ML19340A015

U.S. Nuclear Regulatory Commission'S Report of the Regulatory Audit Performed Between March 4, 2019, Through September 10, 2019, for Nuscale Power, LLC, Regarding Nuscale'S Power Module Leakage Flow Instability Analysis - Public Memo

ML19340A015
Person / Time
Site:	NuScale
Issue date:	12/20/2019
From:	Vera Amadiz M NRC/NRR/DNRL/NRLB
To:	Michael Dudek NRC/NRR/DNRL/NRLB
Vera Amadiz M, 415-5861
References
Download: ML19340A015 (12)

Text

December 20, 2019 MEMORANDUM TO: Michael I. Dudek, Chief New Reactor Licensing Branch Division of New and Renewed Licenses Office of Nuclear Reactor Regulation FROM: Marieliz Vera, Project Manager /RA/

New Reactor Licensing Branch Division of New and Renewed Licenses Office of Nuclear Reactor Regulation

SUBJECT:

U.S. NUCLEAR REGULATORY COMMISSIONS REPORT OF THE REGULATORY AUDIT PERFORMED BETWEEN MARCH 4, 2019, THROUGH SEPTEMBER 10, 2019, FOR NUSCALE POWER, LLC, REGARDING NUSCALES POWER MODULE LEAKAGE FLOW INSTABILITY ANALYSIS On January 6, 2017, NuScale Power, LLC (NuScale) submitted a Design Certification (DC) application, for a Small Modular Reactor, to the U.S. Nuclear Regulatory Commission (NRC)

(Agencywide Documents Access and Management System (ADAMS) Accession Number ML17013A229). The NRC staff started its detailed technical review of NuScales DC application on March 15, 2017.

The NRC staff conducted an audit to examine NuScales comprehensive vibration assessment program (CVAP) analysis specifically regarding the leakage flow instability analysis of reactor internals components to resolve the open item related to Request for Additional Information (RAI) 9408, Question 03.09.02-76. The audit was initiated on March 4, 2019, and ran through September 10, 2019, in accordance with the audit plan in ADAMS (ML19060A271).

The purpose of the audit was to: (1) gain a better understanding of the NuScale design; (2) verify information; (3) identify information that may require docketing to support the basis of the licensing or regulatory decision; and (4) review related documentation and non-docketed information to evaluate conformance with regulatory guidance and compliance with NRC regulations.

CONTACT: Marieliz Vera, NRR/DNRL 301-415-5861

M. Dudek 2 The NRC staff conducted the audit via access to NuScales electronic reading room. The audit was conducted in accordance with the NRC Office of New Reactors (NRO) Office Instruction NRO-REG-108, Regulatory Audits.

The publicly available version of the audit report is enclosed with this memorandum.

Docket No.52-048

Enclosure:

1. Audit Summary - (Non-Proprietary)

2. Audit Summary - (Proprietary) cc: NuScale DC ListServ

M. Dudek 3

SUBJECT:

U.S. NUCLEAR REGULATORY COMMISSIONS REPORT OF THE REGULATORY AUDIT PERFORMED BETWEEN MARCH 4, 2019, THROUGH SEPTEMBER 10, 2019, FOR NUSCALE POWER, LLC, REGARDING NUSCALES POWER MODULE LEAKAGE FLOW INSTABILITY ANALYSIS DATED: DECEMBER 20, 2019 DISTRIBUTION:

PUBLIC NRLB R/F MDudek, NRR MVera, NRR SBailey, NRR TLupold, NRR TScarbrough, NRR RidsNrrOd RidsNrrDnrl RidsNrrDnrlNrlb RidsOgcMailCenter RidsAcrsMailCenter RidsOpaMailResource RidsNrrLACSmithResource NuScale DC Listserv ADAMS Accession No.: ML19340A015 *via email NRR-106 OFFICE DNRL/NRLB: PM DNRL/NRLB: LA DEX/EMIB: BC DNRL/NRLB: PM NAME MVera CSmith SBailey MVera DATE 11/18/2019 12/05/2019 12/20/2019 12/20/2019 OFFICIAL RECORD

U.S. NUCLEAR REGULATORY COMMISSIONS REPORT OF THE REGULATORY AUDIT PERFORMED BETWEEN MARCH 4, 2019, THROUGH SEPTEMBER 10, 2019, FOR NUSCALE POWER, LLC, REGARDING NUSCALES POWER MODULE LEAKAGE FLOW INSTABILITY ANALYSIS NRC Audit Team:

