ML25036A114

LLC - Submittal of Density Wave Oscillation Safety Case

ML25036A114
Person / Time
Site:	05200050
Issue date:	02/05/2025
From:	Griffith T NuScale
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
LO-178898
Download: ML25036A114 (1)

Text

LO-178898 NuScale Power, LLC 1100 NE Circle Blvd., Suite 200 Corvallis, Oregon 97330 Office 541.360.0500 Fax 541.207.3928 www.nuscalepower.com February 05, 2025 Docket No.52-050 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk One White Flint North 11555 Rockville Pike Rockville, MD 20852-2738

SUBJECT:

NuScale Power, LLC Submittal of Density Wave Oscillation Safety Case The purpose of this letter is to docket the Density Wave Oscillation Safety Case requested by the NRC in support of the Standard Design Approval Application review.

This letter makes no regulatory commitments and no revisions to any existing regulatory commitments.

If you have any questions, please contact Wren Fowler at 541-452-7183 or at sfowler@nuscalepower.com.

Sincerely, Thomas Griffith Manager, Licensing NuScale Power, LLC Distribution:

Mahmoud Jardaneh, Chief, New Reactor Licensing Branch, NRC Getachew Tesfaye, Senior Project Manager, NRC Dennis Galvin, Project Manager, NRC David Drucker, Senior Project Manager, NRC : Density Wave Oscillation Safety Case, Nonproprietary

LO-178898 NuScale Power, LLC 1100 NE Circle Blvd., Suite 200 Corvallis, Oregon 97330 Office 541.360.0500 Fax 541.207.3928 www.nuscalepower.com Density Wave Oscillation Safety Case, Nonproprietary

NuScale Non-Proprietary Page 1 of 8 Density Wave Oscillation Safety Case Executive Summary Three pillars comprise the safety case for density wave oscillation (DWO) in the (NPM): analyses, real-time monitoring and control of operating conditions, and physical inspections. The pillars establish a holistic approach to providing reasonable assurance of adequate protection for the public by addressing the steam generator (SG) lifecycle of design, operation, and inspection. This holistic approach is not reliant on any individual pillar (e.g., analyses), thereby providing a defense-in-depth approach to reasonable assurance of adequate protection for the public.

DWO is a thermal-hydraulic instability that imparts mechanical and thermal loads on the SG. DWO loads are analyzed to ensure structural integrity is maintained because a DWO transient in the SG is specified as an American Society of Mechanical Engineers (ASME) Service Level A event. The upper limit of time spent in a region where DWO may occur is established by analysis and monitored by the plant staff to ensure components are maintained within design limits. Evaluating the SG for DWO transient time follows the same principles used in the evaluation of other ASME components (e.g., the reactor pressure vessel) for transients (e.g., reactor heatup to hot shutdown). The evaluation of SG as-built components must ensure acceptable margin to ASME limits considering fatigue usage resulting from the complete design transient inventory over the expected operating life of the plant.

The stability map for real-time monitoring determines when the NPM operates within a region susceptible to DWO by using an approach temperature. If the DWO approach temperature at a feedwater flow rate is determined to be in the region of potential DWO instability, then the amount of time spent in the potentially unstable region is deducted from the time remaining in the design basis.

Thus, the time counted in potential DWO is tracked like other cyclic fatigue usage to ensure that components are maintained within design limits (i.e., Standard Design Approval Application (SDAA)

Part 4, Technical Specification 5.5.3). Time spent in the instability region does not indicate that DWO is occurring, but that there is reduced margin to DWO.

Confirmation of SG integrity is provided by physical inspections of the SG. Physical inspections of the SG tubes is required by the Technical Specifications, and examination of the inlet flow restrictors (IFRs) is required in SDAA Part 2, Chapter 5. The SG tubes are examined on a frequency that is adequate to capture degradation. This approach is analogous to existing licensing renewals where aging management programs and plans are defined with a frequency more than adequate to identify significant degradation such that the degraded condition can be addressed in a timely manner.

An SG tube failure (SGTF) is highly unlikely and would require the analyses, real-time monitoring and control, and physical inspections pillars to fail. However, an SGTF does not result in core damage and the radiological doses are a fraction of the acceptance criteria (SDAA Part 2, Chapter 15). In addition, monitoring of secondary-side system radiation levels and primary-to-secondary leakage rates allow an operator to reasonably identify the condition and take appropriate action.

