Overview of SQUG generic implementation procedure (GIP)

Transcription

1 Nuclear Engineering and Design 123 (1990) North-Holland 225 Overview of SQUG generic implementation procedure (GIP) Richard G. Starck II MPR Associates, Inc., 1050 Connecticut Avenue, N. w.. Washington, D.C , USA George Gary Thomas Stevenson and Associates, 9217 Midwest Avenue, Cleveland, OH 44125, USA Received 1 December 1988 The regulatory criteria for licensing of nuclear power plants require that certain safety-related equipment and systems be designed to function during and/or following a postulated, design basis earthquake. Since many older, operating nuclear power plants were designed and constructed prior to the issuance of the current seismic qualification criteria, the NRC has questioned whether the seismic adequacy of the essential equipment has been sufficiently demonstrated and documented. In response to this concern, a group of affected plant owners formed the Seismic Qualification Utility Group (SQUG). With support from the Electric Power Research Institute (EPRI), SQUG has undertaken a program to demonstrate the seismic adequacy of essential equipment by the use of available seismic experience data for similar equipment. The Generic Implementation Procedure (Glp), provides a generic means of applying this experience to evaluate the seismic adequacy of equipment. This paper is an overview of the technical approach, generic procedures, documentation criteria, and engineering guidance provided in the GIP. 1. Introduction The Seismic Qualification Utilities Group, (SQUG) has developed the "Generic Implementation Procedure for Seismic Verification of Nuclear Plant Equipment" (GIP) for use by its members. The GIP is divided into two major sections. Part I provides SQUG's position relative to certain licensing issues which are addressed in the NRC's Generic Letter 87-02, "Verification of Seismic Adequacy of Mechanical and Electrical Equipment in Operating Reactors, Unresolved Safety Issue (USI) A-46" [1]. Part I also addresses the licensing issues related to the NRC's generic Safety Evaluation Report (SER) and supplements to the SER. Part II is a detailed teclmical document containing criteria and guidance for plant-specific implementation. Part II also contains the requirements for the SQUG training course to teach the seismic review team members how to apply the GIP. This paper is primarily an overview of Part II. The purpose of the GIP is to provide the technical approach, generic procedures, format and description of documentation criteria which can be used to evaluate the seismic adequacy of active mechanical and electrical equipment needed to bring the plant to a safe shutdown condition following a safe shutdown earthquake (SSE). The specific equipment covered in this procedure for which seismic experience data has been obtained are the 20 classes of conventional power plant equipment listed in table 1. In addition to these equipment classes seismic evaluation criteria are also included for relays, for tanks and heat exchangers, and for cable and conduit raceway systems. In early SQUG was fonned for the purpose of collecting seismic experience data as a means to assess the seismic adequacy of equipment in operating plants. The primary source of experience data collected by SQUG was from non-nuclear facilities which have experienced earthquakes. Another source of seismic experience data was from shake table tests performed since the mid-1970s. The GIP provides a generic means of applying this experience to evaluate the seismic adequacy of equipment. The GIP implements this SQUG approach and includes the technical approach, generic procedures, documentation criteria, and engineering guidance. Engineering judgment is the major tool used by the Seismic Review Team(s), SRT(s), during the screening walkdown, to evaluate the seismic adequacy of the /90/$ Elsevier Science Publishers B.V. (North-Holland)

2 226 R.G. Starck l/, G.G. Thomas / SQUG Generic Implementation Procedure Table 1 Equipment classes o Other 1 Motor control centers 2 Low voltage switchgear 3 Medium voltage switchgear 4 Transfonners 5 Horizontal pumps 6 Vertical pumps 7 Fluid-operated valves SA Motor-operated valves 8B Solenoid-operated valves 9 Fans 10 Air handlers 11 Chillers 12 Air compressors 13 Motor generators 14 Distribution panels 15 Batteries and racks 16 Battery chargers and inverters 17 Engine-generators 18 Instrument racks 19 Temperature sensors 20 Control and instrumentation panels and cabinets equipment. The GIP was purposely written not to be overly prescriptive in order that sound judgment could be applied during these walkdowns. This judgment must be first hand and well founded. and based either on SQUG experience, SRT individual member experience, or on the plant-specific construction, analysis, or test documentation. 