ML24346A283
| ML24346A283 | |
| Person / Time | |
|---|---|
| Site: | 05200050 |
| Issue date: | 12/11/2024 |
| From: | NuScale |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML24346A130 | List:
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| References | |
| LO-175762 | |
| Download: ML24346A283 (1) | |
Text
Response to SDAA Audit Question Question Number: A-15.4.1-2S Receipt Date: 05/23/2024 Question:
Audit question A-15.4.1-2 was received by NuScale on April 29, 2023. NuScale provided a response on May 7, 2024. On May 23, 2024, NuScale received the following written feedback from the NRC:
Instead of providing event-specific discussion of loss-of-power scenarios, the SDA FSAR consolidated discussion in the new FSAR Section 15.0.0.6.5 as stated in the response.
However, the applicability of FSAR Section 15.0.0.6.5 to the events discussed in FSAR Sections 15.4.1, 15.4.2, and 15.4.3 is unclear. The response cites the FSAR Section 15.0.0.6.5.2 to disposition loss of the normal DC power system (EDNS). This FSAR section states The response to the loss of EDNS is the same as the response to a loss of normal AC except that the control rods do not drop immediately and MPS actuations do not occur based on a set time from the loss of EDNS. For the Chapter 15 analyses that do not credit immediate control rod drop or MPS actuations based directly on loss of normal AC power, loss of EDNS is equivalent to a loss of normal AC power. Loss of normal AC power is not evaluated for these events because AC power is required for the initiating event to occur. However, it is unclear why loss of the normal DC power system would be equivalent to loss of normal AC power in these cases. Provide an explanation of why loss of EDNS is equivalent to loss of normal AC power for 15.4.1, 15.4.2, and 15.4.3 events (excluding CRA misalignment) or provide an evaluation of these loss of EDNS scenarios.
Response
This response is a supplement to the May 7, 2024 response to address the feedback from May 23, 2024; the May 7, 2024 response remains unchanged in the electronic reading room (eRR).
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Final Safety Analysis Report (FSAR) Section 15.0.0.6.5.1 provides a bulleted list of the effects of a loss of normal alternating current (AC) power. Similarly, FSAR Section 15.0.0.6.5.2 provides a bulleted list of the effects of a loss of normal direct current (DC) power (EDNS).
Comparison of the two lists confirms the FSAR Section 15.0.0.6.5.2 statement that The response to the loss of EDNS is the same as the response to a loss of normal AC except that the control rods do not drop immediately and MPS actuations do not occur based on a set time from the loss of EDNS.
When ignoring the control rod response to the loss of motive power as described, the effect of either a loss of normal AC or a loss of EDNS is a turbine trip and a complete loss of feedwater flow (due to loss of power to feedwater pumps or loss of power to control feedwater pumps). A turbine trip (both with and without a complete loss of feedwater flow) is analyzed in FSAR Section 15.2.1. The results of the turbine trip (e.g., FSAR Tables 15.2-4 through 15.2-6) are a rapid increase in pressurizer pressure that causes a reactor trip within the first 10 seconds.
Similarly, a complete loss of feedwater flow is analyzed in FSAR Section 15.2.7. The results of the complete loss of feedwater flow (e.g., FSAR Table 15.2-17) are a rapid increase in pressurizer pressure that causes reactor trip just after the first 10 seconds. The results of these two events (i.e., in FSAR Sections 15.2.1 and 15.2.7) show that a turbine trip or a complete loss of feedwater flow (or a combination of both) results in a rapid pressure increase and an early reactor trip on the high pressurizer pressure signal.
In the case of one of the rod withdrawal events in FSAR Sections 15.4.1, 15.4.2 or 15.4.3 with a loss of normal AC or a loss of EDNS, a reactor trip on the high pressurizer pressure signal would result in insertion of the control rods and therefore termination of the rod withdrawal initiating event early in the transient. Note that the FSAR Section 15.4.1 event includes cases from low power or subcritical conditions before the turbine is online. Although the turbine is not online and does not trip in response to the loss of power (either AC or EDNS) in these cases, the effects of the complete loss of feedwater described above are still applicable.
In contrast, consider the limiting rod withdrawal events in FSAR Sections 15.4.1, 15.4.2, and 15.4.3 in FSAR Tables 15.4-1, 15.4-4, and 15.4-10, respectively. These limiting cases are relatively slow rod withdrawals where reactor trip does not occur for a minute or more. If a loss of normal AC power or a loss of EDNS was assumed, a reactor trip on high pressurizer pressure would terminate these cases earlier in the reactivity insertion while reactor power was lower. Loss of normal AC power or loss of EDNS are therefore less limiting than assuming continued power availability.
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Note that if the control rod response to the loss of normal AC power is considered, then the loss of normal AC power scenario will result in even earlier rod insertion and termination of the rod withdrawal initiating event. This would change the conclusion that the loss of normal AC power and loss of EDNS event progressions are consistent, but does not impact the conclusion that loss of normal AC power or loss of EDNS are less limiting than assuming continued power availability.
Also note that assuming loss of normal AC power or loss of EDNS at reactor trip (i.e., instead of at event initiation) has no impact on the analysis in FSAR Sections 15.4.1, 15.4.2, and 15.4.3 because the event acceptance criteria are fuel related parameters that are rapidly improved by the reactor trip before relevant primary system parameters are impacted by a potential turbine trip or complete loss of feedwater (or combination of both) caused by loss of power (either AC or EDNS).
In conclusion, the loss of normal AC power and the loss of EDNS have the same impact on rod withdrawal events in FSAR Sections 15.4.1, 15.4.2 or 15.4.3 (when ignoring the control rod response to the loss of normal AC power). Either loss of power scenario is non-limiting for the rod withdrawal events in FSAR Sections 15.4.1, 15.4.2 or 15.4.3 compared to the power available scenarios that are provided in the FSAR.
No changes to the SDAA are necessary.
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