NRC Generic Letter 1991-04
NUCLEAR UNITED STATESi NUCLEA R REGULATORY COMMISSION, SWASHINGTON, D.C. 25April 2, 1991TO: ALL HOLDERS OF OPERATING LICENSES OR CONSTRUCTION PERMITS FORNUCLEAR POWER REACTORSSUBJECT: CHANGES IN TECHNICAL SPECIFICATION SURVEILLANCE INTERVALS TOACCOMMODATE A 24-MONTH FUEL CYCLE (Generic Letter 91-04)Improved reactor fuels allow licensees to consider an increase in the durationof the fuel cycle for their facilities. The staff has reviewed requests forindividual plants to modify surveillance intervals to be compatible with a24-month fuel cycle. Enclosure 1 provides generic guidance for preparing suchlicense amendment requests.Technical specifications (TS) that specify an 18-month surveillance intervalcould be changed to state that these surveillances are to be performed onceper refueling interval. The notation for surveillance intervals would then bechanged to include the definition of a "Refueling Interval" with the existing"R" notation for surveillances that are generally performed during a refuelingoutage. The frequency for the interval indicated by this notation would alsobe changed from 18 months to "At least once every 24 months." The provision toextend surveillances by 25 percent of the specified interval would extend thetime limit for completing these surveillances from the existing limit of22.5 months to a maximum of 30 months.The interval for conducting steam generator (SG) inservice inspections (ISIs)is worthy of special consideration in extending the surveillance intervals tobe compatible with a 24-month fuel cycle. The frequency of SG tube rupturesand their possible effect on safety has prompted the staff to provide analternative with which to extend the existing 24-month interval requirementsfor ISIs. This alternative provides conservative methods for verifying SGintegrity, including increasing the number of tubes in the sample for aninspection based upon the TS category of the results from the previousinspection. In some cases, the results of previous inspections may warrantanalyzing SG tube integrity and reducing the TS limit on leakage between theprimary and secondary coolant systems. Finally, an extension of the inspectioninterval would not be appropriate if the results of the previous inspection ofSG tube integrity were in the lowest (C-3) category.Licensees must address instrument drift when proposing an increase in the sur-veillance interval for calibrating instruments that perform safety functionsincluding providing the capability for safe shutdown. The effect of theincreased calibration interval on instrument errors must be addressed becauseinstrument errors caused by drift were considered when determining safetysystem setpoints and when performing safety analyses. Enclosure 2 describesinformation required to address the effect that instrument drift caused by anincreased calibration interval can have on safety.For other 18-month surveillances, licensees should evaluate the effect onsafety of the change in surveillance intervals to accommodate a 24-month fuel9103270310 U d+ it '?1'4LIDR *Ooze o50caoos p qot- Generic Letter 91-04 ~ -2 -' _ April 2, 1991cycle. This evaluation should support a conclusion that the effect on safetyis small. In addition, licensees should confirm that historical maintenanceand surveillance data do not invalidate this conclusion. Licensees shouldconfirm that the performance of surveillances at the bounding surveillanceinterval limit provided to accommodate a 24-month fuel cycle would not inval-idate any assumption in the plant licensing basis. In consideration of theseconfirmations, the licensees need not quantify the effect of the change in sur-veillance intervals on the availability of individual systems or components.The TS changes do not alter the required 24-month testing interval for perform-ing Type B and C tests under the current testing requirements of Appendix J toPart 50 of Title 10 of the Code of Federal Regulations (10 CFR Part 50). TheU.S. Nuclear Regulatory Commission (NRC) is considering changes to Appendix Jthat would accommodate a 24-month fuel cycle. However, it is anticipated thatlicensees will desire an extension of the current 24-month testing interval ofAppendix J to accommodate a 24-month fuel cycle instead of a unit shutdown toperform these tests. An increase in the testing interval for Type B and Ctests will require a request for an exemption from the Appendix J requirements.Licensees desiring an exemption from the 24-month testing interval shouldprovide supporting leak testing data to demonstrate that the requested testinterval would not provide unacceptable results. Enclosure 3 provides guidanceon the information needed to support a request for an exemption