05000361/LER-2012-001
| San Onofre Nuclear Generating Station (Songs) | |
| Event date: | 02-09-2012 |
|---|---|
| Report date: | 04-03-2012 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3612012001R00 - NRC Website | |
On February 9, 2012, an engineering evaluation determined that during a postulated Probable Maximum Precipitation (PMP) event, the Turbine Driven Auxiliary Feedwater Pump (TDAFWP) turbine [TRB] could potentially trip on overspeed due to excessive condensation in the steam supply line, rendering the TDAFWP [P] inoperable. This condition existed on both Units 2 and 3. The portion of the TDAFWP turbine steam supply piping located in an external trench is susceptible to flooding. If the piping were to become submerged during a PMP event, excessive condensation of steam in the piping is possible due to the heat transfer properties of the as-found pipe insulation. As a result, water slugging and turbine overspeed could occur if the AFW system [BA] is started under an Emergency Feedwater Actuation Signal (EFAS). Technical Specification (TS) Limiting Condition for Operation (LCO) 3.7.5 requires all three AFW trains to be operable in Modes 1, 2, and 3, and Mode 4 when the Steam Generator (SG) [SG] is relied upon for heat removal. TS 3.7.5 Condition B for one AFW train inoperable requires restoration of the AFW train to Operable status within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />. The identified condition existed since 1993; therefore, one AFW train was inoperable for longer than allowed by TS. This event is reportable under 10 CFR 50.73(a)(2)(i)(B) - operation or condition prohibited by TS.
B. INITIAL CONDITIONS
At the time of discovery on February 9, 2012, both units were shutdown. SONGS Unit 2 was in Mode 6 for a refueling outage and Unit 3 was in Mode 5 (cold shutdown). There were no additional inoperable structures, systems, or components at the start of the event that contributed to the event.
C. BACKGROUND INFORMATION
The AFW system supplies feedwater to the SGs during normal plant startup and shutdown. Upon the loss of normal feedwater [SJ], the EFAS automatically actuates the AFW system to supply feedwater to the SGs for decay heat removal and to cooldown the Reactor Coolant System (RCS) [AB]. The AFW pumps independently take suction from the condensate storage tank.
AFW system redundancy and diversity is provided by two independent motor driven ac-powered AFW pumps, one normally aligned to each SG, and a third turbine driven AFW pump capable of supplying either SG. The two ac-powered trains use train aligned diesel generators as a backup power source. Each of the three pumps can provide 100 percent of the required feedwater capacity to the SGs.
The steam supply piping to the TDAFWP turbine is taken from each of the two main steam lines between the containment penetrations and the main steam isolation valves. A portion of the steam supply line to the TDAFWP turbine runs in an external trench. The design function of this line is to provide an assured source of steam supply, the motive force to operate the TDAFWP.
The AFW system is designed to perform its function following a feedwater line break combined with a loss of offsite power and a single active failure of TDAFWP. With all ac power unavailable (blackout) resulting in a loss of the two motor driven pumps, the TDAFWP is credited for providing feedwater to the SGs.
The AFW system is designed to remain functional during and/or following adverse environmental conditions, such as abnormal high water or flooding.
TS LCO 3.7.5 requires all three AFW trains to be Operable in Modes 1, 2, and 3, and Mode 4 when the SG is relied upon for heat removal. TS 3.7.5 Condition B for one AFW train inoperable requires restoration of the AFW train to Operable status within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />.
D. DESCRIPTION OF OCCURRENCE
In November 2010, a design review determined that during a PMP event, the AFW steam line trenches could fill with water submerging the TDAFWP turbine steam supply piping. The steam supply must deliver dry steam upon turbine startup to prevent overspeed due to water slugging and speed control instabilities.
A system of drains along the length of the supply header and the inlet to the turbine stop valve is designed to keep the supply header warm and free of condensate. During a PMP event, condensation of the steam in the steam supply piping could exceed the steam moisture removal capability of the attached steam line condensate drains. The response of the AFW turbine control system to accommodate the changing supply steam conditions may not be fast enough to prevent a TDAFWP turbine overspeed trip.
