Information Notice 1993-27, Level Instrumentation Inaccuracies Observed During Normal Plant Depressurization: Difference between revisions

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{{#Wiki_filter:UNITED STATES NUCLEAR REGULATORY
{{#Wiki_filter:UNITED STATES


COMMISSION
NUCLEAR REGULATORY COMMISSION


===OFFICE OF NUCLEAR REACTOR REGULATION===
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555 April 8, 1993 NRC INFORMATION


NOTICE 93-27: LEVEL INSTRUMENTATION
WASHINGTON, D.C. 20555 April 8, 1993 NRC INFORMATION NOTICE 93-27:   LEVEL INSTRUMENTATION INACCURACIES OBSERVED


INACCURACIES
DURING NORMAL PLANT DEPRESSURIZATION
 
===OBSERVED DURING NORMAL PLANT DEPRESSURIZATION===


==Addressees==
==Addressees==
All holders of operating
All holders of operating licenses or construction permits for nuclear power


licenses or construction
reactors.
 
permits for nuclear power reactors.


==Purpose==
==Purpose==
The U.S. Nuclear Regulatory
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
 
Commission (NRC) is issuing this information
 
notice to alert addressees
 
to inaccuracies
 
in reactor vessel level indication
 
that occurred during a normal depressurization
 
of the reactor coolant system at the Washington
 
Nuclear Plant Unit 2 (WNP-2) and to the fact that errors in level indication
 
may result in a failure to automatically
 
isolate the residual heat removal (RHR) system under certain conditions.
 
It is expected that recipients
 
will review the information
 
for applicability
 
to their facilities


and consider actions, as appropriate, to avoid similar problems.
notice to alert addressees to inaccuracies in reactor vessel level indication


However, suggestions
that occurred during a normal depressurization of the reactor coolant system


contained
at the Washington Nuclear Plant Unit 2 (WNP-2) and to the fact that errors in


in this information
level indication may result in a failure to automatically isolate the residual


notice are not NRC requirements;
heat removal (RHR) system under certain conditions. It is expected that
therefore, no specific action or written response is required.Background


As discussed
recipients will review the information for applicability to their facilities


in NRC Information
and consider actions, as appropriate, to avoid similar problems. However, suggestions contained in this information notice are not NRC requirements;
therefore, no specific action or written response is required.


Notice 92-54, "Level Instrumentation
Background


Inaccuracies
As discussed in NRC Information Notice 92-54, "Level Instrumentation


Caused by Rapid Depressurization," and Generic Letter 92-04,"Resolution
Inaccuracies Caused by Rapid Depressurization," and Generic Letter 92-04,
"Resolution of the Issues Related to Reactor Vessel Water Level


of the Issues Related to Reactor Vessel Water Level Instrumentation
Instrumentation in BWRs Pursuant to 10 CFR 50.54(f)," noncondensible gas may


in BWRs Pursuant to 10 CFR 50.54(f)," noncondensible
become dissolved in the reference leg of water level instrumentation and lead


gas may become dissolved
to false indications of high level after a rapid depressurization event.


in the reference
Reactor vessel level indication signals are important because these signals


leg of water level instrumentation
are used for actuating automatic safety systems and for guidance to operators


and lead to false indications
during and after an event. While Information Notice 92-54 dealt with


of high level after a rapid depressurization
potential consequences of rapid system depressurization, this information


event.Reactor vessel level indication
notice discusses level indication errors that may occur during normal plant


signals are important
cooldown and depressurization.


because these signals are used for actuating
==Description of Circumstances==
On January 21, i993, during a plant cooldown following a reactor scram at


automatic
WNP-2, "notching" of the level indication was observed on at least two of four


safety systems and for guidance to operators during and after an event. While Information
channels of the reactor vessel narrow range level instrumentation. "Notching


Notice 92-54 dealt with potential
is a momentary increase in indicated water level. This increase occurs when a


consequences
gas bubble moves through a vertical portion of the reference leg and causes a


of rapid system depressurization, this information
temporary decrease in the static head in the reference leg. The notching at


notice discusses
9304020319      PD9Z    3X          MoC


level indication
J              3pzu              13 oqat


errors that may occur during normal plant cooldown and depressurization.
P-                o'ik


Description
tv


of Circumstances
IN 93-27 April 8, 1993 WNP-2 was first observed on channel NC" at a pressure  of approximately


On January 21, i993, during a plant cooldown following
827 kPa (120 psig]. Channel IBS experienced notching  starting at


a reactor scram at WNP-2, "notching" of the level indication
approximately 350 kPa [50 psig]. At these pressures, the level error was on


was observed on at least two of four channels of the reactor vessel narrow range level instrumentation. "Notching is a momentary
the order of 10 to 18 centimeters (4 to 7 inches] and persisted for


increase in indicated
approximately one minute.


