from different hazards. However, the sources of those values are not mentioned. | ||
: a. Provide clarification on the peer reviews referred to in Section 3.1.2 of Enclosure 1 of the submittal. | : a. Provide clarification on the peer reviews referred to in Section 3.1.2 of Enclosure 1 of the submittal. Include the peer review reports or details of the peer reviews including information on the peer-review guidance that was followed, the specific part(s) of the ASME/ANS PRA Standard, and finding-level Facts and Observations (F&Os) and their corresponding resolution. | ||
Include the peer review reports or details of the peer reviews including information on the peer-review guidance that was followed, the specific part(s) of the ASME/ANS PRA Standard, and finding-level Facts and Observations (F&Os) and their corresponding resolution. | : b. Explain the relevance to the current application of the "statement of compliance" against the SRs in Parts 5 and 6 of the 2009 ASME/ANS PRA Standard provided in Attachment C to Enclosure 4 of the submittal. | ||
: b. Explain the relevance to the current application of the "statement of compliance" against the SRs in Parts 5 and 6 of the 2009 ASME/ANS PRA Standard provided in Attachment C to Enclosure 4 of the submittal. | : c. Provide in a tabular format, information on the source for the GDF and LERF estimates for each hazard listed in Table 1 and Section 6 of Enclosure 4 of the submittal. If the source is a PRA model, include information on the status | ||
: c. Provide in a tabular format, information on the source for the GDF and LERF estimates for each hazard listed in Table 1 and Section 6 of Enclosure 4 of the submittal. | |||
If the source is a PRA model, include information on the status | of the technical adequacy determination such as whether a full-scope peer-review against the ASME/ANS PRA Standard has been performed per an RG 1.200-endorsed peer review process, and the version of the Standard against which the peer review was performed. Include any findings-level F&Os which has not been closed per an NRG-approved process, any subsequent modifications to the associated PRA model indicating which modifications are considered "upgrades," and the results of any follow-on or focused scope peer-reviews. | ||
Include any findings-level F&Os which has not been closed per an NRG-approved process, any subsequent modifications to the associated PRA model indicating which modifications are considered "upgrades," and the results of any follow-on or focused scope peer-reviews. | : d. If the seismic CDF and LERF values are from a seismic PRA model, justify not exercising that model for the current submittal. If the values are not derived from a seismic PRA model, describe the approach used to determine them and the technical justification for the selected approach. | ||
: d. If the seismic CDF and LERF values are from a seismic PRA model, justify not exercising that model for the current submittal. | : e. Clarify whether the CDF and LERF values in Table 1 and Section 6 of Enclosure 4 of the submittal represent mean estimates. If not, provide mean estimates for each hazard per RG 1.174 or justify the use of point estimates for risk-informed decision-making. | ||
If the values are not derived from a seismic PRA model, describe the approach used to determine them and the technical justification for the selected approach. | : 7. Section 1.6 and Attachment C of Enclosure 4 of the submittal cites Part 6 of the 2009 ASME/ANS PRA Standard (ASME RA-Sa-2009) and states that "the bounding and demonstrably conservative analyses show that [Delta] CDF and [Delta] LERF are acceptably low for hazards challenging the [Containment Truss) CT design." The intent of Section 6-2.1 of the 2009 ASME/ANS Standard is the exclusion of an external hazard further consideration in a risk analysis. Further, Section 6-1.2 does not apply to earthquakes. The submittal is requesting a change to the plant's licensing basis. | ||
: e. Clarify whether the CDF and LERF values in Table 1 and Section 6 of Enclosure 4 of the submittal represent mean estimates. | RG 1.17 4, Revision 2, states that "[t]racking changes in risk (both quantifiable and non-quantifiable) that are due to plant changes would provide a mechanism to account for the cumulative and synergistic effects of these plant changes .... " Further, seismic events are identified as being directly relevant to the risk analysis. Based on the foregoing discussion: | ||
If not, provide mean estimates for each hazard per RG 1. | : a. Explain the relevance of citing Part 6 of the 2009 ASME/ANS PRA Standard to the current application. | ||
: 7. Section 1.6 and Attachment C of Enclosure 4 of the submittal cites Part 6 of the 2009 ASME/ANS PRA Standard (ASME RA-Sa-2009) and states that "the bounding and demonstrably conservative analyses show that [Delta] CDF and [Delta] LERF are acceptably low for hazards challenging the [Containment Truss) CT design." The intent of Section 6-2.1 of the 2009 ASME/ANS Standard is the exclusion of an external hazard further consideration in a risk analysis. | : b. Clarify whether the results of the risk calculations, which represent the quantified risk to the facility from the proposed permanent configuration of the dome trusses, will be included in the baseline risk of the plant for future risk-informed applications to determine the cumulative impact of changes to the licensing basis per the guidance in RG 1.17 4. | ||
Further, Section 6-1.2 does not apply to earthquakes. | : c. Clarify the intent of submitting the "bounding" and the "demonstrably conservative" analyses as part of the submittal including the licensee's expectations of the scope of the staff's review and considering the response to item (b) of this request. | ||
The submittal is requesting a change to the plant's licensing basis. RG 1.17 4, Revision 2, states that "[t]racking changes in risk (both quantifiable and non-quantifiable) that are due to plant changes would provide a mechanism to account for the cumulative and synergistic effects of these plant changes .... " Further, seismic events are identified as being directly relevant to the risk analysis. | : 8. The fragility of the truss is used in both the "bounding" and "demonstrably conservative" analyses presented in Enclosure 4 of the submittal. Details of the conservative deterministic failure margin (CDFM) calculation used to determine the high confidence of low probability of failure (HCM, LPF) capacity of the truss are provided in Section 5. 7 of | ||
Based on the foregoing discussion: | |||
: a. Explain the relevance of citing Part 6 of the 2009 ASME/ANS PRA Standard to the current application. | Enclosure 5 of the submittal. According to that information, the CDFM uses the site-specific ground motion response spectrum (GMRS). According to the licensee's submittal of the reevaluated hazard in response to Near Term Task Force (NTIF) | ||
: b. Clarify whether the results of the risk calculations, which represent the quantified risk to the facility from the proposed permanent configuration of the dome trusses, will be included in the baseline risk of the plant for future risk-informed applications to determine the cumulative impact of changes to the licensing basis per the guidance in RG 1.17 4. c. Clarify the intent of submitting the "bounding" and the "demonstrably conservative" analyses as part of the submittal including the licensee's expectations of the scope of the staff's review and considering the response to item (b) of this request. 8. The fragility of the truss is used in both the "bounding" and "demonstrably conservative" analyses presented in Enclosure 4 of the submittal. | Recommendation 2.1 (ADAMS Accession No. ML14090A275), which is referred to as "Ref. 6.1" in Enclosure 5 of the submittal, the peak ground acceleration (PGA) is 0.14 g for the GMRS. This value is used in the capacity calculation in Section 6.4.2 of of the submittal. That value appears to be based on the mean hazard curve. The CDFM approach, as described in Table 2-5 of the Electric Power Research Institute (EPRI) report NP-6041-SL, Revision 1, requires the use of the 84 percent non-exceedance hazard curve. | ||
Details of the conservative deterministic failure margin (CDFM) calculation used to determine the high confidence of low probability of failure (HCM, LPF) capacity of the truss are provided in Section 5. 7 of | Section 2.1.3 of Enclosure 4 of the submittal provides the calculation for the change in (or delta) CDF due to the seismic hazard. The calculation provided uses the difference between the GDF for the current configuration of the truss (with thermal modifications) and that from a configuration that includes all modifications which would be required to "fully meet seismic and thermal design requirements". The same approach for calculating the delta GDF is applied to the "bounding" and "demonstrably conservative" analyses. | ||
According to that information, the CDFM uses the site-specific ground motion response spectrum (GMRS). According to the licensee's submittal of the reevaluated hazard in response to Near Term Task Force (NTIF) Recommendation 2.1 (ADAMS Accession No. | The analysis described in Section 2 of Enclosure 4 of the submittal assumes that the initiators of interest, seismic and thermal events, are independent. However, seismic events can result in consequential events such as loss-of-coolant accidents (LOCAs) which can, in turn, result in thermal loading of the trusses. Such consequential events have been excluded from the "bounding" and "demonstrably conservative" analyses without justification. | ||
That value appears to be based on the mean hazard curve. The CDFM approach, as described in Table 2-5 of the Electric Power Research Institute (EPRI) report NP-6041-SL, Revision 1, requires the use of the 84 percent non-exceedance hazard curve. Section 2.1.3 of Enclosure 4 of the submittal provides the calculation for the change in (or delta) CDF due to the seismic hazard. The calculation provided uses the difference between the GDF for the current configuration of the truss (with thermal modifications) and that from a configuration that includes all modifications which would be required to "fully meet seismic and thermal design requirements". | : a. Provide the HCLPF and median failure probability for the unmodified and modified configuration of the trusses following the methodology in Table 2-5 of the EPRI report NP-6041-SL and Section 5 of EPRI report TR-103959, Revision 1, or justify the calculation in Section 5.7 of Enclosure 5 of the submittal. | ||
The same approach for calculating the delta GDF is applied to the "bounding" and "demonstrably conservative" analyses. | : b. Describe the modifications that are credited for the trusses in Units 1 and 2 for the calculations based on the truss "modified to fully meet seismic and thermal design requirements," such as in Table 5 of Section 2.1.3 of Enclosure 4 of the submittal. | ||
The analysis described in Section 2 of Enclosure 4 of the submittal assumes that the initiators of interest, seismic and thermal events, are independent. | : c. Provide a seismic event tree to capture the structural failure of the truss along with other possible consequential events, including LOGAs, due to seismic initiators. | ||
However, seismic events can result in consequential events such as loss-of-coolant accidents (LOCAs) which can, in turn, result in thermal loading of the trusses. Such consequential events have been excluded from the "bounding" and "demonstrably conservative" analyses without justification. | : d. Provide qualitative or quantitative technical justification for the exclusion of any sequences from the seismic event tree that are not expected to impact the risk assessment for the trusses. Include information on any generic component fragilities used in the process. | ||
: a. Provide the HCLPF and median failure probability for the unmodified and modified configuration of the trusses following the methodology in Table 2-5 of the EPRI report NP-6041-SL and Section 5 of EPRI report TR-103959, Revision 1, or justify the calculation in Section 5.7 of Enclosure 5 of the submittal. | : e. Provide the re-quantified seismic GDF for the "bounding" and "demonstrably conservative" analyses including any changes due to the responses to parts (a) through (d) of this request or justify the need to not perform such a re-quantification. | ||
: b. Describe the modifications that are credited for the trusses in Units 1 and 2 for the calculations based on the truss "modified to fully meet seismic and thermal design requirements," such as in Table 5 of Section 2.1.3 of Enclosure 4 of the submittal. | : 9. Section 2.1.4 of Enclosure 4 of the submittal discusses the convolution approach wherein the individual plant examination of external events (IPEEE) "plant fragility" is used. The same approach is also cited in Section 5.2.1 of Enclosure 4 of the submittal. | ||
