Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
July 5, 1991
Information Notice No. 91-43: RECENT INCIDENTS INVOLVING RAPID
INCREASES IN PRIMARY-TO-SECONDARY LEAK
RATE
Addressees:
All holders of operating licenses or construction permits for
pressurized-water reactors (PWRs).
Purpose:
This information notice is intended to inform addressees of recent
incidents involving very rapid increases in the primary-to-secondary leak
rate. One of these incidents was followed by a steam generator tube
rupture (SGTR). The leakage during these incidents increased at rates
that were significantly higher than would be predicted on the basis of
Figure 1 of Bulletin 88-02, "Rapidly Propagating Fatigue Cracks in Steam
Generator Tubes." It is expected that recipients will review the
information for applicability to their facilities and consider actions,
as appropriate, to minimize the probability of SGTR events. However,
suggestions contained in this information notice do not constitute NRC
requirements; therefore, no specific action or written response is
required.
Description of Circumstances:
Mihama Unit 2 (Japan)
Mihama Unit 2 is a 19-year old PWR built by Mitsubishi and based on a
Westinghouse Electric Corporation two-loop design. The Mihama Unit 2
steam generators (SGs) are based on the Westinghouse Model 44 design. At
12:24 hours on February 9, 1991, plant personnel received an "attention"
signal from the SG blowdown monitor (R-19). The attention signal
setpoint was at 60 counts per minute (cpm), compared to the normal
reading of 35 cpm. At 12:33 hours plant personnel received an attention
signal from the air ejector monitor (R-15). The attention signal
setpoint for the R-15 was at 900 cpm, compared to a normal reading of 800
cpm. At 13:00 hours, plant personnel sampled the blowdown from SGs A and
B. Results were obtained at 13:20 hours indicating a radioactivity
concentration only slightly higher than normal in SG A, and no detectable
concentration in SG B.
At 13:40 hours, an R-15 "counting rate alarm" (alarm setpoint: 2000 cpm)
was sounded. At 13:45 hours, the R-19 counting rate alarm (alarm
setpoint: 400 cpm) was sounded. At this time, plant personnel manually
started a third charging pump because of decreased pressure and water
level in the pressurizer. At 13:48 hours, personnel began to manually
reduce reactor power at a rate of 4.2 percent per minute. At 13:50
hours, the R-15 "counting rate high" alarm (alarm
9106280018
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July 5, 1991
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setpoint: 1 X 106 cpm) was sounded, followed by reactor trip on "low
pressurizer water level," turbine trip, generator trip, and actuation of
safety injection on low pressure and low water level in the pressurizer.
Leakage from the primary to the secondary was essentially terminated at
14:48 hours. Plant personnel brought the plant to cold shutdown at 02:30
hours on February 10, 1991.
Following this SGTR event*, the utility investigated the rupture and
found that it was a complete circumferential failure of tube R14C45 in SG
A, at the upper-most support plate. The utility found that the failure
mechanism was high cycle fatigue caused by fluid-elastic vibration. By
design, all tubes in rows 11 and greater are supposed to be supported by
anti-vibration bars (AVBs). However, the subject tube was not found to
be so supported because of a reported "incorrect insertion" of the
adjacent AVBs.
Maine Yankee Atomic Power Station
Maine Yankee is a PWR designed by Combustion Engineering, Incorporated,
and was licensed in 1973. Between 14:00 hours on December 12, 1990, and
00:57 hours on December 17, 1990, the rate of primary-to-secondary
leakage gradually increased from 0.0006 gallons per minute (gpm) to 0.008
gpm as determined from grab samples from the condenser air ejector.
During this period, the licensee took grab samples at approximately
4-hour intervals. The licensee analyzed a grab sample taken at 02:34
hours on December 17, 1990, and found that leakage in SG 1 had jumped to
0.017 gpm, with a corresponding reading of 75,000 cpm on the air ejector
radiation monitor. At 03:40 hours, the licensee began reducing power at
a rate of 5 percent per hour. At 04:50 hours, the radiation monitor
reading increased from 75,000 to over 400,000 cpm in less than 1 minute.
Using this and the previous leak rate, the licensee quickly estimated a
leak rate of 0.11 gpm. This estimate exceeded the licensee's
administrative limit of 0.07 gpm, and the licensee increased the rate of
power reduction to 50 percent per hour. The licensee analyzed a grab
sample taken at 05:21 hours and confirmed a leakage rate of 0.105 gpm.
At 06:07 hours, the reading from the air ejector radiation monitor jumped
to 600,000 cpm. Based on a grab sample taken at 06:36 hours, the
calculated leak rate was 1.4 gpm, which exceeded the technical
specification leak rate limit of 0.15 gpm. The plant reached hot standby
status at 06:53 hours and cold shutdown status at 03:10 hours on December
18, 1990.
Subsequent investigation established the source of the leak to be a
4-inch long axial crack at the apex of the row 6, line 43 U-bend. The
licensee described this location as a "steam blanketed region" where the
batwing supports restrict flow permitting a steam void to form and
contaminants to be deposited on the tube surface. On the basis of the
size of the crack found, the staff believes that the early identification
and response to the rapidly increasing leak rate was the key factor in
averting an SGTR event before plant shutdown.
*An SGTR event is defined in this information notice as a
primary-to-secondary leak exceeding the normal charging pump capacity of
the primary system.
