05000455/LER-2013-003
| Byron Station, Unit 2 | |
| Event date: | 08-15-2011 |
|---|---|
| Report date: | 11-25-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(B), System Actuation |
| 4552013003R00 - NRC Website | |
A. Plant Condition Prior to Event Event Date/Time: August 15, 2011 / 1815 hours0.021 days <br />0.504 hours <br />0.003 weeks <br />6.906075e-4 months <br /> CST Unit 2 - Mode 1 - Power Operations, Reactor Power 100 percent Reactor Coolant System [AB]: Normal operating temperature and pressure.
B. Description of Event
On September 26, 2013, with Byron Station Unit 2 operating in Mode 1 at 100 percent reactor power, it was identified that a previous condition existed where the Unit 2, A-Train (2A), Diesel Generator (DG) Ventilation Fan did not automatically start in support of a DG surveillance.
During the start of the semi-annual Engineering Safeguards Feature (ESF) Surveillance for the 2A DG on August 17, 2011, Equipment Operators (E0s) in the field identified that the 2A DG Ventilation Fan had failed to auto start. Upon failure of the fan to start, operators entered Technical Specification 3.8.1, "AC Sources - Operating", Condition B, for the 2A DG. Subsequently, the Reactor Operators took the Main Control Board (MCB) control switch for the ventilation fan to Pull-to-Lock (PTL) position and back to the After-Trip position and the fan started. The LCO was exited at 0958 on August 17, 2011.
The 2A DG Ventilation Fan circuitry had been modified on August 15, 2011 to install a time delay relay on the ventilation fan High Differential Pressure (D/P) trip to address a High Energy Line Break (HELB) operability question for the DGs. Operations performed the Post Maintenance Test (PMT) for this relay and returned the 2A DG to service at 1730 on August 15, 2011. The PMT initiated a trip on High D/P on the 2A DG Ventilation Fan. This trip creates a seal-in condition in the circuitry that must be reset by taking the fan's MCB switch to PTL in order for the fan to be started. However, the fan's MCB control switch was not taken to PTL after the PMT. Additionally, the active High D/P trip seal-in circuit logic, which has no local or remote indication, was not recognized by the Operating Crew following the PMT, nor was it identified as a consequence within the Modification Work Package or within the Post Maintenance Test work instruction steps.
The 2A DG was inoperable for the 2-day period when the supporting ventilation fan was not capable of auto starting. As a result, Technical Specification required actions for this past inoperability were missed. This condition resulted in a violation of TS 3.8.1, "AC Sources — Operating", Condition B, and is a reportable condition in accordance with 10 CFR 50.73 (a)(2)(i)(B) as any event or condition that was prohibited by Technical Specifications.
C. Cause of Event
The cause was that the Post Maintenance Test work instructions did not identify the need to reset the High D/P trip signal by placing the Ventilation Fan Control Switch in PTL and did not validate that the ventilation fan would restart after a High D/P Trip.
Operations personnel inappropriately credited HELB analysis to justify past operability for the two days the 2A DG Ventilation Fan auto start was defeated. This assumption resulted in an LER for this issue not being submitted at the time of the event.
D. Safety Significance
There is reasonable expectation that the 2A DG was capable of performing its design function as discussed under the Updated Final Safety Analysis Report (UFSAR) without the fan auto start function being operable.
UFSAR Section 8.3.1.1.2.2 states, "Short term unavailability of the diesel-generator room ventilation fans and dampers is bounded by the HELB analysis (See Subsection 9.4.5.2.1.3.e)". UFSAR 9.4.5.2.1.3.e provides the following Safety Evaluation: "Loss of diesel-generator room ventilation in the event of a HELB in the Turbine Building will not affect the safe shutdown capability of the unit for the short duration of the transient. These rooms are exposed to elevated temperatures until operator action is taken to restore room ventilation.
The 1A, 1B and 2B DG Ventilation Fans had previously been modified with this same time delay modification and these fans were verified to have their High D/P trip interlocks properly reset outside the PMT work instructions. Each of these DGs was operable and available to support Unit 2 operations for the entire period during which the auto start interlock was not operable on the 2A DG.
Reviews concluded there were no cross-train instances of equipment being inoperable that would constitute a loss of safety function during the period that the 2A DG was inoperable Based the analyses above, the significance of this issue is considered low.
E. Corrective Actions
On August 17, 2011 Reactor Operators took the Main Control Board (MCB) control switch for the 2A DG ventilation fan to Pull-to-Lock (PTL) position and back to the After-Trip position and verified that the fan started.
Each Byron Station work group responsible for generating or reviewing PMT work instructions (Engineering, Operations, and Maintenance Work Planning) communicated and reinforced the need to ensure the as-left condition of components/systems being tested is proper to meet its design function.
Operations performed reviews of pending critical modifications to ensure that PMTS specified via work instructions provide sufficient guidance to test and properly restore systems or components to the correct as-found condition.
F. Previous Occurrences
There have been no previous occurrences of this nature in the previous three years.