ML24215A119
| ML24215A119 | |
| 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: ML24215A119 (1) | |
Text
Response to SDAA Audit Question Question Number: A-5.4.1.2-5 Receipt Date: 09/15/2023 Question:
Section 5.4.1.2 in the SDAA states, Proven chemical or mechanical cleaning methods and techniques in use in the existing pressurized water reactor (PWR) fleet inform the buildup removal. It is unclear what is meant by inform the buildup removal.
Please discuss how existing PWR chemical and mechanical cleaning techniques will inform the buildup removal for the NuScale US460 SG tubes.
Response
During the US600 standard design Design Certification Approval (DCA), NuScale conducted a review of standard cleaning methods for the secondary side of steam generators in the existing fleet and evaluated their applicability to the NuScale design.
Because the physical configuration of the NuScale steam generator (SG) is significantly different from existing fleet pressurized water reactors (PWRs), the water lance mechanical cleaning routinely done at those PWRs is ineffective for the NuScale design. Required periodic chemical cleaning is expected in order to remove corrosion product deposits from the interior of the SG tubes. The specifics of the chemical cleaning techniques require finalization, but the design of the SGs is highly compatible with chemical cleaning.
Existing plants must clean their SGs in-place, requiring chemical cleaning injection ports on high pressure feedwater lines; the NuScale feedwater and main steam plenums will be directly accessible during refueling operations, allowing for direct injection, circulation, and removal of cleaning solution. Additionally, the materials of construction of the NuScale SG are the same as NuScale Nonproprietary NuScale Nonproprietary
those in the existing fleet, which have demonstrated good material compatibility with currently-available cleaning agents over the course of many successful chemical cleanings.
The attached markup to Standard Design Approval Application (SDAA) Section 5.4.1.2 removes the sentence quoted in the audit question to remove confusion.
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 5.4-3 Draft Revision 2 helical tubes are seamless with no intermediate welds. The helical tubes terminate at the feed and steam plenum tubesheets, where the tubes are secured to the tubesheet by expansion and are welded to the tubesheet on the secondary side. Crevices are minimized among the SG tubes, the tube supports, and tubesheets to limit the buildup of corrosion products. There are minimal quantities of corrosion products because the SG tube-to-tubesheet contact is within the primary coolant environment. Expansion of the tube within the tubesheet bore minimizes crevices depths and mitigates exposure of the low alloy steel tubesheet to corrosion products. Expansion of each tube is completed at both the steam and feed plenum tubesheets.
The SG has no secondary side crevices that could concentrate corrosion products or impurities accumulated during the steam generation process. In the once-through SG design there is no bulk reservoir of water at the inlet plena where the accumulation or concentration of corrosion products could occur. There is no SG blowdown to remove deposits in the once-through SG design based on the geometry of the design and flow characteristics that do not allow accumulation of corrosion products within a fluid reservoir. Therefore, a blowdown system would only serve to divert FW flow from the SG and would not remove corrosion products or impurities. Based on these factors, there is no SG blowdown system included in the NPM design.
Audit Question A-5.4.1.2-5 Secondary coolant impurities and corrosion products may deposit directly on the interior tube surfaces as a scale or film, or be removed from the SG tubes by carryover. The concentration of corrosion products and impurities is low based on selection of materials for the condensate system and chemistry control requirements. Periodic cleaning performed during outage periods removes buildup of corrosion product films on the secondary surfaces of the SG tubes.
Proven chemical or mechanical cleaning methods and techniques in use in the existing pressurized water reactor (PWR) fleet inform the buildup removal.
Secondary side SG surfaces are corrosion resistant, either nickel alloy, stainless steel, or stainless steel clad, which removes the concern for degradation of SG components by cleaning solutions. Connecting an appropriate system directly to the MS and FW disconnect flanges during an outage accomplishes cleaning of the SG tubes.
Heated primary coolant from the reactor core exits the riser and flows down the outer annulus across the SG tubes where heat is transferred to secondary coolant inside the SG tubes. Small flow paths in the upper and lower risers permit a small amount of reactor coolant to bypass the top of the riser and flow into the SG tube bundle region. These flow paths ensure sufficient boron mixing in the reactor coolant during DHRS-driven conditions where the riser is not submerged following non-loss-of-coolant accident (LOCA) transients. The primary coolant continues to flow down through the annular downcomer below the SG tubes into the lower reactor vessel plenum, where it reenters the reactor core. Further discussion of the RCS is in Section 5.1, RCS and Connecting Systems, and the RCS loop flow is in Figure 5.1-3.