to the Type Band C test interval requirements in Appendix J.The enclosed guidance is provided to support proposed TS changes and a requestfor an exemption to Appendix J requirements for licensees that plan to adopt a24-month fuel cycle. Proposed amendments that deviate from this guidance willlengthen the time required to complete the review. Please contact the NRCproject manager or the contact indicated below if you have questions on thismatter.Any response to the NRC suggestion for TS changes is voluntary. Therefore, anyaction taken in response to the guidance provided in this generic letter isnot a backfit under 10 CFR 50.109. Likewise, an Office of Management andBudget clearance is not required.Sincerely,Jaes 6. PartlowAs ociate Director for ProjectsOffice of Nuclear Reactor RegulationEnclosures:As statedContact: Tom Dunning, OTSB/NRR(301) 492-11894 * -.,.,A
Generic Letter 91-04Enclosure 1GUIDANCE ON PREPARATION OF A LICENSE AMENDMENT REQUEST FORCHANGES IN SURVEILLANCE INTERVALS TO ACCOMMODATE A 24-MONTH FUEL CYCLEDISCUSSIONLicensees are planning to use improved reactor fuels because of the significanteconomic benefits associated with a longer fuel cycle. A longer fuel cycleincreases the time interval between refueling outages and the performance ofthe associated technical specification (TS) surveillance requirements. Thisguidance addresses TS changes to accommodate a 24-month fuel cycle for thosesurveillances that are performed at each 18-month or other refueling outageinterval.The NRC staff has reviewed a number of requests to extend 18-month surveil-lances to the end of a fuel cycle and a few requests for changes in surveil-lance intervals to accommodate a 24-month fuel cycle. The staff has foundthat the effect on safety is small because safety systems use redundant electri-cal and mechanical components and because licensees perform other surveillancesduring plant operation that confirm that these systems and components canperform their safety functions. Nevertheless, licensees should evaluate theeffect on safety of an increase in 18-month surveillance intervals to accommo-date a 24-month fuel cycle. This evaluation should support a conclusion thatthe effect on safety is small. Licensees should confirm that historical plantmaintenance and surveillance data support this conclusion. Also, licenseesshould confirm that assumptions in the plant licensing basis would not beinvalidated on the basis of performing any surveillance at the boundingsurveillance interval limit provided to accommodate a 24-month fuel cycle.In consideration of these confirmations, the licensees need not quantify theeffect of the change in surveillance intervals on the availability ofindividual systems or components.TECHNICAL SPECIFICATION CHANGESLicensees should propose TS changes to accommodate a 24-month fuel cycle bymodifying 18-month surveillances to indicate that they are to be performedat least once each REFUELING INTERVAL." In addition, the proposedchanges should modify the surveillance interval notation in Table 1.1 in theDefinitions Section of the TS to include the term "REFUELING INTERVAL" alongwith the "R" notation to define the frequency for surveillances that are spec-ified to be performed once each refueling interval. (Capitalization of theterm "refueling interval" is used in the TS to designate a defined term.) Theproposed TS change should modify the frequency for this surveillance intervalnotation from "At least once per 18 months" to "At least once per 24 months" todefine the nominal frequency for surveillances that are specified to be per-formed each refueling interval or with the "R"' notation. The bounding timeinterval for these surveillances would then be 30 months under the provision ofTS 4.0.2 that allows a surveillance to be extended by 25 percent of thespecified interval.Licensees may omit the TS qualification that an 18-month surveillance is to beperformed "during shutdown' when specifying the surveillance interval asI. ..at least once per REFUELING INTERVAL." Because the terms "Hot" and"Cold Shutdown" are defined in the TS as operating modes or conditions, theadded restriction to perform certain surveillances during shutdown may be mis-interpreted. This restriction ensures that a surveillance would only be Generic Letter 91-04 -2 --' Enclosure Iperformed when it is consistent with safe plant operation. However, this con-sideration is valid for other surveillances that are performed during poweroperation, plant startup, or shutdown, but is not addressed by restricting theconduct of these surveillances.The staff concludes that the TS need not restrict surveillances as only beingperformed during shutdown. Nevertheless, safety dictates that when refuelinginterval