Information Notice 1993-89, Potential Problems with BWR Level Instrumentation Backfill Modifications
UNITED STATES NUCLEAR REGULATORY
COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555 November 26, 1993 NRC INFORMATION
NOTICE 93-89: POTENTIAL
PROBLEMS WITH BWR LEVEL INSTRUMENTATION
BACKFILL MODIFICATIONS
Addressees
All holders of operating
licenses or construction
permits for boiling water reactors (BWRs).
Purpose
The U.S. Nuclear Regulatory
Commission (NRC) is issuing this information
notice to alert addressees
to potential
problems that have been identified
by licensees
involving
hardware modification
to the reactor vessel water level instrumentation
system. 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.Description
of Circumstances
NRC Bulletin (NRCB) 93-03, "Resolution
of Issues Related to Reactor Vessel Water Level Instrumentation
in BWRs," issued on May 28, 1993, requested
that licensees
implement
hardware modifications
necessary
to ensure the level instrumentation
system design is of high functional
reliability
for long-term operation.
In response to this bulletin, all BWR licensees
with the exception of Big Rock Point, which does not use cold reference
leg instrumentation, have either implemented
modifications
or have committed
to implement
modifications.
The majority of these licensees
have decided to install a reference
leg backfill system to supply a continuous
flow of water from the control rod drive (CRD) hydraulic
system through the reference
legs to preclude migration of dissolved
noncondensible
gases down the legs. In August 1993, a potential problem was found at the Susquehanna
nuclear power plant during the design of this backfill modification.
Discussion
It was postulated
at Susquehanna
that a manual isolation
valve in one of the reference
legs (see Figure 1) could be closed by operator error. Closure of this valve would result in pressurization
of that reference
leg to CRD system pressure and erroneous
indication
of low reactor water level and high reactor pressure on all instrumentation
associated
with that reference
leg. The transient
resulting
from pressurization
of the most limiting reference
leg 9311190454
1 KE-e 93-o 93 Ill
K.,_ IN 93-89 November 26, 1993 includes reactor scram and opening of all safety relief valves (SRVs) due to the false high reactor pressure.
The SRVs would remain open and depressurize
the reactor until the valves are closed by operator action, or actual reactor pressure falls below approximately
446 kPa [50 psig] at which time the valves can no longer stay open. Reactor depressurization
and loss of inventory through the SRVs, in combination
with the false low water level signal on the affected reference
leg, would result in closure of the main steam isolation valves, actuation
of high-pressure
and low-pressure
emergency
core cooling system (ECCS) and containment
isolation.
Low-pressure
ECCS injection
would commence after the low-pressure
permissive
is satisfied.
This permissive
would be satisfied
in this scenario, allowing the low pressure ECCS injection valves to open, because only one of the pressure transmitters
is affected and the logic would still be satisfied.
A single failure could defeat this logic, however, preventing
all low-pressure
ECCS Injection.
The low-pressure
permissive
can be bypassed in the control room to open the injection
valves for all four low pressure core spray (LPCS) pumps. The Susquehanna
licensee has informed the NRC that it has physically
disabled the manual isolation valves to prevent misoperation
of these valves; in addition, the valves are not readily accessible
as they are located 6.1 meters [20 feet] above the floor.This event was recently analyzed for the LaSalle plant by Commonwealth
Edison.The analysis indicates
that the low-pressure
permissive
for opening the low-pressure ECCS injection
valve would be defeated for the LaSalle design due to the false high pressure signal, thus preventing
ECCS injection
from the affected division.
If a single failure is assumed in the relay for the low-pressure permissive
on the other division, no low-pressure
ECCS injection would be available.
Because the induced plant transient
is potentially
so severe, LaSalle has designed its backfill modification
to make the injection point for the backfill system on the reactor side of the manual isolation valve and excess flow check valve, thereby precluding
the potential
for pressurization
of the reference
leg through the backfill system.Commonwealth
Edison took a different
design approach for its Dresden and Quad Cities plants. The backfill system design for Dresden and Quad Cities injects into the reference
leg on the instrument
rack side of the manual isolation
valve and excess flow check valve. Additional
administrative
controls were developed
to ensure that the isolation
valve would not be inadvertently
closed. The licensee analyzed the inadvertent
closure of the manual isolation
valve for the Dresden and Quad Cities plants and concluded that, while multiple SRVs would open, the resultant
plant transient
could be mitigated
by appropriate
operator actions. Without operator actions, the low-pressure ECCS would be available
for event mitigation;
however, a single failure in the instrumentation
system could defeat the low-pressure
permissive
for opening the low-pressure
ECCS Injection
valves and result in no low-pressure ECCS being available
for this transient.
