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Seismic Qualification of NPP Structures, Systems and Equipment Components

Marek Tengler

Nuclear Research Institute in Řež, November 21–25, 2011

Seismic Engineering Knowledge Transfer Seminar

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2/41 Seismic Qualification of NPP Structures, Systems and Equipment Components

STEVENSON AND ASSOCIATES Office in Czech Republic a. s., Vejprnicka 56, CZ 318 00 Pilsen, Czech Republic

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– Overview of Existing Standards – Principles of Equipment Seismic Qualification (SQ) – Analysis Methods – Experimental Methods – Methodology of SQ including GIP and Practical Examples

TOPICS



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1. OVERVIEW OF EXISTING STANDARDS

National Nuclear Law,

Convention on Nuclear Safety

Documents of the National Nuclear Authority,

relevant IAEA documents

Other relevant national and international codes, norms and standards, industrial standards

as IEC, IEEE, ASME, PNAE, KTA etc.

Hierarchy of Legislation, Codes, Norms and Standards Related to Seismic Qualification



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2. OVERVIEW OF EXISTING STANDARDS (Cont’d)

~4500 Other Standards Cited in Regulatory Documents

Reference: NUREG/CR‐5973, PNL‐8462 Rev. 3, “Codes and Standards and Other Guidance Cited in Regulatory Documents,” Published August 1996.



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2. SEISMIC QUALIFICATION – MOTHERHOOD STD.

IEC 980:1989 IEEE Std 344-2004

ASME QME-1-2007

Consequential applicable standards of seismic qualification:

‐ Partial standards of tribal standards: IEC series 60068‐2, 60068‐3 (standards for mechanical and vibration resistance)

‐ Specific standards: IEC 255‐21‐3, C37.98‐1987, IEEE Std 382‐2006, IEEE Std 317‐1983 etc.



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2. SEISMIC QUALIFICATION – INTRODUCTION

Within the seismic section of the EQ program, all safety‐related equipment must prove its seismic adequacy to withstand the effects of the earthquake corresponding to the maximum design earthquake (SSE, S2, SL‐2). One part of the seismic adequacy verification is the demonstration the equipment is capable to withstand the cumulative degradation effect of five project design earthquakes (OBE, S1, SL‐1), which must not affect the resistance of the equipment to the impact of the maximum design earthquake (SSE, S2, SL‐2).

The seismic qualification must assure the equipment will hold its capability to perform the required safety functions during and/or after a seismic event keeping such a state that corresponds to the end of its qualified life.



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2. SEISMIC QUALIFICATION – SEISMIC CLASIFICATION

The equipment of an NPP safety systems is grouped into seismic classes (sub‐classes) according to the following definitions (General definition):

1 (A) – full functionality is required up to and including the maximum design earthquake level (SSE, S2, SL‐2).

1 (B) – only mechanical integrity is required (i.e. strength and leak tightness) in accordance with relevant strength standards and regulations; partial failures of the functionality are admitted up to and including the maximum design earthquake level (SSE, S2, SL‐2).

1 (C) – only stability is required, i.e. to avoid seismic interactions with other SSC (to keep the stable position mostly); partial failures of the functionality as well as the mechanical integrity are admitted up to and including the maximum design earthquake level (SSE, S2, SL‐2).



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2. SEISMIC QUALIFICATION – SEISMIC INPUT

Seismic conditions of the buildings are represented by required response spectra (RRS) of the locations, on which the equipment subject to qualification is installed.

Figures on next page show an example of the RRS (smoothed) for maximum design earthquake (SSE, S2, SL‐2).

The smoothing of calculated RRS can be done using the method described in US NRC RG 1.122.

The spectra shall correspond to the significant places (floor or structures) situated inside the seismic classified civil‐structures (buildings) where classified equipment to be installed.



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2. SEISMIC QUALIFICATION – SEISMIC INPUT (Cont’d)

RRS envelope, horizontal direction. Earthquake level SSE, S2, SL‐2.

RRS envelope, vertical direction. Earthquake level SSE, S2, SL‐2



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The accelerations needed to determine the seismic excitation with the intensity of the project design earthquake (OBE, S1, SL‐1) are derived from the shown RRS of maximum design earthquake as the one half of the acceleration RRS‐SSE (S2, SL‐2) for the specified frequency.