surveillances are performed during power operation, licensees giveproper regard for their effect on the safe operation of the plant. If the per-formance of a refueling interval surveillance during plant operation wouldadversely affect safety, the licensee should postpone the surveillance untilthe unit is shut down for refueling or is in a condition or mode that is con-sistent with the safe conduct of that surveillance.The NRC provided an updated basis for TS 4.0.2 in Generic Letters (GLs) 87-09,"Sections 3.0 and 4.0 of Standard Technical Specifications on the Applicabilityof Limiting Conditions for Operation and Surveillance Requirements," and 89-14,"Line-Item Improvements in Technical Specifications -Removal of the 3.25 Limiton Extending Surveillance Intervals." However, the TS changes to accommodate alonger fuel cycle will also alter the basis for TS 4.0.2. Therefore, licenseesshould update the Bases Section of TS 4.0.2 to be consistent with these TSchanges and particularly with respect to the safe conduct of refueling intervalsurveillances. The changes to the associated paragraph of the previous guid-ance on the Bases Section of TS 4.0.2 are underlined, as follows:It also provides flexibility to accommodate the length of a fuelcycle for surveillances that are specified to be performed atleast once each REFUELING INTERVAL. It is not intended that thisprovision be used repeatedly as a convenience to extend surveil-lance intervals beyond that specified for surveillances that arenot performed once each REFUELING INTERVAL. Likewise, it is notthe intent that REFUELING INTERVAL surveillances be performedduring power operation unless it is consistent with safe plantoperation. The limitation of Specification 47.0. is ..Licensees should incorporate these changes to the Bases Section of TS 4.0.2where they have been modified as identified in GLs 87-09 and 89-14. Otherwise,the Bases Section should be updated to reflect the intent of this guidance.The proposed amendment request should include a copy of the updated BasesSection for TS 4.0.2.The surveillance interval for performing the second inservice inspection ofsteam generators is currently specified with a bounding time limit of 24 calen-dar months after the previous inspection. The interval for subsequent inspec-tions may be extended to a maximum of 40 months if the results from two consec-utive inspections, excluding the preservice inspection, are within the C-1Category or if two consecutive inspections demonstrate that previously observeddegradation has not continued and no additional degradation has occurred. How-ever, for plants having inspection results in the C-2 Category from inspectionsof steam generators (SGs) during either of the two previous inspections, thebounding interval for the next inspection would be 24 months from the lastinspection.
Generic Letter 51-04-3 -Enclosure 1A 24-month inspection interval may not coincide with the next refueling outagewhen operating on a 24-month fuel cycle, particularly if any outage time isaccumulated over the duration of the fuel cycle or if startup for the next fuelcycle is delayed following the completion of a SG inspection. Therefore, thestaff developed an alternative to compensate for any delay that could cause theinterval for SG inspections to occur near the end of a 24-month fuel cycle butbefore the refueling outage. The alternative includes the following: (1) anincrease in the sample size of tubes examined during the previous inspection,(2) a suitable analysis of the integrity of SG tubes if the results of eitherof the two previous inspections were in the C-2 Category, and (3) a morerestrictive limit for leakage between the primary and secondary coolant systemsfor operation beyond 24 months after the previous inspection. These consider-ations provide an acceptable basis with which to permit the next inspectioninterval to be compatible with the 30-month bounding limit for refueling inter-val surveillances if the results of either of the two previous inspections werenot in the C-3 Category.The current TS requirements permit inspections to be conducted at 40-monthintervals if the results of two previous inspections are in the C-1 Category.However, this may not provide a practical alternative for facilities that wouldoperate on a 24-month fuel cycle. The inspection results would be in the C-2Category if only one defective tube were found during either of the two prev-ious inspections, and this would preclude the use of the provision for extend-ing the inspection interval to 40 months.The alternatives for the TS section on SG inspection frequencies are shownunderlined based on the current STS requirements.4.4.5.3 Inspection Frequencies -The above required