ML24326A126
| ML24326A126 | |
| Person / Time | |
|---|---|
| Site: | 05200050 |
| Issue date: | 11/21/2024 |
| From: | NuScale |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML24326A092 | List: |
| References | |
| LO-175900 | |
| Download: ML24326A126 (1) | |
Text
Response to SDAA Audit Question Question Number: A-16.3.3-2 Receipt Date: 04/29/2024 Question:
NUREG-1431, Rev. 5.0, Table 3.3.1-1 Footnote (a) states:
(a) With rod control system capable of rod withdrawal or one or more rods not fully insert[ed].;
and modifies the MODES 3, 4, and 5 Applicability of reactor trip system Functions:
- 1. Manual Trip
- 5. Source Range Neutron Flux
- 19. Reactor Trip Breakers (RTBs)
- 20. Reactor Trip Breaker Undervoltage and Shunt Trip Mechanisms
- 21. Automatic Trip Logic SDAA Part 4, Rev. 1, Table 3.3.1-1 Footnotes (a) and (b) state:
(a) When capable of withdrawal of more than one CRA.
(b) When capable of withdrawal of more than one CRA and the RCS temperature [is] above the T-3 interlock.
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Footnote (a) modifies the equivalent MODE 2 and 3 applicability of MPS Functions 1.a RTS on High Power Range Linear Power 3.a RTS on High Intermediate Range Log Power Rate 4.a RTS on High Source Range Count Rate 5.a RTS on High Source Range Log Power Rate 7.a RTS on High Pressurizer Pressure 10.a RTS on High Pressurizer Level 11.a RTS on Low Pressurizer Level 16.a RTS on Low Low RCS Flow 17.a RTS on High Main Steam Pressure [MODE 2 only]
19.a RTS on Low Low Main Steam Pressure 22.a RTS on High Narrow Range Containment Pressure 25.a RTS on Low AC Voltage to EDAS Battery Chargers 26.a RTS on High Under-the-Bioshield Temperature Footnote (a) modifies the equivalent MODE 2 applicability and Footnote (b) modifies the equivalent MODE 3 applicability of MPS Functions (based on markups in the Feb 29, 2024, re-revised audit response to item (3) of audit question A-16.3.3.1-5) 1.b DWSI on High Power Range Linear Power 3.b DWSI on High Intermediate Range Log Power Rate 4.b DWSI on High Source Range Count Rate NuScale Nonproprietary NuScale Nonproprietary
5.b DWSI on High Source Range Log Power Rate 7.d DWSI on High Pressurizer Pressure 9.a RTS on Low Low Pressurizer Pressure 9.b DWSI on Low Low Pressurizer Pressure 10.b DWSI on High Pressurizer Level 11.e DWSI on Low Pressurizer Level 16.d DWSI on High Main Steam Pressure 19.c DWSI on Low Low Main Steam Pressure 22.e DWSI on High Narrow Range Containment Pressure 25.e DWSI on Low AC Voltage to EDAS Battery Chargers 26.e DWSI on High Under-the-Bioshield Temperature Footnote (b) also modifies the MODE 1 (equivalent to W-STS MODES 1 and 2) of MPS Function 2.b DWSI on High Power Range Positive and Negative Rate The MODES 2 and 3 Applicability of LCO 3.3.2, RTS Logic and Actuation [two divisions]
states:
MODES 2 and 3 when capable of CRA withdrawal.
SDAA Part 4, Rev. 1, Table 3.3.4-1, Manual Actuation Functions, Footnote (a) states:
(a) When capable of CRA withdrawal.
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and modifies the MODES 2 and 3 Applicability of manual Function
- 1. Reactor Trip System.
Also, Table 1.1-1 Footnote (a), which applies to the definition of MODE 3, Safe Shutdown, states:
(a) Any CRA capable of withdrawal, or any CVCS or CFDS connection to the module not isolated.
