Undetected Installation Errors in Main
Steam Line Pipe Tunnel Differential Temperature-Sensing Elements at
Boiling Water Reactors
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
January 27, 1989
Information Notice No. 89-10: UNDETECTED INSTALLATION ERRORS IN MAIN
STEAM LINE PIPE TUNNEL DIFFERENTIAL
TEMPERATURE-SENSING ELEMENTS AT
BOILING WATER REACTORS
Addressees:
All holders of operating licenses or construction permits for boiling water
reactors (BWRs).
Purpose:
This information notice is being provided to alert addressees to potential
problems from undetected installation errors in main steam line (MSL) pipe
tunnel differential temperature-sensing elements at BWRs. 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 do not constitute
NRC requirements; therefore, no specific action or written response is
required.
Description of Circumstances:
On July 27, 1988, Pennsylvania Power and Light Company (PP&L) declared all of
the differential temperature (DT) instruments in both Units 1 and 2, used for
the detection of leaks in the MSL pipe tunnel at the Susquehanna Steam
Electric Station (SSES) inoperable. In both units the inlet and outlet
temperature detector (thermocouple) locations were found reversed such that
the follower DT instrumentation modules would sense a zero or negative DT in
the event of a steam line leak. In Unit 2, the temperature detectors were
found not only reversed, but installed in the wrong location. The Unit 2
detectors were found in the vicinity of the air cooler units, a location that
would have substan-tially reduced (non-conservatively) the available signal to
the follower DT modules. As a result of these deficiencies, none of the DT
protective instru-mentation would have detected a steam line leak, and thus
they would have been unable to perform their design function.
On June 24, 1987, Niagara Mohawk Power Corporation declared two of four
channels of the main steam tunnel DT instrumentation at Nine Mile Point (NMP)
Unit 2 in-operable. The temperature-sensing elements (thermocouples) were
improperly located such that the cold leg thermocouples, which are supposed to
monitor steam tunnel ventilation inlet air temperature were actually sensing
steam tunnel ambient air temperature. Operations personnel detected this
problem
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January 27, 1989
Page 2 of 3
while the reactor was at less than 2 percent power. They observed significant
differences in the DT indications for the four channels. Two channels with
properly located thermocouples indicated approximately 15�F DT and the other
two channels with improperly located thermocouples indicated approximately 0 DT.
Discussion:
In both the SSES and NMP-2 plants, the steam tunnel is equipped with tempera-
ture detectors that are used to initiate steam line isolation upon detection
of a steam leak, based on either a rise in steam tunnel ambient air
temperature or an increase in the temperature differential between the tunnel
ventilation inlet and outlet. These steam tunnel differential temperature and
ambient tem-perature instruments provide redundant methods for detecting leaks
and isolating the MSLs. The installation errors described above reduced the
designed redun-dancy of the MSL leak detection system.
PP&L attributes the root cause of DT thermocouple location errors in both
units to vague initial system descriptions and design-basis documents that did
not clearly establish locations for the DT thermocouples. As a consequence,
the installation documents for both units inappropriately specified the
connection of the thermocouples to the DT instruments in such a manner that
the inlet and outlet device locations were interchanged. This reversal caused
the follower DT instruments to always read zero and downscale when in fact
they should have read above zero and upscale for both normal power operation
and in the event of a leak. The SSES-2 problem was further compounded by the
fact that the thermo-couples for sensing inlet air temperature were not located
in the air inlet but rather in the fan cooler room for the steam tunnel
cooling system. As a con-sequence, even if the thermocouples were connected
properly, the increase in DT in the event of a steam leak would have been
substantially less than the in-strumentation trip set value. The scope of the
PP&L start-up and surveillance tests was too narrow to identify the location
errors. Although these tests verified that the instrumentation would respond
to temperature and DT, neither test verified that the inputs were of the
proper magnitude for a given steam leak or detected that they were reversed.
During these tests and during years of routine surveillance, neither the
technicians nor the operators recognized the zero or negative DT reading as
being abnormal.
Niagara Mohawk attributes the root cause of the DT thermocouple location
errors for two channels to a design deficiency. The two mislocated
thermocouples were installed away from the inlet air stream, and therefore
they sensed steam tunnel ambient air temperature instead of ventilation inlet
air temperature. This reading provided a non-conservative input into the
steam tunnel differential temperature isolation logic.
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January 27, 1989
Page 3 of 3
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 Regional Administrator of the appropriate regional
office.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contact: Carl H. Woodard, RI
(215) 337-5261
Attachment: List of Recently Issued NRC Information Notices
.. Attachment
IN 89-10
January 27, 1989
Page 1 of 1
LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
_____________________________________________________________________________
Information Date of
Notice No._____Subject_______________________Issuance_______Issued to________
89-09 Credit for Control Rods 1/26/89 All holders of OLs
Without Scram Capability or CPs for test and
in the Calculation of the research reactors.
Shutdown Margin
89-08 Pump Damage Caused by 1/26/89 All holders of OLs
Low-Flow Operation or CPs for nuclear
power reactors.
89-07 Failures of Small-Diameter 1/25/89 All holders of OLs
Tubing in Control Air, Fuel or CPs for nuclear
Oil, and Lube Oil Systems power reactors.
Which Render Emergency Diesel
Generators Inoperable
89-06 Bent Anchor Bolts in 1/24/89 All holders of OLs
Boiling Water Reactor or CPs for BWRs
Torus Supports with Mark I steel
torus shells.
89-05 Use of Deadly Force by 1/19/89 All holders of OLs
Guards Protecting Nuclear for nuclear power
Power Reactors Against reactors.
Radiological Sabotage
89-04 Potential Problems from 1/17/89 All holders of OLs
the Use of Space Heaters or CPs for nuclear
power reactors and
test and research
reactors.
89-03 Potential Electrical 1/11/89 All fuel cycle and
Equipment Problems major nuclear
materials
licensees.
89-02 Criminal Prosecution of 1/9/89 All holders of a
Licensee's Former President U.S. NRC specific
for Intentional Safety license.
Violations
_____________________________________________________________________________
OL = Operating License
CP = Construction Permit
..
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