The licensee also determined
that this design presented
an unreviewed
safety question because it would increase the probability
of a previously
analyzed accident, and submitted
an application
to amend its license pursuant to 10 CFR 50.90. The NRC is currently
reviewing
the licensee submittal.
IN 93-89 November 26, 1993 Other minor problems with the backfill system have been encountered
when installing
the system and returning
the instrumentation
to service after installation
was complete.
At the Perry plant, a problem occurred when the licensee was in the process of venting one of the instrument
lines following the installation
of the modification.
The job plan directed the operation
of the wrong valve, and instead of opening the vent valve the technician
opened the isolation
valve, allowing air into the reference
leg. As a result, the instrumentation
associated
with the high pressure core spray system (HPCS) was inoperable
until it was re-filled
and vented. Similar events have occurred at other plants due to procedural
inadequacy
or lack of attention
to detail.Related Generic Communications
- NRC Information
Notice 92-54, "Level Instrumentation
Inaccuracies
Caused by Rapid Depressurization," July 24, 1992.* Generic Letter 92-04, "Resolution
of the Issues Related to Reactor Vessel Water Level Instrumentation
in BWRs Pursuant to 10 CFR 50.54(f)," August 19, 1992.* NRC Information
Notice 93-27, 'Level Instrumentation
Inaccuracies
Observed During Normal Plant Depressurization," April 8, 1993.* NRC Bulletin 93-03, Resolution
of Issues Related to Reactor Vessel Water Level Instrumentation
in BWRs,' May 28, 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 Attachments:
1. Simplified
Sketch of Backfill Modification
2. List of Recently Issued NRC Information
Notices
rfl tO CONDENSATE
POT REACTOR VESSEL (QA BOUNDARY U DRYWELL REACTOR BLDG 7 MANUAL L ISOLATION C EXCESS FLOW$ CHECK VALVE REFERENCE
LEG CRD CHARGING WATER HEADER INSTRtUm RACK VARIABLE LEG C FIGURE 1 -SIMPLIFIED
SKETCH OF BACKFILL MODIFICATION
e0 (cI-Ij3 ED w, A4...achment
2 IN 93-89 November 26, 1993 LIST OF RECENTLY ISSUED NRC INFORMATION
NOTICES Information
Date of Notice No. Subject Issuance Issued to 93-88 93-87 93-86 93-85 93-84 Status of Motor-Operated
Valve Performance
Pre-diction Program by the Electric Power Research Institute Fuse Problems with Westinghouse
7300 Printed Circuit Cards Identification
of Iso-topes in the Production
and Shipment of Byproduct Material at Non-power Reactors Problems with X-Relays in DB- and DHB-Type Circuit Breakers Manu-factured by Westinghouse
Determination
of Westing-house Reactor Coolant Pump Seal Failure 11/30/93 11/04/93 10/29/93 10/20/93 10/20/93 All holders of OLs or CPs for nuclear power reactors.All holders of OLs or CPs for nuclear power reactors.All holders of OLs or CPs for test and research reactors.All holders of OLs or CPs for nuclear power reactors.All holders of OLs or CPs for pressurized
water reactors (PWRs).93-83 Potential
Loss of Spent Fuel Pool Cooling Following
A Loss of Coolant Accident (LOCA)10/07/93 All holders for boiling (BWRs).of OLs or CPs water reactors 93-82 93-81 Recent Fuel and Core Performance
Problems in Operating
Reactors Implementation
of Engineering
Expertise on Shift 10/12/93 10/12/93 All holders of OLs or CPs for nuclear power reactors and all NRC-approved
fuel suppliers.