: c. Provide a seismic event tree to capture the structural failure of the truss along with other possible consequential events, including LOGAs, due to seismic initiators. | Therefore, the "convolution" approach is used for both the "bounding" and "demonstrably conservative" analyses. Section 2.1.4 of Enclosure 4 of the submittals states that "[a] | ||
: d. Provide qualitative or quantitative technical justification for the exclusion of any sequences from the seismic event tree that are not expected to impact the risk assessment for the trusses. Include information on any generic component fragilities used in the process. e. Provide the re-quantified seismic GDF for the "bounding" and "demonstrably conservative" analyses including any changes due to the responses to parts (a) through (d) of this request or justify the need to not perform such a re-quantification. 9. Section 2.1.4 of Enclosure 4 of the submittal discusses the convolution approach wherein the individual plant examination of external events (IPEEE) "plant fragility" is used. The same approach is also cited in Section 5.2.1 of Enclosure 4 of the submittal. | more accurate characterization of the CT {completion time] risk can be obtained by combining the IPEEE seismic and the CT bounding results." | ||
Therefore, the "convolution" approach is used for both the "bounding" and "demonstrably conservative" analyses. | : a. Justify the applicability of the IPEEE "plant fragility" for the "convolution" approach. | ||
Section 2.1.4 of Enclosure 4 of the submittals states that "[a] more accurate characterization of the CT {completion time] risk can be obtained by combining the IPEEE seismic and the CT bounding results." a. Justify the applicability of the IPEEE "plant fragility" for the "convolution" approach. | : b. Justify the assumption that the truss failure and selected plant level contribution approach, such as the IPEEE "plant fragility", can be considered independent events. | ||
: b. Justify the assumption that the truss failure and selected plant level contribution approach, such as the IPEEE "plant fragility", can be considered independent events. | 1O. Sections 5.2.1 through 5.2.3 of Enclosure 4 of the submittal provides the risk calculations for the "demonstrably conservative" analysis. Those sections provide "event trees" for various initiating events impacting the trusses. The split fractions at each node are the failure probabilities, based on an assessment described in Section 5 of Enclosure 4 of the submittal, of that particular equipment from the falling truss members. | ||
Those sections provide "event trees" for various initiating events impacting the trusses. The split fractions at each node are the failure probabilities, based on an assessment described in Section 5 of Enclosure 4 of the submittal, of that particular equipment from the falling truss members. The final result is termed the "conditional core damage probability" and multiplied with the initiating event frequency. | The final result is termed the "conditional core damage probability" and multiplied with the initiating event frequency. However, the "event trees" represent the component failure probabilities and do not appear to describe the sequences which would potentially lead to core damage following a seismic event resulting in failure of the truss. The event tree does not appear to be modeled based on the logic in the internal events PRA model. In addition, human actions related to the impact of the failure of each component, including the impact of the seismic event on such actions do not appear to have been considered. Therefore, the "event tree" does not appear to provide the conditional core damage probability (CCDP). It appears that deriving such a CCDP would require integration of the component failure probabilities in Sections 5.2.1 through 5.2.3 of Enclosure 4 of the submittal with a plant-specific PRA model (internal events and/or seismic). | ||
However, the "event trees" represent the component failure probabilities and do not appear to describe the sequences which would potentially lead to core damage following a seismic event resulting in failure of the truss. The event tree does not appear to be modeled based on the logic in the internal events PRA model. In addition, human actions related to the impact of the failure of each component, including the impact of the seismic event on such actions do not appear to have been considered. | : a. Describe the logic used to develop the "event trees" in Sections 5.2.1 through 5.2.3 of Enclosure 4 of the submittal including the approach used to determine the "top events", and to capture the sequences which can potentially lead to core damage following truss failure due to the selected initiating events. Provide a comparison of the "event trees" in Sections 5.2.1 through 5.2.3 of Enclosure 4 of the submittal with similar event trees from the licensee's internal events PRA model. | ||
Therefore, the "event tree" does not appear to provide the conditional core damage probability (CCDP). It appears that deriving such a CCDP would require integration of the component failure probabilities in Sections 5.2.1 through 5.2.3 of Enclosure 4 of the submittal with a plant-specific PRA model (internal events and/or seismic). | : b. Considering the factors identified above, provide justification that the analysis in Sections 5.2.1 through 5.2.3 of Enclosure 4 of the submittal can either represent or bound any integration of the failure probabilities into a PRA model. Alternately, provide the mean seismic CDF and significant cutsets from such an integration. | ||
: a. Describe the logic used to develop the "event trees" in Sections 5.2.1 through 5.2.3 of Enclosure 4 of the submittal including the approach used to determine the "top events", and to capture the sequences which can potentially lead to core damage following truss failure due to the selected initiating events. Provide a comparison of the "event trees" in Sections 5.2.1 through 5.2.3 of Enclosure 4 of the submittal with similar event trees from the licensee's internal events PRA model. b. Considering the factors identified above, provide justification that the analysis in Sections 5.2.1 through 5.2.3 of Enclosure 4 of the submittal can either represent or bound any integration of the failure probabilities into a PRA model. Alternately, provide the mean seismic CDF and significant cutsets from such an integration. | : c. The failure probabilities from the assessment described in Section 5 of Enclosure 4 of the submittal that are used for the "event trees" do not change based on the seismic acceleration. It is expected that at a certain threshold seismic acceleration level the fragility of the individual components will | ||
: c. The failure probabilities from the assessment described in Section 5 of Enclosure 4 of the submittal that are used for the "event trees" do not change based on the seismic acceleration. | |||
It is expected that at a certain threshold seismic acceleration level the fragility of the individual components will | dominate the corresponding failure. The convolution with the IPEEE performed in Section 5.2.1 of Enclosure 4 of the submittal appears to be performed to account for such cases. However, the convolution is based on the "plant fragility" and there is no comparison of the fragility of each impacted component against the "plant fragility." Justify that use of the "plant fragility" is bounding for all the components that are found to be impacted by truss failure in the assessment. | ||
: 11. Attachment 8 to Enclosure 4 of the submittal provides a description of the human reliability analysis (HRA) used for the "demonstrably conservative" analysis by inclusion in the "event trees" used in Sections 5.2.1 through 5.2.3 of Enclosure 4 of the submittal. | : 11. Attachment 8 to Enclosure 4 of the submittal provides a description of the human reliability analysis (HRA) used for the "demonstrably conservative" analysis by inclusion in the "event trees" used in Sections 5.2.1 through 5.2.3 of Enclosure 4 of the submittal. | ||
The description in Attachment B states that the operators will have more than 30 minutes to perform feed and bleed (F&B). Further, the analysis also uses "simplified | The description in Attachment B states that the operators will have more than 30 minutes to perform feed and bleed (F&B). Further, the analysis also uses "simplified | ||
[performance shaping factor] PSF adjustments" based on the seismic acceleration level. However, Section 5.2 of Enclosure 4 of the submittal states that the "PORVs [pilot~ operated relief valve]" top "event" uses the baseline internal events value for F&B human error probability (HEP). a. Justify the time available to initiate F&B used for the HRA analysis including a summary of any relevant engineering evaluations. | [performance shaping factor] PSF adjustments" based on the seismic acceleration level. | ||
: b. Justify the multipliers used for the PSF adjustments including the basis and methodology used to derive the multipliers for different seismic intensities. | However, Section 5.2 of Enclosure 4 of the submittal states that the "PORVs [pilot~ | ||
: c. Clarify whether the baseline internal events HEP for F&B is used across all seismic acceleration levels in the analysis in Sections 5.2.1 through 5.2.3 of Enclosure 4 of the submittal and if so, justify such an approach given the analysis in Attachment B to Enclosure 4 of the submittal. | operated relief valve]" top "event" uses the baseline internal events value for F&B human error probability (HEP). | ||
: d. The baseline HEP for F&B is expected to assume the availability of instrument air for the PORVs. The determination of the impact of truss failure on instrument air availability does not appear to have been performed as part of the assessment described in Section 5 of Enclosure 4 of the submittal for the "demonstrably conservative" analysis. | : a. Justify the time available to initiate F&B used for the HRA analysis including a summary of any relevant engineering evaluations. | ||
However, it appears that instrument air is considered unavailable during a seismic event. Describe and justify the assumption regarding the availability of instrument air following truss failure. e. If the response to part (d) of this request for additional information (RAI) credits the planned nitrogen supply modifications for F&B to justify the use of the baseline HEP, justify the credit taken for the planned nitrogen supply modification as a method that maintains appropriate safety margins as mentioned in Section 3.1 of Enclosure 1 of the submittal. | : b. Justify the multipliers used for the PSF adjustments including the basis and methodology used to derive the multipliers for different seismic intensities. | ||
: f. Attachment B to Enclosure 4 of the submittal states that removing the dependency for instrument air reduces the HEP. However, the reduction seems to be based on a simple removal of the step in the procedure | : c. Clarify whether the baseline internal events HEP for F&B is used across all seismic acceleration levels in the analysis in Sections 5.2.1 through 5.2.3 of Enclosure 4 of the submittal and if so, justify such an approach given the analysis in Attachment B to Enclosure 4 of the submittal. | ||
("step 36 in the current procedure") | : d. The baseline HEP for F&B is expected to assume the availability of instrument air for the PORVs. The determination of the impact of truss failure on instrument air availability does not appear to have been performed as part of the assessment described in Section 5 of Enclosure 4 of the submittal for the "demonstrably conservative" analysis. However, it appears that instrument air is considered unavailable during a seismic event. Describe and justify the assumption regarding the availability of instrument air following truss failure. | ||
for restoration of instrument air. Justify the HEP accounting for any actions and/or steps necessary to be performed for entering into and following the procedure for using the nitrogen supply subsequent to not restoring instrument air including the consideration of | : e. If the response to part (d) of this request for additional information (RAI) credits the planned nitrogen supply modifications for F&B to justify the use of the baseline HEP, justify the credit taken for the planned nitrogen supply modification as a method that maintains appropriate safety margins as mentioned in Section 3.1 of Enclosure 1 of the submittal. | ||