TR-101310-NP, Rev. 0, US460 Standard Design Approval Technical Specifications Development, beginning on page 10, discusses the use of the phrase When capable of withdrawal of more than one CRA as described above. The discussion in the report states (emphasis added):
Footnotes referring to capability to withdraw a control rod assembly (CRA), are modified to indicate the requirement applies when capable of withdrawing more than a single CRA. This change is necessary to allow energization of a portion of the control rod drive system (CRDS) in MODE 3 when preparing for module disassembly.
This capability is required to verify the CRA is disconnected from its extension rod. The CRDS design and administrative control ensure that no more than one CRA may be manipulated, and the definition of, and limits on SHUTDOWN MARGIN specified in LCO 3.1.1 ensure the plant remains safely shutdown. A description of the functional design of the CRDS is provided in FSAR Section 4.6. This change results in changes to the Bases discussions for the associated Functions.
Footnotes limiting the Applicability of the Demineralized Water System Isolation are added to only require OPERABILITY when RCS temperature is above the T-3 interlock. The RCS T-3 bypass is active when RCS temperature is less than approximately 340 degrees F. This change also resulted in addition of a footnote to Table 3.3.3-1, Engineered Safety Features Actuation System Logic and Actuation Functions.
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Some other footnotes required modification because of the combination of allowances described above.
- 1. The applicant is requested to justify (a) not using the W-STS phrase or one or more rods not fully inserted in the GTS; (b) the differences between the W-STS phrase With rod control system capable of rod withdrawal and each instance of the GTS phrases Any CRA capable of withdrawal and When capable of withdrawal of more than one CRA; and (c) using the GTS phrase Any CRA capable of withdrawal in some cases instead of the phrase When capable of withdrawal of more than one CRA.
- 2. Please confirm that Table 3.3.1-1 Footnote (b) applies to Function 9.a; or should it be Footnote (a)?
On page B 3.3.1-29 in the Applicable Safety Analyses, LCO, and Applicability section of the Bases, Function 9.a is discussed, as follows (emphasis added):
The RTS and ESFAS Low Low Pressurizer Pressure setpoint is approximately 1200 psia.
Actual setpoints are established in accordance with the Setpoint Program.
Four Low Low Pressurizer Pressure reactor trip channels are required to be OPERABLE when operating in MODE 1 and in MODES 2 and 3 when capable [of] withdrawing more than a single CRA above the T-3 interlock. In MODES 2 and 3 with no capability of withdrawing more than one CRA or below T-3, and in MODES 4 and 5 the function is fulfilled because the CRAs are inserted.
The last sentence denoted by italics needs clarification because with RCS temperature below the T-3 interlock (approx. 340 degrees F):
(a) the unit is in MODE 3 and cannot be in MODE 2; and NuScale Nonproprietary NuScale Nonproprietary
(b) all 24 CRAs are not necessarily inserted if the control rod drive system (CRDS) is capable of withdrawing one or more CRAs.
- 3. Explain how RCS temperature being below the T-3 interlock is relevant to being capable of withdrawing one or more CRAs.
Note that LCO 3.4.2 Minimum Temperature for Criticality (345 degrees F), is only applicable in MODE 1; so RCS temperature being < T-3 in MODE 3 does not mean that one CRA could not be withdrawn and additional CRAs could not be capable of being withdrawn.
- 4. In the above quoted discussion from technical report TR 101310-NP, the second paragraph, last sentence refers to the addition of Table 3.3.3-1 Footnote (d), which modifies the MODE 3 applicability of LCO 3.3.3 logic and actuation Function 9, Pressurizer Line Isolation Actuation. Footnote (d) states (d)
With RCS temperature above the T-3 interlock.
Please explain in the Bases why automatic isolation of the pressurizer spray line and the pressurizer high point vent line (LCO 3.3.1 Function 8.d, Pressurizer Line Isolation on Low Pressurizer Pressure, which is also not required below T-3) is not needed to satisfy any of the safety analyses of design basis events and AOOs (postulated high energy line breaks from the pressurizer vapor space) with RCS temperature < 340 degrees F (T-3 interlock).