2. Seismic evaluation personnel The responsibilities and qualifications of the individuals who implement this procedure are defined in the GIP. The method of organizing the individuals for implementing the procedure is up to the discretion of the utility. The utility may either assign responsibilities to existing utility departments, assign dedicated SRT(s) from within the utility, or hire contractor personnel to be the SRT(s). The scheduling of the screening walkdowns are also at the utility's discretion (but with agreement from the NRC) and may be performed over a short or long time period. The GIP requires that there be at least two seismic capability engineers to perform the actual equipment evaluations and certify the results of the screening walkdown for each item of equipment evaluated. All seismic capability engineers on the SRT must be in agreement with the conclusion reached by the team. In addition to these engineers, other personnel can be used to assist the seismic capability engineers. Systems or operations engineers, plant guides, and radiation protection personnel are also needed during the plant walkdown. The seismic capability engineers are relied upon to use the generic SQUG experience and their own experience while exercising engineering judgment in conducting and certifying the results of the screening walkdown. They must be able to perform additional analyses and calculations as required or make recommendations for any additional evaluations or physical modifications to equipment when verifying its seismic adequacy. Collectively the SRT must have knowledge of the performance of equipment, systems, and structures during strong motion earthquakes, nuclear plant walkdown experience, knowledge of nuclear design standards, and experience in seismic design, seismic analysis, and test qualification practices at nuclear power plants. Each individual seismic capability engineer must possess a portion of the collective experienced discussed, he must be a degreed engineer or equivalent, and have at least five years' experience in earthquake engineering applicable to nuclear power plants. At least one member per SRT must be a licensed professional engineer. Other engineers can be used to assist the seismic capability engineers. They may perform background work to obtain information necessary for screening. They can locate and assist in evaluating existing seismic qualification data. They may also perform calculations as needed under the seismic capability engineers' supervision. The systems or plant operations engineers are responsible to supply information about plant layout, operations, function of the individual equipment, instrumentation, and control systems. The qualifications of these engineers is that they must be able to perform their individual responsibilities and have experience at the specific plant being evaluated. The systems engineers are also responsible for developing the list of equipment required for safe shutdown. These engineers must be degreed engineers or equivalent and have experience in nuclear plant systems engineering. In addition to the seismic evaluation personnel described above, relay evaluation personnel must be selected to perform the relay screening and evaluation. The relay evaluation team is responsible for selecting the relays whose malfunction would adversely affect the plant safe shutdown system. Electrical engineering is the primary engineering discipline involved. However,

3 R.G. Starck II, G.G. Thomas / SQUG Generic Implementation Procedure 227 this activity may involve other engineering disciplines as well. The lead relay evaluation team member should be a degreed, experienced electrical engineer who is familiar with the relay screening and evaluation procedure. In addition to the above qualifications, the seismic capability engineers and lead relay evaluation engineers are required to successfully complete a SQUG training course. 3. Identification of safe shutdown equipment The procedure for identifying equipment within the scope of the USI A-46 review consists of three major steps. The first step is to identify major system alternatives to bring an operating reactor to a safe shutdown condition and maintain it there during the 72-hour period following an SSE. This is accomplished by determining which systems are available to control the following four safe shutdown functions: reactor reactivity, reactor coolant pressure, reactor coolant inventory, and decay heat removal. These functions focus on controlling the nuclear, thermal, and hydraulic performance of the reactor and the reactor coolant system. The safe shutdown capability should remain intact while off-site power is unavailable for a minimum of 72 hours following an earthquake. No other accidents are postulated to occur. The second step in identifying equipment for a USI A-46 review is to select one of the safe shutdown alternatives as the preferred method. The method selected should not be dependent upon a single item of equipment whose failure, due either to seismic loads or to random failure, would prevent safe shutdown by the identified means. Failure means loss of the active functional capability of the equipment, not its structural integrity. This single equipment failure criterion can be met by a second independent train of equipment. Alternately, a single