water level. This increase occurs when a gas bubble moves through a vertical portion of the reference
Beginning at a pressure of approximately 240 kPa [35 psig], the level


leg and causes a temporary
indication from channel IC' became erratic and, as the plant continued to


decrease in the static head in the reference
depressurize, an 81-centimeter (32-inch] level indication error occurred.


leg. The notching at 9304020319 PD9Z X 3 MoC J 3pzu 13 oqat P- o'itv k
This depressurization was coincident with the initiation of the shutdown


IN 93-27 April 8, 1993 WNP-2 was first observed on channel NC" at a pressure of approximately
cooling system. The 81-centimeter [32-inch] level error was sustained


827 kPa (120 psig]. Channel IBS experienced
gradually recovered over a period of two hours. The licensee postulatedand was


notching starting at approximately
this large error in level indication was caused by gas released in the    that


350 kPa [50 psig]. At these pressures, the level error was on the order of 10 to 18 centimeters
reference leg displacing approximately 40 percent of the water volume. The


(4 to 7 inches] and persisted
licensee also postulated that the slow recovery of correct level indication
 
for approximately
 
one minute.Beginning
 
at a pressure of approximately
 
240 kPa [35 psig], the level indication
 
from channel IC' became erratic and, as the plant continued
 
to depressurize, an 81-centimeter
 
(32-inch]
level indication
 
error occurred.This depressurization
 
was coincident
 
with the initiation
 
of the shutdown cooling system. The 81-centimeter
 
[32-inch]
level error was sustained
 
and was gradually
 
recovered
 
over a period of two hours. The licensee postulated
 
that this large error in level indication
 
was caused by gas released in the reference
 
leg displacing
 
approximately
 
40 percent of the water volume. The licensee also postulated
 
that the slow recovery of correct level indication


was a result of the time needed for steam to condense in the condensate
was a result of the time needed for steam to condense in the condensate


chamber and refill the reference
chamber and refill the reference leg. The licensee inspected the IC"
  reference leg and discovered leakage through reference leg fittings. This


leg. The licensee inspected
leakage may have been a contributing factor for an increased accumulation


the IC" reference
dissolved noncondensible gas in that reference leg.                        of


leg and discovered
The licensee determined that the type of errors observed in level indication


leakage through reference
during this event could result in a failure to automatically isolate a leak


leg fittings.
the RHR system during shutdown cooling. The design basis for WNP-2 includes in


This leakage may have been a contributing
postulated leak in the RHR system piping outside containment while the plant a


factor for an increased
Is in the shutdown cooling mode. For this event, the shutdown cooling suctlon


accumulation
valves are assumed to automatically isolate on a low reactor vessel water


of dissolved
level signal to mitigate the consequences of the event. For the January


noncondensible
1993 plant cooldown, the licensee concluded that, with the observed errors21, level indication, the shutdown cooling suction valves may not have          in


gas in that reference
automatically isolated the RHR system on low reactor vessel water level


leg.The licensee determined
designed. The licensee has implemented compensatory measures for future as;
                                                                          plant


that the type of errors observed in level indication
cooldowns to ensure that a leak that occurs in the RHR system during shutdown


during this event could result in a failure to automatically
cooling operation would be isolated promptly. These measures include


isolate a leak in the RHR system during shutdown cooling. The design basis for WNP-2 includes a postulated
touring


leak in the RHR system piping outside containment
the associated RHR pump room hourly during shutdown cooling and backfilling


while the plant Is in the shutdown cooling mode. For this event, the shutdown cooling suctlon valves are assumed to automatically
the water level instrument reference legs after entry into mode 3 (hot


isolate on a low reactor vessel water level signal to mitigate the consequences
shutdown). The licensee is also evaluating measures to minimize leakage


of the event. For the January 21, 1993 plant cooldown, the licensee concluded
the IC' reference leg.                                                   from


that, with the observed errors in level indication, the shutdown cooling suction valves may not have automatically
Discussion


isolated the RHR system on low reactor vessel water level as;designed.
The event described above is different than events previously reported because


The licensee has implemented
of the large magnitude and sustained duration (as opposed to momentary


compensatory
notching) of the level error that occurred during normal plant cooldown.


measures for future plant cooldowns
large sustained level error is of concern because of the potential for     A


to ensure that a leak that occurs in the RHR system during shutdown cooling operation
complicating long-term operator actions. In addition, the scenario of


would be isolated promptly.
postulated leak in the RHR system evaluated by WNP-2 suggests that some a


