Bulletin 93-03: Resolution of Issues Related to Reactor Vessel Water Level Instrumentation in BWRs
OMB No.: 3150-0012 NRCB 93-03 UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 May 28, 1993 NRC BULLETIN 93-03: RESOLUTION OF ISSUES RELATED TO REACTOR VESSEL WATER LEVEL INSTRUMENTATION IN BWRs Addressees All holders of operating licenses or construction permits for boiling water reactors (BWRs) with the exception of Millstone, Unit 1, and Big Rock Point. Purpose The U.S. Nuclear Regulatory Commission (NRC) is issuing this bulletin to (1) notify addressees about new information concerning level indication errors that may occur during plant depressurization, (2) request that all addressees take certain action(s), and (3) require that all addressees report to the NRC if and to what extent the requested actions will be taken and notify the NRC when actions associated with this bulletin are complete. Background As discussed in NRC Information Notice 92-54, "Level Instrumentation Inaccuracies Caused by Rapid Depressurization," and Generic Letter 92-04, "Resolution of the Issues Related to Reactor Vessel Water Level Instrumentation in BWRs Pursuant to 10 CFR 50.54(f)," the staff is concerned that noncondensible gases may become dissolved in the reference leg of BWR water level instrumentation and lead to a false high level indication after a rapid depressurization event. Generic Letter 92-04 requested that addressees determine the impact of potential level indication errors after a rapid depressurization event on how the plants are operated. Generic Letter 92-04 also requested that addressees take short term compensatory measures to mitigate the consequences of potential level indication errors after a rapid depressurization event and provide the staff with plans for long term corrective actions, including any proposed hardware modifications. The generic letter requested that addressees implement the long term corrective actions during the first refueling outage commencing after November 19, 1992. The industry, through the BWR Owners Group (BWROG), requested a delay in the implementation of the long term corrective actions until a de-gas test program could be completed. The test program was intended to gather data to support the design of any necessary hardware modifications. On December 2, 1992, the staff agreed to extend the deadline for the submission of addressee plans for the long term actions to July 1993, with implementation at the earliest opportunity. 9305280173. NRCB 93-03 May 28, 1993 Page 2 of 6 Description of Circumstances During a normal plant cooldown on January 21, 1993, operators at the Washington Public Power System, Unit 2 (WNP-2), observed a sustained level indication error of 0.81 meters [32 inches] that gradually recovered over a period of approximately 2 hours. The licensee determined that errors of this type could result in failure to automatically isolate a leak in the residual heat removal (RHR) system during shutdown cooling operation. On April 8, 1993, the staff issued Information Notice 93-27, "Level Instrumentation Inaccuracies Observed During Normal Plant Depressurization," to discuss level indication errors that may occur during normal plant depressurization. Discussion Following the event reported by the licensee at WNP-2, the NRC staff requested the BWROG to evaluate the effect of level indication errors on events, such as reactor pressure vessel (RPV) drain-down, initiated from low-pressure conditions. Several paths have the potential to drain the RPV. Operator misalignment of one or more valves can establish a flow path resulting in a drain-down of the RPV. Several events of this type have occurred at operating BWRs. Automatic isolation signals based on low RPV level are normally credited for terminating these events. However, automatic isolation of the RHR system, and other systems, will not occur if there are large level errors in multiple instruments. In response to the staff request, the BWROG submitted a report, "Supplementary Information Regarding RPV Water Level Errors due to Noncondensible Gas in Cold Reference Legs of BWRs," to the NRC on May 20, 1993. The BWROG determined that the most limiting drain-down event is an RPV drain-down to the suppression pool through the low-pressure coolant injection suction flow path. The BWROG report indicated that, for this event, the core could reach 1100 C [2000 F] in as little as 16 minutes if there is no makeup to the coolant system. On the basis of the assessment of the NRC staff and the information provided by BWROG, the staff concluded that additional compensatory measures are needed for normal cooldown evolutions. Although the interim procedures currently in place are appropriate for events initiated from full power, they are not adequate for providing protection against events initiated during cooldown when automatic safety systems may be defeated by level instrumentation inaccuracies. In addition, BWROG has completed a reference leg de-gas test program. Although the data are