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July 5, 1991
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Three Mile Island Unit 1 (TMI-1)
Three Mile Island Unit 1 is a PWR designed by Babcock and Wilcox (B&W)
with once-through type steam generators and was licensed in 1974. On
March 6, 1990, TMI-1 was operating at 75 percent steady power, in a
30-hour hold for restart physics testing. The licensee was first alerted
to the onset of primary-to-secondary leakage by an alarm from the
condenser air ejector radiation monitors at 08:23 hours. A post-incident
review of the radiation monitor data indicates that the activity actually
began to increase above its normal steady value at 08:01 hours. At
around 08:50 hours, the radiation monitor reading had increased from its
initial value of 50 cpm to about 50,000 cpm, 5 times the alarm setpoint.
At 09:00 hours, the licensee's preliminary estimates of leak rate were
0.5 gpm based on mass balance estimates for the reactor coolant system
(RCS) and between 0.5 and 0.75 gpm based on the decreased level in the
make-up tank. These estimates were below the plant technical
specification limit of 1.0 gpm. At 09:12 hours, a plant shutdown was
commenced at the rate of 2 percent per minute, and the radiation monitor
readings began to decrease. The plant reached hot shutdown status at
10:42 hours. On March 7, 1990, the plant reached cold shutdown status by
07:30 hours.
The licensee later determined from activity measurements that leakage had
reached 1.1-1.8 gpm before plant shutdown was commenced. However,
approximately 2.5-3 hours are needed to obtain results from this method,
and, thus, this information was not available to the operators prior to
the decision to commence plant shutdown.
After shutting down the plant, the licensee found the source of the leak
to be a 360 degree circumferential crack in tube 1 of row 77 in the
"lane-wedge" region at the lower face of the upper tubesheet. The staff
believes that the early identification and response to the rapidly
increasing leak rate was key to preventing a much larger leak (if not an
SGTR event) before plant shutdown. Similar fatigue cracks in other B&W
steam generators have caused larger leaks, but not SGTRs, because these
cracks were confined within the tubesheets or support plates.
Discussion:
Earlier incidents involving rapidly increasing primary-to-secondary leak
rates were the subject of NRC Bulletin 88-02 and NRC Information Notice
88-99, "Detection and Monitoring of Sudden and/or Rapidly Increasing
Primary-to-Secondary Leakage." Bulletin 88-02 was issued in response to
the July 15, 1987, SGTR event at the North Anna Power Station Unit 1.
The NRC staff requested, in item C.1 of Bulletin 88-02, that an enhanced
primary-to-secondary leak rate monitoring program be implemented at
certain PWRs (i.e., PWRs with Westinghouse steam generators, carbon steel
support plates, and denting corrosion) until a potential fatigue problem
at these plants could be resolved. The staff requested this enhanced
leak rate monitoring program to ensure that licensees could detect and
respond to a rapidly increasing leak rate caused by high cycle fatigue
before an SGTR occurs. The effectiveness of this program was to be
evaluated against the assumed time-dependent leak rate curve given in
Figure 1 of the bulletin. This curve was based on the estimated rate of
increase in leakage before the SGTR
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July 5, 1991
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event at North Anna Unit 1. This curve yields an estimated 63 hours for
leak rates to increase from 20 gallons per day (gpd) to 500 gpd.
During discussions with numerous industry representatives, the staff has
found that Figure 1 of Bulletin 88-02 is widely used throughout the
industry as a benchmark and/or performance measure for plant-specific
leak rate monitoring programs. This is true even at plants which were
not subject to the actions requested by the bulletin.
However, there have been several incidents since issuance of Bulletin
88-02 where the rate of leakage increase occurred more rapidly than would
be predicted on the basis of Figure 1 of the bulletin. The leak rates at
Mihama Unit 2, Maine Yankee, and Three Mile Island Unit 1 escalated from
very low levels to more than 500 gpd over time periods ranging between
one hour and six hours. An earlier leakage incident at Indian Point Unit
3, described in NRC Information Notice 88-99, developed over a similar
time span. Thus, these incidents are indicative of the limitations of
Figure 1 of the bulletin as a benchmark and/or performance measure for
plant-specific leak rate monitoring programs.
Leak rate monitoring programs can provide for early detection and
response to rapidly increasing leak rates and, thus, can be an effective
approach for minimizing the frequency of steam generator tube ruptures.
This can be achieved by having, as close as possible, real time
information on leak rate and rate of increase of leak rate on which to
act. Data from the air ejector radiation monitors, for example, are
displayed continuously in the control room and have been shown to provide
a relatively good time response to rapidly increasing leakage. Use of
these data, in conjunction with appropriate alarm setpoints, can quickly
alert the operators to a rapid increase in leak rate and the need for
confirmatory leakage measurements and/or the need to shut down the plant.
Nitrogen-16 (N-16) monitors on the steamlines are coming into increasing
use in the U.S. industry as a supplemental method for monitoring
primary-to-secondary leakage. These monitors also exhibit good time
response to changes in the leakage rate. Data from the N-16 monitors can
be continuously displayed in the control room directly in terms of
leakage rate and can be alarmed.
No specific action or written response is required by this information
notice. If you have any questions about this matter, please contact the
technical contact listed below or the appropriate NRR project manager.
Charles E. Rossi
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contact: E. Murphy, NRR
(301) 492-0710
Attachment: List of Recently Issued Information Notices
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