ML24326A135
| ML24326A135 | |
| 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: ML24326A135 (1) | |
Text
Response to SDAA Audit Question Question Number: A-16.3.6.3-2 Receipt Date: 06/17/2024 Question:
In followup to A-16.3.6.3-1 response.
- 1. Why is LCO Subsection 3.6.3, "Containment Closure" needed to ensure an inadvertent loss of reactor coolant inventory is precluded in MODE 3 below 345 F with the module being passively cooled?
- 2. What scenarios of module coolant inventory loss to the reactor building are of concern and must be prevented by LCO 3.6.3? Should the relevant section(s) of FSAR Chapter 15 be included in the References section of B 3.6.3? Should the discussion in Section 3.3.17 of TR-101310-NP Draft Revision 1 describe the specific scenario that LCO 3.6.3 is established to prevent?
- 3. The second paragraph of the Background section of Subsection B 3.6.3 indicates that LCO 3.7.1, "Main Steam Isolation Valves (MSIVs)," and LCO 3.7.2, "Feedwater solation" are relied upon to maintain reactor coolant mass in support of decay heat removal. Please explain how these LCOs help maintain the reactor coolant mass.
- 4. To confirm that STSB reviewers understanding of the operation of passive cooling is correct, please comment on the following description of the operation of passive cooling with corrections to any inaccuracies.
In MODE 3 and passively cooled, with reactor coolant hot temperature at or above 345 F, passive cooling may be in operation using either the DHRS with reactor coolant pressure boundary intact, or at least one open RVV and at least one open RRV, which opens the reactor vessel to the CNV, in which case all containment penetrations connecting the CNV to the reactor building must be closed by LCO 3.6.3.
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Below 345 F, passive cooling may be in operation using one or both RVVs open and one or both RRVs open, which has a limited cooldown rate, and/or establishing CNV water level >
45 ft, which provides a higher rate of cooldown until the reactor vessel (RV) and CNV reach reactor building pressure with CNV and RV water temperature below 200 F. The CNV must be isolated as required by LCO 3.6.3 until this state is reached to ensure CNV water level stays >
45 ft and reactor coolant inventory is sufficient to keep the core covered---collapsed riser level is above the top of the core. SR 3.6.2.1 would apply to the automatic containment isolation valves used to isolate containment with coolant temperature 200 F. With coolant temperature <
200 F, the containment penetrations may be isolated manually using these automatic valves or by manual valves, or other devices, such as a blind flange, which is what LCO 3.6.3 requires.
Response
Item 1.
The containment vessel (CNV) may be opened with the Nuclear Power Module (NPM) in MODE 3 and PASSIVELY COOLED. In this configuration the CNV is susceptible to the same inventory loss concerns associated with CNV closure during MODE 4. The applicability of Generic Technical Specification (GTS) 3.6.3 has been extended to MODE 3 and PASSIVELY COOLED to ensure controls are in place to maintain water inventory in the containment during an extended loss of alternating current (AC) power.
Item 2.
MODE 3 and PASSIVELY COOLED, and MODE 4 with the upper module assembly seated on the lower containment vessel flange, require module liquid inventory to maintain core coverage and transfer decay heat from the reactor fuel to the ultimate heat sink. Containment closure ensures the inventory will remain available to perform this function during an extended loss of AC power or during delays in the transfer of the module between the operating location and the containment closure tool.
The scenarios addressed by GTS 3.6.3 are not discussed in Standard Design Approval Application Chapter 15. Generic Technical Specification 3.6.3 ensures containment closure initial conditions for the extended loss of AC power analysis discussed in Standard Design Approval Application Section 9.2.5.
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NuScale revises Section 3.3.17 of TR-101310-NP to include applicability of MODE 3 and passively cooled and to provide additional discussion of the scenarios that GTS 3.6.3 is established to address.
Item 3.
Main steam and feedwater isolation valves maintain reactor coolant mass by isolating the steam generators and limiting release of reactor coolant in the event of steam generator tube failure.
Item 4.
The following information is provided to aid the NRC staffs understanding of the relationship of passive cooling to GTS 3.6.1 and GTS 3.6.3.
Generic Technical Specification LCO 3.6.1 requires Containment Integrity when a module is in MODE 3 with the module not PASSIVELY COOLED. In this condition the emergency core cooling system and decay heat removal system are required to be OPERABLE and could automatically actuate, resulting in a pressure and temperature transient into containment.