train of equipment can be used if there is redundant equipment in this train, e.g., two pumps in parallel, two isolation valves in series, etc. Manual operation of equipment which is normally power-operated is considered an acceptable means of providing back-up operation. Procedures should be in place for operating the equipment selected for safe shutdown and plant operators should be trained in their use. The third step in identifying equipment for a US! A-46 review is to identify the individual items of equipment contained in the preferred safe shutdown path and its backup. The scope of equipment includes the equipment classes listed in table 1 plus tanks and heat exchangers. Two lists of equipment should be developed. The first should contain only the active mechanical and electrical equipment which operate or change state to accomplish a safe shutdown function. Tanks and heat exchangers in the safe shutdown systems should also be added to this list. The equipment on this list should be evaluated for seismic adequacy during the screening verification and walkdown, as described later in this paper. The second list should contain any electrically powered or controlled equipment in these safe shutdown systems which could inadvertently start, stop, or change state due to relay chatter. The relays in the control circuits of the equipment in this second list should be evaluated for the potential of relay chatter as described later. The equipment in these two safe shutdown equipment lists should include the sub-systems and supporting systems needed by the equipment for it to operate, e.g. power supplies, control systems, cooling systems, lubrication systems, instrumentation, etc. The scope of cable and conduit raceways to be evaluated includes all raceway systems which support cable and conduit for safe shutdown equipment. However, to simplify the evaluation, most of the raceway systems in the plant are evaluated in this program rather than only those which support safe shutdown cable and conduit. 4. Screening verification and walkdown The purpose of the screening verification and walkdown is to screen out from further consideration those items of safe shutdown equipment which can be shown to meet certain criteria. This purpose is accomplished through an inspection of each item of safe shutdown equipment using trained experienced engineers that use engineering judgment based upon the GIP guidelines. There are four basic criteria that must be satisfied in order to verify acceptance of candidate equipment during the screening walkdown. The first criterion is that the seismic capacity response spectra must generally envelop the seismic demand spectra over the frequency range of interest. The second criterion is that the candidate equipment must be similar to equipment in the existing seismic experience data bases. The third criterion is that the anchorage capacity, installation, and rigidity must be adequate. The last criterion is that seismic spatial interactions must not cause equipment to fail to perform its required safe shutdown function. The

4 228 R.G. Starck II, G.G. Thomas / SQUG Generic Implementation Procedure results of the screening walkdown are documented on the Screening Verification Data Sheets (SVDS). This documentation is discussed later in this paper. The evaluation of cable and conduit raceway systems is similar to the above approach except that quantitative evaluation criteria are applied only to the most seismically vulnerable portions of these systems Seismic capacity vs. seismic demand In comparing the seismic capacity of an item of equipment to the seismic demand, the frequency range of interest is from 1 to 33 Hz or from the equipment's fundamental natural frequency to 33 Hz. It is acceptable for the SRT to estimate the equipment's fundamental natural frequency if necessary by either referring to the candidate equipment's test or analysis documentation or by the collective judgment of the SRT. The equipment seismic capacity response spectrum is defined by either the 5% damped earthquake experience bounding spectrum, the equipment generic equipment ruggedness spectra (GERS), or component-specific test or analysis documentation. The earthquake experience bounding spectrum was developed during the SQUG program based on the successful performance of many items of equipment in numerous earthquakes. This bounding spectrum represents approximately two-thirds of the estimated average free-field ground motion to which the data base plants were actually exposed for those sites with estimated mean peak ground accelerations in excess of about OAg. The two-thirds factor is used to account for possible additional building amplification of nuclear plant structures compared with the data base plants which were exposed to strong motion earthquakes. This allows the SRT to directly compare the design basis earthquake ground response spectra (GRS), for the plant being reviewed, to the earthquake experience bounding spectrum for equipment with frequencies in excess of about 8 Hz which are mounted below about 40 ft above grade; this