These measures include touring the associated
safety


RHR pump room hourly during shutdown cooling and backfilling
systems may not automatically actuate should an event occur while the reactor


the water level instrument
is in a reduced pressure condition. Generic Letter 92-04 requested, in


reference
that licensees determine the impact of potential level indication errors  part, on


legs after entry into mode 3 (hot shutdown).
IN 93-27 April 8, 1993 automatic safety system response during licensing basis transients and


The licensee is also evaluating
accidents. The information in this notice indicates that sustained level


measures to minimize leakage from the IC' reference
instrument inaccuracies can occur during a normal reactor depressurization.


leg.Discussion
Therefore, events occurring during low pressure conditions may also be


The event described
complicated by level indication errors.


above is different
This information notice requires no specific action or written response. If


than events previously
you have any questions about the information in this notice, please contact


reported because of the large magnitude
the technical contact listed below or the appropriate Office of Nuclear


and sustained
Reactor Regulation (NRR) project manager.


duration (as opposed to momentary notching)
Brian K. Grimes, Director
of the level error that occurred during normal plant cooldown.


A large sustained
Division of Operational Events Assessment


level error is of concern because of the potential
Office of Nuclear Reactor Regulation


for complicating
Technical contact:  Amy Cubbage, NRR


long-term
(301) 504-2875 Attachment:


operator actions. In addition, the scenario of a postulated
===List of Recently Issued NRC Information Notices===


leak in the RHR system evaluated
Attachnent
 
by WNP-2 suggests that some safety systems may not automatically
 
actuate should an event occur while the reactor is in a reduced pressure condition.
 
Generic Letter 92-04 requested, in part, that licensees
 
determine
 
the impact of potential
 
level indication
 
errors on
 
IN 93-27 April 8, 1993 automatic
 
safety system response during licensing
 
basis transients
 
and accidents.
 
The information
 
in this notice indicates
 
that sustained
 
level instrument
 
inaccuracies
 
can occur during a normal reactor depressurization.
 
Therefore, events occurring
 
during low pressure conditions


may also be complicated
IN 93-27 April 8, 1993 Pge I of I


by level indication
dc


errors.This information
LIST OF RECENTLY ISSUED                            ME0
                                    HRC INFORMATION NOTICES


notice requires no specific action or written response.
inroruauon                                                                              a- Inforuti0n                                    u te OT


If you have any questions
Notice No.            Subject                Issuance    Issued to


about the information
93-26          Grease Solidification          04/07/93  All holders of OLs or CPs


in this notice, please contact the technical
Causes Molded Case                        for nuclear power reactors.


contact listed below or the appropriate
Circuit Breaker


Office of Nuclear Reactor Regulation (NRR) project manager.Brian K. Grimes, Director Division of Operational
Failure to Close


===Events Assessment===
) 93-25          Electrical fenetration
Office of Nuclear Reactor Regulation


Technical
Assembly Degradation


contact: Amy Cubbage, NRR (301) 504-2875 Attachment:
04/01/93  All holders of OLs or Cps
List of Recently Issued NRC Information


Notices
for nuclear power reactors.


Attachnent
93-24          Distribution of                03/31/93  All holders of operator and


IN 93-27 April 8, 1993 Pge I of I ME0 a-dc LIST OF RECENTLY ISSUED HRC INFORMATION
Revision 7 of NUREG-1021,                 senior operator licenses at


===NOTICES inroruauon===
*Operator Licensing                      nuclear power reactors.
Inforuti0n


Notice No.93-26)93-25 93-24 93-23 93-22 93-21 Subject Grease Solidification
Examiner Standards'
  93-23           Veschler Instruments          03/31/93   All holders of OLs or CPs


Causes Molded Case Circuit Breaker Failure to Close Electrical
Model 252 Switchboard                    for nuclear power reactors.


fenetration
Meters


===Assembly Degradation===
93-22          Tripping of Ilockner-          03/26/93  All holders of Ots or CPs
Distribution


of Revision 7 of NUREG-1021,*Operator
toeller Molded-Case                      for nuclear power reactors.


Licensing Examiner Standards'
Circuit Breakers due to


===Veschler Instruments===
Support Level Failure
Model 252 Switchboard


Meters Tripping of Ilockner-toeller Molded-Case
93-21          Sumary of NRCStaff            03/25/93  All holders of Ots or CPs


Circuit Breakers due to Support Level Failure Sumary of NRC Staff Observations
Observations Compiled                    for light water nuclear


Compiled'during Engineering
'during Engineering Audits                power reactors.


