ML23205A231
| ML23205A231 | |
| Person / Time | |
|---|---|
| Site: | 99902078, 05200050 |
| Issue date: | 07/24/2023 |
| From: | NuScale |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML23205A216 | List: |
| References | |
| LO-146777 | |
| Download: ML23205A231 (1) | |
Text
NuScale Nonproprietary Response to SDAA Audit Question Question Number: A-19.1-24 Receipt Date: 05/01/2023 Question:
Sensitivity studies listed in Chapter 19 of the FSAR do not provide sufficient information for the staff to understand how the sensitivity was performed and why it addresses an underlying uncertainty or assumption. The staff needs this information on the docket to support its safety finding on the insights from the PRA. For the following sensitivity studies, provide additional description in the SDA FSAR on the magnitude of the change (e.g., by an order of magnitude, from 4.5E-05 to 1.0E-03) to support use of the results of the sensitivity studies for the staffs safety findings. These sensitivity studies support the uncertainties listed in Tables 19.1-14 and 19.1-15 that staff relies on to determine that the uncertainty was adequately addressed for the uses of an SDA PRA.
For full power, internal events:
- Increase LOOP initiating event frequency
- Decrease LOOP initiating event frequency
- Increase SGTF initiating event frequency
- Increase secondary line break initiating event frequency
- Increase CVCS line break outside containment initiating event frequency
- Increase failure probability of passive heat removal
- Increase failure probability of ECCS low differential pressure (RRVs)
For full power, external events:
- Increase fraction of external floods that result in a LOOP -
- Increase probabilities of not recovering offsite power in the hurricane high winds PRA
- Increase frequency of a hurricane induced LOOP For LPSD:
- Increase failure probability of CES For Multiple Module:
- Reduce MMAFs so that NPM-equipment is less correlated
- Decrease MMPSF for module-specific HFEs NuScale Nonproprietary
NuScale Nonproprietary
Response
NuScale has added the requested information to FSAR Table 19.1-22, Sensitivity Studies, for the events listed in the audit issue. In addition, to support the Staffs safety finding, NuScale added similar information to five other events not identified by the NRC. NuScale also corrected an editorial error in the Factor Change in LRF column for one of the identified events (under the Multiple Module heading).
To address a concern that was identified during the May 11, 2023 clarification call on Audit Issue A-19.1-3, NuScale edited the ECCS low differential pressure opening mode entry in FSAR Table 19.1-15, Design-Specific Sources of Level 1 Model Uncertainty, to clarify that it corresponds to the ECCS low differential pressure (RRVs) event described in FSAR Table 19.1-22.
Markups of the affected changes, as described in the response, are provided below:
NuScale Nonproprietary
Audit Issue A-19.1-24 NuScale US460 SDAA NuScale Final Safety Analysis Report Table 19.1-15: Design-Specific Sources of Level 1 Model Uncertainty Uncertainty Description Level 1 Assumption Effect on Model Source General Design state Design changes are likely as the The PRA model reflects the current state of The PRA model is updated to remain consistent with the design evolves beyond standard design for the standard design. maturing design. As such, this is judged not to be a design. significant source of model uncertainty.
Initiating Event Analysis List of initiating Comprehensive list of internal The PRA model captures potential initiating The PRA model includes a wide range of initiating events events initiating events, including events; based on a thorough review of potential to capture potential accident progression scenarios; the potential initiators from other initiating events. There is not a size of LOCA that initiators cover LOCAs, SGTFs, secondary line breaks, modules. exceeds the capability of the ECCS (e.g., reactor loss of electric power, and transients. As such, this is vessel rupture). judged not to be a significant source of model uncertainty.
Operating Frequencies for initiating events Generic data and plant-specific analyses are It is judged that initiating event frequencies are not higher experience and with no plant experience. representative of the initiating event frequencies.
than generic data; the design reflects opportunities to data improve SSC based on operating experience. Although generic data are used, a lognormal distribution with an error factor of 10 is used to bound the uncertainty.
19.1-115 Sensitivity studies provided in Table 19.1-22 were performed to address Initiating event frequency uncertainty.
Availability and Initiating event frequency Plant availability is assumed to be 100 percent. The initiating event frequencies are conservative (i.e., they capacity factor adjustment for capacity factor. are not weighted by the fraction of time the plant is at power.)
SGTF Frequency for an SGTF in a A study is performed to estimate the frequency of A sensitivity study (provided in Table 19.1-22 illustrates helical steam generator with no an SGTF based on a probabilistic physics of that an increase in the frequency of an SGTF has no plant experience. failure approach. impact on the results. As such, this is judged not to be a significant source of model uncertainty.
Secondary line Frequency for a secondary line A study is performed to analyze system design to A sensitivity study provided in Table 19.1-22 illustrates that Probabilistic Risk Assessment breaks break with no plant experience. estimate the frequency of a secondary line break. an increase in the frequency of a secondary line break has no impact on the results. As such, this is judged not to be a significant source of model uncertainty Accident Sequence Analysis and Success Criteria Passive decay Reliability and effectiveness of Experimental testing data and design-specific A sensitivity study provided in Table 19.1-22 illustrates that Draft Revision 1 heat removal passive decay heat removal analysis reflect system success criteria and there is little effect on CDF with order of magnitude systems with no plant experience. reliability, including availability of the UHS. increase in passive heat removal failure probability.