For the equipment which is connected with a pipeline system in a very good manner, like temperature sensors, valve actuators etc., and which require the demonstration of their functionality, a specific technique of the seismic qualification needs to be applied. Such equipment acts as the pipeline components and they are subjected to very hard seismic loads. These loads are generated in the place of the equipment as the seismic response of the pipeline system. Amplified excitation forces are of the discrete nature with a single dominant frequency. To qualify the equipment connected with pipeline systems the seismic excitation derived from the RIM curve must be additionally applied to generic seismic qualification RRS tests.



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Required Input Motion (RIM) curve (see IEEE Std 382). Level SSE, S2, SL‐2.



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2. SEISMIC QUALIFICATION – METHODS

To demonstrate the seismic adequacy of seismic category 1 structures, systems and equipment components the following methods are used:

(a) seismic analyses (main pipelines, main mechanical components, anchorage of equipment),

(b) seismic tests (active mechanical components, electrical and I&C components),

(c) earthquake experience and indirect procedures (small bore pipes, HVAC ducts, additional approach to verify seismic adequacy of equipment components as mounted using the GIP‐VVER procedure).



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4. QUALIFICATION BY SEISMIC ANALYSIS

Purpose: Te determined critical response parameters of equipment to be evaluated for operational and seismic loads by calculation.

General methods of response calculation:

‐ “Hand” calculus (simply equation);

‐ Finite Element Method;

‐ Combination of both above mentioned.



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4. QUALIFICATION BY SEISMIC ANALYSIS (Cont’d)

Methods of the calculation of seismic response

- Static analysis – for stiff components with natural frequency above 33 Hz;

‐ Equivalent static analysis – for simply components

‐ Response spectra method – complex components – linear dynamic behavior assumed

‐ Time‐history method ‐ complex components – linear and non‐linear dynamic behavior assumed



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Assessment of base parameters of component capacity

- Integrity of pressure boundary (housing, nozzles);

‐ Capacity of internals;

‐ Supports (supporting structure) capacity;

‐ Anchor/fixture resistance.



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Assessment of performance capability of component

In addition, for documentation of performance capability (functionality) of active mechanical components are evaluated parameters that affecting their performance of demanded safety function:

- Total relative displacements of moving and static parts to assess their collisions during induced seismic motions – depletion of design spacings between parts.

‐ Total reaction forces and overall displacements in point of parts placing – jaming of bearings.

‐ Total deformation in places of a contact of sealing areas – violation of pressure‐tightness.

- Other specific parameters relating to evaluated component.



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4. QUALIFICATION BY SEISMIC ANALYSIS ‐ EXAMPLE

Math FE model of the flap valve ‐ DN 1000



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4. QUALIFICATION BY SEISMIC ANALYSIS – EXAMPLE (Cont’d)

Seismic excitation ‐ RIM accordingly IEEE Std 382‐2006



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Maximum resulting distribution of total displacements at excitation in Z direction, max. 11.4 mm



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Evaluation – Maximum Displacements

Place of maximum displacement Loading Max. displ. X

[mm] Max. displ. Y

[mm] Max. displ. Z

[mm] Max. displ

[mm] Max. total

displacement [mm]

Allowable displacement

[mm]

NOC 0,0 0,0 0,3 0,3

SSE X 1,5 2,0 1,5 2,6

SSE Y 4,7 5,6 2,9 5,7 Disc of flap

SSE Z 1,1 1,6 11,3 11,4

11,5 20,0

Evaluation of sliding bearings in flap shaft

Loading Fy [kN]

Fz [kN]

Stress [MPa]

Max. stress [MPa]

Allowable stress [MPa]

NOC 1,43 3,83 1,29

SSE X 5,00 1,07 1,62

SSE Y 12,25 1,24 3,90

SSE Z 3,30 11,42 3,77

5,19 30,0




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5. QUALIFICATION BY SEISMIC TEST ‐ GENERALLY

The seismic tests of equipment in NPPs are generally preferred methods for the qualification program of

– Active technological equipment (e.g. valves and their actuators)

– Electrical equipment (e.g. switchgears)

– I&C equipment (cabinets and panels)

– Sensitive equipment components like relays, contactors, circuit breakers, transmitters, sensors etc.

