05000269/LER-2011-006, Regarding Pressurizer Heater Capacity on-Compliant with Technical Specification 3.4.9
| ML11286A316 | |
| Person / Time | |
|---|---|
| Site: | Oconee  |
| Issue date: | 10/11/2011 |
| From: | Gillespie T Duke Energy Carolinas |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| LER 11-006-00 | |
| Download: ML11286A316 (6) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2692011006R00 - NRC Website | |
text
T. PRESTON GILLESPIE, JR.
Vice President MrEnergy Oconee Nuclear Station Duke Energy ON01 VP / 7800 Rochester Hwy.
Seneca, SC 29672 864-873-4478 864-873-4208 fax T. Gillespie@duke-energy. com October 11,2011 U.S. Nuclear Regulatory Commission Document Control Desk Washington, DC 20555
Subject:
Duke Energy Carolinas, LLC Oconee Nuclear Station Units 1 and 2 Docket Nos.: 50-269, 50-270 Licensee Event Report 269/2011-06, Revision 0 Problem Investigation Program No: 0-10-08094 Gentlemen:
Pursuant to 10 CFR 50.73 Sections (a)(1) and (d), please find attached Licensee Event Report (LER) 269/2011-06, Revision 0, regarding the inoperability of the Unit 1 and Unit 2 pressurizers due to insufficient emergency-powered heater capacity that exceeded the three (3) days allowed by Technical Specification (TS) 3.4.9 Condition 'C.' Consequently, this report is being submitted in accordance with 10 CFR 50.73(a)(2)(i)(B), as an operation prohibited by the station's Technical Specifications.
Other than the commitment to submit a LER supplement, there are no regulatory commitments contained in this report. Duke Energy Carolinas, LLC, expects to provide the supplement within 60 days, but does not consider that time frame to be a commitment.
Any questions regarding the content of this report should be directed to Stephen C. Newman, Oconee Nuclear Station Regulatory Compliance Group, at 864-873-4388.
Sincerely, T. Preston Gillespie, Jr.
Vice President Oconee Nuclear Station Attachment wN (24, www. duke-energy. corn
Document Control Desk October 11, 2011 Page 2 cc:
Mr. Victor McCree Administrator, Region II U.S. Nuclear Regulatory Commission Marquis One Tower 245 Peachtree Center Ave., NE, Suite 1200 Atlanta, GA 30303-1257 Mr. John Stang Project Manager U.S. Nuclear Regulatory Commission Office of Nuclear Reactor Regulation Washington, DC 20555 Mr. Andrew Sabisch NRC Senior Resident Inspector Oconee Nuclear Station INPO (Word File via E-mail)
Abstract
During an extent of condition review associated with the temperature induced trip of Standby Shutdown Facility (SSF) Pressurizer (PZR) heater panel boards inside containment, Duke Energy determined that during a loss-of-offsite-power (LOOP) event, the power supply breakers in each unit's east penetration room, supplying PZR Heater Groups A and E through K, may trip on thermal overload due to the penetration room elevated ambient temperature.
Technical Specification (TS) 3.4.9 requires a minimum of 400 kW of PZR heaters to be OPERABLE and capable of being powered from an emergency power supply. However, the above described review determined that during a LOOP event, the total available emergency-powered capacity of the heaters did not comply with TS 3.4.9 for Unit 1 and Unit 2. In the past three years, there is reasonable evidence that the condition may have existed for longer than the TS 3.4.9 Condition 'C' 72-hour completion time. Consequently, this report is being submitted in accordance with 10 CFR 50.73(a)(2)(i)(B), as an operation prohibited by TSs. Since Unit 3 had an excess of 400 kW pressurizer heater capacity under these conditions, this LER does not apply to Unit 3.
Preliminary Safety Significance results showed that the condition reported in this LER were more than minimal, however, there was no consequence as a result of this condition in that, historically, during the period of vulnerability, an actual LOOP event had not occurred that challenged the ability of the heaters to perform their design function. As a corrective action, an alternate configuration of breakers and in-service heaters was developed and implemented that restored compliance with TS 3.4.9.
