05000456/LER-2012-004
| Docket Numbersequential Revmonth Day Year Year Month Day Year Braidwood Station, Unit 2 05000457Number No. | |
| Event date: | 07-07-2012 |
|---|---|
| Report date: | 12-13-2012 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat |
| 4562012004R01 - NRC Website | |
NRC FORM
A. Plant Operating Conditions Before the Event:
Event Date:�June 3, 2012 Unit: 1 MODE: 1 Reactor Power: 99 percent Unit: 2 MODE: 1 Reactor Power: 99 percent Unit 1 Reactor Coolant System [AB]:�Normal operating temperature and pressure Unit 2 Reactor Coolant System:� Normal operating temperature and pressure No structures, systems or components were inoperable at the start of this event that contributed to the event.
Pertinent Ultimate Heat Sink [BSI Information:
Technical Specifications (TS) 3.7.9, "Ultimate Heat Sink (UHS)," Condition A requires that with the UHS inoperable, the unit be placed in Mode 3 in 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> (Required Action A.1) and Mode 5 in 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br /> (Required Action A.2). Surveillance Requirement (SR) 3.7.9.2 verifies that the average water temperature of the UHS is degrees F every 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. This temperature is measured at the discharge of the operating Essential Service Water (SX) [131] system pumps. With the average water temperature of the UHS >100 degrees F, the UHS must be declared inoperable in accordance with Condition A. With the UHS inoperable, Condition A requires that both units be placed in Mode 3, "Hot Standby," within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and Mode 5, "Cold Shutdown," within 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />.
The basis of this TS requirement is that the UHS provides a heat sink for processing and operating heat from safety related components during a transient or accident, as well as during normal operation. This is done by utilizing the SX and the Component Cooling Water (CC) [CC] systems.
The UHS consists of an excavated essential cooling pond integral with the main cooling pond, and the piping and valves connecting the pond with the SX system and pumps. The two principal functions of the UHS are the dissipation of residual heat after reactor shutdown, and dissipation of residual heat after an accident.
The basic performance requirements are that a 30-day supply of water be available, and that the design basis temperatures of safety related equipment not be exceeded. The UHS is sufficiently oversized to permit a minimum of 30 days of operation with no makeup.
B. Description of Event:
From July 4 through July 6, 2012, prolonged, unprecedented hot weather and drought conditions affected the northern Illinois area. High temperatures during the daytime in conjunction with little cooling at night and little precipitation resulted in elevated water temperatures in Braidwood Station's UHS. There were no controllable measures that could be taken to immediately reduce the temperature of the UHS.
The station employs a cooling pond temperature prediction computer model to estimate future UHS temperature based on current local weather forecasts. At 0600 on July 7, 2012, the model made the first prediction indicating that the cooling pond would exceed 100 degrees F.
At 1556 on July 7, 2012, the average discharge temperature of the running essential service water pumps exceeded 100 degrees F. TS 3.7.9 Condition A was entered. Without enforcement discretion, at 2156 on July 7, 2012, Braidwood Station, Units 1 and 2, would have been required to he m Mode Enforcement Discretion was requested to extend the Completion Time of Required Action A.1 by an additional 18 hours2.083333e-4 days <br />0.005 hours <br />2.97619e-5 weeks <br />6.849e-6 months <br /> (i.e., change the current six hour Completion Time to 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />) and to increase the limit of the average water temperature of the UHS in SR 3.7.9.2 to 102 degrees F to allow continued operation of Braidwood Station, Units 1 and 2, with the average water temperature of the UHS > 100 degrees F but Enforcement Discretion would end if the average water temperature of the UHS exceeds 102 degrees F or with the average water temperature of the UHS At 1705 on July 7, 2012, approval of the request was verbally granted by the NRC, and on July 10, 2012, the written request for the Notice of Enforcement Discretion (NOED) was submitted. Compensatory actions in support of the NOED included limiting work on equipment, ensuring specific equipment and systems were protected and remained available, and as an operating compensatory measure, monitoring the Unit 2 refueling water storage tank to ensure it remained less than or equal to 95 degrees F.
At 2018 on July 7, 2012, the UHS temperature was below 99.9 degrees F, and TS 3.7.9 Condition A was exited.
Since TS 3.7.9 allows six hours to shut down Units 1 and 2 and the units were in Condition A for less than six hours, there was not a condition prohibited by the plant's TS.
As stated in the November 8, 2012 Non-Cited Violation received for this event, the NRC considers the UHS to be comprised of a single system, and exceeding the UHS temperature limit rendered the UHS system inoperable for both units. Therefore, this event is reportable under 10 CFR 50.73(a)(2)(v)(B), any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to remove residual heat.
Cause of Event
The cause of this event was due to the prolonged hot weather in the area. 2012 brought unprecedented hot weather (it tied a record triple-digit streak of three consecutive days from August 1947 and July 1911) and drought condition to the northern Illinois area. High temperatures during the daytime, in conjunction with little cooling at night and little precipitation, resulted in elevated UHS water temperatures. There are no controllable measures that can be taken to immediately reduce the temperature of the UHS, in that reduction of the heat input by derating the units would have a negligible short-term effect on the temperature of the UHS.
D. Safety Consequences:
There were no actual safety consequences impacting plant or public safety as a result of this event.
As the UHS was considered to be inoperable for the period of time the temperature exceeded 100 degrees F, this is considered a condition that could have prevented the fulfillment of a safety function.
There was potential minimal safety consequence due to extending the Completion Time of Required Action A.1 by an additional 18 hours2.083333e-4 days <br />0.005 hours <br />2.97619e-5 weeks <br />6.849e-6 months <br /> and increasing the average water temperature of the UHS from an insignificant impact on the loss of coolant accident (LOCA) and non-LOCA results. Additionally, component assessments were performed and determined that components served by UHS would continue to perform satisfactorily despite a two degree F increase in UHS temperature. No adverse influences on risk were identified through examination of the probabilistic risk analysis model for the plant. Additionally, despite the hot weather conditions at the time, the average water temperature of the UHS exceeded the 100 degrees F limit for less than five hours.
E. Corrective Actions:
There are no corrective actions for this event.
The conditions requiring the requested NOED are not typical and have not occurred in the past twelve years since a license amendment was implemented to increase UHS temperature to 100 degrees F. Currently available equipment margins and in one case operational compensatory actions were required to support the increase to 102 degrees F, which are not supportive of submittal of a permanent change to the TS through a License Amendment Request. This was discussed with the NRC during the NOED request, and the NRC agreed that a follow-up amendment is not needed.
F. Previous Occurrences:
No previous, similar Licensee Event Reports were identified at the Braidwood Station.
3. Component Failure Data:
Manufacturer Nomenclature Model Mfg. Part Number N/A N/A N/A N/A
NRC/