Generic Technical Specification LCO 3.6.3 requires Containment Closure when a module is in MODE 3, and PASSIVELY COOLED. Although reactor coolant temperature might be above 345 degrees Fahrenheit when passive cooling is established, the module will cool down and stabilize at a lower temperature and pressure as a result of aligning the heat sink.
The requirement for containment closure versus containment integrity is appropriate because the impact of any engineered safety features actuation is less when starting from a lower energy state. The operational flexibility to use administrative controls to allow access into the containment during this condition allows for activities, such as withdrawal of the in-core detector instrumentation, with the module located in its operating bay. The ability to close containment manways and continue the containment and reactor pressure vessel disassembly sequence without delaying to perform full leak testing of a manway lessens the time required to perform reactor pressure vessel disassembly. The administrative controls allowed by GTS 3.6.3 are expected to be of short duration and include requirements for the module to be cooled down to a steady state reactor coolant system temperature low enough to allow safe personnel access.
A module may be in MODE 3 under two different circumstances, when reactor coolant temperature is above 345 degrees Fahrenheit with PASSIVE COOLING established, and when all reactor coolant temperatures are less than 345 degrees Fahrenheit, regardless of the status NuScale Nonproprietary NuScale Nonproprietary
of PASSIVE COOLING. An example of the first condition would be actuation of at least one train of emergency core cooling or one train of decay heat removal from full power with the reactor coolant system still at normal operating temperature. This would be a transient condition as the module will soon cool to less than 345 degrees Fahrenheit. An important nuance of the second condition is that surveillance testing of the decay heat removal and emergency core cooling systems may be ongoing to establish their operability while operating in MODE 3 with all reactor coolant temperatures less than 345 degrees Fahrenheit.
The description of the operation of passive cooling provided by the NRC staff has been revised as shown in the markup below.
In MODE 3 and passively cooled, with reactor coolant hot temperature at or above 345 F, passive cooling may be in operationsatisfied using either theby at least one train of DHRS in operation with the reactor coolant pressure boundary intact, or at least one open RVV and at least one open RRV, which opens the reactor vessel to the CNV, in which case all containment penetrations connecting the CNV to the reactor building must be closed by LCO 3.6.3. When in this condition, the administrative control provision of LCO 3.6.3 prevents opening of containment penetrations. When ECCS or DHRS are aligned, reactor coolant temperature decreases to less than 345 F.
In MODE 3 belowBelow 345 F, passive cooling may be in operation using one or both RVVs open and one or both RRVs open (i.e., at least one train of ECCS in service),
which has a limited cooldown rate, one or more trains of DHRS in service, and/or establishing CNV water level > 45 ft, which provides a higher rate of cooldown until the reactor vessel (RV) and CNV reach reactor building pressure with CNV and RV water temperature below 200 F. The CNV must be isolated as required by LCO 3.6.3 until this state is reached while cooling down or when the reactor coolant has reached a lower steady state temperature to ensure CNV water level stays > 45 ft and reactor coolant inventory is sufficient to keep the core covered---collapsed riser level is above the top of the core. SR 3.6.2.1 would apply to the automatic containment isolation valves used to isolate containment with coolant temperature 200 F. With coolant temperature <
200 F, the containment penetrations may be isolated manually using these automatic valves, or by manual valves, or other devices, such as a blind flange, which is what in accordance with LCO 3.6.3 requires.
Markups of the affected changes, as described in the response, are provided below:
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US460 Standard Design Approval Technical Specifications Development TR-101310-NP Draft Revision 1
© Copyright 2024 by NuScale Power, LLC 14 Audit Question A-16-5 3.3.15 Modification of Limiting Condition offor Operation 3.5.3, Ultimate Heat Sink The ultimate heat sink (UHS) is redesigned consistent with the other changes to the plant design and analyses, primarily the increased RTP and a reduction in the number of reactors in the design to a maximum of six modules. The redesign resulted in reanalysis and redefinition of the UHS and caused changes in the credited functions of the UHS in LCO 3.5.3.