Enclosure 5 of the submittal compared the supported equipment and the supporting mechanism (e.g., anchors) against the design criteria using the GMRS and thermal loading. However, the risk assessment is not confined to the GMRS or the design basis. According to Enclosure 2 of the submittal, the licensee is also committing to a modification to the containment spray pipe support. It is unclear if a fragility for the equipment supported by the trusses and the corresponding supports was determined which in turn, can be used to obtain the risk from corresponding failures across the seismic hazard curve as well as the thermal loading. Further, the mitigating systems employed for the thermal loading mitigation calculations provided in Section 2.2.4 of Enclosure 4 of the submittal are based solely on the random failure probability without consideration of failure due to the thermal or seismic hazard (such as through an operating reactor gate). The thermal loading mitigation calculations are applicable to both the "bounding" and the "demonstrably conservative" analyses. | : f. Attachment B to Enclosure 4 of the submittal states that removing the dependency for instrument air reduces the HEP. However, the reduction seems to be based on a simple removal of the step in the procedure ("step 36 in the current procedure") for restoration of instrument air. Justify the HEP accounting for any actions and/or steps necessary to be performed for entering into and following the procedure for using the nitrogen supply subsequent to not restoring instrument air including the consideration of | ||
Section 2.2.2 of Enclosure 4 of the submittal uses information from EPRI report 302000079, Revision 3, "Pipe Rupture Frequencies for Internal Flooding Probabilistic Risk Assessments," to determine the initiating frequency for steam line breaks and feedwater line breaks inside containment. | |||
The initiating frequencies are then used in both the "bounding" and the "demonstrably conservative" analyses. | dependencies in intra- and inter-procedure actions or provide and use a different value. | ||
Section 5.1 ("PWR [pressurized-water reactor] High-Energy Piping Systems") | : 12. The seismic and/or thermal failure of the equipment supported from the trusses in the "bounding" analysis in Section 2 of Enclosure 4 of the submittal may increase the overall risk contribution. Enclosure 5 of the submittal compared the supported equipment and the supporting mechanism (e.g., anchors) against the design criteria using the GMRS and thermal loading. However, the risk assessment is not confined to the GMRS or the design basis. According to Enclosure 2 of the submittal, the licensee is also committing to a modification to the containment spray pipe support. It is unclear if a fragility for the equipment supported by the trusses and the corresponding supports was determined which in turn, can be used to obtain the risk from corresponding failures across the seismic hazard curve as well as the thermal loading. | ||
of the cited EPRI report states that the feedwater piping " ... system boundary considered in this evaluation consists of the piping from the low-pressure heaters ... up to the outboard containment isolation valves." Similarly, the high pressure steam piping is considered to be " ... upstream of the [high pressure] | Further, the mitigating systems employed for the thermal loading mitigation calculations provided in Section 2.2.4 of Enclosure 4 of the submittal are based solely on the random failure probability without consideration of failure due to the thermal or seismic hazard (such as through an operating reactor gate). The thermal loading mitigation calculations are applicable to both the "bounding" and the "demonstrably conservative" analyses. | ||
HP turbine throttle valve and extends to the outboard containment isolation valves." Therefore, the initiating frequencies for the piping from the cited EPRI report are for breaks outside containment and are inapplicable to the risk assessment in the submittal. | Section 2.2.2 of Enclosure 4 of the submittal uses information from EPRI report 302000079, Revision 3, "Pipe Rupture Frequencies for Internal Flooding Probabilistic Risk Assessments," to determine the initiating frequency for steam line breaks and feedwater line breaks inside containment. The initiating frequencies are then used in both the "bounding" and the "demonstrably conservative" analyses. Section 5.1 ("PWR | ||
: a. Provide a quantitative assessment of the impact on the submitted risk calculations of the failure of the equipment supported by the trusses due to the seismic hazard and thermal loading or justify not including such impacts for the "bounding" as well as the "demonstrably conservative" cases. b. Provide an estimate, preferably using quantitative approaches such as the median fragility, of the extent to which the defense-in-depth due to containment sprays and containment air recirculation cooling systems is preserved beyond the plant's seismic design basis. c. Provide the results of the thermal analysis using relevant initiating frequencies for the breaks of interest. | [pressurized-water reactor] High-Energy Piping Systems") of the cited EPRI report states that the feedwater piping " ... system boundary considered in this evaluation consists of the piping from the low-pressure heaters ... up to the outboard containment isolation valves." Similarly, the high pressure steam piping is considered to be " ... upstream of the | ||
If the updated initiating frequencies are partitioned by break size, provide the technical justification and methodology for the partitioning. | [high pressure] HP turbine throttle valve and extends to the outboard containment isolation valves." Therefore, the initiating frequencies for the piping from the cited EPRI report are for breaks outside containment and are inapplicable to the risk assessment in the submittal. | ||
Consider the responses to previous parts of this request in the re-quantification. | : a. Provide a quantitative assessment of the impact on the submitted risk calculations of the failure of the equipment supported by the trusses due to the seismic hazard and thermal loading or justify not including such impacts for the "bounding" as well as the "demonstrably conservative" cases. | ||
Include any initiating frequency determined from the development of the seismic event tree as requested in RAI 8. 13. Section 2.5 of RG 1.17 4, Revision 2, discusses uncertainties in risk analysis and states that " ... comparison of the PRA results with the acceptance guidelines must be based on an understanding of the contributors to the PRA results and on the robustness of the assessment of those contributors and the impacts of the uncertainties." Section 9 of Enclosure 4 to the submittal states that "[t]he uncertainties are addressed by the simple bounding case and sensitivity analyses applied in this evaluation | : b. Provide an estimate, preferably using quantitative approaches such as the median fragility, of the extent to which the defense-in-depth due to containment sprays and containment air recirculation cooling systems is preserved beyond the plant's seismic design basis. | ||
... " However, neither the impact of the quantifiable uncertainties in the seismic hazard and seismic fragility is captured in the submittal nor is it demonstrated that the "qualitative factors" in Section 4 of Enclosure 4 to the submittal adequately capture such uncertainties. | : c. Provide the results of the thermal analysis using relevant initiating frequencies for the breaks of interest. If the updated initiating frequencies are partitioned by break size, provide the technical justification and methodology for the partitioning. Consider the responses to previous parts of this request in the re-quantification. Include any initiating frequency determined from the development of the seismic event tree as requested in RAI 8. | ||
The "thermal sensitivity analysis" in Section 2.2.6 of Enclosure 4 to the submittal expands the initiating event frequency but does not address the uncertainties in those frequencies. | : 13. Section 2.5 of RG 1.17 4, Revision 2, discusses uncertainties in risk analysis and states that " ... comparison of the PRA results with the acceptance guidelines must be based on an understanding of the contributors to the PRA results and on the robustness of the assessment of those contributors and the impacts of the uncertainties." Section 9 of Enclosure 4 to the submittal states that "[t]he uncertainties are addressed by the simple bounding case and sensitivity analyses applied in this evaluation ... " However, neither the impact of the quantifiable uncertainties in the seismic hazard and seismic fragility is captured in the submittal nor is it demonstrated that the "qualitative factors" in Section 4 of Enclosure 4 to the submittal adequately capture such uncertainties. The "thermal sensitivity analysis" in Section 2.2.6 of Enclosure 4 to the submittal expands the initiating event frequency but does not address the uncertainties in those frequencies. Further, the submittal does not include a discussion of the sources of uncertainty and their impact for the "demonstrably conservative analysis". | ||
Further, the submittal does not include a discussion of the sources of uncertainty and their impact for the "demonstrably conservative analysis". | Justify the lack of sensitivity studies to capture the impact of key sources of uncertainties on the analyses presented in the submittal, considering the guidance in NUREG-1855, Revision 1. Alternately, describe, with justification, the approach used and provide the results (GDF, delta GDF, LERF, and delta LERF) from the following considering the response to RAI 7c: | ||
Justify the lack of sensitivity studies to capture the impact of key sources of uncertainties on the analyses presented in the submittal, considering the guidance in NUREG-1855, Revision 1. Alternately, describe, with justification, the approach used and provide the results (GDF, delta GDF, LERF, and delta LERF) from the following considering the response to RAI 7c: a. A sensitivity study to address the uncertainty in the seismic hazard and in the seismic fragility for the "bounding case", b. A sensitivity study to address the uncertainty in the initiating events identified for the thermal analysis and in the thermal fragility for the "bounding case." Include justification for the uncertainty bounds and distribution selected for the thermal fragility for use in the sensitivity study. c. A sensitivity study to address the uncertainty in the calculation inputs, such as the seismic hazard, the seismic fragility, the assessment results described in Section 5 of Enclosure 4 of the submittal, and the HEP, for the "demonstrably conservative case" of seismic evaluation. | : a. A sensitivity study to address the uncertainty in the seismic hazard and in the seismic fragility for the "bounding case", | ||
Include justification for the uncertainty bounds and distribution selected for the assessment results and the HEP for use in the sensitivity study. d. A sensitivity study to address the uncertainty in the calculation inputs, such as initiating events identified for the thermal analysis, in the thermal fragility, in the assessment results, and the HEP, for the "demonstrably conservative case" of thermal evaluation. | : b. A sensitivity study to address the uncertainty in the initiating events identified for the thermal analysis and in the thermal fragility for the "bounding case." | ||
Include justification for the uncertainty bounds and distribution selected for the thermal fragility, assessment results, and the HEP for use in the sensitivity study. In addressing the above requests, consider the responses to RAls 5, 10, and 14. 14. Section 6 of Enclosure 4 of the submittal discusses the LERF calculation which uses a CLERP [conditional large early release probability] | Include justification for the uncertainty bounds and distribution selected for the thermal fragility for use in the sensitivity study. | ||
of 0.2 based on the information for different hazards. The CLERP is then applied to the results for the change in GDF from the "bounding" and the "demonstrably conservative" analyses. | : c. A sensitivity study to address the uncertainty in the calculation inputs, such as the seismic hazard, the seismic fragility, the assessment results described in Section 5 of Enclosure 4 of the submittal, and the HEP, for the "demonstrably conservative case" of seismic evaluation. Include justification for the uncertainty bounds and distribution selected for the assessment results and the HEP for use in the sensitivity study. | ||
It appears that the impact of the truss failure on LERF via component failures has not been considered. | : d. A sensitivity study to address the uncertainty in the calculation inputs, such as initiating events identified for the thermal analysis, in the thermal fragility, in the assessment results, and the HEP, for the "demonstrably conservative case" of thermal evaluation. Include justification for the uncertainty bounds and distribution selected for the thermal fragility, assessment results, and the HEP for use in the sensitivity study. | ||
Further, it ts expected that instrument air will be required for containment isolation valves and the determination of the impact of truss failure on instrument air availability does not appear | In addressing the above requests, consider the responses to RAls 5, 10, and 14. | ||