still preliminary, initial results of the test program show that large errors in the indications from the level instrumentation are possible. This information and the event at WNP-2 confirm that the noncondensible gas problem is real and not theoretical, and that the problem applies even to slow depressurizations. Therefore, for longer term operation this problem needs to be addressed promptly with hardware modifications and immediately with compensatory measures for cooldown conditions. . NRCB 93-03 May 28, 1993 Page 3 of 6 Millstone, Unit 1, is exempt from this bulletin because Northeast Utilities, the licensee, has already implemented a hardware modification to prevent the buildup of noncondensible gases in the RPV level instrumentation reference legs. Big Rock Point is exempt from this bulletin because the RPV level instrumentation system installed at that facility is not susceptible to the de-gas problem described in this bulletin. Requested Actions 1. Short Term Compensatory Actions (a) Within 15 days of the date of this bulletin, each licensee is requested to implement the following measures to ensure that potential level errors caused by reference leg de-gassing will not result in improper system response or improper operator actions during transients and accident scenarios initiated from reduced pressure conditions (Mode 3): (1) Establish enhanced monitoring of all RPV level instruments to provide early detection of level anomalies associated with de-gassing from the reference legs. (2) Develop enhanced procedures and additional restrictions and controls for valve alignments and maintenance that have a potential to drain the RPV during Mode 3. (3) Alert operators to potentially confusing or misleading level indication that may occur during accidents or transients initiating from Mode 3. For example, a drain-down event could lead to automatic initiation of high-pressure emergency core cooling systems (ECCS) without automatic system isolation or low- pressure ECCS actuation. Facilities that are in cold shutdown during this 15 day period are requested to complete the above actions within 15 days of the date of this bulletin or prior to startup, whichever is later. (b) By July 30, 1993, each licensee is requested to complete augmented operator training on loss of RPV inventory scenarios during Mode 3, including RPV drain-down events and cracks or breaks in piping. Facilities that are in cold shutdown as of July 30, 1993, are requested to complete this action prior to startup from that shutdown. All of the short term actions described above shall remain in effect until the hardware modifications described below have been implemented. 2. Hardware Modifications Each licensee is requested to implement hardware modifications necessary to ensure the level instrumentation system design is of high functional. NRCB 93-03 May 28, 1993 Page 4 of 6 reliability for long-term operation. This includes level instrumentation performance during and after transient and accident scenarios initiated from both high pressure and reduced pressure conditions. The hardware modifications discussed here are the same as the modifications requested in Generic Letter 92-04. Since the level instrumentation plays an important role in plant safety and is required for both normal and accident conditions, the staff requests that these modifications be implemented at the next cold shutdown beginning after July 30, 1993. If a facility is in cold shutdown on July 30, 1993, each licensee is requested to implement these modifications prior to starting up from that outage. Reporting Requirements Written reports are required as follows: (1) Addressees choosing not to take the requested short term actions must submit a report within 15 days of the date of this bulletin containing a description of the proposed alternative course of action, the schedule for completing it, and a justification for any deviations from the requested actions. (2) By July 30, 1993, all addressees must submit a report providing: (a) the description of the short term compensatory actions taken, and (b) a description of the hardware modifications to be implemented at the next cold shutdown after July 30, 1993. If an addressee chooses not to take the requested actions specified in the Hardware Modifications section, the report shall contain a description of the proposed alternative course of action, the schedule for completing it, and a justification for any deviations from the requested actions. (3) Within 30 days of completion of the requested hardware modifications, a report confirming completion and describing the modification implemented. Address the required written reports to the U.S. Nuclear Regulatory Commission, ATTN: Document Control Desk, Washington, D.C. 20555, under oath or affirmation under the provisions of Section 182a, Atomic Energy Act of 1954, as amended, and 10 CFR 50.54(f). In addition, submit a copy to the appropriate regional administrator. Backfit Discussion The level errors that could result from the