Audits or Inspections
or Inspections of Licen- see Erosion/Corrosion


of Licen-see Erosion/Corrosion
Programs


Programs Thermal Fatigue Cracking of Feedwater
2
  93-20          Thermal Fatigue Cracking       03/24/93  All holdefs of Os or CPs    ° _
                  of Feedwater Piping to                    for PFRs supplied by


===Piping to Stem Generators===
)                Stem Generators                           Westinghouse or Combustion
Slab Hopper Bulging Portable Moisture-Density
 
===Gauge User Responsibilities===
during Field Operations
 
u te OT Issuance Issued to 04/07/93 All holders of OLs or CPs for nuclear power reactors.04/01/93 All holders of OLs or Cps for nuclear power reactors.03/31/93 All holders of operator and senior operator licenses at nuclear power reactors.03/31/93 All holders of OLs or CPs for nuclear power reactors.03/26/93 All holders of Ots or CPs for nuclear power reactors.03/25/93 All holders of Ots or CPs for light water nuclear power reactors.93-20)93-19 93-18 03/24/93 All holdefs of Os or CPs for PFRs supplied by Westinghouse
 
or Combustion


Engineering.
Engineering.


03/17/92 All nuclear fuel cycle licensees.
co


03/10/93 All U.S. Nuclear Regulatory
clo


Couuission
001 o  0
                                                                                                  0
                                                                                        c2 o    LwU)
  93-19          Slab Hopper Bulging            03/17/92  All nuclear fuel cycle


licensees
licensees.                  II


that possess moisture-density
93-18          Portable Moisture-Density      03/10/93  All U.S. Nuclear Regulatory


gauges.2° _clo co o 001 c 2 o II 2 u z J qq:3 0 0 a.LwU):)4 aL.OL -Operating
Gauge User Responsibilities              Couuission licensees that


License CP
during Field Operations                  possess moisture-density              a.


* Construction
gauges.


Permit
:)4
                                                                                        2 u


IN 93-27 April 8, 1993 automatic
OL - Operating License                                                                      z  aL.


safety system response during licensing
CP


basis transients
* Construction Permit


and accidents.
J qq


The information
:3


in this notice indicates
IN 93-27 April 8, 1993 automatic safety system response during licensing basis transients and


that sustained
accidents. The information in this notice indicates that sustained level


level instrument
instrument inaccuracies can occur during a normal reactor depressurization.


inaccuracies
Therefore, events occurring during low pressure conditions may also be


can occur during a normal reactor depressurization.
complicated by level indication errors.


Therefore, events occurring
This information notice requires no specific action or written response. If


during low pressure conditions
you have any questions about the information in this notice, please contact


may also be complicated
the technical contact listed below or the appropriate Office of Nuclear


by level indication
Reactor Regulation (NRR) project manager.


errors.This information
Original signed by


notice requires no specific action or written response.
Brian K.Grime:
                                    Brian K. Grimes, Director


If you have any questions
Division of Operational Events Assessment


about the information
in this notice, please contact the technical
contact listed below or the appropriate
Office of Nuclear Reactor Regulation (NRR) project manager.Original signed by Brian K. Grime: Brian K. Grimes, Director Division of Operational
===Events Assessment===
Office of Nuclear Reactor Regulation
Office of Nuclear Reactor Regulation


Technical
Technical contact:   Amy Cubbage, NRR
 
contact: Amy Cubbage, NRR (301) 504-2875 Attachment:
List of Recently Issued NRC Information
 
Notices*See previous concurrence
 
*OGCB:DORS:NRR


===JLBirmingham===
(301) 504-2875 Attachment:
04/01/93*SRXB:DSSA:NRR
List of Recently Issued NRC Information Notices


ACubbage 03/19/93*C/OGCB:DORS:NRR
*See previous concurrence


GHMarcus 04/01/93*C/SRXB:DSSA:NRR
*OGCB:DORS:NRR *C/OGCB:DORS:NRR                             *TECH:ED


RJones 03/26/93*TECH:ED RSanders 03/18/93*D/DSSA:NRR
JLBirmingham  GHMarcus                                      RSanders


AThadani 03/26/93 Document name: 93-27.IN
04/01/93      04/01/93                                      03/18/93
*SRXB:DSSA:NRR *C/SRXB:DSSA:NRR    *D/DSSA:NRR


IN 93-XX March XX, 1993 errors on automatic
ACubbage      RJones              AThadani


safety system response during licensing
03/19/93      03/26/93            03/26/93 Document name:  93-27.IN


basis transients
IN 93-XX


and accidents.
March XX, 1993 errors on automatic safety system response during licensing basis transients


The information
and accidents. The information in this notice indicates that sustained level


in this notice indicates
instrument inaccuracies can occur during a normal reactor depressurization.


that sustained
Therefore, events occurring during low pressure conditions may also be


level instrument
complicated by level indication errors.


inaccuracies
This information notice requires no specific action or written response. If


can occur during a normal reactor depressurization.
you have any questions about the information in this notice, please contact