Yuken Wong, Senior Mechanical Engineer (NRC), Audit Lead Stephen Hambric, (NRC Consultant)

Marieliz Vera Amadiz, Project Manager (NRC)

1.0 BACKGROUND

Title 10 of the Code of Federal Regulations (10 CFR) Part 52, Section 47, Contents of applications; technical information, states that:

The application must contain a level of design information sufficient to enable the Commission to judge the applicant's proposed means of assuring that construction conforms to the design and to reach a final conclusion on all safety questions associated with the design before the certification is granted. The information submitted for a design certification must include performance requirements and design information sufficiently detailed to permit the preparation of acceptance and inspection requirements by the [U. S. Nuclear Regulatory Commission] NRC, and procurement specifications and construction and installation specifications by an applicant. The Commission will require, before design certification, that information normally contained in certain procurement specifications and construction and installation specifications be completed and available for audit if the information is necessary for the Commission to make its safety determination.

On March 15, 2017, the U.S. Nuclear Regulatory Commission (NRC) accepted and docketed a standard design certification application (DCA) (Reference 10) submitted by NuScale Power, LLC (NuScale), to certify its small module reactor design.

In its letter RAIO-0219-64485 dated February 8, 2019 (Reference 11). The applicant responded to the NRC staffs Request for Additional Information (RAI) 9408, Question 03.09.02-76. The response included the markup of a revision to Technical Report TR-0716-50439, NuScale Comprehensive Vibration Assessment Program Technical Report. The technical report revision included details of NuScales quantitative screening of several reactor internals for leakage flow instability (LFI) which might experience gap flow through a narrow adjacent annulus. The applicant used experimentally validated methods documented by Inada and Hayama in peer-reviewed technical journals. However, it is not clear to the NRC staff, in the applicants RAI response, how specifically, NuScale applied the Inada methods to the analyses. The purpose of this audit is for the NRC staff to better understand these details to support closing the RAI.

The NRC staff provided the applicant with an audit plan to facilitate the audit (Reference 12). The NRC staff followed the NRO Office Instruction NRO-REG-108 (Revision 0), Regulatory Audits, in performing the audit of the NuScale design specifications.

Using the electronic reading room between March 4, 2019, through September 19, 2019, the NRC staff members from the Mechanical Engineering Branch of the Division of Engineering and Infrastructure in the NRC Office of New Reactors and NRC consultants conducted a regulatory audit of the NuScale reactor internals component leakage flow instability analysis.

The NRC staffs observations and findings are documented in the Audit Results section.

2.0 DOCUMENTS REVIEWED

• Report EC-A010-6392, Evaluation of Leakage Flow Instability for Reactor Module Components, dated February 4, 2019.

• Several MathCAD calculations of LFI for individual components.

• Engineering Change Notice ECN-A010-7415, Evaluation of Additional Case with Inactive CRDS Supports, dated July 24, 2019.

3.0 AUDIT RESULTS Overview of LFI and Inada Papers Referenced by NuScale Fluid-dynamic forces induced by flow in the gaps between a structure and an external passage can couple with translational and rotational modes of the structure, sometimes to the point where self-excitation or lock-in occurs. Self-excited vibration amplitudes can be very high and cause contact between the structure and passage. Over time, repeated contact can cause wear and/or material fatigue damage.

The time-harmonic relative motion between a structure and a neighboring passage with through flow induces fluid-dynamic forces due to oscillations in flow rate and pressure. These forces drive the dynamic modes of the structure, and add (or subtract) stiffness, mass, and damping to the equations of motion. When the negative damping due to the oscillating fluid inertial forces exceeds the positive damping due to frictional losses and squeeze film viscous effects, negative net damping results.

Physically, negative damping is caused by inertial forces when the net pressure difference across the top and bottom of the entrained structure is in phase with the vibration, and therefore out of phase with structural damping, thereby reducing net damping and increasing vibration.

This increased vibration in turn increases the fluid inertial oscillations, leading to self-excitation.

In general, negative damping occurs at higher flow rates.