The seismic capacity of such equipment in regard of their functionality during and after an earthquake is impossible, difficult or unreliable to evaluate by other methods.



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5. QUALIFICATION BY SEISMIC TEST ‐ DIVISION

SEISMIC INPUT MOTION

– Single‐frequency motion (with assumption the equipment will be subjected to steady vibrations with one dominant frequency, see RIM, or, if the examination of the natural frequencies and the damping values of the equipment is performed);

– Multi‐frequency motion (generally preferred for the verification of the seismic capability of the equipment, the motion simulation is very close to the typical earthquake motion). In multi‐frequency seismic testing two approaches are applied:

o Test with random excitation (input seismic motion applied on the test piece is given by synthetic time history; TRS corresponds to real quake motion);

o Test with complex sine excitation (input seismic motion applied on the test piece is given by the superposition of complex sine waves)



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5. EXPERIMENTAL METHODS – EXCITATION DIRECTIONS

TEST SAMPLE ORIENTATION

– Single‐axis tests (seismic input motion is applied only in one direction);

– Biaxial tests (seismic input motion is applied in two directions)

o Biaxial installation–testing for two independent directions (seismic input motion for each direction is statistically independent);

o Single axis installation–tests for two dependent directions (seismic platform moves on inclined plane);

– Triaxial tests (seismic input motion applied in three directions/axes of a test piece simultaneously).

o Triaxial installation–test performed with simultaneous but independent input waveform into the three preferred axes of the specimen;

o Biaxial installation (vertical/horizontal)–tests with independent simultaneous excitation signals in horizontal and vertical plane.



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5. EXPERIMENTAL METHODS – RRS

Required Response Spectrum (RRS) is the response spectrum issued by the user of the qualified equipment or by the user’s agent as part of the specification for qualification. RRS represents a requirement to be met. They are prepared for all three orthogonal space directions or at least for horizontal and vertical directions.

Required Response Spectra preparation

– Created to cover application for whole building / whole plant;

– Envelope of broadened and smoothed FRS for equipment installation location (equipment anchored to the relevant floor/structure);

– Multiple of broadened and smoothed FRS due to excitation applied only in one direction (factor 1.5);

– Multiple of broadened and smoothed FRS due to equipment installation on other structures or equipment (using amplification factor);

– Combination of previous both.



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5. EXPERIMENTAL METHODS – RRS VS. TRS

Test Response Spectra (TRS) are calculated from the recordings of the actual motion of the shaking table. TRS shall envelop the RRS or the applicable portion of the RRS taking into account the dynamic characteristics of the equipment tested (natural frequency). TRS shall be computed with 1/3 octave (or narrower) bandwidth resolution.

TRS and RRS comparison is made for or all three orthogonal space directions or at least for horizontal and vertical directions and for five OBE (S1, SL‐1) earthquakes followed by one SSE (S2, SL‐2) earthquakes. However instead of 5 OBE (S1, SL‐1) earthquakes the specimen may be subjected to 2 tests corresponding to level SSE (S2, SL‐2).

TRS and RRS are compared which have the same damping value. Recommended damping value is 5% damping. It is acceptable to compare RRS with TRS of higher damping value then is the damping value of RRS, nevertheless, TRS must envelop RRS.

If the resonance phenomena does not exist below 5 Hz, the RRS should be enveloped for frequency values above 3.5 Hz. Bandwidth 1–3.5 Hz, however should be covered up to level provided by testing device. If resonance phenomena exist below 5 Hz, TRS shall envelop RRS from 1 Hz.



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5. EXPERIMENTAL METHODS – RRS VS. TRS (Cont’d)



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6. QUALIFICATION BY EXPERIENCE/INDIRECT METHOD

In 1982 have been established Seismic Qualification Utility Group, wit a purposed of coordinating and founding works on development of study about behavior of mechanical and electrical components in case of destructive earthquakes. First phase of those works finished already in 1978, when have been published report defined 20 equipment classes indentified as inevitable for safe shutdown of nuclear units. The report evaluated features of different equipment classes during severe earthquake and founded criteria of seismic capacity, i.e. caveats, that have been developed for each equipment class. It has been also determined the capacity spectrum of equipment, so called, Bounding Spectrum, BS.