Such failures can be accounted for by exercising the Level 2 or simplified LERF model in conjunction with the component failure probabilities. | : 14. Section 6 of Enclosure 4 of the submittal discusses the LERF calculation which uses a CLERP [conditional large early release probability] of 0.2 based on the information for different hazards. The CLERP is then applied to the results for the change in GDF from the "bounding" and the "demonstrably conservative" analyses. It appears that the impact of the truss failure on LERF via component failures has not been considered. Further, it ts expected that instrument air will be required for containment isolation valves and the determination of the impact of truss failure on instrument air availability does not appear | ||
In light of the factors identified above, provide an estimate of LERF that quantitatively considers the failure of components and systems that can impact containment integrity or justify that the current approach bounds such impacts. 15. Section 5 of Enclosure 4 to the submittal discusses the "demonstrably conservative analysis" and summarizes the results of the assessment performed in support of that analysis. | |||
The discussion cites a proprietary assessment report, Reference 3, in Enclosure 4 to the submittal. | to have been performed as part of the assessment described in Section 5 of Enclosure 4 of the submittal. Such failures can be accounted for by exercising the Level 2 or simplified LERF model in conjunction with the component failure probabilities. In light of the factors identified above, provide an estimate of LERF that quantitatively considers the failure of components and systems that can impact containment integrity or justify that the current approach bounds such impacts. | ||
The following are related to the assessment: | : 15. Section 5 of Enclosure 4 to the submittal discusses the "demonstrably conservative analysis" and summarizes the results of the assessment performed in support of that analysis. The discussion cites a proprietary assessment report, Reference 3, in Enclosure 4 to the submittal. The following are related to the assessment: | ||
: a. Provide Reference 3 in Enclosure 4 to the submittal or the details of the assessment used to include and exclude different structures, system and components (SSCs) as well as determine the probability of failure given truss failure in the "demonstrably conservative" analysis as summarized in Sections 5.1 and 5.2, respectively, of Enclosure 4 to the submittal. | : a. Provide Reference 3 in Enclosure 4 to the submittal or the details of the assessment used to include and exclude different structures, system and components (SSCs) as well as determine the probability of failure given truss failure in the "demonstrably conservative" analysis as summarized in Sections 5.1 and 5.2, respectively, of Enclosure 4 to the submittal. | ||
: b. Based on the discussion in Section 5.1 of Enclosure 4 to the submittal it appears that "perforation/penetration" is the only failure mode considered as part of the assessment. | : b. Based on the discussion in Section 5.1 of Enclosure 4 to the submittal it appears that "perforation/penetration" is the only failure mode considered as part of the assessment. However, the rationale for the selection has not been provided. Discuss the various structural failure modes that were considered in the assessment for the SSCs impacted by falling truss debris and provide the basis for the determination of perforation/penetration as the only failure mode. | ||
However, the rationale for the selection has not been provided. | : c. Describe how qualitative, quantitative, or a combination of both approaches was used in the assessment summarized in Section 5.1 of Enclosure 4 of the submittal. If any equations/formulae were used to determine the ability of falling debris to damage a particular SSC, provide the basis to support the applicability of those equations/formulae to the current assessment. | ||
Discuss the various structural failure modes that were considered in the assessment for the SSCs impacted by falling truss debris and provide the basis for the determination of perforation/penetration as the only failure mode. c. Describe how qualitative, quantitative, or a combination of both approaches was used in the assessment summarized in Section 5.1 of Enclosure 4 of the submittal. | : d. One of the assumptions made in the assessment appears to be that" ... debris targeting critical SSCs are assumed to be oriented in a way that maximizes damage to targeted SSCs." | ||
If any equations/formulae were used to determine the ability of falling debris to damage a particular SSC, provide the basis to support the applicability of those equations/formulae to the current assessment. | : 1. Describe the approach to determine the orientation(s) of the falling truss debris that "maximizes damage to targeted SSC.'' | ||
: d. One of the assumptions made in the assessment appears to be that" ... debris targeting critical SSCs are assumed to be oriented in a way that maximizes damage to targeted SSCs." 1. Describe the approach to determine the orientation(s) of the falling truss debris that "maximizes damage to targeted SSC.'' ii. Justify any assumptions made in determining the orientation(s) of the falling truss debris that "maximizes damage to targeted SSC" and for any other orientations of the falling truss debris. iii. Describe whether and how the probability of a particular orientation of the falling truss debris was determined, and provide details of the approach used to incorporate those probabilities in the "qualitative evaluation of the impact of trusses and cross members" performed in the assessment. | ii. Justify any assumptions made in determining the orientation(s) of the falling truss debris that "maximizes damage to targeted SSC" and for any other orientations of the falling truss debris. | ||
iv. Section 5.1 of Enclosure 4 of the submittal discusses the assessment of the Impact of the truss debris on the polar crane and cites the | iii. Describe whether and how the probability of a particular orientation of the falling truss debris was determined, and provide details of the approach used to incorporate those probabilities in the "qualitative evaluation of the impact of trusses and cross members" performed in the assessment. | ||
The licensee applied additional capacity adjustment factors, such as those for load redistribution and inelastic energy absorption, to the calculations of the equivalent PGA [ground peak acceleration] (licensee uses the term PGAc in the submittal) and indicated that those factors are reasonable for the calculations. | iv. Section 5.1 of Enclosure 4 of the submittal discusses the assessment of the Impact of the truss debris on the polar crane and cites the | ||
The licensees concluded that the PGAc calculated from the equivalent static analysis is higher than that calculated from the elastic analysis. | |||
Using the higher PGAc, the licensee derived the delta CDFs for both the "bounding" and "demonstrably conservative" analyses presented in Enclosure 4 of the submittal. | proprietary assessment report. Justify the conclusion reached for the polar crane hook in Section 5.1 of Enclosure 4 of the submittal considering the potential impact of the truss debris on the cable holding the crane hook. | ||
However, the staff noted that the lower PGAc calculated from the elastic analysis would yield a larger delta GDF result that may be greater | : 16. The NRG staff noted that in Section 6.4.2.1 of Enclosure 5 to the submittal, the licensee discussed the development of seismic fragility for Unit 1 with Limited Modification and Unit 2 Unmodified. The licensee applied additional capacity adjustment factors, such as those for load redistribution and inelastic energy absorption, to the calculations of the equivalent PGA [ground peak acceleration] (licensee uses the term PGAc in the submittal) and indicated that those factors are reasonable for the calculations. The licensees concluded that the PGAc calculated from the equivalent static analysis is higher than that calculated from the elastic analysis. Using the higher PGAc, the licensee derived the delta CDFs for both the "bounding" and "demonstrably conservative" analyses presented in Enclosure 4 of the submittal. However, the staff noted that the lower PGAc calculated from the elastic analysis would yield a larger delta GDF result that may be greater 1E-05 per year. The licensee has not justified why the PGAc calculated from the elastic analysis should not be used. Furthermore, the licensee has not provided the explanation about the differences in applying the capacity adjustment factors in the PGAc calculations. | ||
: a. Clarify the differences in the capacity adjustment factors applied in the PGAc calculations. | : a. Clarify the differences in the capacity adjustment factors applied in the PGAc calculations. | ||
: b. Provide justification for using the higher PGAc from the equivalent static analysis as opposed to that from the elastic analysis. | : b. Provide justification for using the higher PGAc from the equivalent static analysis as opposed to that from the elastic analysis. | ||
: 17. The construction trusses in each unit were originally installed to provide support for the containment dome liner and initial dome concrete pour during original station construction. | : 17. The construction trusses in each unit were originally installed to provide support for the containment dome liner and initial dome concrete pour during original station construction. After the initial concrete pour cured, the truss structures were lowered a few inches away from the containment liner, no longer providing structural support to the dome, and remained in place. The trusses were then used as an attachment point for containment spray piping, ventilation ductwork, post-accident containment ventilation (PACV) piping, and miscellaneous lights and associated conduits. An initial analysis of seismic adequacy was performed by the construction vendor. LAR Section 2.1.1 stated that "the trusses were not included in the original FSAR seismic classification tables. | ||
After the initial concrete pour cured, the truss structures were lowered a few inches away from the containment liner, no longer providing structural support to the dome, and remained in place. The trusses were then used as an attachment point for containment spray piping, ventilation ductwork, post-accident containment ventilation (PACV) piping, and miscellaneous lights and associated conduits. | They were subsequently added to the UFSAR in 2013 as a Seismic Class I structure supporting Class I piping and ductwork." | ||
An initial analysis of seismic adequacy was performed by the construction vendor. LAR Section 2.1.1 stated that "the trusses were not included in the original FSAR seismic classification tables. They were subsequently added to the UFSAR in 2013 as a Seismic Class I structure supporting Class I piping and ductwork." a. LAR Section 2.1.1 indicates that the containment dome truss is now qualified as a "Seismic Class I" structure. | : a. LAR Section 2.1.1 indicates that the containment dome truss is now qualified as a "Seismic Class I" structure. This statement contradicts the information in Section 1.0 of the LAR. Please clarify whether the containment dome trusses are qualified as "Seismic Class I" or the original intended seismic criteria. | ||
This statement contradicts the information in Section 1.0 of the LAR. Please clarify whether the containment dome trusses are qualified as "Seismic Class I" or the original intended seismic criteria. | : 18. LAR Section 2.0 "Detailed Description" states that "The construction trusses were subsequently reanalyzed and walkdowns and reviews of plant photos discovered a discrepancy between the as-built configuration of the trusses and the design drawing that the analysis was based on. Specifically, the lower diagonal bracing framework of the trusses, and the bottom lower diagonal bracing location on the truss, were different than shown on the design drawing. | ||
: 18. LAR Section 2.0 "Detailed Description" states that "The construction trusses were subsequently reanalyzed and walkdowns and reviews of plant photos discovered a discrepancy between the as-built configuration of the trusses and the design drawing that the analysis was based on. Specifically, the lower diagonal bracing framework of the trusses, and the bottom lower diagonal bracing location on the truss, were different than shown on the design drawing. Consequently, these activities and the refinements of the analysis resulted in identifying non-conformances to the design COR, "AISC [American Institute of Steel Construction] | |||
