NRC Generic Letter 1992-04
UNITED STATES
NUCLEAR REGULATORY COMMISSION
WASHINGTON, D. C. 20555 August 19, 1992 TO: ALL BOILING WATER REACTOR (BWR) LICENSEES OF
OPERATING REACTORS
SUBJECT: RESOLUTION OF THE ISSUES RELATED TO REACTOR VESSEL
WATER LEVEL INSTRUMENTATION IN BWRs PURSUANT TO
10 CFR 50.54(F) (GENERIC LETTER NO. 92-04)
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this generic letter to request information regarding the adequacy of and corrective actions for Boiling Water Reactor (BWR) water level instrumentation with respect to the effects of noncondensible gases on system operation.
Background and Safety Considerations As discussed in NRC Information Notice No. 92-54 "Level Instrumentation Inaccuracies Caused by Rapid Depressurization,"
the staff is concerned that noncondensible gases may become dissolved in the reference leg of BWR water level instrumentation and can lead to a false high level indication after a rapid depressurization event. The dissolved gases which accumulate over time during normal operation can rapidly come out of solution during depressurization and displace water from the reference leg. A reduced reference leg level will result in a false high level indication. This is important to safety because water level signals are used for actuating automatic safety systems and for guidance to operators during and after an event.
On July 29, 1992, the NRC staff held a public meeting with the Regulatory Response Group (RRG) of the Boiling Water Reactor Owners Group (BWROG) to discuss the effect of inaccuracies in the reactor vessel level instrumentation system in BWRs. During the meeting, theBWROG and its consultant, General Electric Company (GE), presented the results of analyses assessing the safety implications of the postulated error in level indication. The analyses consisted of two basic parts: (1) an assessment of the mechanism and potential magnitude of errors in the level instruments and (2) a review of the relevant licensing basis transients and accidents to determine the effect of this error on plant response, including post-accident operator actions.
The BWROG analyses indicated that significant errors in level indication can occur as a result of degassing the instrument reference leg if noncondensible gas is dissolved in the reference leg and if the reactor abruptly depressurizes below 450 psig.
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Generic Letter 92-04 - 2 - August 19, 1992 The NRC staff reviewed the BWROG analyses basis accident scenarios which lead to a and selected design vessel water level and has concluded that lowering of the reactor systems will be actuated at pressures well automatic safety for postulated worst-case noncondensible above 450 psig, even the reference legs. Therefore, the NRC is gas concentrations in emergency cooling systems will initiate as confident that all do. In addition, the BWROG discussed diverse they were designed to also initiate ECCS for reactor water level signals which would NRC staff reviewed the backup systems and lowering events. The would be initiated by diverse signals as concluded that the ECCS
analyzed by the BWROG.
After ECCS actuation, reactor water level the operators for long term actions (i.e., indication is used by reactor water level and ensuring adequate maintaining adequate Operators would not utilize only reactor core cooling).
to determine accident mitigation actions vessel level indications but would also utilize other indications such as containment pressure, humidity to determine accident mitigation temperature, and Additionally, events characterized by gradualstrategies.
would lead to a reduced error in the indicated depressurization two or four reference leg columns in each level. There are plant design. The amount of noncondensible plant, depending on each depends primarily upon system leakage gases dissolved in of this, a common mode, common magnitude and geometry. Because is unlikely. Operators would therefore see level indication error indicated level alerting them to a level a mismatch in indication problem.
Finally, emergency procedure guidelines (EPGs)
reactor vessel water level is indeterminate, state that when flood the reactor vessel using at least one operators should unaffected diverse instrumentation (i.e., pump guided by the pressure indication). Reactor operators high containment these situations should they occur. are trained to deal with Upon reviewing the information provided by staff's assessment, the staff concluded that the BWROG and the operation is acceptable. The bases for the interim plant are as follows: 1) the level instrumentationstaff's conclusion initiate safety systems prior to a significant is expected to of the reactor; 2) emergency procedures which depressurization place in conjunction with operator training are currently in result in adequate operator actions; and are expected to
3)
depressurization event resulting in a common an abrupt magnitude level indication error is unlikely. mode, common For longer term operation however, the staff water level instrumentation inaccuracies considers potential an because level indication has safety and controlimportant issue functions in all
Generic Letter 92-04 - 3 - August 19, 1992 modes of BWR operation. Furthermore, since the analyses provided are of a generic nature and the magnitude of possible errors depends strongly upon plant-specific factors such as system leakage and geometry, it is important that the analyses be reviewed promptly by all individual licensees.
Basis for Compliance Determination The level errors that could result from the effects of noncondensible gas may prevent the level instrumentation systems in BWRs from satisfying the following regulations:
(1) General Design Criterion (GDC) 13, "Instrumentation and control," which requires that "Instrumentation shall be provided to monitor variables and systems over their anticipated ranges 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 accident conditions.