- 5. Please consider including in GTS an appropriate LCO and/or SR that addresses the two-rods-withdrawn configuration, which is needed to transition to Mode 4 from Mode 3 when the RPV is depressurized and reactor coolant temperature is < 200 degrees F; or explain why it would not be appropriate. Where do GTS only allow withdrawing one CRA in Modes 2 and 3?
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Response
Item 1.(a)
Footnotes in NuScale Generic Technical Specifications (GTS) Table 3.3.1-1 referring to the capability to withdraw a control rod assembly (CRA), indicate the requirement applies when the capability exists to withdraw more than a single CRA. For the NuScale design, verification that a control rod drive shaft is uncoupled or coupled is performed with the reactor pressure vessel and containment vessel fully assembled using the associated control rod drive mechanism (CRDM). A single CRDM is energized and utilized to attempt to lift an individual control rod to verify the control rod is unlatched before disassembly and latched after reassembly. During that evolution, current traces are used to determine the status of the control rod drive shaft, coupled or uncoupled. The phrasing of the control rod related footnotes in the GTS allow an single CRDM to be energized to support verification of control rod drive shaft coupling, or uncoupling, respectively.
Items 1.(b) and (c)
As discussed in the response to Item 1.(a), the phrasing of the NuScale specific footnote language supports verification of control rod drive shaft coupling.The Westinghouse Standard Technical Specification (WSTS) footnote wording is utilized for cases where the NuScale GTS rod control related footnotes are restricting any type of rod withdrawal.
Item 2.
NuScale revised GTS Table 3.3.1-1 MODE 2 applicability for module protection system (MPS) instrument functions 1.b and 3.b from footnote (b) to footnote (a) in the response to Audit Question A-16.3.3.1-5.
NuScale revises the third paragraph of page B 3.3.1-29 of the GTS to read:
Four Low Low Pressurizer Pressure reactor trip channels are required to be OPERABLE when operating in MODE 1, MODE 2 when capable of withdrawing more than one CRA, and MODE 3 when capable withdrawing more than a single CRA above the T-3 interlock. The Low Low Pressurizer Pressure reactor trip is not required in MODE 2 with no capability of withdrawing NuScale Nonproprietary NuScale Nonproprietary
more than one CRA and MODE 3 with no capability of withdrawing more than one CRA or below the T-3 interlock because the function is fulfilled by [reactor coolant system] boron concentration above the [shutdown margin] requirements established in LCO 3.3.1. The Low Low Pressurizer Pressure reactor trip is not required in MODES 4 and 5 because the function is fulfilled with the CRAs inserted, and because the [reactor coolant system] boron concentration is above the [shutdown margin] requirements established in LCO 3.3.1.
Item 3.
The T-3 interlock is one of the two conditions specified in Footnote (b) of GTS Table 3.3.1-1.
The reactor coolant system (RCS) temperature being below the T-3 interlock is not relevant to being capable of withdrawing one or more CRAs. The T-3 interlock establishes when the operating bypass is removed from the reactor trip.
Item 4.
The pressurizer line isolation function addresses a very specific set of postulated breaks in pressurizer high point vent lines outside of containment. The pressurizer line isolation function is implemented to retain adequate water mass (inventory) in the NuScale Power Module (NPM) so that the collapsed liquid level remains above the top of the active fuel during long-term emergency core cooling system (ECCS) operation. The pressurizer line isolation function is bypassed below the T-3 interlock because postulated events do not present a challenge at lower temperatures. Designers selected the T-3 interlock as a point of convenience to be consistent with other functions and provide operational flexibility. The module protection system provides protection against breaks initiated from low power and shutdown scenarios.
The T-3 interlock corresponds to the minimum temperature for criticality, so if RCS Thot is less than 340 degrees F, the NPM is shutdown, or at least subcritical. The NPM will have cooled from operating conditions, so decay heat will be reduced from the 102% power decay heat evaluated in the long-term ECCS cooling analyses. The lower water temperature corresponds to higher density and therefore a higher mass of RCS water compared to full power operating conditions. Therefore, more inventory loss can be accommodated before isolation. Reactor trip on low pressurizer level is continuously enabled and will actuate after sufficient break flow. The chemical and volume control system isolates on low-low pressurizer level and this signal is NuScale Nonproprietary NuScale Nonproprietary
active and provides protection unless the NPM is cooled to atmospheric pressure conditions with Thot < 200 degrees F (T-2 interlock) or the containment is flooded (L-1 interlock). ECCS will actuate on low-low riser level. Therefore, the module protection system provides protection for breaks postulated to occur during low power and shutdown scenarios.