Specification for the Design, Fabrication, and Erection of Structural Steel for Buildings," April 1963, 6th Edition, for postulated seismic loads. Follow-on inspection of the trusses during initial resolution activities further identified a nonconformance with regard to the available clearance between a limited number of locations on the construction trusses and the containment liner in each Unit." LAR Section 2.1.1 "Construction Trusses" stated that "The trusses were not included in the original FSAR [final safety analysis report] seismic classification tables. They were subsequently added to the UFSAR in 2013 as a Seismic Class I structure supporting Class I piping and ductwork." a. Clarify whether structural assessments and modifications were performed consistent with licensing basis AISC, April 1963, 6th Edition. If other criteria was used, identify the criteria and discuss the basis for selecting criteria other than the licensing basis. 19. Section 3.1.3 of Enclosure 1 of the submittal (page 15 of 26) discusses how sufficient safety margins are maintained with the proposed change to the licensing basis. According to Regulatory Guide (RG) 1. | Consequently, these activities and the refinements of the analysis resulted in identifying non-conformances to the design COR, "AISC [American Institute of Steel Construction] | ||
: a. Clarify, citing relevant portions of the licensee's UFSAR, whether the UFSAR Chapter 14, analyses have adequately considered the following as a result of the abandonment of the containment dome truss in place: i. Impact on the containment available volume and containment total heat sinks due to the truss material. | Specification for the Design, Fabrication, and Erection of Structural Steel for Buildings," | ||
ii. Alternately, provide adequate justification that the UFSAR, Chapter 14, analyses remains bounding. | April 1963, 6th Edition, for postulated seismic loads. Follow-on inspection of the trusses during initial resolution activities further identified a nonconformance with regard to the available clearance between a limited number of locations on the construction trusses and the containment liner in each Unit." | ||
: b. If the truss material is credited for accident and/or containment analyses, Clarify whether the credited material reflects the as-designed or the as-built trusses indicating the difference between the two in case the as-designed truss material is credited. | LAR Section 2.1.1 "Construction Trusses" stated that "The trusses were not included in the original FSAR [final safety analysis report] seismic classification tables. They were subsequently added to the UFSAR in 2013 as a Seismic Class I structure supporting Class I piping and ductwork." | ||
: c. Provide details about the amount of truss material that is to be removed as part of the proposed modifications and justify not reassessing the safety analyses and containment analyses to ensure that the assumptions for those analyses are valid and the acceptance criteria continue to be met. d. Provide quantitative or qualitative technical justification that the collapse of the dome truss does not result in accidents other than those analyzed in the licensee's UFSAR | : a. Clarify whether structural assessments and modifications were performed consistent with licensing basis AISC, April 1963, 6th Edition. If other criteria was used, identify the criteria and discuss the basis for selecting criteria other than the licensing basis. | ||
One of the five principles states that "the proposed change maintains sufficient safety margins." The NRC staff noted that in Table 4-2 of Enclosure 5, the licensee identified the alternative criteria/methods for the licensee's risk-informed approach. | : 19. Section 3.1.3 of Enclosure 1 of the submittal (page 15 of 26) discusses how sufficient safety margins are maintained with the proposed change to the licensing basis. | ||
: a. Section 2 of Enclosure 5 of the March 31, 2017, submittal, indicates that damping factor used in the design of welded steel framed and bolted steel framed structures are 2 percent and 5 percent for the Hypothetical Earthquake in the UFSAR. In Section 5.3, the licensee stated that the damping value used in the analyses is 7 percent for bolted steel with bearing connections. | According to Regulatory Guide (RG) 1.174, Revision 2, assurance of sufficient safety margins following risk-informed licensing basis changes needs to consider whether, | ||
The applicant stated that while each of the individual 18 trusses is a welded planar truss assembly, the transfer of load between the 18 trusses is through a bolted brace system and concluded that the use of 7 percent damping is appropriate. | "[s]afety analysis acceptance criteria in the [licensing basis] LB (e.g., final safety analysis report (FSAR), supporting analyses) are met..." Safety analysis and containment integrity analysis are documented in Chapter 14 of the licensee's UFSAR (ADAMS Accession No. ML16251A166). | ||
: i. Justify that the as-built conditions of each of those 18 truss assemblies are consistent with the assumption that the transfer of load is through a bolted brace system. ii. Explain how the safety margins are impacted by the use of a 7 percent damping factor and whether sufficient safety margins are maintained. | : a. Clarify, citing relevant portions of the licensee's UFSAR, whether the UFSAR Chapter 14, analyses have adequately considered the following as a result of the abandonment of the containment dome truss in place: | ||
: b. Results in Section 6.5.1.2 of Enclosure 5 of the March 31, 2017, submittal, identify locations that would exceed the AISC N690 allowable stress. The licensee explained in Section 5.5 of Enclosure 5 of the March 31, 2017, submittal, that it is acceptable to use a strain-based acceptance criteria. | : i. Impact on the containment available volume and containment total heat sinks due to the truss material. | ||
The NRC staff noted that NUREG-0800, "Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants: LWR Edition," Section | ii. Alternately, provide adequate justification that the UFSAR, Chapter 14, analyses remains bounding. | ||
: b. If the truss material is credited for accident and/or containment analyses, Clarify whether the credited material reflects the as-designed or the as-built trusses indicating the difference between the two in case the as-designed truss material is credited. | |||
: c. Provide details about the amount of truss material that is to be removed as part of the proposed modifications and justify not reassessing the safety analyses and containment analyses to ensure that the assumptions for those analyses are valid and the acceptance criteria continue to be met. | |||
: d. Provide quantitative or qualitative technical justification that the collapse of the dome truss does not result in accidents other than those analyzed in the licensee's UFSAR | |||
: 20. RG 1.17 4, Revision 2, indicates that in implementing risk-informed decision making, licensing basis changes are expected to meet a set of key principles. One of the five principles states that "the proposed change maintains sufficient safety margins." The NRC staff noted that in Table 4-2 of Enclosure 5, the licensee identified the alternative criteria/methods for the licensee's risk-informed approach. | |||
: a. Section 2 of Enclosure 5 of the March 31, 2017, submittal, indicates that damping factor used in the design of welded steel framed and bolted steel framed structures are 2 percent and 5 percent for the Hypothetical Earthquake in the UFSAR. In Section 5.3, the licensee stated that the damping value used in the analyses is 7 percent for bolted steel with bearing connections. The applicant stated that while each of the individual 18 trusses is a welded planar truss assembly, the transfer of load between the 18 trusses is through a bolted brace system and concluded that the use of 7 percent damping is appropriate. | |||
: i. Justify that the as-built conditions of each of those 18 truss assemblies are consistent with the assumption that the transfer of load is through a bolted brace system. | |||
ii. Explain how the safety margins are impacted by the use of a 7 percent damping factor and whether sufficient safety margins are maintained. | |||
: b. Results in Section 6.5.1.2 of Enclosure 5 of the March 31, 2017, submittal, identify locations that would exceed the AISC N690 allowable stress. The licensee explained in Section 5.5 of Enclosure 5 of the March 31, 2017, submittal, that it is acceptable to use a strain-based acceptance criteria. The NRC staff noted that NUREG-0800, "Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants: LWR Edition," Section 3.8.4.111.5, states that the staff should evaluate the justification provided to show that structural integrity will not be affected if the licensee proposes to exceed some of these limits. | |||
: i. Justify the use of the proposed 1.5 percent strain acceptance criterion as it relates to structural integrity. | |||
ii. Explain how the safety margins are impacted by the proposed 1.5 percent strain acceptance criterion and whether sufficient safety margins are maintained. | |||
==REFERENCES:== | ==REFERENCES:== | ||
: 1. Letter NRC 2017-0017, dated March 31, 2017, from Robert Coffey, Site Vice President to NRG regarding the "License Amendment Request 278, Risk-Informed Approach to Resolve Construction Truss Design Code Non-conformances" (ADAMS Accession No. ML17090A511 ). | |||
: 2. "AISC Specification for the Design, Fabrication, and Erection of Structural Steel for Buildings," American Institute of Steel Construction, 6th Edition, April 1963. | |||
: 3. ACI 318-63, "Building Code Requirements for Reinforced Concrete," American Concrete Institute, 1963. | |||
* .. ML18025C043 *via email OFFICE NRR/D0RL/LPL3/PM NRR/DORULPL3/LA NRR/DRA/APLB/RILI (A)* NRR/DSS/SRXB/BC(A) | |||
NAME MChawla SRohrer MReisi Fard JWhitman DATE 01/29/18 01/29/18 01/24/18 01/30/18 OFFICE NRR/OE/ESEB/BC* NRR/DORL/LPL3/BC NRR/D0Rl/LPL3/PM NAME BWittick DWrona MChawla DATE 01/25/18 01/31/18 01/31/18}} | |||
NRR/DORL/LPL3/BC NRR/D0Rl/LPL3/PM NAME BWittick DWrona MChawla DATE 01/25/18 01/31/18 01/31 | |||
Latest revision as of 17:52, 3 February 2020
ML18025C043 | |
Person / Time | |
---|---|
Site: | Point Beach ![]() |
Issue date: | 01/31/2018 |
From: | Mahesh Chawla Plant Licensing Branch III |
To: | Coffey R Point Beach |
Chawla M | |
References | |
EPID L-2017-LLA-0209 | |
Download: ML18025C043 (16) | |
Text
UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, O.C. 20555-0001 January 31, 2018 Mr. Robert Coffey Site Vice President NextEra Energy Point Beach, LLC 6610 Nuclear Road Two Rivers, WI 54241
SUBJECT:
REQUEST FOR ADDITIONAL INFORMATION FOR POINT BEACH NUCLEAR PLANT, UNITS 1 AND 2, REGARDING LICENSE AMENDMENT REQUEST TO RESOLVE NONCONFORMANCES RELATING TO CONTAINMENT DOME TRUSS (EPID L-2017-LLA-0209)
Dear Mr. Coffey:
By letter dated March 31, 2017 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML17090A511), NextEra Energy Point Beach, LLC (NextEra, the licensee) submitted a license amendment request (LAR) for the Point Beach Nuclear Plant PBNP), Units 1 and 2. The proposed LAR uses a risk-informed resolution strategy to resolve legacy design code non-conformances associated with construction trusses in the containment buildings of PBNP, Units 1 and 2.
The U.S. Nuclear Regulatory Commission (NRC) staff conducted the audit on November 29 and 30, 2017, at the NextEra office in Jupiter, Florida, to support the review of the license amendment. The additional information requested within the enclosure to this letter is needed by the NRC staff to complete its review. Please provide a response to the request within 60 days of the date of this letter.
If you have any questions, please contact me, at (301) 415-8371.
Sincerely, Mahesh Chawla, Project Manager Plant Licensing Branch Ill Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket Nos. 50-266 and 50-301
Enclosure:
Request tor Additional Information cc: ListServ
REQUEST FOR ADDITIONAL INFORAMTION FOR LICENSE AMENDMENT REQUEST 278 TO FACILITY OPERATING LICENSE NOS. DPR-24 AND DPR-27 RISK-INFORMED APPROACH TO RESOLVE CONSTRUCTION TRUSS DESIGN CODE NONCONFORMANCES NEXTERA ENERGY POINT BEACH, LLC POINT BEACH NUCLEAR PLANT, UNITS 1 AND 2 DOCKET NOS. 50-266 AND 50-301
- 1. Section 2.4 of Enclosure 1 of the submittal proposes "[a]cceptance of the final modified configuration of the Unit 1 construction truss and associated equipment, and the current configuration of the Unit 2 construction truss and associated equipment, using a risk-informed approach for resolution." The current licensing basis for the trusses is the code of record (COR) against which nonconformance is noted for certain truss members. It appears that the licensing basis change includes the use of a different analysis method and acceptance criteria, such as those listed in Tables 4-1 and 4-2 of Enclosure 5 of the submittal, for certain truss members. Enclosure 3 of the submittal includes the text for the new section (Section A.5.1 O) of the updated final safety analysis report (UFSAR),
which also appears to change the licensing basis for certain truss members to a different code and/or acceptance criteria by incorporating the submittal by reference. However, Section 3.4 of Enclosure 1 of the submittal states that "[t]he alternative evaluation methods and acceptance criteria are not proposed as part of the license basis revision" which appears to be contradictory in that it seeks acceptance of the final proposed configurations of the trusses and associated equipment without any change to the current licensing basis.
