ML24215A118
| ML24215A118 | |
| Person / Time | |
|---|---|
| Site: | 05200050 |
| Issue date: | 08/02/2024 |
| From: | NuScale |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML24215A000 | List:
|
| References | |
| LO-169995 | |
| Download: ML24215A118 (1) | |
Text
Response to SDAA Audit Question Question Number: A-5.4.1.2-4 Receipt Date: 09/15/2023 Question:
Section 5.4 of the SDAA does not appear to discuss stabilizing tubes after plugging. However, Section 5.4.1.2 of the SDAA states, Individual SG tubes may be plugged to prevent adverse interaction with non-plugged tubes. This statement appears to be describing tube plugging as tube stabilizing. The staff notes that plugging removes a tube from service and a plugged tube may or may not be stabilized.
Please discuss whether tubes plugged in the NuScale US460 will be stabilized, if necessary, to prevent adverse interaction with non-plugged tubes.
There is a follow-up question sent on November 17, 2023:
The response to Audit Issue A-5.4.1.2-4 indicates that plugged tubes will be stabilized, if necessary. No changes to the SDAA were proposed.
The staff notes that Acceptance Criterion 1 in DSRS Section 5.4.2.2 (ML15355A535) states, Tubes with unacceptable flaws should also be capable of being stabilized if it is determined that a plugged tube potentially may sever (as a result of continued degradation) and subsequently affect the integrity of an active tube. In addition, Evaluation Finding 2 in DSRS Section 5.4.2.2 states, If unacceptable flaws are identified during the course of an inspection, each tube is accessible and can be removed from service and stabilized, as necessary.
As noted in the Audit Issue A-5.4.1.2-4 question, SDAA Section 5.4.1.2 that states, Individual SG tubes may be plugged to prevent adverse interaction with non-plugged tubes, is incorrectly referring to plugging a tube and should have referred to stabilizing a plugged tube. Therefore, the SDAA does not clearly address the ability to stabilize plugged tubes if necessary.
NuScale Nonproprietary NuScale Nonproprietary
The staff notes that DCA Section 5.4.1.2 states, Individual SG tubes may be plugged, and if necessary, stabilized to prevent adverse interaction with non-plugged tubes. Therefore, it is unclear why this statement was changed for the SDAA.
So that the staff may make a finding under DSRS Section 5.4.2.2 regarding the capability to stabilize tubes, please add or revise the SDAA to clearly state, as in the audit issue response, that plugged tubes will be stabilized, if necessary.
Response
Steam generator (SG) tubes that are plugged while in service may require the use of stabilizers in the NuScale US460 design. The evaluation of the need for SG tube stabilization occurs at the time of tube plugging by the licensee and is the licensees responsibility. Technical specification 5.5.4 in Part 4 of the Standard Design Approval Application (SDAA) outline requirements for the Steam Generator Program, which forms the basis for a licensees course of action for SG tube stabilization.
The attached markup to Section 5.4.1.2 of SDAA addresses SG tube stabilization.
Markups of the affected changes, as described in the response, are provided below:
NuScale Nonproprietary NuScale Nonproprietary
NuScale Final Safety Analysis Report Reactor Coolant System Component and Subsystem Design NuScale US460 SDAA 5.4-5 Draft Revision 2 Audit Question A-5.4.1.2-4 The SG design permits periodic inspection and testing of critical areas and features to assess their structural and pressure boundary integrity when the NPM is disassembled for refueling as shown in Figure 5.4-3. The internal surface of SG tubes is accessible over their entire length for application of nondestructive examination methods and techniques that are capable of finding the types of degradation that may occur over the life of the tubes. Individual SG tubes may be plugged and, if necessary, stabilized to prevent adverse interaction with non-plugged tubes. Access to the internal (secondary) sides of tubesheets affords opportunity for inspection, and for removal of foreign objects. Figure 5.4-3 and Figure 5.4-4 contain illustrations of the steam and feed plena inspection ports.
Classifications and Quality Group designations for design, fabrication, construction and testing of SGS components that form part of the RCPB are in Table 3.2-2. Chapter 3 provides detailed information regarding the design basis and qualification of structures, systems, and components based on these classifications and designations. Figure 6.6-1 shows the ASME BPVC Section III, Class 1 and 2 boundaries for the SGS.
Steam Generator Tube Supports and Steam Generator Supports The seamless helical coil SG tubing is supported by a series of austenitic stainless steel tube support assemblies. The geometric design and materials utilized facilitate fluid flow while minimizing the potential for the generation of corrosive products and buildup. The material and geometry choice precludes two of the most significant historical contributors to tube degradation by the tube supports.
The tube support assemblies are between each column of helical tubes as shown in Figure 5.4-6. The tube support structure is within the primary coolant environment; therefore, no ingress path exists for general corrosion products from the secondary system to deposit on the primary side of the SG. Optimization of the circumferential spacing of the tube supports provides the minimum possible tube free span lengths while still accommodating the transition of the tubes to the steam and FW tube sheets.
Audit Question A-5.4-1 The SG supports and SG tube supports provide support for vibration and seismic loads. As shown in Figure 5.4-5, the SG tube support assemblies attach to upper SG supports welded to the PZRintegral steam baffle plate and inner surface of the RPV, and also interface with lower SG supports welded to the inner surface of the RPV. The use of eight sets of tube support assemblies limit the unsupported tube lengths, which ensures the SG tubes do not experience unacceptable flow-induced vibration (FIV).
As shown in Figure 5.4-5, the lower SG supports permit thermal growth and provide lateral support of the tube supports.