There are three sources of fluid-dynamic damping for a structure oscillating in a narrow channel:

the so-called squeeze film effect, which is always positive and associated with viscous forces acting around the vibrating structure; oscillations in inertial fluid forces, caused by axial velocity differences between the narrowing and expanding parts of the passage induced by the structural motion; and finally oscillations in pressure loss across the passage, due to inlet and exit loss coefficients, and in frictional losses across the length of the passage. The inertial

oscillations are what can cause net negative fluid-dynamic damping, since they can be out of phase with the other damping terms, and also out of phase with structural damping. Inada,

[Reference 1] uses the nondimensional parameter T to quantify the ratio of the inertial (numerator) and resistance (denominator) forces:

Inada combines both the inertial and resistance terms in his definition of the net damping term Ce (Cw, the squeeze film term, is separated out):

So, Ce is negative if the inertial term ym is dominant. For a net negative damping (Ce+Cw<0) to occur, the inertial term must exceed both the resistance (inlet and exit losses and passage frictional losses) and squeeze film (Cw) levels. The integrals in the ym and yr formulae are statistical moments of the 1/h (inertial) and 1/h3 (resistive) distributions along the passage. For an of 0 (straight passage), both moments are equal. For a diverging passage, the ym moments are higher than those of yr (and therefore dominant), and for a converging passage the ym moments are lower than those of yr. LFI is therefore generally associated with diverging gaps.

Note that LFI is also associated with sudden gap changes near the inlet of a passage, causing a pressure drop distribution which can also constructively couple with structural motion (see the diagram from Paidoussis [Reference 2] below). None of the NuScale conditions correspond to this case, except for the SGIFRs, which are evaluated by testing prior to initial plant startup.

Inada and Hayama derived coupled equations of motion for a flat plate in a narrow channel

[Reference 3], generating useful charts to assess possible regions of instability. They partially confirmed their procedure against test data in A study on leakage-flow induced vibrations. Part 2: Stability analysis and experiments for two degree of freedom systems combining translational and rotational motions, [Reference 4]. These test data were limited to the case of coupled translational and rotational motion, and for a flat plate in a narrow annulus. Although different from the conditions for a rod in a hole with purely translational motion (the NuScale cases), the test results seem sufficient to confirm the authors procedure, at least to within reasonable certainty.

Inada also published useful charts to identify unstable conditions [Reference 3], including the one below for an inlet loss coefficient of 1 and exit loss coefficient of 0 (typical of an expanding gap).

is the nondimensional taper of the gap (called the area increment ratio) and the effective

pressure resistance loss factor. Specifically:

= (H)/Hinlet

= L/4Hinlet, where is the passage friction factor, computed using the passage flow Reynolds Number.

Hinlet is the inlet gap width H is the difference between the exit and inlet gap width L is sthe gap length An of 0 (1+ of 1) implies no taper. 1+ greater than 1 implies a diverging gap. A of 1 seems to represent worst-case conditions for possible negative damping. The more deviates from unity, the less likelihood of negative damping.

The chart shows the transition conditions for both negative stiffness (in dashed lines, less of a concern) and negative damping (solid lines, the main concern for LFI). The possibility of negative damping is highest at the lowest frequencies, shown by the T (non-dimensional frequency) = 0 curve. Negative added stiffness which exceeds structural stiffness stops oscillatory motion (the resonance frequency goes to 0), and the internal structure will simply rest against the side of the exterior wall. This is sometimes called a buckling type instability.

Interestingly, if structural rotation is included, with the pivot point at the inlet to the hole, only very low conditions can lead to negative damping, as seen below.

If a mode vibrates with a node line (location of zero translation, but maximum rotation) within the annulus, the conditions for possible negative damping shift upward in the graph toward higher values. However, a rotational mode can also experience negative damping for convergent passages at lower values (see the lower left region in the chart below). In this case, the pivot point for the mode shape is close to the discharge, effectively shifting the relative locations of the peak fluid forces upstream. This condition is unlikely for fundamental beam/rod modes like those in the incore instrument guide tube (ICIGT) and control rod drive (CRD) shaft, however, since they are almost purely translational near any through holes.

Later, Inada added stability diagrams for various conditions [Reference 5]. An example is shown below, and indicates that unstable conditions (negative damping) are more likely at higher flow rates, and lower effective masses (Mr lines). This is, of course, the case with all flow-excited instabilities, such as vortex shedding lock-in with cylinders and other structures.

