ML24346A314
| ML24346A314 | |
| Person / Time | |
|---|---|
| Site: | 05200050 |
| Issue date: | 12/11/2024 |
| From: | NuScale |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML24346A130 | List:
|
| References | |
| LO-175762 | |
| Download: ML24346A314 (1) | |
Text
Response to SDAA Audit Question Question Number: A-19.1-55 Receipt Date: 02/12/2024 Question:
Per the request documented in ML24269A008, dated October 24, 2024, only the relevant portions of the question and response are included.
The staff reviewed ER-116289 Confirmation of Passive Safety System Reliability (PSSR) PRA Report R00 to understand the Passive Safety System Reliability results reported in the SDA Chapter 19. The staff understands that the purpose of ER-116289 is to confirm the conclusions of the PSSR of the ECCS and the DHRS under accident conditions as performed in ER-P010-3777 Revision A, dated 3/30/2016, Passive Safety System Reliability, for the DCA design. The staff understands ER-116289 is a limited scope evaluation to confirm the conclusions and reliability probabilities of the full scope evaluation performed in ER-ER-010-3777 and not to reproduce or recalculate the passive safety system response surface or reliability probabilities. The staff also understands the performance of ECCS and DHRS during ATWS scenarios was not evaluated in both the DCA and the SDA PSSR reports.
ER-116289 confirms the probabilities of the ECCS failing to prevent core damage and of the DHRS failing to protect the reactor coolant pressure boundary. These probabilities are used in determining the overall core damage frequency of a single NPM in the support of the NuScale plant SDA PRA. In addition to the above-mentioned reports, the staff also reviewed:
EC-120490 Safety Class, Safety Related, Revision 0, NPM Long Term Cooling Analysis ER-P010-4713, Revision 0, dated 10/27/2016, Confirmation of Passive System Reliability PRA Report Based on its review, the staff has the following questions:
1.
The error factors for the DHRS train passive heat transfer to the UHS and the ECCS passive heat transfer to the pool in SDA Table 19.1-9, Basic Events Requiring Design Specific Analysis, are different than the DCA and inconsistent with the ER-116289, Section 5, Conclusions. NuScale is requested to confirm the error factors actually used in the SDA PRA, NuScale Nonproprietary NuScale Nonproprietary
discuss the technical basis for the for the error factors that were used, and update the SDA FSAR Chapter 19 as applicable.
3.
Based on its review of SDA Table 19.1-10, Phenomena Affecting Emergency Core Cooling System Passive Performance and SDA Table 19-11, Phenomena Affecting DHRS Passive Performance:
a.
Table 19.1-10 includes a new entry, Actuation Setpoints, compared to the same table in the DCA. The new entry reads, Actuating the ECCS on a lower level delays the time in which recirculation may be established. However, actuating the ECCS on a lower level can also have the effect of reducing RPV level, which is not stated. NuScale is requested to update the SDA FSAR to add the impact on RPV level for this new entry.
b.
The staff did not find the minimum Cv for the ECCS reactor vent valve and the reactor circulation valve documented in any docketed information. Please identify the location for such information in docketed documents or provide FSAR markups with this information.
- 6. ((
2(a),(c) NuScale is requested to provide this information in the ERR.
Response
- 1. While error factors for the US600 Design Certification Application (DCA) passive safety system reliability (PSSR) are calculated in ER-P010-3777, Passive Safety System Reliability Probabilistic Risk Assessment Report, the US460 Standard Design Approval Application (SDAA) approach follows a more generalized practice. The expanded error factors documented in NuScales US460 SDAA are intended to account for a potentially broader range of uncertainties.
Section 3.6 of ER-102070, Data Analysis Notebook, which is available in the SDAA Audit Section 19.1-19.3 eRR, describes the methodology NuScale uses to determine error factors, including those for decay heat removal system (DHRS) and emergency core cooling system (ECCS) passive heat transfer basic events. Table 19.1-9 of the FSAR, Basic Events Requiring Design-Specific Analysis, reflects the values from Table F-1 of ER-102070. These values bound the distributions identified in ER-P010-3777 and included in ER-116289, Confirmation of Passive Safety System Reliability PRA Report. NuScale Nonproprietary NuScale Nonproprietary
3a. ((
}}2(a),(c), ECI NuScale revised FSAR Table 19.1-10 to capture this effect.
3b. Section 6.3.2.2 of the FSAR provides ECCS valve flow coefficients. The reactor recirculation valves have a minimum flow coefficient of 55; the reactor vent valve and diffuser, as a combined unit, have a minimum flow coefficient of 375.
- 6. Assumption 2.2.9 of ER-116289 identifies an RPV functional pressure capacity of ((
}}2(a),(c), ECI Additionally, while the RPV design pressure is 2200 psia, reactor coolant system pressure is permitted to reach 110 percent of design pressure in anticipated operational occurrences (or 2420 psia), in accordance with Section III of the American Society of Mechanical Engineers code. This is not an acceptance criterion for the PSSR analysis, but it reinforces that the limiting PSSR case for DHRS does not challenge RPV pressure integrity.
Markups of the affected changes, as described in the response, are provided below: NuScale Nonproprietary NuScale Nonproprietary
NuScale Final Safety Analysis Report Probabilistic Risk Assessment NuScale US460 SDAA 19.1-107 Draft Revision 2 Audit Question A-19.1-55 Table 19.1-10: Phenomena Affecting Emergency Core Cooling System Passive Performance Parameter Significance* Decay power Higher energy production after shutdown increases the long-term ECCS heat removal requirements. CNV convective heat transfer Increased wall-fluid heat transfer decreases pressure in the CNV, reducing the RPV level. RPV initial level A lower initial RPV level reduces the available hydrostatic head for recirculation. Non-condensable gas A lower non-condensable gas inventory increases the condensation rate of steam and decreases pressure in the CNV, which has the net effect of reducing the RPV level. ECCS valve flow An increased pressure drop across the ECCS valves (decreased flow capacity) maintains the RPV at higher pressure, reducing the RPV level. Pool temperature A lower pool temperature increases heat transfer through the CNV and decreases pressure in the CNV, reducing the RPV level. Actuation setpoints Actuating the ECCS on a lower level delays the time in which recirculation may be established and has the effect of reducing the RPV level.
- Note: Parameter significance is provided with respect to the passive reliability of the ECCS to facilitate liquid coolant recirculation to the RPV.}}