ML24326A125
| ML24326A125 | |
| 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: ML24326A125 (1) | |
Text
Response to SDAA Audit Question Question Number: A-16.3.5.1-5 Receipt Date: 04/29/2024 Question:
- 1. SDAA Part 4 Subsection B 3.5.1 Background section, third paragraph, third and fourth sentences state:
The ECCS is also actuated by the MPS [eight] hours after any reactor trip signal to ensure the supplementary boron function described in Specification 3.5.4 occurs. The [eight] hour actuation can be manually bypassed when subcriticality at cold conditions is assured.
According to the improved TS writers guide, when stating time intervals, it is customary to use digits (i.e., 0, 1, 2,, 9) for the number of units of time being stated. Please replace [eight]
with [8]; also bracketing 8 implies an expectation that an applicant for a combined license or an operating license will propose a different site-specific time interval. If that is not the case, please remove the brackets around the time interval for ECCS actuation of 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> after a reactor trip signal; from these two instances, and also from all other such instances throughout the generic TS and Bases.
Since the title of LCO 3.5.4 is ECCS Supplemental Boron, for consistency, consider changing supplementary to supplemental in the third sentence.
- 2. SDAA Part 4 Subsection B 3.5.1 Background section, fourth paragraph states:
The ECCS actuation solenoid trip valves are supplied battery power to prevent inadvertent actuation in certain loss of power events. If an ECCS actuation signal occurs during one of these events, the solenoid trip valves will be deenergized resulting in immediate RVV opening and RRV opening when the mechanical pressure interlock clears. Although not credited in NuScale Nonproprietary NuScale Nonproprietary
the safety analyses, the electrical power to the solenoid trip valves will be interrupted after 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> if electrical power is still being supplied to the solenoids from the batteries. This initiates ECCS valve actuations with the RRVs opening when the mechanical interlock clears.
Staff understands that the mechanical pressure interlock is the inadvertent actuation block (IAB) RPV CNV differential pressure mechanical interlock of the ECCS reactor recirculation valves (RRVs). The applicant is requested to (a) consider using the phrase inadvertent actuation block (IAB) RPV CNV differential pressure mechanical interlock in the second sentence, then use the same abbreviated phrase (e.g., IAB interlock) when referring to this feature of the RRVs in the remainder of Subsection B 3.5.1; also consider doing this in all references to the RRV IAB interlock in other Bases subsections, and (b) insert a comma after immediate RVV opening in second sentence to ensure the phrase IAB mechanical interlock clears only applies to the RRVs.
Response
Item 1.
NuScale revises the US460 Standard Design Approval Application (SDAA), Part 4, Section B 3.5.1 to reflect the time interval in digits per the improved technical specification writers guide.
Additionally, since the title of US460 SDAA LCO 3.5.4 is ECCS Supplemental Boron, for consistency, NuScale revises the word supplementary to supplemental in the third sentence.
Markups of the affected changes, as described in the response, are provided below.
Item 2.
NuScale revises the US460 SDAA, Section B 3.5.1 to reflect the phrase inadvertent actuation block (IAB) in the second sentence and the acronym IAB in the remainder the section.
Markups of the affected changes, as described in the response, are provided below:
NuScale Nonproprietary NuScale Nonproprietary
ECCS B 3.5.1 NuScale US460 B 3.5.1-1 Draft Revision 2 B 3.5 PASSIVE CORE COOLING SYSTEMS (PCCS)
B 3.5.1 Emergency Core Cooling System (ECCS)
BASES BACKGROUND The ECCS provides decay heat removal for a postulated steam generator tube failure event or Loss of Coolant Accident (LOCA) event that exceeds the makeup capacity of the Chemical and Volume Control System (CVCS). The ECCS is designed to bring the reactor coolant system (RCS) to a low temperature and low pressure safe shutdown condition. ECCS actuation also results in condensate flow causing dissolution of the supplemental boron dissolver contents into the containment inventory as described in Specification 3.5.4.
The ECCS consists of two reactor vent valves (RVVs) located on the reactor head, two reactor recirculation valves (RRVs) located above the reactor flange, and associated controls and instrumentation. The RVVs are connected to the vapor space of the pressurizer region of the reactor vessel. The RRVs penetrate the reactor vessel above the top of the reactor core and open into the downcomer region of the reactor vessel. The ECCS valves form a portion of the reactor coolant pressure boundary.
ECCS actuation occurs when the Module Protection System (MPS) de-energizes solenoid trip valves in the pilot controls of the RVVs and RRVs. MPS is designed to actuate the ECCS on low reactor pressure vessel level. The ECCS is also actuated by the MPS [8eight] hours after any reactor trip signal to ensure the supplementalry boron function described in Specification 3.5.4 occurs. The [8eight] hour actuation can be manually bypassed when subcriticality at cold conditions is assured.
