ML20087N663

SPRA - Spr Audit Questions Redacted

ML20087N663
Person / Time
Site:	Robinson
Issue date:	02/03/2020
From:	Milton Valentin-Olmeda NRC/NRR/DORL/LPMB
To:	Grzeck L - No Known Affiliation
Milton Valentin-Olmeda - 301-661-8104
Shared Package
Ml20086L680	List: ML20087N663
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Download: ML20087N663 (12)

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From: Valentin-Olmeda, Milton To: Grzeck, Lee Cc: Philpott, Stephen

Subject:

Robinson SPRA - SPR Audit Questions Date: Monday, February 03, 2020 12:29:00 PM

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Topic #15 - Documentation of the Seismic PRA

1. A plants safe shutdown earthquake (SSE) provides the design basis for certain structures, systems, and components (SSCs) to remain functional after the occurrence of the SSE to ensure safe shutdown of the plant.

According to the results provided the in December 12, 2019, submittal, there is a non-negligible core damage frequency (CDF) from seismic events at and below the design basis SSE. Based on the results in Table 5.4-4 of the submittal, approximately 13% of the seismic CDF (i.e., seismic CDF approximately of 2E-5 per year) of the plant is from accelerations equal to or less than the SSE. In addition, based on the information in Table 5.4-2 of the submittal, the failure probabilities of the dominant contributors at the SSE are also high. Examples include the >> >> @@

>> @@ @@, the turbine building pounding-induced failure probability of approximately 13% at SSE, the turbine gantry crane failure probability of approximately 13% at SSE, the liquefaction induced failure of the diesel fuel oil tank of approximately 20% at the SSE, and the liquefaction-induced failure of SDAFW of approximately 6% at SSE.

Tables 5.4-2 and 5.5-2 of the December 12, 2019, submittal provide the dominant risk contributors for seismic CDF and seismic LERF, respectively. The dominant risk contributors significantly challenge the plants defense-in-depth and safety margins. The dominant risk contributors result in failures of multiple key safety functions and redundancies in the design even at the SSE.

As noted above, the purpose of the SSE is to support safe shutdown of the plant under seismic event by designing certain SSCs to remain functional at the occurrence of the SSE. The design and capability at SSE are expected to result in negligible risk at or below the SSE (e.g., between 0% and 3% of total seismic 6(&85,7< 5(/$7(' ,1)250$7,21 &(,,  '2 127 5(/($6(

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CDF). In fact, multiple seismic PRAs submitted in response to the 10 CFR 50.54(f), letter, including for Oconee, demonstrate the design and capability of SSCs at a plants SSE. Therefore, the results presented in the submittal appear to challenge the ability to achieve safe shutdown at the plants SSE.

Based on the results presented in the submittal, explain how the design basis SSE for Robinson continues to meet its purpose and intent. The discussion should consider the impact on defense-in-depth, safety margin, and risk from seismic events.

2. Tables 5.4-2 and 5.5-2 of the December 12, 2019, submittal provide the dominant risk contributors for seismic CDF and seismic LERF, respectively. The dominant contributor in each case is SF-TB-CLASS-3-POUND, which, according to Table 5.4-2 of the submittal, represents the failure of the Class III Turbine Building (TB) due to [Reactor Auxiliary Building] RAB pounding induced cracking and splitting of the mezzanine floor slab resulting in loss of structural integrity. The NRC staff reviewed available plant drawings for the mezzanine floor of the TB and the adjacent RAB. Based on this review, it appears that the control room, safeguards room, cable spreading rooms, and Heating Ventilation and Air Conditioning (HVAC) room are all located in the RAB at the elevation of or near the mezzanine floor of the Class III TB. All the rooms mentioned above have a wall facing the Class III TB at the mezzanine floor level. The pounding between the Class III TB and the RAB, severe enough to cause loss of structural integrity of the Class III TB, can impact the function of the control room by causing panels or equipment failure, spurious operation, or delays and complications in personnel evacuation and/or human actions. The submittal and the audit material available to the staff does not provide information about whether and how such impacts on the control room have been evaluated and considered in the seismic PRA.