- a. Clarify the change(s) to the licensing basis being sought by the submittal and provide a tabular comparison of the current licensing basis and the proposed change, including any specific changes to the current COR.
- b. Clarify whether the intent of reference to the submittal in the proposed text of the new Section A.5.1 O of the UFSAR, as shown in Enclosure 3 of the submittal, is to include the alternative evaluation methods and acceptance criteria used in the submittal as part of the proposed new licensing basis or explain which sections of the submittal are intended to be incorporated by reference.
- c. Explain how the cited nonconformances can be reconciled if "{t]he alternative evaluation methods and acceptance criteria are not proposed as part of the license basis revision" as stated in Section 3.4 of Enclosure 1 of the submittal.
Enclosure
- 2. Enclosure 2 of the submittal identifies the regulatory commitments made by the licensee in conjunction with the submittal. One of the commitments states that the licensee will "implement new seismic operating limits applicable to both Units ... Site procedures will be revised .... " Section 3.2 of Enclosure 1 of the submittal provides the proposed "new seismic operating limits" and Section 2.2.2 of the same enclosure provides the maximum ground accelerations in the horizontal and vertical directions for the operational basis earthquake (OBE). It appears that the "new seismic operating limit" for the vertical direction exceeds the corresponding value for the OBE.
- a. Provide the basis and justification for the selection of the "new seismic operating limits."
- b. Clarify the purpose of the "new seismic operating limits" as compared to the QBE limits when the "new seismic operating limit" for the vertical direction appears to exceed the corresponding value for the QBE.
- 3. According to Section 3.1.1 of Enclosure 1 and Section 1.3 of Enclosure 4 of the submittal, the initiating events considered are seismic events and thermal loading arising from postulated accidents. These initiators are used in the risk assessments for both the "bounding" and the "demonstrably conservative" analyses. The focus on these initiators is presumably due to the source of the nonconformances. However, a risk assessment needs to consider events and hazards that can credibly impact the structural integrity of the truss and the equipment supported from it. Provide quantitative or qualitative technical justification for the exclusion of internal and external initiating events other than those already considered in the analysis that could impact the application.
- 4. According to Enclosure 2 and Section 1.2 of Enclosure 4 of the submittal, no modifications are to be performed for the trusses in Unit 2. However, Section 6.4.2.2 of Enclosure 5 of the submittal states that "[t]rimming the first panel point at 14 locations for Unit 1 and 11 locations for Unit 2" while Section 6.5.1 states that "[t]he Unit 1 truss was analyzed in the configuration with limited modifications (six first panel point locations require trimming)."
- a. Clarify, with technical justification for the selection, the modifications that are proposed for the Unit 1 trusses and provide the rationale for not proposing all the modifications for Unit 1 trusses mentioned in Section 6.4.2.2 of Enclosure 5 of the submittal.
- b. Clarify whether any modifications are proposed for the Unit 2 trusses. If no modifications are proposed provide the rationale for not performing the modifications for Unit 2 trusses mentioned in Section 6.4.2.2 of Enclosure 5 of the submittal.
- c. Confirm that the configuration of the Unit 2 trusses that were analyzed to determine the seismic and thermal fragility values, which are used in the submittal are consistent with the configuration of the Unit 2 trusses as proposed in the submittal. If differences are found between the two configurations, provide and propagate the updated values or justify the continued use of the values in Section 6.4.2.2 of Enclosure 5 (and throughout Enclosure 4 ).
- 5. Section 6.4.1 of Enclosure 5 of the submittal discusses the determination of the probability of truss failure as a function of temperature, which is used for the risk calculations in Sections 2 and 5 of Enclosure 4 of the submittal. The discussion states that a probability of failure was "assigned" and provides separate curves for the Unit 1 and Unit 2 trusses.
- a. Describe the process used to "assign" the probability of failure as a function of temperature for the trusses including information from any expert elicitation, any guidance, such as NUREG/CR-6372 and NUREG/CR-6771 (ADAMS Accession Nos. ML080090003 and ML022410135, respectively),
used for the expert elicitation process, and a description of the qualification of the experts.
- b. Section 2.2.1 of Enclosure 4 of the submittal uses a single thermal fragility curve" for both units as opposed to the two different curves presented in Section 6.4.1 of Enclosure 5 of the submittal. Provide the technical justification for using a single curve to represent the "thermal fragility" of the trusses in both units.
- c. Demonstrate, preferably quantitatively, the impact of changing the "assigned" probability of failure as a function of temperature for the trusses on the "bounding" and "demonstrably conservative" risk calculations.
- 6. Section 3.1.2 of Enclosure 1 of the submittal states that "the peer review of the PRA
[probabilistic risk analysis] was conducted ... and the peer review of the seismic and thermal fragility analyses was conducted. " The risk calculations presented in Enclosure 4 of the submittal do not include all the technical elements of a PRA as defined in Regulatory Guide (RG) 1.200, Revision 2 (ADAMS Accession No. ML090410014). Attachment C of Enclosure 4 of the submittal provides a "statement of compliance" against the supporting requirements (SRs) in the 2009 American Society of Mechanical Engineers (ASME)/American Nuclear Society (ANS) PRA Standard for external hazard screening (Part 6 of the Standard) and seismic PRA (Part 5 of the Standard). Further, Table 1 and Section 6 of Enclosure 4 of the submittal include core damage frequency (COF) and large early release frequency (LERF) values from different hazards. However, the sources of those values are not mentioned.
- a. Provide clarification on the peer reviews referred to in Section 3.1.2 of Enclosure 1 of the submittal. Include the peer review reports or details of the peer reviews including information on the peer-review guidance that was followed, the specific part(s) of the ASME/ANS PRA Standard, and finding-level Facts and Observations (F&Os) and their corresponding resolution.
- b. Explain the relevance to the current application of the "statement of compliance" against the SRs in Parts 5 and 6 of the 2009 ASME/ANS PRA Standard provided in Attachment C to Enclosure 4 of the submittal.
- c. Provide in a tabular format, information on the source for the GDF and LERF estimates for each hazard listed in Table 1 and Section 6 of Enclosure 4 of the submittal. If the source is a PRA model, include information on the status
of the technical adequacy determination such as whether a full-scope peer-review against the ASME/ANS PRA Standard has been performed per an RG 1.200-endorsed peer review process, and the version of the Standard against which the peer review was performed. Include any findings-level F&Os which has not been closed per an NRG-approved process, any subsequent modifications to the associated PRA model indicating which modifications are considered "upgrades," and the results of any follow-on or focused scope peer-reviews.
- d. If the seismic CDF and LERF values are from a seismic PRA model, justify not exercising that model for the current submittal. If the values are not derived from a seismic PRA model, describe the approach used to determine them and the technical justification for the selected approach.
- e. Clarify whether the CDF and LERF values in Table 1 and Section 6 of Enclosure 4 of the submittal represent mean estimates. If not, provide mean estimates for each hazard per RG 1.174 or justify the use of point estimates for risk-informed decision-making.
- 7. Section 1.6 and Attachment C of Enclosure 4 of the submittal cites Part 6 of the 2009 ASME/ANS PRA Standard (ASME RA-Sa-2009) and states that "the bounding and demonstrably conservative analyses show that [Delta] CDF and [Delta] LERF are acceptably low for hazards challenging the [Containment Truss) CT design." The intent of Section 6-2.1 of the 2009 ASME/ANS Standard is the exclusion of an external hazard further consideration in a risk analysis. Further, Section 6-1.2 does not apply to earthquakes. The submittal is requesting a change to the plant's licensing basis.
RG 1.17 4, Revision 2, states that "[t]racking changes in risk (both quantifiable and non-quantifiable) that are due to plant changes would provide a mechanism to account for the cumulative and synergistic effects of these plant changes .... " Further, seismic events are identified as being directly relevant to the risk analysis. Based on the foregoing discussion:
- a. Explain the relevance of citing Part 6 of the 2009 ASME/ANS PRA Standard to the current application.
- b. Clarify whether the results of the risk calculations, which represent the quantified risk to the facility from the proposed permanent configuration of the dome trusses, will be included in the baseline risk of the plant for future risk-informed applications to determine the cumulative impact of changes to the licensing basis per the guidance in RG 1.17 4.
- c. Clarify the intent of submitting the "bounding" and the "demonstrably conservative" analyses as part of the submittal including the licensee's expectations of the scope of the staff's review and considering the response to item (b) of this request.
- 8. The fragility of the truss is used in both the "bounding" and "demonstrably conservative" analyses presented in Enclosure 4 of the submittal. Details of the conservative deterministic failure margin (CDFM) calculation used to determine the high confidence of low probability of failure (HCM, LPF) capacity of the truss are provided in Section 5. 7 of
Enclosure 5 of the submittal. According to that information, the CDFM uses the site-specific ground motion response spectrum (GMRS). According to the licensee's submittal of the reevaluated hazard in response to Near Term Task Force (NTIF)
Recommendation 2.1 (ADAMS Accession No. ML14090A275), which is referred to as "Ref. 6.1" in Enclosure 5 of the submittal, the peak ground acceleration (PGA) is 0.14 g for the GMRS. This value is used in the capacity calculation in Section 6.4.2 of of the submittal. That value appears to be based on the mean hazard curve. The CDFM approach, as described in Table 2-5 of the Electric Power Research Institute (EPRI) report NP-6041-SL, Revision 1, requires the use of the 84 percent non-exceedance hazard curve.
Section 2.1.3 of Enclosure 4 of the submittal provides the calculation for the change in (or delta) CDF due to the seismic hazard. The calculation provided uses the difference between the GDF for the current configuration of the truss (with thermal modifications) and that from a configuration that includes all modifications which would be required to "fully meet seismic and thermal design requirements". The same approach for calculating the delta GDF is applied to the "bounding" and "demonstrably conservative" analyses.
The analysis described in Section 2 of Enclosure 4 of the submittal assumes that the initiators of interest, seismic and thermal events, are independent. However, seismic events can result in consequential events such as loss-of-coolant accidents (LOCAs) which can, in turn, result in thermal loading of the trusses. Such consequential events have been excluded from the "bounding" and "demonstrably conservative" analyses without justification.
- a. Provide the HCLPF and median failure probability for the unmodified and modified configuration of the trusses following the methodology in Table 2-5 of the EPRI report NP-6041-SL and Section 5 of EPRI report TR-103959, Revision 1, or justify the calculation in Section 5.7 of Enclosure 5 of the submittal.
- b. Describe the modifications that are credited for the trusses in Units 1 and 2 for the calculations based on the truss "modified to fully meet seismic and thermal design requirements," such as in Table 5 of Section 2.1.3 of Enclosure 4 of the submittal.
