|Davis-Besse Nuclear Power Station, Unit 1|
|Reporting criterion:||10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications|
10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat
|3462017002R00 - NRC Website|
|Person / Time|
|From:||Bezilla M B|
FirstEnergy Nuclear Operating Co
Document Control Desk, Office of Nuclear Reactor Regulation
|Download: ML17333A122 (5)|
comments regarding burden estimate to the Information Services Branch (T-2 F43), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
Energy Industry Identification System (EIIS) codes are identified in the text as [XX].
The Davis-Besse Nuclear Power Station (DBNPS) Emergency Feedwater (EFW) System [BA] consists of two Auxiliary Feedwater (AFW) trains and the Motor Driven Feedwater Pump (MDFP) [BA-P]. The AFW System provides a safety-related source of feedwater to the secondary side of the Steam Generators [AB-SG] in the event of a loss of normal feedwater flow to remove reactor decay heat. The AFW pumps [BA-P] take suction from the condensate storage tanks [KA-T] and pump to the Steam Generator secondary side through the AFW nozzles. The Steam Generators function as a heat sink for core decay heat. The heat load is dissipated by releasing steam to the atmosphere from the Steam Generators via the Main Steam Safety Valves [SB-RV] or Atmospheric Vent Valves [SB-VTV].
The AFW System consists of two steam turbine driven AFW pumps, each of which provides a nominal 100% capacity. The steam turbine driven AFW pumps receive steam from either of the two main steam headers, upstream of the main steam isolation valves. The AFW System supplies water via two headers, each capable of feeding either steam generator. The 100% capacity is sufficient to remove decay heat and cool the unit to Decay Heat Removal System [BP] entry conditions. The AFW System normally receives a supply of water from the Condensate Storage Tanks. A safety grade source of water is also supplied by the Service Water System [B1].
The MDFP train provides feedwater to the steam generators during normal plant startup and shutdown. The non-safety related MDFP train is also designed to provide a backup supply of feedwater to the steam generators in the event of a total loss of both AFW and main feedwater.
Technical Specification (TS) Limiting Condition for Operation (LCO) 3.7.5 requires three EFW trains be Operable in Modes 1 through 3, and in Mode 4 when a Steam Generator is relied upon for heat removal. In Mode 1 with one EFW train inoperable for reasons other than an inoperable steam supply, LCO 3.7.5 Condition B requires the inoperable EFW train be restored to Operable status in72 hours
and in Mode 4 in12 hours
DESCRIPTION OF EVENT:
On September 13, 2017, AFW Train 1 was declared inoperable at1007 hours
to perform scheduled quarterly Surveillance Testing to meet TS Surveillance Requirement 126.96.36.199. At1044 hours
AFW Pump 1 was started, and at 1120 the pump achieved full flow (at least 600 gallons per minute) through the flow test line. Shortly after achieving full pump flow, the turbine inboard bearing temperature began to rapidly increase, and exceeded the bearing metal temperature computer alarm setpoint of 220 degrees F at1126 hours
. After the operating crew investigated the possibility of a faulted temperature instrument, the turbine was tripped locally at1155 hours
. The inboard turbine bearing oil was then sampled, which showed discoloration indicative of bearing degradation. The journal bearing was then disassembled to reveal damage to the journal bearing. The bearing was replaced, and following successful post maintenance testing, AFW Train 1 was declared Operable on September 16, 2017, at0540 hours
comments regarding burden estimate to the Information Services Branch (T-2 F43), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required, to respond to, the information collection.
CAUSE OF EVENT:
The direct cause of the damaged AFW Pump Turbine 1 inboard bearing was due to operation with insufficient lubrication oil. The AFW Pump Turbine journal bearings are lubricated by slinger rings that ride on the turbine shaft located within the bearing assembly. During operation, the slinger rings rotate on the turbine shaft and rotate through the oil contained in the oil reservoir sump. The oil adheres to the slinger ring surfaces and is deposited on the turbine shaft near the bearing as the shaft rotates to provide bearing lubrication. During investigation and disassembly of the damaged inboard bearing, it was identified the existing oil level in the bearing was below the vendor recommended level, but was within the marked acceptable band of the oil level sight glass. The AFW Pump Turbine 1 inboard bearing oil sight glass operational band was marked too low to ensure proper bearing lubrication.
The root cause of the damaged AFW Pump Turbine 1 inboard bearing was that the preventive maintenance work instruction for replacing the sight glass did not include dimensions or guidance for setting required operational bands, and did not contain a requirement to reference the vendor manual to ensure the appropriate operational bands are established. The lack of detail in the work instruction resulted in Maintenance personnel setting the sight glass narrow operational band (minimum and maximum acceptable levels) via skill-of-the-craft when replacing the inboard bearing oil level sight glass.