In addition, the function of the equipment and any personnel in the safeguards, cable spreading, and HVAC room can also be impacted by the pounding between the Class III TB and RAB. The information available to the staff as part of the audit shows that fragilities have been developed for the equipment in these rooms and used with the DUMMY_CD and DUMMY_PDS events. However, it is unclear if the fragilities include the impact from the loads due to pounding in addition to the in-service response spectrum for the rooms. Similarly, it is unclear if the impact of pounding on the component cooling water (CCW) storage tank on the roof of the control room was evaluated as part of the seismically-induced flood sources. In light of this information, please address the following questions:

a. Explain how the potential for loss of control room function via impacts of the pounding between the Class III TB and the RAB on the control room equipment, panels, and operators in the control room to respond to a 6(&85,7< 5(/$7(' ,1)250$7,21 &(,,  '2 127 5(/($6(

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seismic event were determined and evaluated in the seismic PRA. Provide justification for not considering such impacts or include such impacts via a sensitivity study. If a sensitivity study is performed, discuss how the above-mentioned impacts were considered in the sensitivity and provide the results in the form of dominant risk contributors and risk metrics for seismic CDF and LERF.

b. Explain how the impacts of the pounding between the Class III TB and the RAB on the equipment in the safeguards room, cable spreading rooms, HVAC room, and the Hagan racks outside the control room were determined and evaluated in the seismic PRA. Provide justification for not considering such impacts or include such impacts via a sensitivity study. If a sensitivity study is performed, discuss how the above-mentioned impacts were considered in the sensitivity and provide the results in the form of dominant risk contributors and risk metrics for seismic CDF and LERF.

c. Explain, with justification, how the impact of the failure of the Class III TB and liquefaction, including the potential for fatalities, on operator pathways, accessibility and deployment of portable equipment, and any recovery actions was included in any ex-control room operator actions credited in the seismic PRA or include such impacts via a sensitivity study. If a sensitivity study is performed, discuss how the above-mentioned impacts were considered in the sensitivity and provide the results in the form of dominant risk contributors and risk metrics for seismic CDF and LERF.

d. Explain how the impact of pounding on the integrity of the CCW storage tank on the roof of the control room and eventual flooding was evaluated in the seismic PRA. Provide justification for not considering such impacts or include such impacts via a sensitivity study. If a sensitivity study is performed, discuss how the above-mentioned impacts were considered in the sensitivity and provide the results in the form of dominant risk contributors and risk metrics for seismic CDF and LERF.

e. Explain whether the pounding-induced failure of the Class III TB results in a loss of DC power and resulting challenge to containment isolation.

Include a discussion of how the impacts of the pounding between the Class III TB and the RAB on the battery room in the RAB were determined and evaluated in the seismic PRA. Provide justification for not considering such impacts or include such impacts via a sensitivity study. If a sensitivity study is performed, discuss how the above-mentioned impacts were considered in the sensitivity and provide the results in the form of dominant risk contributors and risk metrics for seismic CDF and LERF.

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Topic #16 - Review of Plant Modifications and Licensee Actions, If Any

3. Tables 5.4-2 and 5.5-2 of the December 12, 2019, submittal provide the dominant risk contributors for seismic CDF and seismic LERF, respectively. These dominant risk contributors are important even at relatively low seismic accelerations. The dominant risk contributors result in failures of multiple key safety functions and redundancies in the design even at relatively low seismic accelerations (with relatively high occurrence frequencies). Regulatory Guide (RG) 1.174, Revision 3 (ADAMS Accession No. ML17317A256) provides guidance for consideration of defense-in-depth in risk-informed decisions. The dominant risk contributors impact several defense-in-depth considerations in RG 1.174, Revision 3. These include (i) the substantial reduction (to the point of elimination) of system redundancy, independence, and diversity, (ii) loss of balance among the layers of defense, (iii) reduction in the effectiveness of multiple fission product barriers, (iv) compromising adequate capability of design features, and (v) challenges to the intent of the plants design criteria (see Question 1). The safety margin in various SSCs impacted by the dominant risk contributors is also lost (i.e., there is no contribution) because of the failure modes of the dominant risk contributors (i.e., structural failures and liquefaction).