- c. Provide a seismic event tree to capture the structural failure of the truss along with other possible consequential events, including LOGAs, due to seismic initiators.
- d. Provide qualitative or quantitative technical justification for the exclusion of any sequences from the seismic event tree that are not expected to impact the risk assessment for the trusses. Include information on any generic component fragilities used in the process.
- e. Provide the re-quantified seismic GDF for the "bounding" and "demonstrably conservative" analyses including any changes due to the responses to parts (a) through (d) of this request or justify the need to not perform such a re-quantification.
- 9. Section 2.1.4 of Enclosure 4 of the submittal discusses the convolution approach wherein the individual plant examination of external events (IPEEE) "plant fragility" is used. The same approach is also cited in Section 5.2.1 of Enclosure 4 of the submittal.
Therefore, the "convolution" approach is used for both the "bounding" and "demonstrably conservative" analyses. Section 2.1.4 of Enclosure 4 of the submittals states that "[a]
more accurate characterization of the CT {completion time] risk can be obtained by combining the IPEEE seismic and the CT bounding results."
- a. Justify the applicability of the IPEEE "plant fragility" for the "convolution" approach.
- b. Justify the assumption that the truss failure and selected plant level contribution approach, such as the IPEEE "plant fragility", can be considered independent events.
1O. Sections 5.2.1 through 5.2.3 of Enclosure 4 of the submittal provides the risk calculations for the "demonstrably conservative" analysis. Those sections provide "event trees" for various initiating events impacting the trusses. The split fractions at each node are the failure probabilities, based on an assessment described in Section 5 of Enclosure 4 of the submittal, of that particular equipment from the falling truss members.
The final result is termed the "conditional core damage probability" and multiplied with the initiating event frequency. However, the "event trees" represent the component failure probabilities and do not appear to describe the sequences which would potentially lead to core damage following a seismic event resulting in failure of the truss. The event tree does not appear to be modeled based on the logic in the internal events PRA model. In addition, human actions related to the impact of the failure of each component, including the impact of the seismic event on such actions do not appear to have been considered. Therefore, the "event tree" does not appear to provide the conditional core damage probability (CCDP). It appears that deriving such a CCDP would require integration of the component failure probabilities in Sections 5.2.1 through 5.2.3 of Enclosure 4 of the submittal with a plant-specific PRA model (internal events and/or seismic).
- a. Describe the logic used to develop the "event trees" in Sections 5.2.1 through 5.2.3 of Enclosure 4 of the submittal including the approach used to determine the "top events", and to capture the sequences which can potentially lead to core damage following truss failure due to the selected initiating events. Provide a comparison of the "event trees" in Sections 5.2.1 through 5.2.3 of Enclosure 4 of the submittal with similar event trees from the licensee's internal events PRA model.
- b. Considering the factors identified above, provide justification that the analysis in Sections 5.2.1 through 5.2.3 of Enclosure 4 of the submittal can either represent or bound any integration of the failure probabilities into a PRA model. Alternately, provide the mean seismic CDF and significant cutsets from such an integration.
- c. The failure probabilities from the assessment described in Section 5 of Enclosure 4 of the submittal that are used for the "event trees" do not change based on the seismic acceleration. It is expected that at a certain threshold seismic acceleration level the fragility of the individual components will
dominate the corresponding failure. The convolution with the IPEEE performed in Section 5.2.1 of Enclosure 4 of the submittal appears to be performed to account for such cases. However, the convolution is based on the "plant fragility" and there is no comparison of the fragility of each impacted component against the "plant fragility." Justify that use of the "plant fragility" is bounding for all the components that are found to be impacted by truss failure in the assessment.
- 11. Attachment 8 to Enclosure 4 of the submittal provides a description of the human reliability analysis (HRA) used for the "demonstrably conservative" analysis by inclusion in the "event trees" used in Sections 5.2.1 through 5.2.3 of Enclosure 4 of the submittal.
The description in Attachment B states that the operators will have more than 30 minutes to perform feed and bleed (F&B). Further, the analysis also uses "simplified
[performance shaping factor] PSF adjustments" based on the seismic acceleration level.
However, Section 5.2 of Enclosure 4 of the submittal states that the "PORVs [pilot~
operated relief valve]" top "event" uses the baseline internal events value for F&B human error probability (HEP).
- a. Justify the time available to initiate F&B used for the HRA analysis including a summary of any relevant engineering evaluations.
- b. Justify the multipliers used for the PSF adjustments including the basis and methodology used to derive the multipliers for different seismic intensities.
- c. Clarify whether the baseline internal events HEP for F&B is used across all seismic acceleration levels in the analysis in Sections 5.2.1 through 5.2.3 of Enclosure 4 of the submittal and if so, justify such an approach given the analysis in Attachment B to Enclosure 4 of the submittal.
- d. The baseline HEP for F&B is expected to assume the availability of instrument air for the PORVs. The determination of the impact of truss failure on instrument air availability does not appear to have been performed as part of the assessment described in Section 5 of Enclosure 4 of the submittal for the "demonstrably conservative" analysis. However, it appears that instrument air is considered unavailable during a seismic event. Describe and justify the assumption regarding the availability of instrument air following truss failure.
- e. If the response to part (d) of this request for additional information (RAI) credits the planned nitrogen supply modifications for F&B to justify the use of the baseline HEP, justify the credit taken for the planned nitrogen supply modification as a method that maintains appropriate safety margins as mentioned in Section 3.1 of Enclosure 1 of the submittal.
- f. Attachment B to Enclosure 4 of the submittal states that removing the dependency for instrument air reduces the HEP. However, the reduction seems to be based on a simple removal of the step in the procedure ("step 36 in the current procedure") for restoration of instrument air. Justify the HEP accounting for any actions and/or steps necessary to be performed for entering into and following the procedure for using the nitrogen supply subsequent to not restoring instrument air including the consideration of
dependencies in intra- and inter-procedure actions or provide and use a different value.
- 12. The seismic and/or thermal failure of the equipment supported from the trusses in the "bounding" analysis in Section 2 of Enclosure 4 of the submittal may increase the overall risk contribution. Enclosure 5 of the submittal compared the supported equipment and the supporting mechanism (e.g., anchors) against the design criteria using the GMRS and thermal loading. However, the risk assessment is not confined to the GMRS or the design basis. According to Enclosure 2 of the submittal, the licensee is also committing to a modification to the containment spray pipe support. It is unclear if a fragility for the equipment supported by the trusses and the corresponding supports was determined which in turn, can be used to obtain the risk from corresponding failures across the seismic hazard curve as well as the thermal loading.
Further, the mitigating systems employed for the thermal loading mitigation calculations provided in Section 2.2.4 of Enclosure 4 of the submittal are based solely on the random failure probability without consideration of failure due to the thermal or seismic hazard (such as through an operating reactor gate). The thermal loading mitigation calculations are applicable to both the "bounding" and the "demonstrably conservative" analyses.
Section 2.2.2 of Enclosure 4 of the submittal uses information from EPRI report 302000079, Revision 3, "Pipe Rupture Frequencies for Internal Flooding Probabilistic Risk Assessments," to determine the initiating frequency for steam line breaks and feedwater line breaks inside containment. The initiating frequencies are then used in both the "bounding" and the "demonstrably conservative" analyses. Section 5.1 ("PWR
[pressurized-water reactor] High-Energy Piping Systems") of the cited EPRI report states that the feedwater piping " ... system boundary considered in this evaluation consists of the piping from the low-pressure heaters ... up to the outboard containment isolation valves." Similarly, the high pressure steam piping is considered to be " ... upstream of the
[high pressure] HP turbine throttle valve and extends to the outboard containment isolation valves." Therefore, the initiating frequencies for the piping from the cited EPRI report are for breaks outside containment and are inapplicable to the risk assessment in the submittal.
- a. Provide a quantitative assessment of the impact on the submitted risk calculations of the failure of the equipment supported by the trusses due to the seismic hazard and thermal loading or justify not including such impacts for the "bounding" as well as the "demonstrably conservative" cases.
- b. Provide an estimate, preferably using quantitative approaches such as the median fragility, of the extent to which the defense-in-depth due to containment sprays and containment air recirculation cooling systems is preserved beyond the plant's seismic design basis.
- c. Provide the results of the thermal analysis using relevant initiating frequencies for the breaks of interest. If the updated initiating frequencies are partitioned by break size, provide the technical justification and methodology for the partitioning. Consider the responses to previous parts of this request in the re-quantification. Include any initiating frequency determined from the development of the seismic event tree as requested in RAI 8.
- 13. Section 2.5 of RG 1.17 4, Revision 2, discusses uncertainties in risk analysis and states that " ... comparison of the PRA results with the acceptance guidelines must be based on an understanding of the contributors to the PRA results and on the robustness of the assessment of those contributors and the impacts of the uncertainties." Section 9 of Enclosure 4 to the submittal states that "[t]he uncertainties are addressed by the simple bounding case and sensitivity analyses applied in this evaluation ... " However, neither the impact of the quantifiable uncertainties in the seismic hazard and seismic fragility is captured in the submittal nor is it demonstrated that the "qualitative factors" in Section 4 of Enclosure 4 to the submittal adequately capture such uncertainties. The "thermal sensitivity analysis" in Section 2.2.6 of Enclosure 4 to the submittal expands the initiating event frequency but does not address the uncertainties in those frequencies. Further, the submittal does not include a discussion of the sources of uncertainty and their impact for the "demonstrably conservative analysis".
Justify the lack of sensitivity studies to capture the impact of key sources of uncertainties on the analyses presented in the submittal, considering the guidance in NUREG-1855, Revision 1. Alternately, describe, with justification, the approach used and provide the results (GDF, delta GDF, LERF, and delta LERF) from the following considering the response to RAI 7c:
- a. A sensitivity study to address the uncertainty in the seismic hazard and in the seismic fragility for the "bounding case",
- b. A sensitivity study to address the uncertainty in the initiating events identified for the thermal analysis and in the thermal fragility for the "bounding case."
Include justification for the uncertainty bounds and distribution selected for the thermal fragility for use in the sensitivity study.
- c. A sensitivity study to address the uncertainty in the calculation inputs, such as the seismic hazard, the seismic fragility, the assessment results described in Section 5 of Enclosure 4 of the submittal, and the HEP, for the "demonstrably conservative case" of seismic evaluation. Include justification for the uncertainty bounds and distribution selected for the assessment results and the HEP for use in the sensitivity study.
- d. A sensitivity study to address the uncertainty in the calculation inputs, such as initiating events identified for the thermal analysis, in the thermal fragility, in the assessment results, and the HEP, for the "demonstrably conservative case" of thermal evaluation. Include justification for the uncertainty bounds and distribution selected for the thermal fragility, assessment results, and the HEP for use in the sensitivity study.
In addressing the above requests, consider the responses to RAls 5, 10, and 14.