effects of noncondensible gases in the level indication reference legs may prevent the level instrumentation systems in BWRs from satisfying the following regulations: (1) General Design Criterion (GDC) 13, "Instrumentation and control," of Appendix A to 10 CFR Part 50 which states: "Instrumentation shall be provided to monitor variables and systems over their anticipated ranges. NRCB 93-03 May 28, 1993 Page 5 of 6 for normal operation, for anticipated operational occurrences, and for accident conditions as appropriate to assure adequate safety." Existing instrumentation may not accurately monitor reactor vessel water level under normal cooldown or accident conditions. (2) GDC 21, "Protection system reliability and testability," which states: "The protection system shall be designed for high functional reliability...commensurate with the safety function to be performed." The instrumentation may not be reliable during and following normal depressurization and rapid depressurization. (3) GDC 22, "Protection system independence," which states: "The protection system shall be designed to assure that the effects of natural phenomena, and of normal operating, maintenance, testing, and postulated accident conditions...do not result in loss of the protection function." Degassing may cause a loss of the reactor vessel water level indication function during and following normal depressurization and rapid depressurization. (4) Section 50.55a(h) of Title 10 of the Code of Federal Regulations (10 CFR 50.55a(h)), which requires that protection systems, for those plants with construction permits issued after January 1, 1971, meet the requirements stated in editions of the Institute of Electrical and Electronics Engineers Standard, "Criteria for Protection Systems for Nuclear Power Generating Stations" (IEEE-279). Section 4.20 of IEEE-279 states: "The protection system shall be designed to provide the operator with accurate, complete, and timely information pertinent to its own status and to generating station safety." The water level instrumentation for the reactor vessel may not be accurate during and following normal depressurization and rapid depressurization. The hardware modifications discussed here are the same as the modifications requested in Generic Letter 92-04 and, therefore, the modifications are not considered to be additional backfits beyond those requested in Generic Letter 92-04. The short term compensatory actions requested by this bulletin are considered necessary to ensure that the addressees are in compliance with existing NRC rules and regulations. Therefore, this bulletin is being issued as a compliance backfit under the terms of 10 CFR 50.109(a)(4). A notice of opportunity for public comment on this bulletin was not published in the Federal Register because of the urgent nature of the short term compensatory actions requested by this bulletin and because the hardware modifications requested are the same as those previously requested in Generic Letter 92-04. Paperwork Reduction Act Statement This bulletin contains information collection requirements that are subject to the Paperwork Reduction Act of 1980 (44 U.S.C. 3501 et seq.). These requirements are covered by Office of Management and Budget clearance number 3150-0012, which expires June 30, 1994. The estimated average number of burden hours is 200 hours per licensee response, including the time for. NRCB 93-03 May 28, 1993 Page 6 of 6 reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for further reducing reporting burden, to the Information and Records Management Branch (MNBB-7714), U.S. Nuclear Regulatory Commission, Washington, D.C. 20555; and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-3019, (3150-0012), Office of Management and Budget, Washington, D.C. 20503. Compliance with the following request for information is purely voluntary. The information would assist NRC in evaluating the cost of complying with this bulletin: (1) the licensee staff time and costs to perform requested inspections, corrective actions, and associated testing (2) the licensee staff time and costs to prepare the requested reports and documentation (3) the additional short-term costs incurred as a result of the inspection findings such as the costs of the corrective actions or the costs of down time (4) an estimate of the additional long-term costs which will be incurred in the future as a result of implementing commitments such as the estimated costs of conducting future inspections or increased maintenance If you have any questions about this matter, please contact the technical contact or the lead project manager listed below or the appropriate Office of Nuclear Reactor Regulation project manager. ORIGINAL SIGNED BY James G. Partlow Associate Director for Projects Office of Nuclear Reactor Regulation Technical contact: Amy E. Cubbage (301) 504-2875 Lead project manager: James W. Clifford (301) 504-1323 Attachment: List of Recently Issued NRC Bulletins .
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