(2) GDC 21, "Protection system reliability and testability,"
which requires that "The protection system shall be designed for high functional reliability...commensurate with the safety function to be performed." The instrumentation may not be reliable under rapid depressurization conditions.
(3) GDC 22, "Protection system independence," which requires that "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." The natural phenomena of degassi'ng may cause a loss of the reactor vessel water level indication function following a 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, shall 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 requires that "The protection system shall be designed to provide the operator with accurate,
Generic Letter 92-04 - 4 - August 19, 1992 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 after a rapid depressurization event.
Requested Actions
1. In light of potential errors resulting from the effects of noncondensible gas, each licensee should determine:
a. The impact of potential level indication errors on automatic safety system response during all licensing basis transients and accidents;
b. The impact of potential level indication errors on operator's short and long term actions during and after all licensing basis accidents and transients;
c. The impact of potential level indication errors on operator actions prescribed in emergency operating procedures or other affected procedures not covered in (b).
2. Based upon the results of (1), above, each licensee should
-notify the NRC of short term actions taken, such as:
a. Periodic monitoring of level instrumentation system leakage; and, b. Implementation of procedures and operator training to assure that potential level errors will not result in improper operator actions.
3. Each licensee should provide its plans and schedule for corrective actions, including any proposed hardware modifications necessary to ensure the level instrumentation system design is of high functional reliability for long term operation. Since this instrumentation plays an important role in plant safety and is required for both normal and accident conditions, the staff recommends that each utility implement its longer term actions to assure a level instrumentation system of high functional reliability at the first opportunity but prior to starting up after the next refueling outage commencing 3 months after the date of this letter.
Generic Letter 92-04 - 5. - August 19, 1992 Required Information - -
Because of the importance of plant-specific aspects of this issue and the potential magnitude of the-errors, the staff requires,- pursuant to 10 CFR 50.54.(f) and Section 182 of the Atomic Energy Act, that you provide a response to this letter by September 27,
1992.
Merely committing to evaluate the safety significance as part of the individual plant examination (IPE) program is not an -
acceptable alternative to the actions rdescribed herein; since the licensee should resolve this issue as a matter of compliance.
Backfit Discussion
In accordance with NRC procedures, the actions requested herein are considered a backfit to assure that facilities are in compliance with existing regulatory requirements discussed above.
Thus, a backfit analysis is not-required by 10 CFR 50.109(a)(4)(i),
and the staff performed a-documented evaluation as discussed in
10 CFR 50.109(a) (6) . The-documented evaluation is provided in the preceding discussions.
Burden Information This request is covered by Office of Management and Budget Clearance Number 3150-0011, which expires May. 31, 1994. The estimated average number of burden hours is 200 person hours for each licensee response, including the time required to assess the questions, search data sources, gather and analyze the data, and prepare the required response. These estimated average burden hours pertain only to the identified response-related matters and do not include the time for actual implementation of the requested actions. Comments on the accuracy of this estimate and suggestions to reduce the burden may be directed to Ronald Minsk, Office of Information and Regulatory Affairs (3150-0011), NEOB-
3019, Office of Management and Budget, Washington, D.C. 20503 and to the U.S. Nuclear Regulatory Commission, Information and Records Management Branch, Division of Information Support Services, Office of Information and Resources Management, Washington, D.C. 20555.
Although no specific request or requirement is intended, the following information would be helpful to the NRC in evaluating the cost of complying with this generic letter:
(1) the licensee staff time and costs to perform requested inspections, corrective actions, and associated testing;
(2) the licensee staff's time and costs to prepare the requested reports and documentation;
Generic Letter 92- 04 - 6 - August 19t 1992
(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; and
(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.
Please address your response to this generic letter to the U.S.
Nuclear Reguiatory Commission, Attn: Document Control Desk, Washington, D.C. 20555 pursuant to 10 CFR 50.4(a) of the NRC's regulations.
Sincerely, James G. Partlow Adsociate Director for Projects Office of Nuclear Reactor Regulation Enclosure:
List of recently issued generic letters.
Technical Contact:
Timothy E. Collins, NRR
.. (301) 504-2897
ENCLOSURE I
LIST OF RECENTLY ISSUED GENERIC LETTERS
Generic Date of Subiect Issuance Issued To Letter No.
ALTERNATIVE REQUIREMENTS 07/31/92 ALL LWR LICENSEES
90-02
1 FOR FUEL ASSEMBLIES IN THE AND APPLICANTS
SUPPLEMENT
DESIGN FEATURES SECTION OF
TECHNICAL SPECIFICATIONS
SAFETY EVALUATION REPORT 05/22/92 ALL USI A-46
87-02 NO. 2 ON SQUG GENERIC LICENSEES WHO
SUPPLEMENT 1 IMPLEMENTATION PROCEDURE, ARE SQUG MEMBERS
REVISION 2.