The level of detail in the GTS Bases discussion of the Pressurizer Line Isolation function is consistent with the level of detail in the discussion of other MPS instrumentation in the GTS Bases. The last paragraph on GTS page B 3.3.1-28 states:
In MODE 3 below the T-3 interlock isolation of the pressurizer lines is accomplished by other isolation signals. In MODES 4 and 5 the pressurizer line isolation safety function is accomplished.
No additional discussion of the Pressurizer Line Isolation function below the T-3 interlock is considered necessary.
Item 5.
The reactor enters MODE 4 to move the NPM from the operating bay to the containment vessel flange tool, where the lower containment vessel is removed, then relocated to the reactor vessel flange tool, where the lower reactor pressure vessel is removed. While in MODE 4 there is no mechanism to withdraw any control rods, and there is no control rod position indication available. Operators verify control rods are fully inserted and unlatched, and operators verify the RCS boron concentration is greater than the shutdown margin requirements established in LCO 3.3.1 prior to entering MODE 4.
The last sentence on GTS page B 3.3.1-19 states:
A single CRDM may be energized in MODE 2 or 3 using an alternate power source while conducting CRA coupling and decoupling.
NuScale Generic Technical Specification Surveillance Requirement SR 3.1.1.1 ensures shutdown margin considers the reactivity effects of CRA position.
Markups of the affected changes, as described in the response, are provided below:
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MPS Instrumentation B 3.3.1 NuScale US460 B 3.3.1-30 Draft Revision 2 BASES APPLICABLE SAFETY ANALYSES, LCO, and APPLICABILITY (continued)
- c. Low Low Pressurizer Pressure - Reactor Trip and Demineralized Water System Isolation (Table 3.3.1-1 Functions 9.a, b)
The Low Low Pressurizer Pressure trip is designed to protect against RCS line breaks outside of containment and protect the RCS subcooled margin against flow instability events.
The RTS and ESFAS Low Low Pressurizer Pressure setpoint is approximately 1200 psia. Actual setpoints are established in accordance with the Setpoint Program.
Four Low Low Pressurizer Pressure reactor trip channels are required to be OPERABLE when operating in MODE 1, MODE 2 when capable of withdrawing more than one CRA, and MODE 3 when capable of withdrawing more than a single CRA above the T-3 interlock. The Low Low Pressurizer Pressure reactor trip is not required in MODE 2 with no capability of withdrawing more than one CRA and MODE 3 with no capability of withdrawing more than one CRA or below the T-3 interlock because the function is fulfilled by RCS boron concentration above the SDM requirements established in LCO 3.3.1. The Low Low Pressurizer Pressure reactor trip is not required in MODES 4 and 5 because the function is fulfilled with the CRAs inserted, and because the RCS boron concentration is above the SDM requirements established in LCO 3.3.1.Four Low Low Pressurizer Pressure reactor trip channels are required to be OPERABLE when operating in MODE 1 and in MODES 2 and 3 when capable withdrawing more than a single CRA above the T-3 interlock. In MODES 2 and 3 with no capability of withdrawing more than one CRA or below T-3, and in MODES 4 and 5 the function is fulfilled because the CRAs are inserted.
Four Low Low Pressurizer Pressure DWSI channels are required to be OPERABLE when operating in MODES 1, 2, and MODE 3 above the RCS temperature T-3 interlock. In MODE 3 below the RCS temperature T-3 interlock actuation the reactor is shutdown.
The high subcritical multiplication signal and the low RCS flow signal provide the required protective functions below the RCS T-3 interlock temperature. In MODES 4 and 5 the demineralized water system is isolated from the reactor module.