inservice inspections ofsteam generator tubes shall be performed at the following frequencies:a. The first inservice inspection shall be performed after 6 EffectiveFull Power Months but within 24 calendar months of initialcriticality. Subsequent inservice inspections shall be performed atintervals of not less than 12 nor more than 24 calendar months afterthe previous inspection. If 20 percent of the tubes were inspectedand the results were in the C-1 Category or if 40 percent of thetubes were inspected and were in the C-2 Category during the previouinspection, the next inspection may be extended up to a maximum of30 months in order to correspond with the next refueling outage ifthe results of the two previous Inspections were not in the C-3Category. However, if the results of either of the previous twoinspections were in C-2 Category an engineering assessment shall beperformed before operation beyond 24 months and shall provide assur-ance that all tubes will retain adequate structural margins againstburst throughout normal operating, transient, and accident conditionsuntil the end of the fuel cycle or 30 months, which ever occurs first.If two consecutive inspections, not including the preservice inspec-tion, result in all inspection results falling within the C-1 Cate-gory or if two consecutive inspections demonstrate that previouslyobserved degradation has not continued or no additional degradationhas occurred, the inspection interval may be extended to a maximum ofonce per 40 months;
Generic Letter 91-04 -4- Eniosure 1b. If the results of the inservice inspection of a steam generatorconducted in accordance with Table 4.4.2 at 40-month intervals fallinto Category C-3, the inspection frequency shall be increased to atleast once per 20 months. The increase in inspection frequency shallapply until the subsequent inspections satisfy the criteria ofSpecification 4.4.5.3a.; the interval may then be extended to amaximum of once per 30 or 40 months, as applicable;c. (no change to unscheduled inservice inspection requirements.); andd. The provisions of Specification 4.0.2 do not apply for extending thefrequency for performing inservice inspections as specified inSpecifications 4.4.5.3a. and b.The staff added TS 4.4.5.3d. to clarify its position that the provision ofTS 4.0.2 does not apply to extend SG inspection intervals because TS 4.4.5.3a.addresses those conditions under which the 24-month surveillance interval forSG tube inspections may be extended and TS 4.4.5.3b. addresses conditions underwhich the surveillance interval for inspections shall be reduced to at leastonce every 20 months.Licensees should update the Bases Section of TS 3/4.5.4 to clarify the intentof the engineering assessment of SG tube integrity addressed in the aboveaddition to TS 4.4.5.3a. by adding the following:An engineering assessment of steam generator tube integritywill confirm that no undue risk is associated with plantoperation beyond 24 months of the previous steam generatortube inspection. To provide this confirmation, the assess-ment would demonstrate that all tubes will retain adequatestructural margins against burst during all normal operating,transient, and accident conditions until the end of the fuelcycle. This evaluation would include the following elements:1. An assessment of the flaws found during the previousinspection.2. An assessment of the maximum flaw size that can be expect-ed before the end of the current fuel cycle or 30 months,which ever comes first, and the corresponding structuralmargins relative to the criteria of Regulatory Guide 1.121,"Bases for Plugging Degraded PWR Steam Generator Tubes."3. An update of the assessment model, as appropriate, basedon comparison of the predicted results of the steam generatortube integrity assessment with actual inspection results fromprevious inspections.Along with the above alternative for inspection intervals, the followingaddition to the TS section on reactor coolant system operational leakage isrequired and shown underlined for the current STS requirements.
Generic Letter 91-04 -5 -Enclosure 13.4.6.2 Reactor Coolant System leakage shall be limited to:a. & b. (No change.)c. 1 GPM total reactor-to-secondary leakage through all steam generatorsnot isolated from the Reactor Coolant System and [500] gallons perday through any one steam generator not isolated from the ReactorCoolant System. For plant operation beyond 24 months from theprevious steam generator tube inspection when the results of eitherof the two previous inspections are in the C-2 Category as definedby Specification 4.4.5. , the leakage through any one steam generatornot isolated from the Reactor Coolant System shall not exceed 100gallons per day,d. & e. (No change.)