COMPILATION OF THE CURRENT 03/19/92 ALL NUCLEAR POWER
92-03 LICENSING BASIS: REQUEST PLANT APPLICANTS
FOR VOLUNTARY PARTICIPATION AND LICENSEES
IN PILOT PROGRAM
REACTOR VESSEL STRUCTURAL 3/06/92 ALL HOLDERS OF OP
92-01 LICENSES OR CONST.
REVISION 1 INTEGRITY, 10CFR50.54(f)
PERMITS FOR NUCLEAR
PWR PLANTS (EXCEPT
YANKEE ATOMIC FOR
RESOLUTION OF GENERIC 03/06/92 ALL HOLDERS OF OP
92-02 ISSUE 79, UNANALYZED REACTOR LICENSES OF CONST.
VESSEL (PWR) THERMAL STRESS PERMITS FOR PWRs DURING NATURAL CONVECTION
COOLDOWN
REACTOR VESSEL STRUCTURAL NOT ISSUED ALL HOLDERS OF OP
92-01 INTEGRITY, 10CFR50.54(f) Revision LICENSES OR CONST.
Listed PERMITS FOR NUCLEAR
Above PWR PLANTS (EXCEPT
YANKEE ATOMIC FOR
CONSIDERATION OF VALVE 02/14/92 ALL LICENSEES OF OP
- 89-10
MISPOSITIONING IN BWRs NUC PWR PLANTS AND
SUPPLEMENT 4 HOLDERS OF CONSTRUC.
PERMITS FOR PWR
PLANTS
- NOTE: 89-10 Supp. 4 -
Accession No. 9202070037 has been changed to 9202250311.
Generic Letter 92- 04 - 6 - August l19, 1992
(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; and
(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.
Please address your response to this generic letter to the U.S.
Nuclear Regulatory Commission, Attn: Document Control Desk, Washington, D.C. 20555 pursuant to 10 CFR 50.4(a) of the NRC's regulations.
Sincerely, James G. Partlow Associate Director for Projects Office of Nuclear Reactor Regulation Enclosure:
List of recently issued generic letters.
Technical Contact:
Timothy E. Collins, NRR
(301) 504-2897 EDITED: J. MAIN
DATE: 8/13 /92 SRXB:DST* SRXB:DST* SICB/DST* D:DST* C:DOBA*
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8/\1%/92 8/ /92 8/1 /92 DISTRIBUTION
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Generic Letter 92- - 6 -
(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.
Please address your response to this generic letter to the U.S.
Nuclear Regulatory Commission, Attn: Document Control Desk, Washington, D.C. 20555 pursuant to 10 CFR 50.54 of the NRC's regulations.
Sincerely, James G. Partlow Associate Director for Projects Office of Nuclear Reactor Regulation Enclosure:
List of recently issued generic letters.
Technical Contact:
Timothy E. Collins, NRR
(301) 504-2897 EDITED: J. MAIN
DATE: 8/13/92 SRXB:DST* SRXB:DST* SICB/DST* D:DST* C:DOBA*
TCollins:Bah RJones SNewberry AThadani GMarcus
8/17/92 8/17/92 8/17/92 8/18/92 8/18/92 AD:PNRR 0 GC
- See previous concurrence JPartlow SILewis WRussell
8/ /92 8/ /92 8/Y / 92 DISTRIBUTION
SRXB R/F SNewberry AThadani GMarcus JPartlow SLewis Collins WRussell Collins R/F RJones
Generic Letter 92- - 6 -
Please address your response to this generic letter to the U.S.
Nuclear Regulatory Commission, Attn: Document Control Desk, Washington, D.C. 20555 pursuant to 10 CFR 50.54 of the NRC's regulations.
Sincerely, James G. Partlow Associate Director for Projects Office of Nuclear Reactor Regulation Enclosure:
List of recently issued generic letters.
Technical Contact:
Timothy E. Collins, NRR
(301) 504-2897 EDITED: J. MAIN .
DATE: 8/13/92 a,
SRXB:DST* SRXB:DST* SICV/DST* D:DS'r C:DOBA
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Generic letter 92- - 6 -
Please address your response to this generic letter to the U.S.
Nuclear Regulatory Commission, Attn: Document Control Desk, Washington, D.C. 20555 pursuant to 10 CFR 50.54 of the NRC's regulations.
Sincerely, James G. Partlow Associate Director for Projects Office of Nuclear Reactor Regulation Enclosure:
List of recently issued generic letters.
Technical Contact:
Timothy E. Collins,
(301) 504-2897 SRXB: DS1b ig DST SI3 D:DST C: DOBA
TCollins:Bah J6nes SN rry AThadani GMarcus
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