All holders of OLs or CPs for nuclear power reactors.OL -Operating
License CP -Construction
Permit
IN 93-89 a-> November 26, 1993 Other minor problems with the backfill system have been encountered
when installing
the system and returning
the instrumentation
to service after installation
was complete.
At the Perry plant, a problem occurred when the licensee was in the process of venting one of the instrument
lines following the installation
of the modification.
The Job plan directed the operation
of the wrong valve, and instead of opening the vent valve the technician
opened the isolation
valve, allowing air into the reference
leg. As a result, the instrumentation
associated
with the high pressure core spray system (HPCS) was inoperable
until it was re-filled
and vented. Similar events have occurred at other plants due to procedural
inadequacy
or lack of attention
to detail.Related Generic Communications
- NRC Information
Notice 92-54, "Level Instrumentation
Inaccuracies
Caused by Rapid Depressurization," July 24, 1992.* Generic Letter 92-04, "Resolution
of the Issues Related to Reactor Vessel Water Level Instrumentation
in BWRs Pursuant to 10 CFR 50.54(f)," August 19, 1992.* NRC Information
Notice 93-27, 'Level Instrumentation
Inaccuracies
Observed During Normal Plant Depressurization," April 8, 1993.* NRC Bulletin 93-03, 'Resolution
of Issues Related to Reactor Vessel Water Level Instrumentation
in BWRs," May 28, 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.orig /s/'d by BKGrimes Brian K. Grimes, Director Division of Operating
Reactor Support Office of Nuclear Reactor Regulation
Technical
contact: Amy Cubbage, NRR (301) 504-2875 Attachments:
1. Simplified
Sketch of Backfill Modification
2. List of Recently Issued NRC Information
Notices* SEE PREVIOUS CONCURRENCE
SRXB:DSSA*
OGCB:DORS*
TECH ED.* SRXB:DSSA*
SRXB:DSSA*
D:DSSA* OGCB:DORS*
ACUBBAGE PWEN RSANDERS WLYON RJONES ATHADANI GMARCUS 1 11/16/93 11/15/93 11/16/93 11/18/93 11/20/93 11/23/93 11/2.493 DOCUMENT NAME: 93-89.IN
IN 93-xx November 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 Attachments:
1. Simplified
Sketch of Backfill Modification
2. List of Recently Issued NRC Information
Notices* SEE PREVIOUS CONCURRENCE
SRXB:DSSA*
OGCB:DORS*
TECH ED.*ACUBBAGE PWEN RSANDERS 11/15/93 11/16/93 11/15/93 SRXB:DSSA*
WLYON 11/16/93 SRXB:DSSA*
RJONES 11/18/93 D: DSSA*ATHADANI 11/20/93 OGCB:DORS GMARCUS At'i 11/23/93 D: DORS BGRIMES 11/ /93 DOCUMENT NAME: BWRWTLVL.WEN
instrumentation
associated
with the high pressure core spray system (HPCS) was inoperable
until it was re-filled
and vented. Similar events have occurred at other plants due to procedural
inadequacy
or lack of attention
to detail.Related Generic Communications
- NRC Information
Notice 92-54, "Level Instrumentation
Inaccuracies
Caused by Rapid Depressurization," July 24, 1992.* Generic Letter 92-04, "Resolution
of the Issues Related to Reactor Vessel Water Level Instrumentation
in BWRs Pursuant to 10 CFR 50.54(f)," August 19, 1992.* NRC Information
Notice 93-27, "Level Instrumentation
Inaccuracies
Observed During Normal Plant Depressurization," April 8, 1993.* NRC Bulletin 93-03, "Resolution
of Issues Related to Reactor Vessel Water Level Instrumentation
in BWRs," May 28, 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 Attachments:
1. Simplified
Sketch of Backfill Modification
2. List of Recently Issued NRC Information
Notices EDITED BY: R. Sanders DATED: 11/15/93 SRXB:DSSA*
OGCB:DORS*
SRXB:DSSA*
SRXB:DSSA*
D:DSSA OGCB:DORS
D:DORS ACUBBAGE PWEN WLYON RJONES ATHAqkNI GMARCUS BGRIMES 11/15/93 11/16/93 11/16/93 11/18/93 11/?V/93 11/ /93 11/ /93* SEE PREVIOUS CONCURRENCE