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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 8901230427 .. IN 89-10 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. .. IN 88-10 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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