BACKGROUND This event is reportable per 10 CFR 50.73(a)(2)(i)(B) as an operation prohibited by Technical Specifications. At the time of this event, both Unit 1 and Unit 2 were operating in Mode 1 at approximately 100% power. No systems, structures, or components (SSCs) were out of service that contributed to this event.
The pressurizer [PZR] heaters [HTR] replace heat lost during normal steady state operation, raise the pressure to normal operating pressure during Reactor Coolant System (RCS)[AC] heatup from the cooled-down condition, and restore system pressure following transients. The heaters are arranged into four banks, which are then divided into 11 groups. Pressurizer heater input may decrease over the course of an operating cycle due to tripped breakers [52] or burnt elements. A minimum required heater capacity capable of being powered from an emergency power source is necessary to offset these losses and ensure that RCS pressure can be maintained.
TS 3.4.9, "Reactor Coolant System - Pressurizer," requires a minimum of 400 kilowatts (kW) of pressurizer heaters be Operable and capable of being powered from an emergency power supply. If pressurizer heater capacity capable of being powered from an emergency power supply is less than 400 kW, TS 3.4.9 Condition
'C' specifies a 72-hour completion time (CT) to restore the pressurizer heater capacity to a minimum of 400 kW.
If the CT cannot be met, TS 3.4.9 Condition 'D' requires the unit be shutdown to Mode 4 conditions with RCS temperature - 3250F within 30 hours3.472222e-4 days <br />0.00833 hours <br />4.960317e-5 weeks <br />1.1415e-5 months <br />.
EVENT DESCRIPTION
On July 8, 2011, during an extent of condition review associated with the temperature response trip of Standby Shutdown Facility (SSF)[NB] pressurizer panel boards [BD] inside containment, an unanalyzed condition was discovered relative to the load current rating of power supply breakers that supply Pressurizer Heater Groups
'A' and 'E' through 'K.' The power supply breakers to these pressurizer heater groups are located in motor control centers (MCCs)[MCC] in the East Penetration Room for each unit and are molded case circuit breakers rated for 50 0C ambient conditions. However, during a loss-of-offsite-power (LOOP) event, the maximum ambient temperature in each unit's East Penetration Room could rise to approximately 60°C (140 0F) due to loss of the 'C' and 'D' chillers [CHU]. Thus, these pressurizer heater power supply breakers, on all three units, may trip on thermal overload due to the elevated ambient temperature in the East Penetration Rooms. Duke Energy determined that the loss of pressurizer heater groups during a LOOP event results in violation of the TS LCO 3.4.9.b requirement for a minimum of 400 kW pressurizer heater capacity for Unit 1 (233 kW) and Unit 2 (341 kW). Unit 3 (439 kW) does not violate the minimum pressurizer heater capacity during a LOOP event. This adverse condition was documented in Problem Investigation Program (PIP) Report No. 0-11-08094.
A reportability review of the above described condition, completed on August 11, 2011, determined that in the past three years, there is reasonable evidence that the condition existed for longer than the TS 3.4.9 Condition
'C' 72-hour CT. Consequently, this adverse condition is reportable as a condition prohibited by Technical Specifications pursuant to 10 CFR 50.73(a)(2)(i)(B).
CAUSAL FACTORS The Root Cause report for the adverse condition described in this LER has not been finalized. Thus, only the apparent cause and corrective actions to restore compliance with TS 3.4.9.b are provided below. As necessary, following acceptance of the Root Cause report and associated corrective actions, an LER supplement will be submitted.
For the purpose of this LER, the apparent cause for the adverse condition described is assumed to be a design deficiency.
CORRECTIVE ACTIONS
As a measure to restore compliance with TS 3.4.9.b, on July 11, 2011, selected pressurizer heater breakers were opened in Unit 1 and Unit 2 Reactor Buildings that reduced the total heater load on each associated supply breaker in the MCCs located in the East Penetration Rooms such that the resulting load current will ensure the MCC supply breakers do not trip on thermal overload during a LOOP event. The alternate configuration of heater breakers and in-service pressurizer heaters restored pressurizer heater capacity to Operable status on Unit 1 and Unit 2.