Therefore, events occurring
(one of) the technical contact(s) listed below or the appropriate Office of


during low pressure conditions
Nuclear Reactor Regulation (NRR) project manager.


may also be complicated
Brian K. Grimes, Director


by level indication
Division of Operational Events Assessment


errors.This information
notice requires no specific action or written response.
If you have any questions
about the information
in this notice, please contact (one of) the technical
contact(s)
listed below or the appropriate
Office of Nuclear Reactor Regulation (NRR) project manager.Brian K. Grimes, Director Division of Operational
===Events Assessment===
Office of Nuclear Reactor Regulation
Office of Nuclear Reactor Regulation


Technical
Technical contact:  Amy Cubbage, NRR


contact: Amy Cubbage, NRR (301) 504-2875 Attachment:
(301) 504-2875 Attachment:   List of Recently Issued NRC Information Notices
List of Recently Issued NRC Information


Notices*See previous c OGCB:DORS:NRR
*See previous cconcurrence


===JLBirmingham===
OGCB:DORS:NRR C/OGCB:DORS:NRR      D/DORS:NRR         *TECH:ED
03"' 3 gY.Z-*SRXB:DSSA:NRR


ACubbage 03/19/93 concurrence
JLBirmingham    GHMarcusas          BKGrimesrMk        RSanders


C/OGCB:DORS:NRR
03"'  gY.Z-
      3        °f 1 /93C tW        03/ /931'          03/18/93
*SRXB:DSSA:NRR *C/SRXB:DSSA:NRR    *D/DSSA:NRR


GHMarcusas
===ACubbage        RJones              AThadani===
03/19/93        03/26/93            03/26/93 Document name:  RVLEVEL.IN


°f 1 /93C tW*C/SRXB:DSSA:NRR
\  I


RJones 03/26/93 D/DORS:NRR
This information notice requires no specific action or written response.    If


BKGrimesrMk
you have any questions regarding the information in this notice, please


03/ /931'*D/DSSA:NRR
contact the technical contact listed below or the appropriate Office of


AThadani 03/26/93*TECH:ED RSanders 03/18/93 Document name: RVLEVEL.IN
Nuclear Reactor Regulation (NRR) project manager.


\ I This information
Brian K. Grimes, Director


notice requires no specific action or written response.
Division of Operating Reactor Support


If you have any questions
Office of Nuclear Reactor Regulation
 
regarding
 
the information
 
in this notice, please contact the technical
 
contact listed below or the appropriate
 
Office of Nuclear Reactor Regulation (NRR) project manager.Brian K. Grimes, Director Division of Operating


===Reactor Support Office of Nuclear Reactor Regulation===
Technical contact: Amy Cubbage, NRR
Technical


contact: Amy Cubbage, NRR (301) 504-2875 Attachment:  
(301) 504-2875 Attachment:   List of Recently Issued NRC Information Notices
List of Recently Issued NRC Information


Notices Document name: RVLEVEL.IN
Document name: RVLEVEL.IN


*SEE PREVIOUS CONCURRENCE
*SEE PREVIOUS CONCURRENCE


OGCB:DORS:NRR
OGCB:DORS:NRR     C/OGCB:DORS:NRR    D/DORS:NRR      *TECHED:ADM


C/OGCB:DORS:NRR
JLBirmingham      GHMarcus            BKGrimes          RSanders


D/DORS:NRR
03//903/                    93        03    /93        03/ /93
*SRXB:DSSA:NRR            B:DSSA:NR& D/DSSA:NR


JLBirmingham
ACubbage          RJ nes                Thadani


GHMarcus BKGrimes 03//903/ 93 03 /93*SRXB:DSSA:NRR
03/ /93            0 t;//93           03 X/931


B:DSSA:NR
IN 93-XX


& D/DSSA:NR ACubbage RJ nes Thadani 03/ /93 0 t;/ /93 03 X/931*TECHED:ADM
March XX, 1993 This information notice requires no specific action or written response.        If


RSanders 03/ /93 IN 93-XX March XX, 1993 This information
you have any questions regarding the information in this notice, please


notice requires no specific action or written response.
contact the technical contact listed below or the appropriate Office of


If you have any questions
Nuclear Reactor Regulation (NRR) project manager.


regarding
Brian K. Grimes, Director


the information
Division of Operating Reactor Support


in this notice, please contact the technical
Office of Nuclear Reactor Regulation
 
contact listed below or the appropriate
 
Office of Nuclear Reactor Regulation (NRR) project manager.Brian K. Grimes, Director Division of Operating
 
===Reactor Support Office of Nuclear Reactor Regulation===
Technical


contact: Amy Cubbage, NRR (301) 504-2875 Attachment:
Technical contact:     Amy Cubbage, NRR
List of Recently Issued NRC Information