is generally the majority of the equipment in the plant. For equipment with a natural frequency less than about 8 Hz, the floor response spectra (FRS) should be compared to 1.5 times the earthquake experience bounding spectrum (i.e., remove the two-thirds factor). For equipment mounted higher than about 40 ft above grade, the FRS should be compared to 1.5 times the earthquake experience bounding spectrum. It is also permissible to use the generic equipment ruggedness spectra (GERS) for the equipment seismic capacity. The GERS were developed based on the results of numerous shake table tests. Likewise, equipment-specific test data or analysis results can be used to establish the equipment seismic capacity. This capacity should generally envelop the demand defined as either 1.5 times the unbroadened realistic mean-center FRS, or 1.0 times the conservative FRS. Conservative FRS are those to which the plant was originally licensed or FRS generated in accordance with current NRC Regulatory Guides and Standard Review Plan. If the equipment is mounted below about 40 ft above grade, the GERS can then be compared to 2.25 times the unbroadened 5% damped GRS. Procedures are also in place for verifying in-line equipment that account for amplifications that may occur through the piping (or other) system to which the candidate equipment is attached Equipment similarity In order to verify that the candidate equipment is similar to equipment in the data bases, the SRT determines if the "intent" of certain equipment class caveats are met. The caveats describe the bounds of similar equipment in the earthquake experience and testing data bases. When using the earthquake experience bounding spectrum for defining the equipment seismic capacity, the class caveats are based on the Senior Seismic Review and Advisory Panel (SSRAP) report [2]. These caveats were developed from the SQUG earthquake experience data base. The GERS caveats are based on the SSRAP and EPRI reports [2,3]. In order to use the GERS, the equipment must also satisfy all of the class caveats for the bounding spectrum. To satisfy the caveat requirements, the SRT must have a thorough understanding of the caveat origins documented in the GIP reference reports. The class caveats do not include every possible situation that may compromise the equipment's seismic ruggedness. The SRT is expected to exercise its engineering judgment in addressing these uncommon situations. There are two generic caveats that must be satisfied in order to meet the GIP requirements. The first is that seismically sensitive relays essential to safe shutdown must be evaluated. The second is that the equipment anchorage must have adequate capacity, and stiffness and must be properly installed. The relay and anchorage evaluations are discussed later in this paper Equipment anchorage In performing the anchorage evaluation, the SR T reviews the plant's existing documentation, including drawings, specifications, calculations, and typical de-

5 R.G. Starck II, G.G. Thomas / SQUG Generic Implementation Procedure 229 tails. During the screening walkdown the number and size of the anchorage are verified. All anchorage is visually inspected to check for proper installation, unless justification is provided and documented. A bolt tightness check for expansion anchor bolts is required for certain situations. The tightness check is performed by torquing the expansion anchors to a specified level. This tightness check may be performed prior to the walkdown. To check that the anchorage capacity is greater than the demand, a demand acceleration level is determined. The demand acceleration level for rigid equipment is the zero period acceleration, ZPA, of the applicable response spectrum. The demand acceleration for flexible equipment is the peak of the applicable response spectrum over the frequency range of interest (equipment fundamental frequency to 33 Hz). The applicable response spectra for equipment located below about 40 ft above grade is either 1.25 X 1.5 X GRS, 1.25 X realistic mean-centered FRS, or 1.0 X conservative FRS. Above about 40 ft above grade, the applicable response spectra is either 1.25 X realistic mean-centered FRS, or 1.0 X conservative FRS. In cases where there are two applicable horizontal spectra, one North-South and one East-West, the SRT should use either the higher of the two spectra, the actual acceleration levels in each direction, or the spectra which is aligned in the direction of weak anchorage. It is appropriate for most equipment to use the 5 % damped spectra to define seismic demand. In the cases of simple, lightly loaded, typically welded instrument racks and more massive equipment, a lower level of damping should be used. There are two anchorage evaluation methods, developed by EPRI [4], that have been incorporated into the GIP, either one of them can be used. The first is the conservative method where minimum anchorage requirements for certain equipment type are given. The conservative anchorage requirements are based on the equipment having 50% more anchor bolts than necessary and using