Generic Letter 91-04Enclosure 2GUIDANCE FOR ADDRESSING THE EFFECT OF INCREASEDSURVEILLANCE INTERVALS ON INSTRUMENT DRIFTAND SAFETY ANALYSIS ASSUMPTIONSDiscussionThe U.S. Nuclear Regulatory Commission (NRC) staff determined that licenseesshould address the issue of instrumentation errors caused by drift in order tojustify an increase in surveillance intervals to accommodate a 24-month fuelcycle. Licensees must evaluate the effects of an increased calibration inter-val on instrument errors in order to confirm that drift will not result ininstrument errors that exceed the assumptions of the safety analysis. Instru-ment drift affects the capability of a system to perform its safety functionand is a consideration for determining safety system setpoints. The amount ofinstrument drift that occurs over a long interval between calibrations may notbe readily available from the instrument vendor. However, operating experienceand available vendor data can provide insights on the increase in instrumenterrors that could occur with an increased calibration interval. These insights,with a program to monitor and assess the long-term effects of instrument drift,can provide the basis for increasing the refueling outage related calibrationintervals for instruments that perform safety functions.Justification for Increased Calibration IntervalsLicensees should address a number-of issues to provide an acceptable basis forincreasing the calibration interval for instruments that are used to performsafety functions. The NRC staff has identified a specific action that licen-sees should address for each of these issues in order to justify a proposedincrease in the calibration interval. A summary of the applicable issue isprovided after each of the following actions.1. Confirm that instrument drift as determined by as-found andas-left calibration data from surveillance and maintenance recordshas not, except on rare occasions, exceeded acceptable limits fora calibration interval.The surveillance and maintenance history for instrument channels should demon-strate that most problems affecting instrument operability are found as aresult of surveillance tests other than the instrument calibration. If thecalibration data show that instrument drift is beyond acceptable limits onother than rare occasions, the calibration interval should not be increasedbecause instrument drift would pose a greater safety problem in the future.2. Confirm that the values of drift for each instrument type(make, model, and range) and application have been determinedwith a high probability and a high degree of confidence. Providea summary of the methodology and assumptions used to determinethe rate of instrument drift with time based upon historicalplant calibration data.The licensee should have a body of as-found and as-left calibration data thatpermits the determination of the rate of instrument drift with time over thecalibration interval. This data should allow the determination of instrumentdrift for those instruments that perform safety functions.
Generic Letter 91-04 -2 --Entlosure-23. Confirm that the magnitude of instrument drift has beendetermined with a high probability and a high degree of confidencefor a bounding calibration interval of 30 months for each instrumenttype (make, model number, and range) and application that performsa safety function. Provide a list of the channels by TS sectionthat identifies these instrument applications.The magnitude of the instrument drift error that occurs over a longer intervalis an important consideration to justify an extension of the calibration inter-val for instruments that perform safety functions. Licensees need to identifythe applications where the calibration interval for these instruments dependsupon the length of the fuel cycle and could be as long as 30 months (the exten-sion limit for this calibration interval). Licensees should determine theprojected value of the instrument drift error that could occur over a 30-monthinterval for each of these applications.4. Confirm that a comparison of the projected instrumentdrift errors has been made with the values of drift used inthe setpoint analysis. If this results in revised setpointsto accommodate larger drift errors, provide proposed TS changesto update trip setpoints. If the drift errors result in a revisedsafety analysis to support existing setpoints, provide a summaryof the updated analysis conclusions to confirm that safety limitsand safety analysis assumptions are not exceeded.Licensees should ensure that the projected value of instrument drift for anincreased calibration interval is consistent with the values of drift errorsused in determining safety system setpoints. These setpoints ensure that theconsequences of accidents and anticipated transients are bounded within theassumptions of the safety analysis. If the allowance for instrument drift thatwas used to establish trip setpoints for safety systems would be exceeded,licensees should establish new trip setpoints for safety systems. InstrumentSociety of America (ISA) Standard, ISA-A67.04-1982, "Setpoints for NuclearSafety-Related Instrumentation Used in Nuclear Power Plants," provides amethodology for evaluating instrument drift. The NRC endorsed this standardin Regulatory Guide 1.105, "Instrument Setpoints for Safety-Related Systems."If a new setpoint must be used to ensure that safety actions will beinitiated consistent with the assumptions of the safety analysis, this willrequire a TS revision to reflect a new trip setpoint value. If the combinationof instrument drift errors and current trip setpoints is not consistent withexisting safety analysis assumptions, licensees should perform a new safetyanalysis to confirm that safety limits will not be exceeded with the increaseddrift associated with longer calibration intervals.5. Confirm that the projected instrument errors caused bydrift are acceptable for control of plant parameters to effecta safe shutdown with the associated instrumentation.Licensees should determine the effect of instrument errors on control systemsused to effect a safe shutdown. Licensees must confirm that the instrumenterrors caused by drift will not affect the capability to achieve a safe plantshutdown.