Notices Document name: INFONOT2.RVL
(301) 504-2875 Attachment:    List of Recently Issued NRC Information Notices


OGCB:DORS:NRR
Document name:     INFONOT2.RVL


===JLBirmingham===
OGCB:DORS:NRR        C/OGCB: DORS:NRR 0/DORS:NRR         TECHED LADM
03/ /93 C/OGCB: DORS:NRR GHMarcus 03/ /93 0/DORS:NRR


BKGrimes 03/ /93 TECHED LADM JMain 03/8 /93 SRXB:DSSA~:NlRR
JLBirmingham        GHMarcus          BKGrimes           JMain


ACubbagqAtf-~
03/ /93             03/ /93          03/ /93            03/8 /93 SRXB:DSSA~:NlRR      C/SRXB:DSSA:NRR  D/DSSA:NRR
03/lcj/93 C/SRXB:DSSA:NRR


RJones 03/ /93 D/DSSA:NRR
ACubbagqAtf-~        RJones           AThadani


AThadani 03/ /93}}
03/lcj/93            03/ /93          03/ /93}}


{{Information notice-Nav}}
{{Information notice-Nav}}

Revision as of 03:20, 24 November 2019

Level Instrumentation Inaccuracies Observed During Normal Plant Depressurization
ML031080007
Person / Time
Site: Beaver Valley, Millstone, Hatch, Monticello, Calvert Cliffs, Dresden, Davis Besse, Peach Bottom, Browns Ferry, Salem, Oconee, Mcguire, Nine Mile Point, Palisades, Palo Verde, Perry, Indian Point, Fermi, Kewaunee, Catawba, Harris, Wolf Creek, Saint Lucie, Point Beach, Oyster Creek, Watts Bar, Hope Creek, Grand Gulf, Cooper, Sequoyah, Byron, Pilgrim, Arkansas Nuclear, Braidwood, Susquehanna, Summer, Prairie Island, Columbia, Seabrook, Brunswick, Surry, Limerick, North Anna, Turkey Point, River Bend, Vermont Yankee, Crystal River, Haddam Neck, Ginna, Diablo Canyon, Callaway, Vogtle, Waterford, Duane Arnold, Farley, Robinson, Clinton, South Texas, San Onofre, Cook, Comanche Peak, Yankee Rowe, Maine Yankee, Quad Cities, Humboldt Bay, La Crosse, Big Rock Point, Rancho Seco, Zion, Midland, Bellefonte, Fort Calhoun, FitzPatrick, McGuire, LaSalle, Fort Saint Vrain, Shoreham, Satsop, Trojan, Atlantic Nuclear Power Plant, Crane  Entergy icon.png
Issue date: 04/08/1993
From: Grimes B
Office of Nuclear Reactor Regulation
To:
References
IN-93-027, NUDOCS 9304020319
Download: ML031080007 (8)
v • d • e


UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555 April 8, 1993 NRC INFORMATION NOTICE 93-27: LEVEL INSTRUMENTATION INACCURACIES OBSERVED

DURING NORMAL PLANT DEPRESSURIZATION

Addressees

All holders of operating licenses or construction permits for nuclear power

reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information

notice to alert addressees to inaccuracies in reactor vessel level indication

that occurred during a normal depressurization of the reactor coolant system

at the Washington Nuclear Plant Unit 2 (WNP-2) and to the fact that errors in

level indication may result in a failure to automatically isolate the residual

heat removal (RHR) system under certain conditions. It is expected that

recipients will review the information for applicability to their facilities

and consider actions, as appropriate, to avoid similar problems. However, suggestions contained in this information notice are not NRC requirements;

therefore, no specific action or written response is required.

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)," noncondensible gas may

become dissolved in the reference leg of water level instrumentation and lead

to false indications of high level after a rapid depressurization event.

Reactor vessel level indication signals are important because these signals

are used for actuating automatic safety systems and for guidance to operators

during and after an event. While Information Notice 92-54 dealt with

potential consequences of rapid system depressurization, this information

notice discusses level indication errors that may occur during normal plant

cooldown and depressurization.

Description of Circumstances

On January 21, i993, during a plant cooldown following a reactor scram at

WNP-2, "notching" of the level indication was observed on at least two of four

channels of the reactor vessel narrow range level instrumentation. "Notching

is a momentary increase in indicated water level. This increase occurs when a

gas bubble moves through a vertical portion of the reference leg and causes a

temporary decrease in the static head in the reference leg. The notching at

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IN 93-27 April 8, 1993 WNP-2 was first observed on channel NC" at a pressure of approximately

827 kPa (120 psig]. Channel IBS experienced notching starting at

approximately 350 kPa [50 psig]. At these pressures, the level error was on

the order of 10 to 18 centimeters (4 to 7 inches] and persisted for

approximately one minute.