The ability of the steam supply line to provide dry steam to the TDAFWP turbine is dependent on the heat transfer coefficient of the insulation installed on the line as it passes through an open external, potentially flooded, trench. Assuming the steam piping insulation was 3-inch thick mineral wool as specified in the plant design drawings, a 2011 engineering evaluation determined the insulation sufficiently protected the submerged line, and concluded that the TDAFWP remained operable during a PMP event. The analysis demonstrated that the drain system was properly sized to remove condensate in the steam line during a PMP event, and therefore, water slugging and turbine overspeed would not occur, if the TDAFWP was started under an EFAS. A walkdown was also performed at that time to verify the insulation was intact and did not contain gaps. The incorrect insulation material was not detected during this walkdown.
On February 1, 2012, during walkdowns associated with Unit 2 AFW trench drain system modifications, engineering identified that AFW steam piping insulation was not mineral wool. Investigation determined the as-found insulation on the Unit 2 AFW steam pipe was calcium silicate, and the Unit 3 insulation was a combination of fiberglass, calcium silicate, cerablanket, and mineral wool.
On February 9, 2012, an engineering evaluation determined that the 2011 evaluation was not valid for the as-found condition of the AFW steam pipe insulation in both Units 2 and 3. In the 2011 engineering evaluation, mineral wool thermal conductivity data was obtained from the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) handbook (RP-721), The Effect of Moisture Content on the Thermal Conductivity of Insulation Materials Used on District Heat and Cooling Pipes.
Although thermal conductivity values for wet calcium silicate and wet cerablanket are not provided in the ASHRAE handbook, the handbook did contain a limited amount of data for the thermal conductivity of wet fiberglass insulation at lower temperatures. A comparison of the conductivity of wet fiberglass extrapolated to higher temperatures versus wet mineral wool determined that fiberglass insulation was less effective when submerged in preventing condensate formation within the AFW steam supply line. Therefore, with the as-found condition, the TDAFWP turbine could potentially trip on overspeed and the TDAFWP would An Apparent Cause Evaluation determined that the incorrect insulation was most likely installed in 1993 during 10-year Inservice Inspection (ISI) activities. This work involved removal of insulation on the AFW steam piping to support visual inspections of the piping. The isometric drawings and construction specifications correctly identified the insulation as 3-inch mineral wool; however, the work orders did not require or specify the use of the drawings or specifications for re-installation of the insulation.
F. CORRECTIVE ACTIONS
At the time of discovery on February 9, 2012, both units were shutdown. The insulation was replaced with mineral wool prior to plant startup. An Apparent Cause Evaluation was performed, as described above.
No additional corrective actions were necessary because the cause was identified as a legacy issue. The work practices in 1993 would not be acceptable based on current work standards.
G. SAFETY ASSESSMENT
There was no actual safety significance since there has been no flooding events and no rainfalls on the order of a PMP event at SONGS that have impacted the operability of the TDAFWP during the time period the identified condition existed. In terms of potential safety significance, it is postulated that during a PMP event, the TDAFWP turbine would trip on overspeed due to excessive condensation, rendering the TDAFWP inoperable and placing the plant in a condition prohibited by TS. In the past three years, at least one motor driven AFW pump was operable in both Units 2 and 3 during Modes 1 through 4. Therefore, with the TDAFWP inoperable, the AFW system would remain functional and capable of fulfilling its safety function with at least one redundant and independent motor driven AFW train.
The incremental core damage probability (ICDP) and incremental large early release probability (ILERP) was calculated to be 2.0E-7 and 1.0E-8, respectively. This very small risk increase is attributed to two factors: (1) the combination of events which need to occur to impact the unavailability of the TDAFWP in response to normal/light rainfall events is very low, (2) the frequency of heavier rainfalls on the order of a PMP event is extremely low at SONGS.
Based on the above, the safety significance of this event is considered minimal.
H. ADDITIONAL INFORMATION
There have been no similar events in the past three years at SONGS with the same underlying cause.