ANALYSIS OF EVENT:
During investigation into the reasons for the damaged AFW Pump Turbine 1 inboard bearing, it was identified on September 26, 2017, that low inboard bearing oil level had likely existed since completion of the previous quarterly surveillance test on June 21, 2017. Bearing oil samples are taken routinely following the quarterly surveillance test for analysis, and the investigation revealed the inboard bearing was not refilled after sampling on June 21, 2017, because the inboard bearing oil level was within the acceptable band indicated on the sight glass.
A risk assessment was performed for the approximately 86 days AFW Pump Turbine 1 was unavailable until the bearing was repaired on September 15, 2017. The assessment considered there was reasonable assurance AFW Train 1 would have successfully completed the first total hour of operation, but may have failed sometime during the latter23 hours
The plant risk associated with the extended unavailability for AFW Train 1 was determined to be very low, with a delta Core Damage Frequency (CDF) of 7.90E-07 and a delta Large Early Release Frequency (LERF) of 2.22E-08 for internal events. A sensitivity study to include the impact of a common cause failure was performed, resulting in a delta CDF of 8.09E-07, which is still below the threshold to be considered very low plant risk. However, when qualitatively evaluated for External Events (Seismic, Fire), this event is overall considered to be of low safety significance. Additional evaluation of the ability of AFW Pump Turbine 1 to continue to operate for the required mission time in the as-found condition is ongoing, and results of this evaluation will be provided in a supplement to this report.
Because AFW Train 1 was inoperable for approximately 87 days with the plant operating in Mode 1, and Technical Specification 3.7.5 allows operation to continue only for 3 days for this condition, this issue represents a condition prohibited by the plant's Technical Specifications, which is reportable in accordance comments regarding burden estimate to the Information Services Branch (T-2 F43), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
05000 - 346
3. LER NUMBER
002 00 2017 Reportability Discussion: (Continued) with 10 CFR 50.73(a)(2)(i)(B). This Licensee Event Report is being submitted 60 days from the discovery of this reportable issue on September 26, 2017, in accordance with 10 CFR 50.73(a)(1) and 10 CFR 50.4(a)).
Additionally, during the time AFW Train 1 was inoperable, AFW Train 2 was inoperable for maintenance and testing on multiple occasions. Therefore, this issue is also reportable as an event or condition that could have prevented fulfillment of a safety function of system needed to remove residual heat per 10 CFR 50.73(a)(2)(v)(B). At the time of the AFW Train 1 bearing damage on September 13, 2017, AFW Train 2 was Operable; therefore, no loss of safety function existed at the time of discovery, and no immediate notification to the NRC per 10 CFR 50.72 was required.
AFW Pump Turbine 1 inboard bearing was replaced, and following successful post maintenance testing, declared Operable on September 16, 2017, at0540 hours
. As part of this maintenance work, the AFW Pump Turbine 1 inboard bearing oil level sight glass minimum and maximum dimensions were properly marked based on slinger ring submergence in accordance with the turbine vendor manual.
The AFW Pump Turbine 2 inboard bearing sight glass minimum and maximum dimensions have been verified to be consistent with the as-left AFW Pump Turbine 1 inboard bearing dimensions, and the AFW Pump Turbine 1 and 2 outboard bearing sight glasses have been re-marked to the appropriate levels based on measurements made external to the bearing.
The Lubrication Manual Data Sheets for the AFW Pump Turbines have been revised to include bearing oil sight glass minimum and maximum level dimensions, and the AFW Pump Turbine Preventive Maintenance Activities have been revised to reference the Lubrication Manual for marking the minimum and maximum dimensions on the bearing oil sight glasses.
The AFW Pump Turbine outboard bearing sight glass level markings will be verified to be correct by direct observation of one turbine's oil slinger rings submergence.
For Safety-Related and critical equipment, the basis for oil sight glass markings will be validated and captured in the Lubrication Manual, and field sight glass markings will be verified to be in accordance with the information captured in the Lubrication Manual. Additionally, Preventive Maintenance Activities that implement sight glass replacements for Safety-Related and critical equipment will be revised as necessary to include guidance to mark the new sight glass in accordance with the Lubrication Manual.
PREVIOUS SIMILAR EVENTS:
There have been no Licensee Event Reports (LERs) at the DBNPS in the past three years related to inoperability of the Auxiliary Feedwater System or low bearing oil level.