Beginning at a pressure of approximately 240 kPa [35 psig], the level

indication from channel IC' became erratic and, as the plant continued to

depressurize, an 81-centimeter (32-inch] level indication error occurred.

This depressurization was coincident with the initiation of the shutdown

cooling system. The 81-centimeter [32-inch] level error was sustained

gradually recovered over a period of two hours. The licensee postulatedand was

this large error in level indication was caused by gas released in the that

reference leg displacing approximately 40 percent of the water volume. The

licensee also postulated that the slow recovery of correct level indication

was a result of the time needed for steam to condense in the condensate

chamber and refill the reference leg. The licensee inspected the IC"

reference leg and discovered leakage through reference leg fittings. This

leakage may have been a contributing factor for an increased accumulation

dissolved noncondensible gas in that reference leg. of

The licensee determined that the type of errors observed in level indication

during this event could result in a failure to automatically isolate a leak

the RHR system during shutdown cooling. The design basis for WNP-2 includes in

postulated leak in the RHR system piping outside containment while the plant a

Is in the shutdown cooling mode. For this event, the shutdown cooling suctlon

valves are assumed to automatically isolate on a low reactor vessel water

level signal to mitigate the consequences of the event. For the January

1993 plant cooldown, the licensee concluded that, with the observed errors21, level indication, the shutdown cooling suction valves may not have in

automatically isolated the RHR system on low reactor vessel water level

designed. The licensee has implemented compensatory measures for future as;

plant

cooldowns to ensure that a leak that occurs in the RHR system during shutdown

cooling operation would be isolated promptly. These measures include

touring

the associated RHR pump room hourly during shutdown cooling and backfilling

the water level instrument reference legs after entry into mode 3 (hot

shutdown). The licensee is also evaluating measures to minimize leakage

the IC' reference leg. from

Discussion

The event described above is different than events previously reported because

of the large magnitude and sustained duration (as opposed to momentary

notching) of the level error that occurred during normal plant cooldown.

large sustained level error is of concern because of the potential for A

complicating long-term operator actions. In addition, the scenario of

postulated leak in the RHR system evaluated by WNP-2 suggests that some a

safety

systems may not automatically actuate should an event occur while the reactor

is in a reduced pressure condition. Generic Letter 92-04 requested, in

that licensees determine the impact of potential level indication errors part, on

IN 93-27 April 8, 1993 automatic safety system response during licensing basis transients and

accidents. The information in this notice indicates that sustained level

instrument inaccuracies can occur during a normal reactor depressurization.

Therefore, events occurring during low pressure conditions may also be

complicated by level indication errors.

This information notice requires no specific action or written response. If

you have any questions about the information in this notice, please contact

the technical contact listed below or the appropriate Office of Nuclear

Reactor Regulation (NRR) project manager.

Brian K. Grimes, Director

Division of Operational Events Assessment

Office of Nuclear Reactor Regulation

Technical contact: Amy Cubbage, NRR

(301) 504-2875 Attachment:

List of Recently Issued NRC Information Notices

Attachnent

IN 93-27 April 8, 1993 Pge I of I

dc

LIST OF RECENTLY ISSUED ME0

HRC INFORMATION NOTICES

inroruauon a- Inforuti0n u te OT

Notice No. Subject Issuance Issued to

93-26 Grease Solidification 04/07/93 All holders of OLs or CPs

Causes Molded Case for nuclear power reactors.

Circuit Breaker

Failure to Close

) 93-25 Electrical fenetration

Assembly Degradation

04/01/93 All holders of OLs or Cps

for nuclear power reactors.

93-24 Distribution of 03/31/93 All holders of operator and

Revision 7 of NUREG-1021, senior operator licenses at

  • Operator Licensing nuclear power reactors.

Examiner Standards'

93-23 Veschler Instruments 03/31/93 All holders of OLs or CPs

Model 252 Switchboard for nuclear power reactors.

Meters

93-22 Tripping of Ilockner- 03/26/93 All holders of Ots or CPs

toeller Molded-Case for nuclear power reactors.

Circuit Breakers due to

Support Level Failure

93-21 Sumary of NRCStaff 03/25/93 All holders of Ots or CPs

Observations Compiled for light water nuclear

'during Engineering Audits power reactors.

or Inspections of Licen- see Erosion/Corrosion

Programs

2

93-20 Thermal Fatigue Cracking 03/24/93 All holdefs of Os or CPs ° _

of Feedwater Piping to for PFRs supplied by

) Stem Generators Westinghouse or Combustion

Engineering.

co

clo

001 o 0

0

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93-19 Slab Hopper Bulging 03/17/92 All nuclear fuel cycle

licensees. II

93-18 Portable Moisture-Density 03/10/93 All U.S. Nuclear Regulatory

Gauge User Responsibilities Couuission licensees that

during Field Operations possess moisture-density a.

gauges.

