ML24346A325
| ML24346A325 | |
| 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: ML24346A325 (1) | |
Text
Response to SDAA Audit Question Question Number: A-19.1-65 Receipt Date: 05/06/2024 Question:
Low Temperature Over Pressure (LTOP) events were screened from the LPSD PRA. To prevent LTOP, as discussed in SDA FSAR Section Chapter 5.2.2, there is a Technical Specification (TS) for an automatic isolation of the CVCS injection line on high pressurizer water level to preclude RCS inventory water solid conditions. The ECCS reactor vent valves provide overpressure protection during low-temperature conditions as depicted in SDA Table 5.2-
- 5. These setpoints are significantly different from the DCA. For example, in the SDA, the LTOP setpoint at 175F is 1750 psia compared to 380 psia in the DCA. LCO 3.4.10, Low Temperature Overpressure Protection (LTOP) Valve, states that each closed reactor vent valve (RVV) shall be OPERABLE in Mode 3. The staff understands when LTOP is required, the CNV is being flooded in preparation for the module to be moved and therefore, the free volume in the CNV is decreasing. The staff is concerned about the CNV integrity in the event of LTOP actuation.
During an audit call on 04/30/24, NuScale explained that the NPM will never be operated or refueled with the CNV completely filled with water (i.e., water solid) and therefore, the CNV will always have open volume. This will ensure that the staffs concern is addressed. However, the staff cannot locate information and associated controls to prevent a CNV completely filled with water in the FSAR. The discussion in Chapter 6.2.1.1.2 focuses on precluding reduced inventory operation and the staff is unable to extend the discussion, as currently written, to address the LTOP concern. Therefore, NuScale is requested to:
1.
Identify the FSAR location other than Chapter 6.2.1.1.2 that specify that the NPM will never be operated or refueled with the CNV completely filled with water and identify the associated controls OR 2.
Provide documents in the eRR that include this information and provide FSAR markups (e.g., amendment to the discussion in Chapter 6.2.1.1.2) to specify that the NPM will never be operated or refueled with the CNV completely filled with water including identifying the associated controls NuScale Nonproprietary NuScale Nonproprietary
Response
The maximum fill level of the containment vessel (CNV) is ((2(a),(c) per Table 6-3, CNV Design Data, in SD-112568, Containment System Design Description, Revision 1, which is available in the SDAA Audit Section 19.1-19.3 electronic reading room. Overfilling of the CNV is prevented in several ways:
The containment flooding process is automatic, and stops when containment water level reaches the pre-programmed level. Once the pre-programmed level is reached, both containment flooding and drain system (CFDS) pumps stop and the CFDS interface valve closes. Automatic control of CFDS operations in flooding mode prevents the CFDS pumps from overfilling the CNV.
There is no operational reason to fill the CNV above (( }}2(a),(c) As described in Section 6.2.1.1.2 of the Final Safety Analysis Report (FSAR), the NuScale Power Module is in a partially flooded condition during movement for refueling. Operators monitor module protection system parameters (e.g., containment water level) in the control room to confirm CNV water level prior to transport.