a factor of safety of 4 for the anchor bolts. The equipment should have at least 6 anchor bolts. A visual inspection of the anchorage is also required for this evaluation. The second anchorage assessment method is the screening anchorage evaluation. A more rigorous inspection procedure for the anchorage, including a bolt tightness check on a sampling of bolts, is required for this method. This method is based upon a factor of safety of 3 to failure for expansion bolts. Allowable stress limits are also given for both welds and cast-inplace bolts. A series of tables and charts is available to compare anchorage capacity to demand or calculations may be performed by hand or by computer, with two IBM-PC based computer programs available for use during the walkdown. During the anchorage evaluation, the SRT also determines if the anchorage is adequately rigid. The major concerns are that flexible anchorages may shift the natural frequency of the equipment below that of the experience data base and equipment may lift up and then bang down during an earthquake. In order to ensure that there is sufficient anchorage rigidity in equipment containing essential relays (i.e. relays required to function during and/or immediately after an earthquake), the bolts for that equipment must be checked for tightness and must use a factor of safety of at least 4. Base isolation systems for equipment must also be evaluated for seismic adequacy Seismic interaction The GIP requirement for seismic interaction is for the SR T to identify any interaction with other nearby equipment or structures that could adversely affect the equipment's safe shutdown function. There are three interaction issues within the scope of the GIP. The first issue is proximity, which includes impact from adjacent equipment due to relative motion. The second issue is structural failure and falling. These issues are commonly called II over I issues when applied to newer nuclear facilities, and include impact from the failure of overhead and adjacent equipment, structures, or architectural features. The last issue is flexibility of attached lines which includes distribution lines attached to safe shutdown equipment. If an item of safe shutdown equipment contains an essential relay that could chatter, any impact with adjacent equipment or structures is not acceptable. Prescriptive limits for interaction are not included in the GIP; rather. guidance is given in the form of several examples that emphasize engineering judgment. Housekeeping issues are also addressed Screening walkdown completion At the completion of the screening verification and walkdown, each equipment item should be identified as either verified or an outlier. It is important to note that outliers are not in general deficiencies, they are simply equipment items that could not be screened. Reasons for identifying an equipment item as an outlier are either that demand exceeds capacity, the equipment fails to satisfy the intent of the class caveats, the

6 230 R.G. Starck II, G.G. Thomas / SQUG Generic Implementation Procedure equipment anchorage capacity is exceeded by demand or has excessive flexibility, there is a seismic interaction which could affect the item of equipment, or the information necessary for verification was unavailable during the screening walkdowd Screening walkdown guidance The GIP also provides guidance to improve. the efficiency and success of the screening walkdowll. One appendix in the GIP describes the preparations prior to the walkdown to maximize walkdown effectiveness. A second appendix describes an approach which can be used to perfonn the screening evaluation. This approach is based on the experience gained in performing the trial plant reviews. 5. Outlier identification and resolution At the completion of the screening process, the SRT is responsible for documenting any outlier concern(s) on the Outlier Seismic Verification Sheet (OSVS). The reasons for outliers were identified previously in this paper. The SRT may also suggest a method to reconcile the outlier. The utility management determines who perfonns the outlier resolution and the best approach for resolu tion. Outlier resolution is subject to review by the NRC. Options available for outlier resolution discussed in the GIP include expansion of the earthquake or testing data bases, more rigorous evaluation techniques, mod ification of the equipment to bring it within the scope of the data bases, replacement of the equipment, in situ vibration testing, shake table testing. or the evaluation of infonnation not available during the screening pro cess. In order to document the outlier resolution, the utility, through their licensing organization, should determine the method to be used to resolve each outlier and the schedule for its resolution. The utility should track, and implement, the outlier resolution using their existing licensing procedures. The NRC should be ad vised of the identification and resolution of outliers just like any other safety issue. 