Generic Letgter 91-04-3 -Enclosure 26. Confirm that all conditions and assumptions of the setpointand safety analyses have been checked and are appropriatelyreflected in the acceptance criteria of plant surveillanceprocedures for channel checks, channel functional tests, andchannel calibrations.Licensees should take care to avoid errors or oversights when establishingacceptance criteria for plant surveillance procedures that are derived fromthe assumptions of the safety analysis and the results of the methodology fordetermining setpoints. The NRC staff experience is that licensees haveencountered problems when asked to confirm that instrument drift and othererrors and assumptions of the safety and setpoint analyses are consistent withthe acceptance criteria included in plant surveillance procedures. This reviewshould include channel checks, channel functional tests, and the calibration ofchannels for which surveillance intervals are being increased.7. Provide a summary description of the program for monitoringand assessing the effects of increased calibration surveillanceintervals on instrument drift and its effect on safety.Finally, licensees should have a program to monitor calibration results andthe effect on instrument drift that will accompany the increase in calibrationintervals. The program should ensure that existing procedures provide datafor evaluating the effects of increased calibration intervals. The datashould confirm that the estimated errors for instrument drift with increasedcalibration intervals are within the limits projected.In summary, licensees can provide a justification for increased surveillanceintervals for instrument channel calibration by addressing each of the itemsnoted herein.
Generic Letter 91-04 Enclosure 3GUIDANCE ON INFORMATION NEEDED TO SUPPORT AN EXEMPTION TO APPENDIX JOF 10 CFR PART 50 TO ACCOMMODATE A 24-MONTH FUEL CYCLE
BACKGROUND
The NRC staff is developing changes to Appendix J to 10 CFR Part 50 to resolvea number of problems that have been encountered with this regulation. Thesechanges include revising the surveillance interval for performing Type B and Cleak testing in order to accommodate a longer fuel cycle. However, until suchchanges are incorporated in the regulation, the current 24-month surveillanceinterval for Type B and C tests would likely require a plant shutdown to per-form Appendix J leak testing before the completion of a 24-month fuel cycle.To temporarily solve this limitation, the NRC staff has prepared guidance oninformation needed to support an exemption to the requirements of Appendix Juntil the regulation is revised to accommodate a 24-month fuel cycle.DISCUSSIONThe NRC staff guidance on revising surveillance intervals specified in TS pro-vides a bounding time limit of 30 months to accommodate a 24-month fuel cycle.The exemption to Appendix J will require that a 25-percent increase in the24-month surveillance interval for Type B and C leak tests be granted in orderto be compatible with the change in the TS surveillance intervals that accommo-date a 24-month fuel cycle. The NRC staff concludes that licensees shouldaddress two issues to justify an exemption to Appendix J in order for the NRCto grant a request to extend the surveillance interval of Type B and C testsup to 30 months.The first issue is a possible reduction in the combined leakage limit forType B and C tests based upon an increase in the margin to the TS allowableleakage limit which is proportional to the proposed increase in the surveil-lance interval. The acceptance criterion for Type B and C leak tests is acombined leakage rate for all penetrations and valves that are subject toType B and C tests that shall be less than 0.6 of La, where La is the maximumallowable leakage rate that is specified in the TS. This constitutes a marginof 40 percent of La. Licensees should use leak test data to demonstrate thatthis margin will be preserved with the proposed increase in the test interval,or should consider proposing an acceptance criterion limit of less than 0.6 ofLa for Type B and C tests as a TS change.The second issue is that there should be a reasonable basis for licensees toconclude that containment leakage will be maintained within acceptable limitsbased upon the extrapolation of the results of past Type B and C leak tests toaccount for an increase in the surveillance interval up to 30 months. Theexemption request should include a summary of the methodology used and resultsobtained that support this conclusion.In summary, licensees can provide a justification for an exemption toAppendix J by addressing each of the issues noted herein.