As stated in Section 2.3 of SD-112568, structures, systems, and components located below the 55 ft elevation are designed for submergence, but those above that elevation have no such requirement. Filling the CNV above (( }}2(a),(c) risks damaging equipment in containment. For these reasons, the Probabilistic Risk Assessment (( }}2(a),(c) In response to the clarification call with the NRC and NuScale held on 06/26/2024, NuScale has updated FSAR Section 9.3.7, Containment Flooding and Drain System, to describe the automatic shutoff function of the CFDS during flooding operations. NuScale has updated FSAR Section 19.1.6.1.2, Low Power and Shutdown Initiating Events, to describe how the automatic shutoff function of the CFDS during flooding operations ensures CNV integrity in the event of a low temperature overpressure protection actuation. Markups of the affected changes, as described in the response, are provided below: NuScale Nonproprietary NuScale Nonproprietary
NuScale Final Safety Analysis Report Containment Flooding and Drain System NuScale US460 SDAA 9.3-52 Draft Revision 2 with administrative controls in plant procedures, prevent inadvertent CFDS makeup to an operating NPM. The CFDS pump discharges water removed from the CNV during draining to the containment drain separator tank, which allows entrained gases to be vented before the water is returned to the reactor pool. The drain separator tank vents gases to the RBVS exhaust system through a radiation monitor and a high-efficiency particulate air filter. The CFDS suction line includes an anti-siphon hole above the minimum level for a fuel handling accident listed in Table 9.2.5-1, to prevent siphoning of the ultimate heat sink below this level. 9.3.7.2.1 System Operation Containment Draining following Refueling The CFDS drains a CNV following refueling. To facilitate CNV draining, the service air system pressurizes the CNV to provide additional net positive suction head for the CFDS pump. Containment Flooding in Preparation for Refueling To flood a CNV, an operator aligns a CFDS pump to take suction from the reactor pool and discharge to the selected NPM. To minimize thermal stress on NPM components, an operator initiates flooding only after reactor pool bulk temperature is above a specified minimum temperature. If an off-normal condition requires flooding a CNV with elevated reactor pressure vessel temperatures, personnel connect a temporary skid-mounted heater to the CFDS flow path using provided connections. Audit Question A-19.1-65 During normal flooding operation, when the water level in containment reaches the target level, the running CFDS pump stops automatically and the applicable CFDS interface valve closes automatically, ensuring containment does not overfill and an open volume is maintained in the CNV. Addition of Coolant Inventory into a Containment Vessel during a Beyond-Design-Basis Event During a beyond-design-basis event, the CFDS can inject water from the reactor pool into an NPM. When not in operation, the CFDS pump suction lines are normally full and vented to facilitate readiness for emergency containment flooding operations. Before initiating emergency containment flooding, CNV pressure must be low enough for the CFDS pumps to inject into the CNV.
NuScale Final Safety Analysis Report Probabilistic Risk Assessment NuScale US460 SDAA 19.1-75 Draft Revision 2 as not applicable to the POS. For example, during POS1, POS6, and POS7, the configuration of the module is similar to normal operation, and initiating events considered for full power are applicable to LPSD. By contrast, most at-power initiating events can be screened for the remaining plant operating states. The flooded CNV allows LOCA inside containment events to be screened. In POS4, the module is disassembled and the core is open to the reactor pool, which passively provides both decay heat removal and inventory control; thus, all internal initiating events are screened. In POS3, the module is completely disconnected and unaffected by any at-power initiating events. Coolant recirculation by the CVCS is in place for portions of POS2 and POS5 when the ECCS valves are closed; however, CVCS line break outside and inside containment are modeled for the full duration of the POS in accordance with the convention of assuming constant plant conditions in a POS. Table 19.1-48 summarizes the full power initiating events and their applicability during LPSD. Potential initiating events that are unique to the LPSD mode relate to low temperature overpressurization, module drop, mechanical damage during fuel movement, and alternate system alignments. Audit Question A-19.1-65 To maintain the RCS integrity safety function during cold RCS conditions, the low temperature overpressure protection (LTOP) function is enabled when the RCS temperature is below the LTOP enable temperature. Because a gas or steam bubble is maintained in the PZR, the PZR is not water-solid and a pressurization transient must first fill the PZR before the RPV would experience brittle fracture. Pressurization could be caused by an uncontrolled coolant injection (e.g., inadvertent CVCS injection) or heat addition (e.g., inadvertent pressurizer heater actuation) at low RCS temperature. Low temperature overpressure events are screened based on the number of failures that must occur in order for a pressurization event to occur, the short time period in which the pressurization event could occur when LTOP is enabled, the amount of time that the pressurization event must continue before the RPV is challenged, the presence of automatic MPS signals and alarms, and the high reliability of the RVVs to open on demand. As discussed in Section 9.3.7, Containment Flooding and Drain System, automatic CFDS flooding operation ensures an open volume in the CNV, and therefore ensures CNV integrity in the event of an LTOP actuation. The NPM transport is unique to the NuScale refueling process, as discussed in Section 9.1. Accordingly, the initiator IE-RBC---DROP associated with failure of the RBC is included in the LPSD risk assessment. This initiator is applicable to POS3. Table 19.1-50 provides the frequency for IE-RBC---DROP, which is}}