Information Notice 2002-06, Design Vulnerability in BWR Reactor Vessel Level Instrumentation Backfill Modification
| ML013470481 | |
| Person / Time | |
|---|---|
| Issue date: | 01/18/2002 |
| From: | Beckner W Division of Regulatory Improvement Programs |
| To: | |
| Koshy T, NRR/OD, 415-1176 | |
| References | |
| TAC MB3218 IN-02-006 | |
| Download: ML013470481 (6) | |
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555-0001 January 18, 2002 NRC INFORMATION NOTICE 2002-06: DESIGN VULNERABILITY IN BWR REACTOR
VESSEL LEVEL INSTRUMENTATION BACKFILL
MODIFICATION
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 a potential design vulnerability in a 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.
Background
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 or 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. The back-fill system
precludes migration of dissolved noncondensible gases down the legs. In August 2001, a
design vulnerability in this backfill modification was found at the Pilgrim Nuclear Power Station.
Description of Circumstances
It was discovered that, under certain conditions, the safety-related reactor vessel water level
indicators that provide certain safety-related initiation signals could give nonconservative water
level readings. During a reactor trip at Pilgrim, both trains of the emergency core cooling
system (ECCS) reactor water level instruments indicated higher than actual levels. These
instrument inaccuracies were noted after operator actions were taken to isolate the CRD
system charging water header supply valve, CRD 301-25, to insert a control rod that had not
settled at position 00. Closure of the charging water header isolates the ECCS level indicator
reference-leg backfill system (see Attachment 1). The erroneous readings were attributed to
the reference legs draining back through the reference-leg keepfill system to the CRD hydraulic
control units (HCUs). With the CRD charging header valve shut, the reactor scram reset, and
reactor pressure rising, the reference legs drained back to the HCUs. This occurs when the
CRD system pump is secured or the charging header isolation valve is closed, the source of the
reference-leg keepfill system, is at a slightly lower pressure than the reactor so that the
differential pressure across the keepfill system check valve is not sufficient to fully seat the
check valves. The draining of the reference legs results in a transient false reference-leg
indication until the check valves seat and the reference legs refill via the condensate pots or
flow is reestablished through the CRD reference leg backfill system. Pilgrim concluded that this
phenomenon can occur with either the isolation of the reference leg backfill system or possibly
with the loss of the running CRD pump.
During the reactor scam at Pilgrim, this event resulted in both the wide-range vessel level
instruments indicating higher than actual water level with the A train reaching a maximum
deviation of 26 inches and the B train reaching a maximum of 11 inches. These erroneous high
readings occurred for approximately 30 minutes. During this time, the automatic emergency
core cooling system initiation function would not have occurred on low-low water level as
designed. This false reading is due to the relative locations of the reactor vessel instrument
nozzles and the differential pressure corresponding to the ECCS initiation setpoint.
Related Generic Communications:
NRC Information Notice 93-89 Potential Problems With BWR Level Instrumentation
Backfill Modifications, November 26, 1993 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 notice, please contact the technical contacts listed below or
the appropriate Office of Nuclear Reactor Regulation (NRR) project manager.
/RA/
William D. Beckner, Program Director
Operating Reactor Improvements Program
Division of Regulatory Improvement Programs
Office of Nuclear Reactor Regulation
Technical contacts: R. Arrighi, Region I Thomas Koshy, NRR
508-747-0565 301-415-1176 E-mail: rja1@nrc.gov E-mail: txk@nrc.gov
Attachments:
1. Sketch of Backfill Modification
2. List of Recently Issued NRC Information Notices
- See previous concurrence
OFFICE RORP TECH ED REG I:RI DSSA/SRXB (A) SC:RORP PD:RORP
NAME TKoshy PKleene* RArrighi* JWermiel* TKoshy* WDBeckner*
DATE 01/ /2002 12/04/2001 12/17/2001 01/17/2002 01/17/2002 01/18/2002
Attachment 1 IN 2002-06 RWL Reference Leg Backfill
Condensing Chamber
Reference Leg Reference Leg
Variable Leg Variable Leg
LT LT
Needle Valve
Charging Water Line 301-25 Scram Vlvs CRD Pumps
Attachment 2 LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
_____________________________________________________________________________________
Information Date of
Notice No. Subject Issuance Issued to
_____________________________________________________________________________________
2002-05 Foreign Material in Standby 01/17/02 All holders of licenses for nuclear
Liquid Control Storage Tanks power reactors.
2002-04 Wire Degradation at Breaker 01/10/02 All holders of operating licenses
Cubicle Door Hinges for nuclear power reactors.
2002-03 Highly Radioactive Particle 01/10/2002 All holders of operating licenses
Control Problems During Spent for nuclear power reactors, Fuel Pool Cleanout holders of licenses for
permanently shutdown facilities
with fuel onsite, and holders of
licenses for non-power reactors.
2002-02 Recent Experience with 01/08/2002 All holders of operating licenses
Plugged Steam Generator for pressurized-water reactors
Tubes (PWRs), except those who have
permanently ceased operations
and have certified that fuel has
been permanently removed from
the reactor.
2002-01 Metalclad Switchgear Failures 01/08/2002 All holders of licenses for nuclear
and Consequent Losses of power reactors.
Offsite Power
2001-19 Improper Maintenance and 12/17/2001 All holders of operating licenses
Reassembly of Automatic Oil for nuclear power reactors, Bubblers except those who have
permanently ceased operations
and have certified that fuel has
been permanently removed from
the reactor vessel.
2001-18 Degraded or Failed Automated 12/14/2001 All uranium fuel conversion, Electronic Monitoring, Control, enrichment, and fabrication
Alarming, Response, and licensees and certificate holders
Communications Needed for authorized to receive safeguards
Safety and/or Safeguards information. Information notice is
not available to the public
because it contains safeguards
information.
______________________________________________________________________________________
OL = Operating License
CP = Construction Permit