- 14. Section 6 of Enclosure 4 of the submittal discusses the LERF calculation which uses a CLERP [conditional large early release probability] of 0.2 based on the information for different hazards. The CLERP is then applied to the results for the change in GDF from the "bounding" and the "demonstrably conservative" analyses. It appears that the impact of the truss failure on LERF via component failures has not been considered. Further, it ts expected that instrument air will be required for containment isolation valves and the determination of the impact of truss failure on instrument air availability does not appear
to have been performed as part of the assessment described in Section 5 of Enclosure 4 of the submittal. Such failures can be accounted for by exercising the Level 2 or simplified LERF model in conjunction with the component failure probabilities. In light of the factors identified above, provide an estimate of LERF that quantitatively considers the failure of components and systems that can impact containment integrity or justify that the current approach bounds such impacts.
- 15. Section 5 of Enclosure 4 to the submittal discusses the "demonstrably conservative analysis" and summarizes the results of the assessment performed in support of that analysis. The discussion cites a proprietary assessment report, Reference 3, in Enclosure 4 to the submittal. The following are related to the assessment:
- a. Provide Reference 3 in Enclosure 4 to the submittal or the details of the assessment used to include and exclude different structures, system and components (SSCs) as well as determine the probability of failure given truss failure in the "demonstrably conservative" analysis as summarized in Sections 5.1 and 5.2, respectively, of Enclosure 4 to the submittal.
- b. Based on the discussion in Section 5.1 of Enclosure 4 to the submittal it appears that "perforation/penetration" is the only failure mode considered as part of the assessment. However, the rationale for the selection has not been provided. Discuss the various structural failure modes that were considered in the assessment for the SSCs impacted by falling truss debris and provide the basis for the determination of perforation/penetration as the only failure mode.
- c. Describe how qualitative, quantitative, or a combination of both approaches was used in the assessment summarized in Section 5.1 of Enclosure 4 of the submittal. If any equations/formulae were used to determine the ability of falling debris to damage a particular SSC, provide the basis to support the applicability of those equations/formulae to the current assessment.
- d. One of the assumptions made in the assessment appears to be that" ... debris targeting critical SSCs are assumed to be oriented in a way that maximizes damage to targeted SSCs."
- 1. Describe the approach to determine the orientation(s) of the falling truss debris that "maximizes damage to targeted SSC.
ii. Justify any assumptions made in determining the orientation(s) of the falling truss debris that "maximizes damage to targeted SSC" and for any other orientations of the falling truss debris.
iii. Describe whether and how the probability of a particular orientation of the falling truss debris was determined, and provide details of the approach used to incorporate those probabilities in the "qualitative evaluation of the impact of trusses and cross members" performed in the assessment.
iv. Section 5.1 of Enclosure 4 of the submittal discusses the assessment of the Impact of the truss debris on the polar crane and cites the
proprietary assessment report. Justify the conclusion reached for the polar crane hook in Section 5.1 of Enclosure 4 of the submittal considering the potential impact of the truss debris on the cable holding the crane hook.
- 16. The NRG staff noted that in Section 6.4.2.1 of Enclosure 5 to the submittal, the licensee discussed the development of seismic fragility for Unit 1 with Limited Modification and Unit 2 Unmodified. The licensee applied additional capacity adjustment factors, such as those for load redistribution and inelastic energy absorption, to the calculations of the equivalent PGA [ground peak acceleration] (licensee uses the term PGAc in the submittal) and indicated that those factors are reasonable for the calculations. The licensees concluded that the PGAc calculated from the equivalent static analysis is higher than that calculated from the elastic analysis. Using the higher PGAc, the licensee derived the delta CDFs for both the "bounding" and "demonstrably conservative" analyses presented in Enclosure 4 of the submittal. However, the staff noted that the lower PGAc calculated from the elastic analysis would yield a larger delta GDF result that may be greater 1E-05 per year. The licensee has not justified why the PGAc calculated from the elastic analysis should not be used. Furthermore, the licensee has not provided the explanation about the differences in applying the capacity adjustment factors in the PGAc calculations.
- a. Clarify the differences in the capacity adjustment factors applied in the PGAc calculations.
- b. Provide justification for using the higher PGAc from the equivalent static analysis as opposed to that from the elastic analysis.
- 17. The construction trusses in each unit were originally installed to provide support for the containment dome liner and initial dome concrete pour during original station construction. After the initial concrete pour cured, the truss structures were lowered a few inches away from the containment liner, no longer providing structural support to the dome, and remained in place. The trusses were then used as an attachment point for containment spray piping, ventilation ductwork, post-accident containment ventilation (PACV) piping, and miscellaneous lights and associated conduits. An initial analysis of seismic adequacy was performed by the construction vendor. LAR Section 2.1.1 stated that "the trusses were not included in the original FSAR seismic classification tables.
They were subsequently added to the UFSAR in 2013 as a Seismic Class I structure supporting Class I piping and ductwork."
- a. LAR Section 2.1.1 indicates that the containment dome truss is now qualified as a "Seismic Class I" structure. This statement contradicts the information in Section 1.0 of the LAR. Please clarify whether the containment dome trusses are qualified as "Seismic Class I" or the original intended seismic criteria.
- 18. LAR Section 2.0 "Detailed Description" states that "The construction trusses were subsequently reanalyzed and walkdowns and reviews of plant photos discovered a discrepancy between the as-built configuration of the trusses and the design drawing that the analysis was based on. Specifically, the lower diagonal bracing framework of the trusses, and the bottom lower diagonal bracing location on the truss, were different than shown on the design drawing.
Consequently, these activities and the refinements of the analysis resulted in identifying non-conformances to the design COR, "AISC [American Institute of Steel Construction]
Specification for the Design, Fabrication, and Erection of Structural Steel for Buildings,"
April 1963, 6th Edition, for postulated seismic loads. Follow-on inspection of the trusses during initial resolution activities further identified a nonconformance with regard to the available clearance between a limited number of locations on the construction trusses and the containment liner in each Unit."
LAR Section 2.1.1 "Construction Trusses" stated that "The trusses were not included in the original FSAR [final safety analysis report] seismic classification tables. They were subsequently added to the UFSAR in 2013 as a Seismic Class I structure supporting Class I piping and ductwork."
- a. Clarify whether structural assessments and modifications were performed consistent with licensing basis AISC, April 1963, 6th Edition. If other criteria was used, identify the criteria and discuss the basis for selecting criteria other than the licensing basis.
- 19. Section 3.1.3 of Enclosure 1 of the submittal (page 15 of 26) discusses how sufficient safety margins are maintained with the proposed change to the licensing basis.
According to Regulatory Guide (RG) 1.174, Revision 2, assurance of sufficient safety margins following risk-informed licensing basis changes needs to consider whether,
"[s]afety analysis acceptance criteria in the [licensing basis] LB (e.g., final safety analysis report (FSAR), supporting analyses) are met..." Safety analysis and containment integrity analysis are documented in Chapter 14 of the licensee's UFSAR (ADAMS Accession No. ML16251A166).
- a. Clarify, citing relevant portions of the licensee's UFSAR, whether the UFSAR Chapter 14, analyses have adequately considered the following as a result of the abandonment of the containment dome truss in place:
- i. Impact on the containment available volume and containment total heat sinks due to the truss material.
ii. Alternately, provide adequate justification that the UFSAR, Chapter 14, analyses remains bounding.
- b. If the truss material is credited for accident and/or containment analyses, Clarify whether the credited material reflects the as-designed or the as-built trusses indicating the difference between the two in case the as-designed truss material is credited.
- c. Provide details about the amount of truss material that is to be removed as part of the proposed modifications and justify not reassessing the safety analyses and containment analyses to ensure that the assumptions for those analyses are valid and the acceptance criteria continue to be met.
- d. Provide quantitative or qualitative technical justification that the collapse of the dome truss does not result in accidents other than those analyzed in the licensee's UFSAR
- 20. RG 1.17 4, Revision 2, indicates that in implementing risk-informed decision making, licensing basis changes are expected to meet a set of key principles. One of the five principles states that "the proposed change maintains sufficient safety margins." The NRC staff noted that in Table 4-2 of Enclosure 5, the licensee identified the alternative criteria/methods for the licensee's risk-informed approach.
- a. Section 2 of Enclosure 5 of the March 31, 2017, submittal, indicates that damping factor used in the design of welded steel framed and bolted steel framed structures are 2 percent and 5 percent for the Hypothetical Earthquake in the UFSAR. In Section 5.3, the licensee stated that the damping value used in the analyses is 7 percent for bolted steel with bearing connections. The applicant stated that while each of the individual 18 trusses is a welded planar truss assembly, the transfer of load between the 18 trusses is through a bolted brace system and concluded that the use of 7 percent damping is appropriate.
- i. Justify that the as-built conditions of each of those 18 truss assemblies are consistent with the assumption that the transfer of load is through a bolted brace system.
ii. Explain how the safety margins are impacted by the use of a 7 percent damping factor and whether sufficient safety margins are maintained.
- b. Results in Section 6.5.1.2 of Enclosure 5 of the March 31, 2017, submittal, identify locations that would exceed the AISC N690 allowable stress. The licensee explained in Section 5.5 of Enclosure 5 of the March 31, 2017, submittal, that it is acceptable to use a strain-based acceptance criteria. The NRC staff noted that NUREG-0800, "Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants: LWR Edition," Section 3.8.4.111.5, states that the staff should evaluate the justification provided to show that structural integrity will not be affected if the licensee proposes to exceed some of these limits.
- i. Justify the use of the proposed 1.5 percent strain acceptance criterion as it relates to structural integrity.
ii. Explain how the safety margins are impacted by the proposed 1.5 percent strain acceptance criterion and whether sufficient safety margins are maintained.
REFERENCES:
- 1. Letter NRC 2017-0017, dated March 31, 2017, from Robert Coffey, Site Vice President to NRG regarding the "License Amendment Request 278, Risk-Informed Approach to Resolve Construction Truss Design Code Non-conformances" (ADAMS Accession No. ML17090A511 ).
- 2. "AISC Specification for the Design, Fabrication, and Erection of Structural Steel for Buildings," American Institute of Steel Construction, 6th Edition, April 1963.
- 3. ACI 318-63, "Building Code Requirements for Reinforced Concrete," American Concrete Institute, 1963.
- .. ML18025C043 *via email OFFICE NRR/D0RL/LPL3/PM NRR/DORULPL3/LA NRR/DRA/APLB/RILI (A)* NRR/DSS/SRXB/BC(A)
NAME MChawla SRohrer MReisi Fard JWhitman DATE 01/29/18 01/29/18 01/24/18 01/30/18 OFFICE NRR/OE/ESEB/BC* NRR/DORL/LPL3/BC NRR/D0Rl/LPL3/PM NAME BWittick DWrona MChawla DATE 01/25/18 01/31/18 01/31/18