05000346/FIN-2012008-01
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Finding | |
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| Title | Impact of a high energy line break in the turbine building on safety-related electrical equipment located in the switchgear rooms |
| Description | The non-safety-related non-radwaste area ventilation system provides ventilation for the low and high voltage switchgear rooms, as well as the battery rooms. The ventilation supply fan takes suction from a mixing box supplied from turbine building, recirculation flow from the switchgear rooms, and outside air sources. A temperature controller modulates dampers on the three air supplies to ensure an appropriate mix of air in the mixing box. The licensee credited the non-safety-related ventilation system in mitigating the consequences of the event since commercial operation. In 2010 as documented in CR-2010-080802, the licensee identified that a turbine building HELB would close the fire damper associated with the safety-related ventilation system fan such that the ventilation system would not be able to exhaust into the turbine building to cool the switchgear room. In evaluating this issue, the licensee credited the proper operation of the non-safety ventilation system controls, as well as the non-safety fan and its modulating dampers even though the equipment was not designed or qualified to operate under a HELB scenario. The licensee reasoned that since the controls of the non-safety-related system were not exposed to a harsh environment, the system could be credited for mitigating the consequences of a turbine building HELB. The inspectors disagreed with the licensees assessment and were concerned a HELB in the turbine building may result in a hot, moist environment entering into the switchgear room through the two non-radwaste area ventilation system dampers (exhaust and supply) and the safety-related switchgear ventilation system damper located in the turbine building. The non-safety-related modulating damper located in the turbine building supply air duct, which the licensee relied upon for mitigation of a turbine building HELB were qualified for pressure rating of 6 inches of water while the plants HELB analysis showed a peak pressure of 1.06 psig (~30 inches of water). As such, the peak pressure may damage the modulating dampers and the isolation damper preventing their closure based on their lower design operating pressure. Furthermore, depending on the size and location of the break, the temperature of the HELB air entering the switchgear rooms may be lower than 135oF setpoint to close the non-safety-related isolation damper on high temperatures and shut off the operating fan. As a result, these scenarios could prevent the isolation of the switchgear rooms from the turbine building HELB environment. The non-safety-related temperature controller TC-5325, controlling the movements of the modulating dampers for temperature control, was calibrated on February 1, 2012, and found to be out of calibration. The desired output was 8.0 psig with a 0.2 psi tolerance, but the controller was found with a 1.07 psi error. The error would allow more air from the turbine building and less air from the outside entering the switchgear rooms. The result of such error could aggravate the consequences of a turbine building HELB on components located in the switchgear rooms because at range of certain flow rate of steam into the switchgear rooms, the modulating damper could allow greater flow of steam from the turbine building into the switchgear rooms. A review of design specification M-324AQ, Technical Specification for Operational Phase Miscellaneous Electronic Controls, showed the maximum relative humidity for some of the safety-related switchgear equipment located in the low voltage switchgear Room 2 was 90 percent (without condensation). A review of specification E-7Q, Technical Specification for Operational Phase for 480V Unit Substations, and E-8Q, Technical Specification for Operational Phase for AC and DC Motor Control Centers, showed the equipment purchased would be located at a high humidity site and relevant other electrical specifications have a value of 60-85 percent relative humidity associated with the term high humidity site. All remaining electrical equipment in the switchgear rooms was qualified to 100 percent relative humidity, but without condensation. This was a concern because if the temperature of the humid HELB air was greater than the initial temperature of the switchgear room (could be as low as 60oF), the high humidity from the HELB would condense on the sensitive switchgear electrical equipment. This could result in the failure of the sensitive switchgear equipment and consequently in the failure of the electrical distribution system to numerous safety-related components. The licensee initiated CR-2012-12292 and performed a prompt operability determination and also issued Standing Order 2012-011 to caution the operators and instruct them to immediately isolate a steam line break, regardless of the size. The inspectors had several concerns with this evaluation. Specifically, the licensee relied on the operation of installed ventilation duct fire dampers, however, these dampers did not provide a reasonable assurance of operability because (a) the dampers have not been actuated since their installation 37 years ago; (b) the temperature of the air entering the room may not be high enough (165oF to 176oF) to actuate them; and (c) the dampers design pressure was much lower than the pressure expected during a HELB (as low as 0.25 inches of water). In addition, the licensees prompt operability determination credited operator action to isolate the steam break within 3 minutes of the break versus the 10 minutes assumed by the procedure. Moreover, the prompt operability determination did not take into account the possibility of a single failure (e.g., MSIV fails to close), which was described in USAR Section 3.6.2.7.1.16, Environmental Effects Due to HELB in the Turbine Building, as the worst case resulting in the highest mass and energy release into the turbine building and the largest pressure and temperature excursion into the adjoining switchgear rooms. Based on the inspectors concerns with the reliance on equipment that may not function under a turbine building HELB scenario, the licensee implemented ECP 2012-0632 to modify the plant design by permanently closing exhaust damper CV5325B, supply damper CV5325C, and fire doors 519A and 520A. This design change would allow the non-safety-related non-radwaste area ventilation system to function by taking suction from outside air combined with recirculated air from the switchgear rooms. The change would also protect the switchgear rooms from the consequences of a HELB in the turbine building. The inspectors were concerned with two different turbine building HELB scenarios that could affect the safety-related switchgear rooms. The first concern was a large HELB, such as a main steam line break, where the turbine peak pressure would exceed the pressure ratings on the ventilation dampers such that the dampers may not function to isolate the HELB from the switchgear room. The second concern was a smaller HELB that would allow the non-radwaste ventilation system to function as designed, which during winter operation may continue to allow the hot/humid environment to enter the switchgear rooms in order to maintain the ventilation system set point temperature of 70oF. The inspectors received additional information on the operation of the ventilation control equipment, specifically the smoke detectors, that need to be evaluated. Based on the inspectors concerns, the licensee was performing some additional analysis. Since this analysis was not completed prior to the end of the inspection, this issue is considered an unresolved item (URI 05000346/2012008-01) pending the following actions: 1) NRC review of whether the non-safety ventilation system would have functioned as necessary during a smaller HELB in the turbine building; 2) licensee determining which turbine building pipe breaks could produce a peak pressure that would exceed the ratings of the non-safety ventilation system dampers; and 3) an analysis that would determine the effects of a large turbine building HELB on the nonsafety ventilation system and the resultant effects on the safety-related equipment in the switchgear rooms. |
| Site: | Davis Besse  |
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| Report | IR 05000346/2012008 Section 1R21 |
| Date counted | Dec 31, 2012 (2012Q4) |
| Type: | URI: |
| cornerstone | Mitigating Systems |
| Identified by: | NRC identified |
| Inspection Procedure: | IP 71111.21 |
| Inspectors (proximate) | A Dunlop J Neurauter P Pelke D Kimble T Briley J Steffesa Dunlopc Zoia M Yeminy M Jones A Stone A Della Grecag Morris |
| INPO aspect | |
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Finding - Davis Besse - IR 05000346/2012008 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Davis Besse) @ 2012Q4
Self-Identified List (Davis Besse)
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