05000390/LER-2010-002
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. N/A N/A | |
| Event date: | 08-15-2010 |
|---|---|
| Report date: | 10-14-2010 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3902010002R00 - NRC Website | |
The events discussed in Licensee Event Report (LER) 2010-002 began August 15, 2010, while the unit was at 20% thermal power.
II. DESCRIPTION OF EVENT:
A. Event On August 15, 2010 Watts Bar Nuclear Plant (WBN) Unit 1 was at 20% thermal power, preparing for a manual turbine [Energy Industry Identification System (EllS) Code TRB] trip. In accordance with Abnormal Operating Instruction (A01) 17, "Turbine Trip," the manual turbine trip was initiated.
The main turbine was taken off-line to make repairs to the main turbine electro-hydraulic control system (EllS Code TG). Following the turbine trip, high level in the No. 2 heaters (EllS Code HX) occurred due to reverse flow from the No. 3 heater drain tank (EllS Code TK). This reverse flow was caused by low extraction steam pressure in the No. 2 heaters. The No. 2 heaters are located below the No. 3 heater drain tank. Reverse flow occurs when the No. 3 heater drain tank's static head is greater than the extraction steam pressure in the No. 2 heaters. Under these conditions, the No. 2 heater bypass drain (EllS Code DRN) to the condenser (EllS Code COND) cannot maintain proper level by draining the condensing steam from the heaters with the additional flow from the No. 3 heater drain tank.
The high level signals from the No. 2 heaters automatically isolated the intermediate heater string. Isolation of all three heater strings resulted in a "Loss of Normal Feedwater" (EllS Code SJ). In anticipation of losing suction to the Main Feedwater (MFW, EllS Code SJ) pumps (EllS Code P), Operations manually started the Auxiliary Feedwater (AFW, EllS Code SJ) pumps (EllS Code P) and tripped both the MFW Pump A and Condensate (EllS Code SD) Booster Pump A (EllS Code P).
This event is addressed in Tennessee Valley Authority's (TVA's) Corrective Action Program as Problem Evaluation Report (PER) 244876.
B. Inoperable Structures, Components, or Systems that Contributed to the Event.
No structures, components, or systems were inoperable that contributed to this event.
E. Method of Discovery
Operators in the Main Control Room observed indications of the intermediate feedwater heater string isolation and took actions based on these indications.
F. Operator Actions Operations personnel manually initiated startup of the AFW System in response to plant conditions. The plant was stabilized using AFW and the Main Steam (EllS Code SB) dump valves (EllS Code V). Operators followed their procedures, and a reactor trip was not required.
G. Safety System Responses AFW was manually initiated before the automatic signal was generated. Safety system responses were appropriate for the conditions.
During performance of A01-17 by Operations personnel, the manual operator action to place the level control valves (El IS Code LCV) in full bypass of the No. 3 heater drain tank following a turbine trip was not taken in time to prevent reverse flow into the No. 2 heater. This caused increased levels in the No. 2 heater, which led to the subsequent intermediate pressure heater string isolations of all three strings.
IV. ANALYSIS OF THE EVENT
The intermediate pressure heater strings A, B and C are not safety related equipment. Additionally, they are not required to mitigate any of the design basis accidents for which Unit 1 is licensed to safely withstand.
The design of these non-safety heater strings is such that a loss of main feedwater can occur if timely manual action is not taken. As concluded by the cause analysis, the failure to take timely operator action was the cause of this event.
Plant safety systems performed their intended safety functions in response to the manual actuation of AFW. See Section V, "Assessment of Safety Consequences," below for further discussion.
V. ASSESSMENT OF SAFETY CONSEQUENCES
No equipment was placed in operational conditions beyond design limitations of pressure or temperature. No personnel were exposed to adverse safety circumstances during the plant transient, and no personnel injuries were sustained.
The manual AFW actuation on August 15, 2010 was compared to the Updated Final Safety Analysis Report (UFSAR) "Loss of Normal Feedwater Event," UFSAR Section 15.2.8. AFW was manually actuated before the automatic actuation setpoint was reached due to the isolation of the intermediate pressure heater string. This isolation occurred because the No. 3 Heater Drain Tank was dumping to the No. 2 heater, which caused the No. 2 heater level to increase, resulting in heater string isolation. Operators manually isolated MFW and manually actuated AFW in response to these changes. The secondary side steam generator (El IS Code SG) atmospheric relief valves (El IS Code RV) and safety valves were not challenged during the transient. The UFSAR analysis shows that AFW is capable of removing stored and residual heat from the reactor core. The UFSAR assumes that the plant is initially operating at 110.6% of the Nuclear Steam Supply System (NSSS), i.e., thermal, power level while the manual AFW actuation occurred at approximately 20% thermal power. Failure of the Turbine Driven (TD) AFW pump is assumed in the UFSAR. The TD AFW pump, along with both Motor Driven (MD) AFW pumps, started and were available for decay heat removal.
Therefore, the manual AFW actuation is bounded by the UFSAR safety analysis assumptions.
VI. CORRECTIVE ACTIONS- The corrective actions for this condition are being managed within TVA's Corrective Action Program (PER 244876) and therefore are not considered to be regulatory commitments. An overview of the corrective action plan is provided below:
1. Operations procedure A01-17 was revised to prioritize feedwater heater manipulations following a turbine trip. The procedure now requires that bypassing the No. 3 heater drain tank to the condenser shall be performed as soon as the No. 3 heater drain tank pumps are secured.
2. General Operating Instructions GO-2, "Unit Startup From Less Than 4% Reactor Power to 30% Reactor Power," and GO-3, "Normal Power Operation," were revised to ensure full bypass of the No. 3 Heater Drain Tank below 40 percent rated thermal power (RTP).
VII.ADDITIONAL INFORMATION
A. Failed Components
No components failed that contributed to this event.
B. Previous Similar Events
In 2008, Operations personnel did initiate a manual reactor trip in response to the isolation of the feedwater heaters. This previous event was caused by a failed air line on a heater drain tank bypass valve. The corrective actions for the 2008 event would not have prevented the event reported in this LER.
C. Additional Information
The issue of overloading the No. 2 feedwater heater is being evaluated for future design changes.
D. Safety System Functional Failure This event did not involve a safety system functional failure as defined in NEI 99-02, Revision 5.
E. Loss of Normal Heat Removal Consideration The high level signals from the No. 2 heaters automatically isolated the intermediate heater string. Isolation of all three heater strings resulted in a "Loss of Normal Feedwater." In anticipation of losing suction to the MFW Pumps, Operations manually started the AFW pumps and tripped both the MFW Pump A and the Condensate Booster Pump A. Heat removal using the auxiliary feedwater system was adequate to mitigate this event.
VIII. COMMITMENTS
None.