The UHS water level requirements are specified to a new band defined by upper and lower limits that improve containment heat removal behavior. The redesigned UHS and its functions are described in FSAR Section 9.2.5. The new limits are consistent with the safety analyses in the FSAR that credit the UHS function. Similarly, the maximum bulk average pool temperature is increased to align with the safety analyses assumptions. The structure of the Actions in LCO 3.5.3 are changed to reflect the removal of distinct limits that the DCA credited for separate safety functions. This change removed the need for Condition B of the DCA technical specifications, which is now addressed by Condition A. Completion Times remain consistent with the credited functions of the UHS. Subsequent Conditions are renumbered. Corresponding changes are made to the Bases.
Audit Question A-16-5 3.3.16 Addition of Limiting Condition offor Operation 3.5.4, Emergency Core Cooling System Supplemental Boron Audit Question A-16-5 The US460 design adds a passive system that provides soluble boron in dissolvers mounted inside the containment. The dissolvers provide a reservoir of boron that mixes with condensate from the upper inner surfaces of the containment vessel when the ECCS is actuated. Limiting Condition offor Operation 3.5.4 is added to ensure that the quantity of boron available for dissolution when the ECCS actuates conforms to the assumptions in the safety analyses. The boron ensures the reactor remains subcritical after certain events in combination with limiting conditions, and subsequent cooldown of the reactor system. The quantity of boron required is specified in the COLR. The ESB satisfies Criterion 3 of 10 CFR 50.36(c)(2)(ii).
Audit Question A-16-5 3.3.17 Addition of Limiting Condition offor Operation 3.6.3, Containment Closure Audit Question A-16.3.6.3-1, Audit Question A-16.3.6.3-2, Audit Question A-16-5 Limiting Condition offor Operation 3.6.3 is added to ensure that module inventory is preserved during movement of the module between the operating location and the containment closure tool. The LCO requires a module that is in MODE 3 and passively cooled, or in MODE 4, with the upper module assembly seated on the lower containment vessel flange, be maintained closed. The LCO and allowances are patterned on portions of NUREG-1431, LCO 3.9.4 with extensive modifications to align with the NuScale application. Maintaining containment closure ensures that the decay heat removal mechanism required to assure core cooling is maintained during periods when the module is isolated from other systems such as CVCSMODE 3 and
US460 Standard Design Approval Technical Specifications Development TR-101310-NP Draft Revision 1
© Copyright 2024 by NuScale Power, LLC 15 PASSIVELY COOLED, and MODE 4 with the upper module assembly seated on the lower containment vessel flange, require module liquid inventory to maintain core coverage and transfer decay heat from the reactor fuel to the ultimate heat sink.
Containment closure ensures the inventory will remain available to perform this function during an extended loss of alternating current power or during delays in the transfer of the module between the operating location and the containment closure tool. Containment closure must be maintained until the containment is disassembled and the reactor vessel is thermally connected to, or when the containment is disassembled from the UHS via the de-energized ECCS valves. Limiting Condition offor Operation 3.6.3 satisfies Criterion 3 of 10 CFR 50.36(c)(2)(ii).
Audit Question A-16-5 3.3.18 Removal of Limiting Condition offor Operation 3.7.3, In-Containment Secondary Piping Leakage Audit Question A-16-5 Limiting Condition offor Operation 3.7.3 is deleted as no longer necessary because the break exclusion design criteria is applied to the secondary system piping within the containment. The DCA design for secondary system piping met the leak-before-break design criteria of General Design Criteria 4.
US460 Standard Design Approval FSAR Section 3.6 describes the application of design measures to prevent or mitigate postulated dynamic effects associated with postulated rupture of US460 piping. The US460 SDA design of secondary piping inside the containment meets the criteria for exclusion from postulated breaks and cracks provided in NRC Branch Technical Position (BTP) 3-4. Based on this change the US600 Design Certification Application LCO is no longer needed because the piping is excluded from consideration of postulated breaks and cracks.
3.3.19 Other Bases Changes In addition to the specific changes described above, Applicable Safety Analyses sections are modified to reflect changes to the safety analyses, primarily as a result of the increased reactor power. Other changes are made in response to operational analysis feedback to clarify and ease understanding of the requirements.
3.4 Chapter 4, Design Features Section 4.3 Fuel Storage The fuel storage design description is modified to reflect changes to the design and analyses. Key variables are bracketed to allow replacement with actual plant-specific values when design details are finalized by a future applicant that references the NuScale power plant US460 standard design. NuScale is monitoring industry efforts to relocate fuel storage detailed requirements to a COLR-like document and anticipates adopting this practice when the concept matures.