)4

2 u

OL - Operating License z aL.

CP

  • Construction Permit

J qq

3

IN 93-27 April 8, 1993 automatic safety system response during licensing basis transients and

accidents. The information in this notice indicates that sustained level

instrument inaccuracies can occur during a normal reactor depressurization.

Therefore, events occurring during low pressure conditions may also be

complicated by level indication errors.

This information notice requires no specific action or written response. If

you have any questions about the information in this notice, please contact

the technical contact listed below or the appropriate Office of Nuclear

Reactor Regulation (NRR) project manager.

Original signed by

Brian K.Grime:

Brian K. Grimes, Director

Division of Operational Events Assessment

Office of Nuclear Reactor Regulation

Technical contact: Amy Cubbage, NRR

(301) 504-2875 Attachment:

List of Recently Issued NRC Information Notices

  • See previous concurrence
  • OGCB:DORS:NRR *C/OGCB:DORS:NRR *TECH:ED

JLBirmingham GHMarcus RSanders

04/01/93 04/01/93 03/18/93

  • SRXB:DSSA:NRR *C/SRXB:DSSA:NRR *D/DSSA:NRR

ACubbage RJones AThadani

03/19/93 03/26/93 03/26/93 Document name: 93-27.IN

IN 93-XX

March XX, 1993 errors on automatic safety system response during licensing basis transients

and accidents. The information in this notice indicates that sustained level

instrument inaccuracies can occur during a normal reactor depressurization.

Therefore, events occurring during low pressure conditions may also be

complicated by level indication errors.

This information notice requires no specific action or written response. If

you have any questions about the information in this notice, please contact

(one of) the technical contact(s) listed below or the appropriate Office of

Nuclear Reactor Regulation (NRR) project manager.

Brian K. Grimes, Director

Division of Operational Events Assessment

Office of Nuclear Reactor Regulation

Technical contact: Amy Cubbage, NRR

(301) 504-2875 Attachment: List of Recently Issued NRC Information Notices

  • See previous cconcurrence

OGCB:DORS:NRR C/OGCB:DORS:NRR D/DORS:NRR *TECH:ED

JLBirmingham GHMarcusas BKGrimesrMk RSanders

03"' gY.Z-

3 °f 1 /93C tW 03/ /931' 03/18/93

  • SRXB:DSSA:NRR *C/SRXB:DSSA:NRR *D/DSSA:NRR

ACubbage RJones AThadani

03/19/93 03/26/93 03/26/93 Document name: RVLEVEL.IN

\ I

This information notice requires no specific action or written response. If

you have any questions regarding the information in this notice, please

contact the technical contact listed below or the appropriate Office of

Nuclear Reactor Regulation (NRR) project manager.

Brian K. Grimes, Director

Division of Operating Reactor Support

Office of Nuclear Reactor Regulation

Technical contact: Amy Cubbage, NRR

(301) 504-2875 Attachment: List of Recently Issued NRC Information Notices

Document name: RVLEVEL.IN

  • SEE PREVIOUS CONCURRENCE

OGCB:DORS:NRR C/OGCB:DORS:NRR D/DORS:NRR *TECHED:ADM

JLBirmingham GHMarcus BKGrimes RSanders

03//903/ 93 03 /93 03/ /93

  • SRXB:DSSA:NRR B:DSSA:NR& D/DSSA:NR

ACubbage RJ nes Thadani

03/ /93 0 t;//93 03 X/931

IN 93-XX

March XX, 1993 This information notice requires no specific action or written response. If

you have any questions regarding the information in this notice, please

contact the technical contact listed below or the appropriate Office of

Nuclear Reactor Regulation (NRR) project manager.

Brian K. Grimes, Director

Division of Operating Reactor Support

Office of Nuclear Reactor Regulation

Technical contact: Amy Cubbage, NRR

(301) 504-2875 Attachment: List of Recently Issued NRC Information Notices

Document name: INFONOT2.RVL

OGCB:DORS:NRR C/OGCB: DORS:NRR 0/DORS:NRR TECHED LADM

JLBirmingham GHMarcus BKGrimes JMain

03/ /93 03/ /93 03/ /93 03/8 /93 SRXB:DSSA~:NlRR C/SRXB:DSSA:NRR D/DSSA:NRR

ACubbagqAtf-~ RJones AThadani

03/lcj/93 03/ /93 03/ /93


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