EXTENT OF CONDITION The MCCs that feed the power supply breakers in question are limited to MCC XH, Xl, XJ and XK on each unit.
These MCCs feed the pressurizer heater loads and some motor starters [MSTR] for Feedwater [SJ] and Containment Purge [VA] motor-operated valves [ISV]. Only the pressurizer heaters are required to function for a LOOP event. A walk down of the East Penetration Rooms on each unit was performed to identify any additional breakers beyond those in MCCs XH, XI, XJ and XK. In the Unit 1 East Penetration Room, there are two panel boards with molded case circuit breakers, lighting panel board 1Ll0 and power panel board 1KF. In the Unit 2 East Penetration Room, there are two panel boards with molded case circuit breakers, lighting panel board 2L10 and power panel board 2KD. In the Unit 3 East Penetration Room, there are two panel boards with molded case circuit breakers, lighting panel board 3L10 and power panel board 3KF. In all of these panel boards, there are no loads on the panel boards that are required during or after a LOOP event.
In addition to the power supply breakers, the cabling [CBL5] and electrical penetrations [PEN] associated with the pressurizer heaters are also exposed to the elevated temperature in the East Penetration Room. An evaluation of the cables and electrical penetrations determined that both are fully capable of providing their intended function during a LOOP event.
There are other components/equipment (valves [V], instrumentation, electrical penetrations, etc.) located in the East Penetration Rooms that are required to function during or after a Loss of Coolant Accident (LOCA)/LOOP event. As such, this equipment is scoped into the Environmental Qualification (EQ) program and is required to meet the maximum temperature specified for the East Penetration Rooms for accident conditions. Per the ONS EQ Criteria Manual, the maximum temperature for the East Penetration Rooms is 145 0F for accident conditions which bounds the 600C (140 0F) temperature specific for a LOOP event.
SAFETY ANALYSIS
The Applicable Safety Analyses section of the Bases for TS 3.4.9 states that the requirement for emergency power supplies is based on NUREG-0737. The intent is to allow maintaining the reactor coolant in a sub-cooled condition with natural circulation at hot, high pressure conditions for an undefined, but extended, time period after a LOOP. While a LOOP event is an initial condition or coincident event assumed in many accident analyses, maintaining hot, high pressure conditions over an extended time period is not evaluated as part of Updated Final Safety Analysis Report accident analyses. Although the pressurizer heaters are not specifically used in accident analyses, the need to maintain sub-cooling in the long term during a LOOP, is the reason for providing a TS Limiting Condition for Operation.
Duke Energy used a risk-informed approach to determine the risk significance associated with the Technical Specification violation of the Unit 1 and 2 Insufficient Pressurizer Heater Capacity During a LOOP event.
The Incremental Conditional Core Damage Probability (ICCDP) of this event was evaluated by considering the following:
" The duration of the period of vulnerability of a LOOP event with insufficient pressurizer heater capacity.
" Ambient heat losses have not exceeded the capacity of the pressurizer heaters for Unit 2 but have for Unit 1.
" The use of the average maintenance PRA model to represent plant configuration, equipment unavailability, and maintenance activities during this violation.
The ICCDP associated with this event taking into account both Units 1 and 2 was preliminarily determined to be greater than 1.OE-06 using a modified internal events model for the specific scenario of insufficient pressurizer heater capacity. Therefore, this event is considered to have more than minimal safety significance. However, since no LOOP event occurred during the period of vulnerability, there were no actual consequences to the health and safety of the public as a result of this adverse condition.
At the time of this submittal, this safety analysis information had not been finalized and will be updated, as necessary, in a supplement to this LER.
ADDITIONAL INFORMATION
Energy Industry Identification System (EIIS) codes are identified in the text within brackets [].
There were no releases of radioactive materials, radiation exposures or personnel injuries associated with this event.
This event is considered reportable under the Equipment Performance and Information Exchange (EPIX) program as noted in Failure Report No. 1168.
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