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6. QUALIFICATION BY EXPERIENCE/INDIRECT METHOD



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6. QUALIFICATION BY EXPERIENCE ‐ GIPThe scope of equipment covered by the current version of the GIP procedure includes, the following twenty classes of mechanical and electrical equipment:

(1) Motor Control Centers;

(2) Low Voltage Switchgears;

(3) Medium Voltage Switchgears;

(4) Transformers;

(5) Horizontal Pumps;

(6) Vertical Pumps;

(7) Fluid‐Operated Valves;

(8) Motor‐Operated and Solenoid‐ Operated Valves;

(9) Fans (ventilators);

(10) Air Handlers;

(11) Chillers;

(12) Air Compressors;

(13) Motor Generators;

(14) Engine Generators;

(15) Distribution Panels;

(16) Batteries on Racks;

(17) Battery Chargers and Inverters;

(18) Instruments on Racks;

(19) Temperature Sensors;

(20) I&C Panels and Cabinets.



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6. QUALIFICATION BY EXPERIENCE – GIP (Cont’d)

European and particularly VVER‐type relays, switches, transmitters and electric penetrations are significantly different from those included into the original GIP databases. These two classes of equipment are not included in specific modified procedure for European NPP’s, so called GIP‐VVER procedure (or GIP‐International), and their seismic verification shall be based on testing.

In addition to twenty classes listed above, the GIP‐VVER procedure also includes guidelines for simplified analytical seismic evaluation of the following classes of equipment:

(23) Cable Supporting Structures (based mainly on the EPRI methodology);

(24) Tanks, Heat Exchanger, Filters (TANKV computer code, based on the public available documents);

(25) Pipelines and HVAC Ducts (based on the public available documents).

GIP‐VVER also includes two special guidelines to verify adequacy of anchorage and seismic adequacy of non‐bearing masonry walls.



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6. GIP‐VVER PROCEDURE – WALKDOWN PROCEDURE

The GIP as well as the GIP‐VVER or the DOE‐GIP is primarily a screening and walkdown procedure. However, if an equipment item is classified as an outlier, rigorous approaches as testing on shaking table, deep study of input data, sophisticated analysis etc. may be used to verify its seismic adequacy. Generally, four major steps of this procedure when applied evaluation of seismic adequacy of classes of equipment identified above are as follows:

– selection of Seismic Review Team (SRT);

– identification of equipment the seismic adequacy shall be evaluated and set‐up the Seismic Equipment List (SEL);

– screening verification and walkdowns;

– outlier identification and resolution.



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6. GIP‐VVER PROCEDURE – WALKDOWN PROCEDURE (Cont’d)

An engineering judgment is the major tool used by SRT during the screening verification and walkdowns to evaluate seismic adequacy of the equipment. The SRT should include the system engineers, plant operation personnel, experienced and professionally trained seismic capacity engineers, and also personnel to identify and evaluate essential relays (if necessary).

Seismic evaluation engineers should have at least 3 years experience in seismic design or qualification of nuclear safety related structures, systems and components. They should have at least a bachelor’s degree in civil or mechanical engineering and formal instruction in structural dynamic analysis. They should also have completed at least a 3 day course including field analysis in the use of GIP‐VVER methodology in seismic evaluation of nuclear facility safety related SSCs. It is forbidden to use the GIP‐VVER procedure without deep study of corresponding documentation, training and practical experience.



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6. GIP‐VVER PROCEDURE – CRITERIA

The basic criteria to verify seismic adequacy of an equipment item during the screening walkdown are (see also schema & flow):

– seismic capacity greater than seismic demand (by comparison of the corresponding ISRSRLE(SL2,SSE) or GRSRLE(SL2, SSE) to the Bounding Spectrum;

– similarity to the equipment in the seismic experience databases (checking of caveats, based on walkdown and information available from documentation);

– adequate anchorage of equipment (calculations or engineering judgment, based on walkdowns and information available from documentation);

– potential seismic interactions evaluated (based on walkdowns).