Table 19.1-15: Design-Specific Sources of Level 1 Model Uncertainty (Continued)
NuScale US460 SDAA NuScale Final Safety Analysis Report Uncertainty Description Level 1 Assumption Effect on Model Source ECCS low Reliability of the ECCS low The probability of the ECCS low differential A sensitivity study provided in Table 19.1-22 evaluated the differential differential pressure (RRVs) pressure (RRVs) opening mode is assumed to be effect of increasing the failure probability as small. In pressure operating mode with no plant 0.1. addition, a sensitivity study addressed uncertainty in ECCS opening mode experience. actuation because of CCF.
ATWS and Power oscillations during ATWSOnly sequences that exceed peak clad Successful end states in the PRA do not require the core to definition of core sequences. temperature are assumed to result in core remain subcritical. Because this is not a safety issue as damage damage. heat removal is effective, it is not expected to be a source of model uncertainty.
Data Analysis Mission time Use of a 72 hour8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> mission time for Time-dependent component failures generally Use of a 72-hour mission time is consistent with the a passive design. Standard modeled using a 72 hour8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> mission time. guidance for passive reactor designs. This may result in industry PRA practice uses a 24 conservative equipment reliability estimates.
hour mission time.
Testing scheme Plant testing scheme. Standby failure rates assume non-staggered This is conservative assumption; results are slightly testing. conservative in comparison to a staggered testing assumption.
19.1-116 Test and Identification and modeling of test Test and maintenance unavailabilities were The PRA model includes several system test and Maintenance and maintenance unavailability identified from draft technical specifications, maintenance unavailabilities; although generic data are Unavailability events with no plant experience. discussions with operations and design used, a lognormal distribution with an error factor of 10 is engineers, and other PRA models. used to bound the uncertainty.
Unavailabilities are based on generic data.
Component Reliability data with no plant Generic data are assumed to better represent Potential for over or under estimating component reliability; failure data experience. reliability of components. this is captured in the parametric uncertainty results and not expected to be a measurable source of model uncertainty.
Common Cause Only intra-system CCF events Common cause events are considered for The only potential for inter-system CCFs (i.e., between Events considered. intra-system components, based on common different systems that perform a similar function) is coupling mechanisms. Generic NRC data are between the CVCS and CFDS (e.g., pumps). Because Probabilistic Risk Assessment used for common cause alpha factor parameters. operation of these systems requires operator action, the uncertainty of any potential inter-system CCF is effectively captured in a sensitivity study provided in Table 19.1-22, which addresses HEP.
Draft Revision 1
NuScale Final Safety Analysis Report Probabilistic Risk Assessment Audit Issue A-19.1-24 Table 19.1-22: Sensitivity Studies Factor Factor Sensitivity Description Change in Change in CDF LRF Full Power, Internal Events IncreaseDouble the LOOP initiating event frequency 1.3 1.2 Decrease LOOP initiating event frequency by an order of magnitude 0.8 0.9 Increase steam generator tube failure initiating event frequency by more than an order of 1.0 1.0 magnitude, to the generic data value Increase secondary line break initiating event frequency by more than 2 orders of 1.0 1.0 magnitude, to the generic data value IncreaseDouble the LODC initiating event frequency 1.0 1.0 IncreaseDouble the CVCS LOCA initiating event frequency 1.0 1.0 Increase CVCS line break outside containment initiating event frequency by an order of 1.0 3.9 magnitude Increase failure probability of passive heat removal by an order of magnitude 1.0 1.0 Increase failure probability of ECCS low differential pressure (RRVs) by a factor of 5 1.2 1.0 Include ECCS low differential pressure opening for RVVs 0.4 0.3 Decrease probability of post-trip RSV demand by a factor of 50 0.9 1.0 Assume core damage RPV overpressure sequences also result in large release N/A 1.0 All HEPs set to 5th percentile 0.6 0.4 All HEPs set to 95th percentile 2.8 6.4 All CCF set to 0 0.1 <0.1 All CCF set to 95th percentile >1001 >1001 Full Power, External Events Credit CVCS makeup in non-RXB internal floods 0.8 1.0 MinimizeSet fire PRA growth to false, which stops; stop fires before they damage 0.1 <0.01 mitigating equipment MinimizeSet fire PRA growth to true, which ensures; allow fires to damage mitigating 14.3 2.5 equipment IncreaseDouble the fraction of external floods that result in a LOOP 2.0 2.1 Include ECCSpossibility of RVV low differential pressure opening for RVVs in the external 0.4 0.4 flood PRA Increase probabilities of not recovering offsite power in the hurricane high winds PRA by 1.5 1.6 50%
IncreaseDouble the frequency of a hurricane induced LOOP 1.9 2.2 Include ECCSpossibility of RVV low differential pressure opening for RVVs in the 0.4 0.4 hurricane high winds PRA Low Power and Shutdown IncreaseDouble the failure probability of CES in POS6 1.0 1.0 Multiple Module ReduceDecrease MMAFs by an order of magnitude so that NPM-equipment is less 0.6 0.30 correlated Decrease MMPSF for module-specific HFEs by a factor of 5 0.5 <0.01 Note 1: Failures assumed to be "as-is" on loss of MPS NuScale US460 SDAA 19.1-128 Draft Revision 1