05000323/LER-2012-001
| Diablo Canyon Power Plant | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 3232012001R00 - NRC Website | |
I. Plant Conditions
On August 18, 2012, Unit 2 was in Mode 1 (Power Operation) at 100 percent power.
II. Problem Description
A. Background
Diablo Canyon Power Plant (DCPP) Unit 2 has three emergency diesel generators (EDGs) [DG] that provide vital backup power to three electrical buses [BU] to mitigate the consequences of a design basis accident (DBA) whenever normal or off-site power sources are unavailable. DCPP EDGs are designed to function so that a single failure of any EDG will not jeopardize the capability of the remaining EDGs to start and provide power to operate the shutdown systems required to mitigate any DBA condition.
Each EDG has a single fuel oil booster pump (FOBP) [P] driven by a belt connected to the EDG crankshaft extension shaft. The FOBP supplies fuel oil from the diesel engine's day tank [TK] to each cylinder's fuel oil injection pump. The engine fuel oil system [DC] and FOBP are required for diesel operation. However, because of the independence of each diesel and its associated powered bus, a single failure involving one diesel will not cause a failure of any other EDG to perform its designed safety functions.
B. Event Description
On August 18, 2012, the DCPP Unit 2 Operations turbine building (TB) watch discovered a broken EDG 2-3 FOBP belt. Operators declared EDG 2-3 inoperable, entering Technical Specification (TS) 3.8.1, "AC Sources - Operating," Condition B, "One DG inoperable." TS 3.8.1 and its associated TS Bases required EDG 2-3 to be returned to operable within 7 days for unplanned maintenance. Security personnel evaluated the as-found condition of the EDG 2-3 drive belt and concluded that the belt had not been broken due to sabotage. The other five EDGs have different FOBP belts, which cause lower side loading on the pumps. Plant engineers and operators walked down all five other EDGs and found no visible damage of the belts or FOBPs.
Plant personnel evaluated the pump's internal material condition. Normally, the FOBP easily rotates by hand with no drive belt attached. The FOBP would not rotate. Plant staff disassembled the pump and identified significant and uneven wear within the shaft bushing, significant and uneven scoring wear on the pump case faces, and galling on the internal gear [GR] faces, indicative of side loading on the pump. With normal side loading and adequate lubrication, the bushing would not be expected to wear unevenly and lead to shaft skewing. However, with excessive side loading, this would be expected to occur.
DCPP Engineering performed a thorough evaluation of plausible causes of the broken belt and seized FOBP that included discussions with the EDG vendor and FOBP vendor. Based upon the inspection results and vendor discussions, DCPP Engineering concluded that the FOBP drive belt failed because the FOBP seized. The FOBP seized because the excessive drive belt tension caused uneven side loading, bushing wear, and shaft skewing. This prevented pump sheave rotation that resulted in the belt snapping during coast-down on August 3, 2012, at 1304 PDT, following EDG 2-3's satisfactory performance of DCPP Surveillance Test Procedure (STP) M-9A, "Diesel Engine Generator Routine Surveillance Test.
On August 20, 2012, DCPP Maintenance personnel replaced the FOBP and its drive belt. On August 21, 2012, at 0330 PDT, PG&E declared EDG 2-3 operable. PG&E failed to meet TS 3.8.1, Condition B, "One DG inoperable," because EDG 2-3 was not returned to operable status within the required 7-day completion time for unplanned maintenance after EDG 2-3 belt and FOBP failure following the test run on August 3, 2012, at 1304 PDT.
During the time between August 3, 2012, at 1304 PDT, and August 21, 2012, at 0330 PDT, EDG 2-2 was taken out of service and declared inoperable twice to perform surveillance testing. However, EDG 2-2 was still available for use via manual operator action. For those periods that EDG 2-2 was declared inoperable, DCPP Unit 2 only had one operable EDG, when two are required to be operable to perfoun the designed safety function of providing vital emergency power. This condition would have prevented the fulfillment of the EDG's designed safety function of mitigating the consequences of a DBA.
C. Status of Inoperable Structure, Systems, or Components That Contributed to the Event None.
D. Other Systems or Secondary Functions Affected
None.
E. Method of Discovery
The DCPP Unit 2 Operations turbine building watch discovered the broken EDG 2-3 FOBP belt.
F. Operator Actions
DCPP Operators entered TS 3.8.1, "AC Sources - Operating," declaring EDG 2-3 inoperable.
G. Safety System Responses
None.
III. Cause of the Problem
The EDG 2-3 FOBP seized because the vendor-recommended drive belt tension caused excessive side loading and shaft skewing. The drive belt broke because the FOBP seized, preventing sheave rotation, resulting in the drive belt snapping during coast-down of EDG 2-3 following surveillance testing.
IV. Assessment of Safety Consequences
PG&E's quantitative risk analysis for the 18 days between the time EDG 2-3 was last successfully run on August 3, 2012, and the time when EDG 2-3 was declared operable on August 21, 2012, after replacing the FOBP and its drive belt, concluded that the incremental conditional core damage probability was less than the 1.0E-06 risk significance threshold. This event was a Safety System Functional Failure, but it is not considered risk significant and did not adversely affect the health and safety of the public.
V. Corrective Actions
A Immediate Corrective Actions On August 20, 2012, DCPP Maintenance personnel replaced the FOBP and its drive belt.
B. Other Corrective Actions
1) DCPP will reduce the EDG 2-3 drive belt tension specification in procedure MP M-21.7B, "Diesel Engine 2-3 Fuel Oil Booster Pump," to a lower vendor-approved deflection force value.
2) DCPP will establish a periodic replacement strategy for the FOBP.
VI. Additional Information
A. Failed Components
2) The EDG 2-3 FOBP drive belt.
B. Previous Similar Events
None.