The chart shows that dynamic instabilities (flutter in the chart) are unlikely for area increment ratios less than 2.

Finally, Inada expanded the analysis procedure for the case of a cylinder in a short gap

[Reference 1]. The equations are similar, and several new terms related to a circular flow passage are determined to be negligible (such as Couette flow around the annulus, which induces fluid-dynamic forces much smaller than those of the oscillating axial flow). Other terms vanish when considering very short gaps. Inada provides sample calculations for four test conditions, which NuScale uses to verify their implementation of his methods.

Inadas work is consistent with observations made by Mulcahy [References 6 and 7] and Paidoussis [Reference 2], who state that dynamic instability is only possible for translational motion with upstream gap constrictions and/or diverging gap width. Mulcahy also echoes Inadas observations that coupled translational and rotational motion can lead to dynamic instability without the presence of upstream constrictions and/or with converging gap width.

NuScale Calculations The only NuScale structures subject to possible LFI that have sudden converging gap constrictions are the SGIFRs, which will be tested prior to initial startup. All other components with possible leakage flows (CRD shaft, ICIGT, and control rod assembly guide tube (CRAGT))

have constant diameters through the surrounding holes. In the case of upward flow around the CRDS in the pressurizer baffle plate and in the CRD shaft alignment cone, the gap gradually diverges. In the case of downward flow around the CRDS through the pressurizer baffle plate/hanger ring/sleeve, the flow also gradually diverges, with sudden increases in gap width.

Normal operation is for upward flow, so it is therefore unlikely that significant damage could occur over the life of the reactor for downward flow conditions.

Since the Inada stability charts are only for specific inlet and outlet loss coefficients, NuScale computes actual added damping, stiffness, and mass, along with gap flow rates, for all possible ICIGT, CRD shaft, and CRAGT leakage flow paths, assuming translational motion only.

Ignoring rotational motion is acceptable for the fundamental mode shapes of these structures, which are the only shapes which could experience LFI given the low flow rates in a NuScale plant. Although some rotation will occur near the ends of the mode shapes, there will be no pivot points where the motion changes direction within any of the gaps considered.

NuScale assumes all gap lengths are small with respect to hole diameter (per Inadas assumption), and evaluated the possibility for LFI assuming translational motion only. However, per EC-A010-6932, Table 4-3, Input Parameters for LFI analyzed, two passage lengths are long, and deviated from the short gap length assumptions:

• CRDS in sleeve ([ ]); and

• CRDS at pressurizer baffle plate ([ ] less countersink).

NuScale showed that deviating from this assumption is reasonable in ECN-A010-7415, citing research by Paidoussis [Reference 8] that shows that damping increases in longer passages, improving margin against LFI.

Before computing the various terms for the ICIGT, CRD shaft, and CRAGT gap flows, NuScale verified its implementation of the Inada equations against sample calculations provided in A study on leakage flow induced vibration from engineering viewpoint, [Reference 1]. NuScales calculations agree exactly with Inadas. The NRC staff also calculated LFI stability using Inadas methods and confirmed NuScales implementation. Next, NuScale developed conservative parameter estimates for their assessments, including inlet and exit loss coefficients and gap flow rates. Although most gaps are designed to be uniform, NuScale conservatively applies a diverging 25% gap area increase, which is sufficient to account for any installation effects that might cause a non-uniform gap, and the few conditions with diverging flows. The critical term is calculated with loss coefficients based on high Reynolds Number. The ICIGT has very large Beta values ranging from [ ] (due to the tiny annular area), whereas the CRD

shaft cases have smaller values ([ ]). The CRAGT gap has a [ ].

In general, larger values imply more stability.

Inlet and outlet loss coefficient assumptions are derived from Idelchik [Reference 9] and seem reasonable. Inlet losses for rods in holes are generally 0.5 (the typical maximum), with exit losses lower due to the conservative assumption of diverging gaps. The passage friction loss factor is proportional to the friction factor 0.266/Re0.25 for turbulent flow (consistent with standard practice). Structural modal masses and stiffnesses are estimated from the finite element models of the ICIGT, CRD shaft, and CRAGT. The modal masses include fluid loading effects.