The following information can be determined based on Tables 5.4-2 and 5.5-2 of the submittal as well as supporting information available to the staff as part of its audit:

• The dominant contributor in each case is SF-TB-CLASS-3-POUND, which, according to Table 5.4-2 of the submittal, represents the failure of the Class III TB due to RAB pounding induced cracking and splitting of the mezzanine floor slab resulting in loss of structural integrity. Its failure results in (i) a consequential loss-of-offsite power; (ii) failure of the auxiliary feedwater system C (AFW-C), (iii) failure of the steam driven auxiliary feedwater system (SDAFW), (iv) failure of SST-G and SST-F which provide power to the emergency buses E1 and E2, (v) failure of Condensate Storage Tank (CST) which is primary water source for all AFWs, (vi) loss of condenser hotwell (an alternate AFW or service water source), (vii) failure of Dedicated Shutdown Diesel Generator (DSDG) function due to loss of corresponding switchgear, and (viii) loss of Diverse and Flexible Coping Strategies (FLEX) connections and equipment (stored in turbine building).

• The failure of the TB Gantry Crane as well as the Class III TB results in failure of the CST.

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• The settlement induced failure of the SDAFW pump results in an additional failure mode for that system.

• The liquefaction-induced failure of the piping from the diesel fuel oil tank impacts the operation of the EDGs.

• The liquefaction-induced failure of the deepwell pump D eliminates an alternate source of service water and CST makeup.

Information available to the staff as part of its audit includes exploration of several plant modifications, both individually and in combination, to address the dominant risk contributors and re-establish defense-in-depth and safety margin in the plants design and operation. Several of the modifications explored have the potential to reducing risk by more than the amount stated in Table 6-1 of the submittal.

However, none of the modifications, individually or in combination, have been included in the submittal.

a. It appears to the NRC staff that potential modifications exist that directly address the pounding induced failure of the Class III TB. Such modifications include (i) increasing the capacity of the Class III TB against the pounding and shaking loads, (ii) adding structural members between Bays 8 and 12 on the south end of the mezzanine floor of the Class III TB, (iii) using dampers between the Class III TB and the RAB (viscous or rubber dampers) to decrease or eliminate pounding loads, and (iv) using viscous or visco-elastic dampers in the Class III TB to increase its damping thereby decreasing its displacement.

Based on this information;

i. Discuss the identification and evaluation of potential modifications to directly address the dominant risk contribution of the pounding induced failure of the Class III TB, including the examples above.

ii. If such modifications were not evaluated, provide an evaluation of the 6(&85,7< 5(/$7(' ,1)250$7,21 &(,,  '2 127 5(/($6(

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impact from their implementation on the seismic risk profile (risk metrics and dominant risk contributors). Alternately, justify not evaluating and/or pursing such modifications given their potential to significantly increase in the plants defense-in-depth and safety margin capability during a seismic event. Include any cost-benefit analysis performed as part of the justification.

E It appears to the NRC staff that potential modifications exist that address the impact of the dominant risk contributions from the failures of the TB Gantry

>> @@ Crane, >> @@ (i.e., loss of service water and backup source for the >> @@

CST) via deepwell D, SDAFW pump (settlement-induced failure), and the liquefaction-induced failure of the diesel fuel oil tank piping. Based on this; L Discuss the identification and evaluation of potential modifications to address the dominant risk contributors discussed above, individually and in combinations including combination with modifications identified in part (a).

ii. If such modifications were not evaluated, provide an evaluation of the impact from their implementation on the seismic risk profile (risk metrics and dominant risk contributors). Alternately, justify not evaluating and/or pursing such modifications given their potential to significantly increase in the plants defense-in-depth and safety margin capability during a seismic event. Include any cost-benefit analysis performed as part of the justification.