In summary, analyses show that the SG can withstand a conservative amount of time in DWO, real-time monitoring and control provides a means for an operator to track the SG operating time in the region that potentially has DWO, and physical inspections identify potential SG degradation, creating a solid

NuScale Non-Proprietary Page 2 of 8 foundation for the safety case. However, should one of the pillars fail, safety analysis shows there is no safety consequence for an SGTF, and operators are required to comply with relevant Technical Specifications.

What could go wrong?

For the NPM design, operating in the domain where DWO would cause thermal-hydraulic instabilities within the SG tubes results in stresses and wear. Over time, the stresses and wear could result in SGTF of SG tubes. If an SG tube fails, inventory from inside the reactor pressure vessel transfers into the main steam system (MSS).

How likely is it?

Through analysis and testing, NuScale has shown there is ample stable operating space for the SG so as not to challenge the integrity of the reactor coolant pressure boundary (RCPB) due to DWO. However, a conservative amount of time with active DWO is analyzed for the SG design. The design can withstand a significant amount (i.e., >2840 days) of continuous DWO. Operation with continuous DWO is not likely for most of the NPM lifetime.

What are the consequences?

An SGTF results in reactor coolant passing from the primary side of the SG to the secondary side.

Radionuclides contained in the primary coolant are discharged through the tube leak and released through the condenser until the faulted SG is isolated by automatic closure of the main steam isolation valves and main steam isolation bypass valves. The failure of an SG tube is a postulated accident within the design basis of the NPM. The SGTF is an SDAA Part 2, Chapter 15 design basis event analyzed with conservative assumptions. The minimum critical heat flux ratio (MCHFR) is met for an SGTF (SDAA Part 2, Section 15.6.3), and radiological doses are a fraction of the acceptance criteria (SDAA Part 2, Table 15.0-5).

What could go right?

The safety case for DWO is presented in terms of three interrelated pillars. The pillars provide a defense-in-depth approach that is not reliant solely on one pillar. Figure 1 illustrates this approach.

NuScale Non-Proprietary Page 3 of 8 Figure 1 DWO Safety Case Areas and Interrelations Analyses SDAA Part 2, Chapter 3 The ASME Service Level A transients are representative of events that are expected to occur during plant operation. The DWO transient is included in Chapter 3 as a Service Level A event and is required to be considered in the design of the SG. The amount of time spent in thermal-hydraulic conditions where DWO could occur is defined (i.e., 2840 days) and is considered in the mechanical analysis for the SG. The time limit of 2840 days in Table 3.9-1 is conservative and provides margin over the expected 60-year service life of the SG.

The IFR is not part of the RCPB and is not an ASME component, but it will be evaluated to ensure that it can withstand postulated DWO thermal loads. Analyses show that the IFR can withstand postulated dynamic loads of DWO in the SG.

The evaluation of SG as-built components must ensure acceptable margin to ASME limits considering fatigue usage resulting from the complete design transient inventory over the expected operating life of the plant.

NuScale does not intend that steady-state operation of the NPM occurs in an operating domain where DWO is likely to occur; however, the SG may experience some DWO during transient operating conditions. DWO is an anticipated transient assumed within the design basis of the plant.

NuScale Non-Proprietary Page 4 of 8 SDAA Part 2, Chapter 15 NuScale evaluated and provided information related to how an oscillatory instability, including DWO, would impact Chapter 15 analyses. The result of the evaluations demonstrated that existing Chapter 15 transients are either equivalent to or bounding of events that consider DWO. The MCHFR limit is met.

In addition, an SGTF is included in Chapter 15 as a postulated accident. The SGTF analysis evaluates the primary system and secondary system response to the transient and demonstrates that specified acceptable fuel design limits are met and dose consequences are a fraction of acceptable limits. While not anticipated within the life of the plant, should DWO result in an SGTF, the consequences are within acceptance criteria.

SDAA Part 2, Chapter 19 As discussed in the Chapter 19 audit, NPM response to multiple tube failures in comparison to individual tube failures are not significantly different than a single SGTF. A higher flow area from multiple tube failures shortens the time to secondary system isolation. Release potential is terminated with the automatic isolation, and the passive emergency core cooling system precludes core damage by providing decay heat removal.