The ECCS actuation solenoid trip valves are supplied battery power to prevent inadvertent actuation in certain loss of power events. If an ECCS actuation signal occurs during one of these events, the solenoid trip valves will be deenergized resulting in immediate RVV opening, and RRV opening when the inadvertent actuation block (IAB)mechanical pressure interlock clears. Although not credited in the safety analyses, the electrical power to the solenoid trip valves will be interrupted after 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> if electrical power is still being supplied to the solenoids from the batteries. This initiates ECCS valve actuations with the RRVs opening when the mechanical interlock clears.
ECCS B 3.5.1 NuScale US460 B 3.5.1-2 Draft Revision 2 BASES BACKGROUND (continued)
The ECCS mechanical pilot valve design will result in the ECCS valves opening if the RCS and containment pressures approach the same values, regardless of the ECCS actuation signal status. This behavior is a function of the design of the valve actuators. In addition to the solenoid trip valve actuation, the ECCS RRVs are hydraulically interlocked in the closed position until the differential pressure between the RCS and containment vessel is reduced by flow through the RVVs or from a postulated break. Even with an open signal present the RRVs do not actuate open until the differential pressure has fallen to the credited differential pressure. The differential pressure interlock, also referred to as the inadvertent actuation block (IAB),
delays the opening of the RRVs and slows the initial mass and energy discharge into the containment volume.
ECCS actuation and functions, including the RRV IAB feature, do not require electrical power. The solenoid trip valves are designed to actuate upon loss of electrical power. The IAB feature is mechanical and does not require external power, depending only on the pressure sources of the reactor vessel and of the containment environment to function. No operator action is required to establish and maintain long term core cooling after the system is actuated.
RCS vapor is vented from the pressurizer space through the RVVs into the containment vessel when the RVVs are opened. This steam condenses on the inner walls of the containment vessel and flows toward the bottom of the vessel where it accumulates with any other leakage that is in the containment vessel from a postulated break.
Some of the condensate flow is passively routed through the supplemental boron dissolvers and some of the condensate flow is passively routed into the containment mixing tubes.
The RRVs open after a delay due to their mechanical pressureIAB interlock following the RVVs to provide a flow path for this condensate and dissolved supplemental boron from the containment vessel to flow back into the reactor vessel. The design of the reactor and containment vessel geometries and the total RCS liquid volume is such that upon ECCS actuation, liquid levels in both the reactor and containment vessel will stabilize above the top of the core. The containment water level will be higher than the RCS level, providing the driving force for natural circulation flow of cooler RCS water containing additional dissolved boron in containment back into the reactor vessel. This natural circulation flow will maintain core submersion and cooling, and will provide additional boration ensuring the reactor core remains subcritical during limiting design basis events.
ECCS B 3.5.1 NuScale US460 B 3.5.1-3 Draft Revision 2 BASES BACKGROUND (continued)
Heat is transferred to the containment vessel walls by steam condensation on the containment interior, and then removed from the containment vessel by heat conduction through the containment vessel wall. In addition to mass transfer by steam flow, heat is removed by conduction through the reactor vessel walls during ECCS operation because the lower portions of the reactor vessel walls are submerged and wetted by coolant on both sides. Heat is removed from the containment wall through contact with the reactor pool, which acts as the ultimate heat sink (UHS).
The ECCS valves are sized to ensure that sufficient pressure equalization exists to support core cooling when at least one RVV and at least one RRV have opened.
In MODES 1, 2, and 3 the ECCS actuates when the reactor vessel riser level is low indicating a loss of RCS inventory. This actuation occurs at about 470 inches above the bottom of the module assembly, and at about 550 inches above the bottom of the module assembly if the RCS cold temperature is above the T-5 interlock (approximately 440 °F). These setpoints ensure that the ECCS actuates before the upper riser holes are uncovered and upon riser uncover, respectively.
Specification 3.3.1 describes the instrumentation and actuation logic for ECCS actuation. In applicable design basis accident scenarios, the actuation setpoints and the RRV IABmechanical pressure interlock operation are sufficient to ensure the core remains cooled and covered.
In MODE 3 the RVVs provide Low Temperature Over-Pressure(LTOP) protection for the RCS as described in LCO 3.4.10.
In MODE 3 in PASSIVE COOLING, the ECCS is either performing its design function to support the transfer of decay heat from the reactor core to the containment vessel or alternative means of removing decay heat have been established and the system is no longer required to be OPERABLE.
In MODE 4 the ECCS is not required because the ECCS valves are open and de-energized, and the unit is being passively cooled, which ensures decay heat removal is being accomplished. Additionally, in MODE 4 during module relocation between the containment tool and the reactor tool, the de-energized and opened RRVs are open between the UHS water inside the containment and the RCS. In MODE 5, core cooling is accomplished by conduction through the RPV wall to the ultimate heat sink until the upper containment and upper