4. Table 6-1 in Section 6 of the December 12, 2019 submittal mentions a plant modification involving changes to the existing FLEX strategy to provide AFW to the steam generators (SGs). The modification is intended to mitigate the failure mode of the Turbine Building Class 3 caused by building pounding between the RAB and the mezzanine floor portion of the Turbine Building Class 3. The submittal does not provide enough information to understand the details of the modification and its impact on the seismic risk profile for the plant. For that reason;

a. Explain the difference between the modification mentioned in Table 6-1 of the submittal and the sensitivity HR-2a in Table 5.7-1 of the submittal, including the reasons for the vastly different impacts on the risk from seismic events.

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b. Discuss how the modification mentioned in Table 6-1 impacts the defense-in-depth and the risk profile from seismic events. Include an explanation of the ability of the proposed modification to re-establish defense-in-depth and the change in the risk profile due to the modification including the risk metrics, risk contributors, seismic accelerations that contribute to the risk, and the risk contribution from accelerations at and below the plants SSE.

c. Provide a detailed description of the modification including the SSCs that are or will be part of the modification and the design parameters for those SSCs.

d. Discuss the assumptions related to modeling of human actions for the proposed modification. For example, if the SSCs to be used for the modification are portable and highly reliant of operator actions, the ability to access, retrieve, stage, connect, and operate the SSCs appears to be one of the key assumption and source of uncertainty for the efficacy of the modification. Discuss the impact of the assumptions on the risk profile for portable or staged SSC given the impact of structural collapse, including the potential for fatalities, on accessibility and deployment, on human actions, and the potential limitations on the access and deployment of portable equipment due to liquefaction at various site locations.

e. The proposed modification assumes the use of available water sources at the site (e.g., Lake Robinson or alternate water sources, such as existing or new tanks). It appears to be one of the key assumption and source of uncertainty for the efficacy of the modification. Discuss the impact of the uncertainty associated with the availability of water sources on the risk profile given the high probabilities of failures of water sources highlighted by the seismic PRA and liquefaction as well as lateral spreading concerns for any water sources. Discuss how potential failures of water sources were considered. Include discussion of any designs and plant modifications to support the assumption of availability of water sources.

f. Discuss whether decisions and design (seismic and mechanical) parameters for the proposed modification (e.g., use of existing or new tanks; design enhancements for existing tanks or design and location of new tanks; design parameters and location of the pumps; design and location of connectors and hoses; site procedures for deployment of the pumps) have been performed or the status of such decisions and designs.

g. Discuss how consideration of the re-establishment of defense-in-depth for 6(&85,7< 5(/$7(' ,1)250$7,21 &(,,  '2 127 5(/($6(

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the plant in response to seismic events and the impact on the plants seismic risk profile was included in determining the duration for completion of the modification mentioned in Table 6-1.

5. Section 6 of the December 12, 2019 submittal states that a sensitivity for the modification mentioned in Table 6-1 of the submittal resulted in SCDF and SLERF reductions to be approximately 40 percent and 30 percent, respectively.

The submittal does not provide enough information to understand the details to understand the basis for the risk reduction. For that reason;

a. Explain the assumptions for determining the potential risk reduction from the proposed modification. Discuss the consideration of potential failure modes of the proposed modification such as liquefaction, seismically-induced failures of the SSCs involved in the modification, seismically-induced failure of enclosures or structures housing the equipment, failure of human actions, and random failures. The discussion should provide a summary of how the corresponding failure probabilities were determined.

b. If failure modes and events, such as those mentioned in item (a), for SSCs involved in the modification were not considered in the evaluation for determining the potential risk reduction from the proposed modification:

i. Justify their exclusion by performing sensitivity study(ies) and demonstrating that the results can be considered representative of the risk reduction from the modification. Alternately, update the results of the expected risk reduction from the proposed modification by including the failure modes and events for SSCs involved in the modification.

ii. Provide an explanation of how the design of the proposed modification can achieve the reduction estimates in Table 6-1 of the submittal given the unique site-specific failure modes that can impact the efficacy of the proposed modification.