Real-Time Monitoring and Control SDAA Part 2, Chapter 5 Chapter 5 describes the approach temperature operational regions, which allows an applicant to determine when an SG is operating in a region where DWO could occur. The difference between the safety-related reactor coolant system (RCS) hot temperature and safety-related main steam temperature provides real-time monitoring in the main control room of approach temperature and thus SG conditions with respect to DWO onset. Figure 2 shows the DWO approach temperature stability map for the NPM from Figure 5.4-16. Operation within Region 2 precludes DWO. Operation within Region 1 or outside the bounds of applicability of the figure, is when margin to DWO onset is reduced and DWO is possible. Time in Region 1 is tracked to ensure that the SG is maintained within the design limits specified in SDAA Part 2, Table 3.9-1, although DWO onset is not indicated by operation in Region 1.

Operating in Region 1 does not challenge SG integrity provided that the total time spent in the region is less than that conservatively analyzed in SDAA Part 2, Chapter 3.9. It is expected that operators will manipulate the NPM to move operation back into Region 2 when possible (i.e., increase secondary pressure).

NuScale Non-Proprietary Page 5 of 8 Figure 2 DWO Approach Temperature for the NPM SDAA Part 2, Chapter 11 The MSS includes process radiation monitoring and provides operators indication and alarms of rising main steam radiation levels. If a high radiation condition is detected, the MSS radiation monitors provide automatic isolations to ensure that the MSS does not contribute to an unmonitored release. In addition to automatic actions, the operators have the ability to identify an increase in secondary system radiation levels through monitoring, and can take action to correct the abnormality.

SDAA Part 4, Technical Specifications Technical Specification 3.4.5 includes limits for the amount of primary-to-secondary leakage allowed.

Specifically, the US460 design is limited to 150 gallons per day of primary-to-secondary leakage. Should primary-to-secondary leakage exceed the established limit, the NPM is required to be placed in Mode 3 with RCS hot temperature less than 200 degrees Fahrenheit. Monitoring of the primary-to-secondary leakage is required by Technical Specification Surveillance Requirement 3.4.5.2.

Technical Specification 3.4.9 requires SG tube integrity to be maintained. Should SG tube integrity not be maintained, the Technical Specification requires placing the NPM into Mode 3 with passive cooling.

To meet the surveillance requirements of the Limiting Condition for Operation, Surveillance Requirement 3.4.9.1 requires compliance with the SG program.

0 5

10 15 20 0

100 200 300 400 500 600 700 800 900 Approach Temperature (degrees F)

Feedwater Flow (gpm/SG)

Region 2 Region 1

NuScale Non-Proprietary Page 6 of 8 Technical Specification 5.5.3 requires programmatic controls to track operation against the transients in Chapter 3 to ensure components are maintained within the design limits. Approach temperature monitoring provides a means for operations to infer the SG in-tube condition that allows for tracking the time spent in conditions where DWO could occur.

Technical Specifications 5.5.1 and 5.5.2 require the establishment of a radioactive effluent control program and the development of an offsite dose calculation manual. The purpose of the radiological effluent control program includes elements for controlling the total offsite effluent from the operating plant and remedial actions required if established limits are exceeded. The effluent monitoring provided in the US460 design, in combination with programmatic controls, provide assurance that reasonable and appropriate action is taken to protect the health and safety of the public.

Technical Specification 5.5.4 requires an SG Program that includes physical inspections.

Technical Specification 5.5.5 requires programmatic control for monitoring secondary water chemistry to inhibit SG tube degradation.

Physical Inspections SDAA Part 2, Chapter 5 Further supporting the pillars of analyses and real-time monitoring, physical inspections of the SG provide an additional layer of defense-in-depth for reasonable assurance of public health and safety.

Physical inspections of the SG components are outlined in Chapter 5, and Combined Operating License (COL) Item 5.2-4 requires site-specific inspection and testing programs compliant with ASME requirements.

Section 5.4.1.6.1 requires a degradation assessment program, which will address degradation mechanisms, structural integrity performance criteria and appropriate loading conditions for degradation mechanisms, inspections locations, sampling size, and expansion criteria, among other considerations.

An SG Program is required by COL Item 5.4-1. The SG Program will follow NEI 97-06 and applicable Electric Power Research Institute SG guidelines, including the following elements pertinent to assessing SG tube fatigue or wear: assessment of degradation, tube inspection requirements, tube integrity assessment, tube plugging, primary-to-secondary leakage monitoring, and shell side integrity assessment.

