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.
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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.
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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.
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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.
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May 28, 1993
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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.
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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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