Topic #14 - Peer Review of the Seismic PRA, Accounting for NEI 12-13 and Topic #15 -

Documentation of the Seismic PRA

6. The seismic PRA includes an assumption that there is 50% probability of the Class III TB failing towards the Class I TB and 50% probability of the Class III TB 6(&85,7< 5(/$7(' ,1)250$7,21 &(,,  '2 127 5(/($6(

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failing away from the Class I TB (i.e., there is equal probability of the direction of the pounding-induced failure of the Class III TB). If the Class III TB falls towards the Class I TB, the seismic PRA assumes a failure of the steam driven auxiliary feedwater system (SDAFW) which is in the Class I TB. Therefore, the seismic PRA assumes that the SDAFW fails 50% of the time the Class III TB fails. The assumption appears to be a key assumption for the seismic PRA and is identified as such by the peer-review team. It is unclear how the direction of a catastrophic, uncontrolled failure of a building can be determined.

Finding level F&O 24-8 from the SPRA full-scope peer-review provided in Table A-2 in Appendix A of the submittal, challenges the licensees assumption that the SDAFW fails 50% of the time the Class III TB fails. The assumption appears to be a key assumption for the seismic PRA and is identified as such by the peer-review team. In fact, according to information available to the staff as part of its audit, the event TB-CLASS3-1 which represents the 50% assumption contributes approximately 15% to the total seismic CDF. The licensee has provided an explanation based on post peer-review walkdown to justify and retain the assumption in the seismic PRA.

The licensees disposition of F&O 24-8 in Table A-2 in Appendix A of the submittal discusses equipment mounted on the pump exposed to the Class III Turbine Building but does not discuss cables and electrical wiring for control power, the supply piping for the SDAFW from the CST, or the injection piping from the SDAFW to the SG that is outside the containment.

The pounding-induced failure of the Class III TB originates at the mezzanine floor which is where the concrete slab protecting the Class I TB is located. In addition, the Class III TB failure at the mezzanine floor, which includes the failure of the turbine generator pedestal, will lead to catastrophic failure of the Class III TB as well as the turbine itself. It is unclear how, given such failures, the SDAFW is sufficiently protected from the impacts that it can be expected to continue performing its function, regardless of the direction of failure of the Class III TB.

How close or far a catastrophic structural failure will be to an SSC and its components appears to be a highly speculative judgement. Based on this;

a. Provide, with justification, the basis for the assumption that the Class III TB fails and interacts with the Class I TB 50% of the time. Explain what direction(s) the Class III TB would fail if it when it does not interact with the Class I TB.

b. Discuss whether the assessment of the functionality of the SDAFW 6(&85,7< 5(/$7(' ,1)250$7,21 &(,,  '2 127 5(/($6(

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following the failure of the Class III TB when it does not interact with the Class I TB considered the potential failures of (i) cables and electrical wiring for control power, the supply piping for the SDAFW from the CST, or the injection piping from the SDAFW to the SG that is outside the containment, and (ii) the catastrophic structural failure of the Class III TB originating from the mezzanine floor, even if it is away from the Class I TB, on the function of the SDAFW. Include any drawings or pictures, as necessary, to support the justification.

c. Provide the results of a sensitivity analysis which assumes guaranteed failure of SDAFW conditional on the Class III TB failure and discuss the impact on the results presented in the submittal (risk metrics and dominant risk contributors).