COL Item 5.4-1 also requires 100 percent SG tube examination during the first refueling outage after preservice inspection occurs. COL Item 5.4-1 has been updated to require the first NPM to undergo refueling to have at least 20 percent of tubes inspected each outage for the first 72 effective full power months after the initial 100 percent tube inspection of the first refueling outage. Subsequent NPMs will utilize this examination data to determine the extent of examinations required for their refueling outages following the first refueling outage. The NPM proposed refueling cycle is every 1.5 years, and the total bank of allowable operating time with DWO (2840 days) is greater than the maximum time between 100 percent SG tube examinations. This requirement is similar to the AP1000TM (Westinghouse AP1000 Design Control Document Rev. 19, Tier 2 Chapter 14, Initial Test Program, Section 14.4.6, ML11171A364), which specified specific initial test requirements for the first fleet units placed in

NuScale Non-Proprietary Page 7 of 8 service. This additional requirement for the NPM provides further reasonable assurance and aids in establishing operating experience for the SG.

A VT-3 examination is required for the IFRs when they are removed from the SG tube in Section 5.4.1.4.

The examination of the IFRs will determine their mechanical and structural condition, and detection of discontinuities and imperfections, such as wear or erosion.

SDAA Part 4, Technical Specifications Technical Specification 5.5.4 requires 100 percent SG tube inspections during the first refueling outage and 100 percent SG tube inspection by at least every 72 effective full power months after the first 100 percent inspections are completed during the first refueling outage. In total, with six NPMs in operation, an operator would detect abnormal indications during inspections and take reasonable actions to remediate indications prior to SGTF.

The SG Program requires condition monitoring assessments, performance criteria for SG tube integrity, SG tube plugging criteria, and the provisions for SG tube inspections.

Summary DWO is a well-understood phenomenon and is not new to the nuclear industry, but its occurrence in the NPM SG requires different operations and a different safety approach than those used for boiling water reactors. NuScale defined the total amount of time permitted under potential DWO conditions for the NPM and performed preliminary structural analysis required to conclude that SG structural integrity is maintained over the NPM lifetime during potential operation with DWO. The total amount of time allowed in SDAA Part 2, Table 3.9-1 is 2840 days of DWO over the lifetime of the NPM. NuScale does not expect for the SG to be operated in a region where DWO is possible during power operations. However, the approach of this safety case is to accommodate and account for operation with DWO.

To supplement the analytical results, NuScale developed a way to monitor SG conditions in real-time during operations such that operators can track when the plant is in a region that has the potential for DWO. This real-time monitoring uses the DWO approach temperature, which defines a minimum temperature difference, such that any temperature difference above this limit is in a region of stability and any temperature difference below this limit or outside the applicability bounds of the curve is in a region of potential DWO. The approach temperature method creates a stability map that uses safety-related instrumentation to monitor SG stability. If a transient results in the SG operating in the region where the potential for DWO exists (Region 1) or outside the bounds of the stability map, the time spent in that region is tracked to ensure that the SG is maintained within the design limits in SDAA Part 2, Table 3.9-1.

To supplement both the analytical results and real-time monitoring, periodic physical inspections of the SG will identify evidence of wear. The frequency of these inspections is such that wear would be identified in a timely manner to support appropriate corrective actions prior to tube failure.

NuScale Non-Proprietary Page 8 of 8 The safety case for DWO uses three intersecting pillars to provide reasonable assurance of adequate protection for the public, which includes the following:

Analyses in SDAA Part 2, Chapter 3, Chapter 15, and Chapter 19 Real-time monitoring and control with a stability map in SDAA Part 2, Chapter 5 and radiation monitoring in SDAA Part 2, Chapter 11 Physical examinations required by SDAA Part 2, Chapter 5, COL Item 5.2-4 and COL Item 5.4-1 and SDAA Part 4, Technical Specification 5.5.4 NuScale provides these three pillars to demonstrate there are no thermal-hydraulic or structural safety concerns for the NPM during operation with potential DWO. The SG can withstand DWO within the analyzed limits, conditions for DWO will be monitored in real-time, and physical examinations occur on a frequency that is timely enough to identify issues that need to be addressed. Therefore, consistent with its treatment as a postulated accident, SGTF is not anticipated within the lifetime of an NPM. If SGTF occurs, the consequences are well within applicable acceptance limits.