The ICIGT radial hole gaps are [ ], so there is very little gap flow. Squeeze film effects are likely strong enough to justify NuScales assumption of hinged boundary conditions. The ICIGT calculation results show buckling (static) instability is possible, but at much higher speeds than estimated gap velocity. Added damping, however, never becomes negative at any speed.

Since the pressure drop and flow rates are very small and added damping is positive, it is reasonable to rule out LFI for ICIGT structures.

The CRAGT and CRD shaft sleeve pressure drops are also very small. Leakage flow through the gap at the top of the CRAGT is low, and even if the CRAGT began vibrating, it is unlikely that it will cause any significant damage that would affect safe reactor operation. Leakage flow through the upper/lower riser joint is shown to be impossible due to a very low-pressure difference ([ ]) and much higher hold-down forces due to weight and preloading. The pressure difference loading is [ ] of the hold down force. Therefore, LFI is not calculated for the upper/lower riser joint.

In its initial analyses of the CRDS structures, NuScale assumed simply supported boundaries for holes away from the leakage location. The NRC staff does not find the assumption of fixed boundaries at holes with non-negligible gap sizes credible, particularly for the [ ] CRDS radial hole gaps. NuScale addressed how the LFI calculations for the CRDS locations change if free boundaries are assumed in ECN-A010-7415. In its reassessment, NuScale assumed structural damping of [ ] which is acceptable per RG 1.20. Even with all boundaries free, NuScale estimates greater than 100 percent margin against LFI.

Conclusion NuScale has evaluated all RVI with passage flows for LFI using quantitative methods validated against measurements and documented in peer-reviewed journal articles. The NRC staff performed confirmatory calculations using these methods. NuScales assumptions and inputs are conservative, and its analysis results show greater than 100 percent margin against LFI. In spite of the high margins, NuScale will instrument the prototype NPM to monitor any unexpectedly high vibrations due to LFI or other FIV mechanisms. Also, all components analyzed for LFI are part of the long-term inspection program. Therefore, the NRC staff finds NuScales LFI evaluations, to be acceptable.

References

1. Inada, F., A study on leakage flow induced vibration from engineering viewpoint, Paper No. PVP2015-45944, ASME 2015 Pressure Vessel and Piping Conference Volume 4, Fluid-Structure Interaction, Boston, MA, issued July 2015.

2. Paidoussis, M.P., Real life experiences with flow-induced vibration, Journal of Fluids Structures, Vol. 22, pp 741-755, issued 2006.

3. Inada, F., and Hayama, S., A study on leakage-flow induced vibrations. Part 1: Fluid dynamic forces and moments acting on the walls of a narrow-tapered passage, Journal of Fluids Structures, Vol. 4, pp 395-412, issued 1990.

4. Inada, F., and Hayama, S., A study on leakage-flow induced vibrations. Part 2:

Stability analysis and experiments for two degree of freedom systems combining translational and rotational motions, Journal of Fluid Structures, Vol. 4, pp 413-428, issued 1990.

5. Inada, F., A parameter study of leakage-flow induced vibrations, Proceedings of the ASME 2009 Pressure Vessel and Piping Conference, Prague, Czech Republic, issued July 2009.

6. Mulcahy, T.M., One dimensional leakage flow vibration instability, Journal of Fluids Structures, Vol. 2, pp 383-403, issued 1988.

7. Mulcahy, T.M., A review of leakage flow induced vibrations of reactor components, ANL-83-43, issued 1983.

8. Paidoussis, M., Fluid-Structure Interactions, Volume 1: Slender Structures and Axial Flow, 2nd Edition, Elsevier Academic Press, issued 2004.

9. Idelchik, I.E., Handbook of Hydraulic Resistance, 3rd edition, CRC Press, issued 1994.

10. NRC Letter, NuScale Power, LLC, - Acceptance of an Application for Standard Design Certification of a Small Modular Reactor, ADAMS Accession No. ML17074A087, issued March 23, 2017.

11. RAIO-0219-6448, NuScale Power, LLC Response to NRC Request for Additional Information No. 427 (eRAI No. 9408) on the NuScale Design Certification Application, ML19039A194, issued February 8, 2019.

12. NRC, Phase 4 Audit Plan for the Audit of NuScale Power, LLC., Documents related to Reactor Internals Comprehensive Vibration Assessment Program, ML19060A271, issued March 4, 2019.