Undetected Installation Errors in Main Steam Line Pipe Tunnel Differential Temperature-Sensing Elements at Boiling Water Reactors

                                  UNITED STATES
                          NUCLEAR REGULATORY COMMISSION
                             WASHINGTON, D.C.  20555

                                January 27, 1989

                                   STEAM LINE PIPE TUNNEL DIFFERENTIAL
                                   TEMPERATURE-SENSING ELEMENTS AT 
                                   BOILING WATER REACTORS


All holders of operating licenses or construction permits for boiling water 
reactors (BWRs).


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 

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 

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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.


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 

                                   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

89-03          Potential Electrical          1/11/89        All fuel cycle and
               Equipment Problems                           major nuclear 

89-02          Criminal Prosecution of       1/9/89         All holders of a
               Licensee's Former President                  U.S. NRC specific
               for Intentional Safety                       license.
OL = Operating License
CP = Construction Permit 

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