6. Relay functionality review The GIP also includes seismic evaluation criteria for relays used in the circuits of safe shutdown equipment. The concern with relays is that their contacts may chatter during an earthquake and cause the equipment they control to operate improperly. The methodology for evaluating the seismic func tionality of relays is based on a two part screening process. The first part will identify a minimum set of plant systems, and associated electrical relays, which are required to function to maintain the plant in a safe condition during and after an earthquake. This screen ing process is intended to significantly reduce the number of systems and electrical circuits which are consid ered essential to plant safety in an earthquake and, therefore, to reduce the number of relay types whose seismic functionality would have to be demonstrated under current licensing criteria. The second part of the evaluation process uses relay and test data to assess the seismic ruggedness of the essential relays used in these plants without the need for expensive, type-by-type testing as is the current practice. Taken together, these two screening approaches are expected to make the relay functionality verification under USI A-46 manageable and significantly more cost effective than would be the case under current licensing criteria, while at the same time provide good assurance that the affected plants can safely shut down and maintain safe shutdown conditions during and after a major earthquake. These screening processes are discussed in more detail in reference [5]. 7. Tanks and heat exchangers review Detailed evaluation procedures are presently being developed by EPRI and SQUG for tanks and heat exchangers. These procedures are expected to be included in the GIP in Cable tray and conduit raceway review Detailed evaluation procedures are presently being developed by SQUG for cable tray and conduit raceways. These procedures are expected to be included in the GIP in Documentation Required documentation in the GIP is kept to a minimum since evaluations are based on the use of sound judgment being exercised by qualified engineering professionals.

7 RG. Starck II, G.G. Thomas / SQUG Generic Implementation Procedure 231 The required documentation for the selection of safe shutdown equipment includes a description of the method selected for safe shutdown, the equipment selected (documented on the Safe Shutdown Equipment List, SSEL), and the plant operating procedures used for achieving and a summary of maintaining safe shutdown. The SSELs must be certified by the systems engineer selecting the safe shutdown equipment. The required documentation in the GIP for the relay evaluation includes the SSEL used to initiate the procedure, identification of essential relays, screening and evaluation results, walkdown results, and outliers and corrective actions, if any. The results of the relay evaluation are certified by the lead relay evaluator. The results of the screening verification and walkdown are documented on Screening Verification Data Sheets (SYDS). The SYDS are tables tbat only include the basic requirements of the GIP; capacity greater than demand, caveats satisfied, anchorage adequate, and no unacceptable seismic interaction. These sheets must be certified by all seismic capability engineers on the SRT that evaluated the equipment item. The GIP recommends the use of an IBM-PC based data base management system for completing both SSELs and SVDSs. In addition to the SVDS. non-mandatory but recommended Seismic Evaluation Work Sheets (SEWS) are included for each equipment class in an appendix of the GIP. The SEWS are intended to be used as checklists and for recording informal notes by the walkdown engineers to refer to at a later time if that became necessary. The Outlier Seismic Verification Sheets, (OSVS) described above are required documentation along with the documentation used for the outlier's final resolution. The resumes of the Seismic Capability Engineers on each SRT are required documentation in the GIP. 10. Conclusions The GIP provides the framework and guidance based on the experience gained by SQUG for qualified seismic review team members to verify the seismic adequacy of safe shutdown equipment. This paper provided a brief overview of the GIP requirements. References [1] Generic Letter 87-02, Verification of seismic adequacy of mechanical and electrical equipment in operating reactors, Unresolved Safety Issue (USI) A-46, U.S. Nuclear Regulatory Commission, Washington, D.C. (Feb. 1987). [2] SSRAP Report, Use of seismic experience data to show ruggedness of equipment in nuclear power plants, Senior Seismic Review and Advisory Panel (April 1990). [3] EPRI NP-5223, Generic seismic ruggedness of nuclear plant equipment, ANCO Engineers, Inc., Culver City, California (July 1989). [4] EPRI NP-5228, Seismic verification of nuclear plant equipment anchorage, URS Corporation/John A. Blume and Associates, Engineers, San Francisco, California (May 1987). [5] J. Betlack and R. Carritto, Procedure for evaluating nuclear power plant relay seismic functionality, MPR Associates, Inc., Washington, DC (July 1987).