Topic #15 - Documentation of the Seismic PRA

7. Based on the information in the submittal, the seismic PRA includes an assumption of failure of the CST 75% of the time the TB Gantry Crane fails. The basis for that assumption is unclear. In addition, it is unclear why the failure of the TB Gantry Crane will not fail the TB itself and whether such a dependency is included in the model. Also, based on the information in the submittal and available to the staff as part of its audit, the seismic PRA eliminated a failure of the CST based on the failure of the Class III TB. The explanation for the elimination is that the failure of the Class I TB conditional on the failure of the Class III TB (which is assumed to occur 50% of the time the Class III TB fails) results in the failure of the SDAFW and therefore, accounts for the failure of the CST. However, the failure of the CST independent of the failure of the Class I TB does not appear to have been accounted in the seismic PRA (i.e., for the 50% of the time the Class III TB fails and does not interact with the Class I TB). It appears that, under the assumptions currently in use in the seismic PRA, the SDAFW should fail either due to interaction of the Class III TB with the Class I TB OR due to the failure of the CST when the Class III TB does not interact with the Class I TB. The common failures (overlaps) would be eliminated via Boolean logic for OR gates. The elimination of the failure of the CST when the Class III TB does not interact with the Class I TB would under-estimate the failure probability of the SDAFW. For these reasons;

a. Provide the basis for assumption of failure of the CST 75% of the time the TB Gantry Crane fails.

b. Discuss the impact, such as through a sensitivity study, of assuming guaranteed failure of CST conditional on the TB Gantry Crane failure on the results presented in the submittal (risk metrics and dominant risk contributors).

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c. Based on the explanation above, justify why the elimination of the failure of the CST when the Class III TB does not interact with the Class I TB would not under-estimate the failure probability of the SDAFW and consequently, impact the results in the submittal (risk metrics and dominant risk contributors). The justification can include the results of a sensitivity that includes the failure of the CST for the fraction where the Class III TB does not interact with the Class I TB.

Topic #15 - Documentation of the Seismic PRA

8. Based on the information available to the staff as part of its audit, the seismic event tree includes an event questioning whether offsite power is available (S-OSP).

This event occurs after the event questioning whether the Class III TB has failed (S-TB-CLASS3). The failure of the Class III TB results in a consequential loss-of-offsite power (LOOP) due to the switchgear in that building. It is unclear why offsite power availability is questioned after the failure of the Class III TB (presumably the down branch of the Class III TB failure event). For that reason, justify that the modeling of the seismic event tree, which includes a split based on the failure probability of offsite power after the failure of the Class III TB, is appropriate to represent the impact of the seismic event on the SSCs at the plant.

Topic #14 - Peer Review of the Seismic PRA, Accounting for NEI 12-13

9. Table A-2 in Appendix A to the submittal provides the results of the full-scope peer-review of the seismic PRA in the form of finding-level Facts and Observations (F&Os) along with the licensees dispositions for each finding-level F&O. Finding-level F&O states that the seismic PRA uses the internal events LERF as basis and a number of the LERF SRs in the internal events PRA are only met at CC-I and no seismic specific changes are made. The licensees disposition includes statements such as [p]erforming a LERF analysis per NUREG/CR-6595 is an acceptable methodology as referenced in Reg. Guide 1.174 Rev 3 and [t]he Robinson LERF analysis employs a methodology fully endorsed by the NRC as an acceptable means of calculating LERF for use in risk-informed applications.

RG 1.174, Revision 3 (ADAMS Accession No. ML17317A256), cited by the licensee, explicitly states (Section C.2.5.2, page 34 of the RG) [emphasis added]

The approach described in NUREG/CR-6595 may be used to quantify LERF only in those cases when the plant is not close to the CDF and LERF acceptance guidelines. The results provided in the submittal and mean values available to the 6(&85,7< 5(/$7(' ,1)250$7,21 &(,,  '2 127 5(/($6(

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staff from the audit information unambiguously exceed the CDF and LERF acceptance guidelines in RG 1.174, Revision 3. Therefore, the licensees disposition for finding-level F&O 24-23, especially the statements cited above, is inconsistent with the guidance, represents a convenient rather than correct interpretation of the guidance, and does not adequately address the concern raised in finding-level F&O 24-23.

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