The GIP‐VVER procedure uses two bounding spectra (BS):

(a) BS attached to PGA = 0.33 g (the same as introduced by SSRAP and used by GIP);

(b) BS attached to PGA = 0.50 g (1.5 times SSRAP BS) for selected VVER equipment classes, which are evidently robust and rugged.



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6. GIP‐VVER PROCEDURE – SCHEMA & FLOW

START

DOES CAPACITY EXCEED DEMAND?

ARE INTERACTIONS ACCEPTABLE?

ARE FOUR ABOVE CRITERIA MET?

IS ANCHORAGE ADEQUATE?

ARE CAVEATS MET?

VERIFIED

CHOOSE ALTERNATE METHODSFOR CAPACITY/DEMAND

CAPACITY/DEMAND OUTLIER

CAVEAT OUTLIER

DETAIL INVESTIGATION

ANCHORAGE OUTLIER

INTERACTION OUTLIER

IDENTIFICATION AND RESOLUTION OF OUTLIERS

CHOOSE ALTERNATE METHODSFOR CAPACITY

(DETAIL INVESTIGATION)

DETAIL INVESTIGATION

YES

YES

YES

YES

YES

NO

NO

NO

NO

NO



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6. GIP‐VVER PROCEDURE – CAPACITY SPECTRA



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6. GIP‐VVER PROCEDURE – CAPACITY VS. DEMAND

A. Comparison with RLE (SL2, SSE) Ground Response Spectra (GRS) 2)

This can be used when the equipment item is mounted below about 12 m above the effective grade and when the natural frequency of equipment is greater than 12 Hz 3)

BS GRSRLE (SL2,SSE) (5% damping) 4)

B. Comparison with RLE (SL2, SSE) In‐Structure Response Spectra (ISRS) 1.5 x BS realistic (median, mean, best estimated) ISRSRLE (SL2,SSE) (5% damping) 4)

Notes: (1) Apply at least one of these two rules, which applicable. (2) The criterion A can be used only with the well rigid building structures as the lower concrete reactor building. Do not use this criterion with evidently flexible building structures. (3) Do not apply the 12 Hz limit for equipment mounted on piping systems (valves, valve operators etc.). (4) These criteria shall be met for all three orthogonal spatial directions.



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6. GIP‐VVER PROCEDURE – SIMILARITY

Similarity of VVER‐type equipment to equipment included in the databases of seismic resistant equipment is the most important keystone of practical application of the GIP‐VVER procedure. Generally, the principal of similarity is based upon comparison of equipment dynamic and physical characteristics. The procedure to establish similarity within an each equipment class includes the following comparisons:

– most probable modes of malfunction (based on recognized behavior of all critical devices);

– predominant resonant and critical frequencies and mode shapes;

– critical damping;

– most important physical equipment characteristics, like equipment size, mass and position (vertical, horizontal, inclined etc.); general making, quality of making, age of equipment; location of the center of gravity, presence and location of cantilevered parts; implementation of heavy and / or moving internal parts; implementation of supports and anchorage; implementation of attached lines, substructures, devices etc.; presence of devices (mechanical or electrical) sensitive to vibrations and shocks.



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6. GIP‐VVER PROCEDURE – ANCHORAGE

The screening approach to verify of equipment anchorage is based upon a combination of inspections, calculations, and engineering judgment.

Inspections consist of measurements and visual evaluations of the equipment and its anchorage, supplemented by use of plant documentation and drawings. Calculations should be performed to compare the anchorage capacity to the corresponding loading (demand) imposed upon the anchorage. Engineering judgment is also an important part in the evaluation of equipment anchorage.

Generally, evaluation the adequacy of equipment anchorage includes:

– anchorage installation inspection,

– anchorage capacity determination,

– anchorage demand determination,

– comparison of capacity to demand.



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6. GIP‐VVER PROCEDURE – SEISMIC INTERACTIONS

The four seismic interaction effects that are considered are:

– proximity (impacts of adjacent equipment or structures on safety‐related equipment due to their relative motion during an earthquake),

– structural failure and falling of overhead or adjacent structures, systems, or equipment components),

– flexibility of attached lines and cables,

– flooding due to earthquake induced failures of tanks or vessels.

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