05000263/LER-2011-006
| Monticello Nuclear Generating Plant | |
| Event date: | 09-02-2011 |
|---|---|
| Report date: | 02-28-2012 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| LER closed by | |
| IR 05000263/2011005 (1 February 2012) | |
| 2632011006R01 - NRC Website | |
On August 26, 2011, with the plant operating in Mode 1 at 100% power, it was determined during the performance of Surveillance Test Procedure 0323-01, Fire Protection System Sprinkler Functional Test, that blockage existed at valve FP-171-10, Sprinkler Inspectors Test Valve. At this time Operations declared the fire suppression system [KP] non-functional. A 14-day fire protection system impairment and continuous compensatory fire watch, with backup suppression, had been previously established at the commencement of Procedure 0323-01 and remained in effect. Follow-up maintenance activities confirmed the valve was plugged and removed the blockage.
When Surveillance Test Procedure 0323-01 was re-performed on August 28, 2011 as the post- maintenance test, it was determined that additional blockage was present in the piping. Further maintenance activities on September 2, 2011, identified significant blockage in the fire sprinkler line upstream of valve FP-171-10 as well as in three vertical risers going to sprinkler heads.
At approximately 1600 hours0.0185 days <br />0.444 hours <br />0.00265 weeks <br />6.088e-4 months <br />, following additional inspections of the Intake Structure fire system piping, Maintenance informed Operations that additional portions of the sprinkler suppression piping were blocked. The fire suppression system is relied upon, in part, to satisfy an approved exemption for the plant to 10 CFR 50 Appendix R, Section III.G.2.b concerning separation of components in the Intake Structure.
EVENT ANALYSIS
This condition is being reported in accordance with 10 CFR 50.73(a)(2)(ii)(B) — Degraded or Unanalyzed Condition.
Radiography was performed at various locations throughout the Intake Structure sprinkler system to determine the extent of blockage. The most remote portion of the sprinkler system was found to be blocked due to an accumulation of what was later determined to be corrosion products.
System design documentation indicates the piping should be sloped downward from the branches to the cross main and continuing back to the sprinkler control valve main drain. Investigation showed that portions of the system were not installed per the design such that the slope did not promote complete draining of the system. Incomplete draining contributed to the overall accumulation of corrosion products.
During system tests or actuations, water surged through the system which both loosened fixed corrosion products and transported existing corrosion products further into remote parts of the system. The corrosion byproducts were further broken down as a function of time, reducing them to fine particles which took on the form of a reddish, brownish, clay-like mud.
Samples of the "mud" were taken at various locations within the system and sent to an independent lab for analysis to determine the makeup/origin of the blockage. The results indicated the blockage was primarily made up of iron related corrosion products.
The Monticello risk assessment group reviewed the event for risk impact. A quantitative bounding PRA analysis was performed by assuming failure of all equipment residing in the Intake Structure, given a fire occurs in the room. The risk associated with degradation of the automatic fire suppression system in the Intake Structure is conservatively below the thresholds of what is considered to be low significance with regard to core damage frequency ( to large early release frequency ( Although the sprinkler system was assumed to have failed in the PRA analysis, alternative methods were functional and available to mitigate a fire in the Intake Structure.
CAUSE
Installation of the Intake Structure fire sprinkler system in 1983 did not comply with design requirements for providing required pipe slope to ensure proper draining. This condition allowed excessive water to remain in the system which then contributed to accelerated internal corrosion and accumulation of corrosion byproducts.
CORRECTIVE ACTION
The Intake Structure sprinkler system was flushed and returned to service following radiographic and boroscopic inspections to confirm removal of the blockage. Planned long term actions include:
- Restore portions of the Intake Structure fire sprinkler system piping not meeting design requirements for slope to compliance with the design requirements.
- Perform periodic internal inspections and periodic testing of the Intake Structure fire sprinkler piping to validate that the sprinklers will perform their intended function.
Corrective actions are being tracked under the Monticello Corrective Action Program.
PREVIOUS SIMILAR EVENTS
An extent of condition evaluation for a 2007 Emergency Diesel Generator sprinkler piping blockage identified a potential for this to occur in the Intake Structure, but while identified, flushing of the sprinkler system had not been performed prior to the 2011 event.
During post maintenance testing in 2009, following reinstallation of the Intake Structure sprinkler piping, no flow was observed in the inspector test valve orifice. Pipe scale was observed in the inspector test valve orifice. At that time, blockage was assumed to be local. Since there were no sprinkler heads downstream of FP-171-10, it was concluded that the system remained functional.
It has since been determined that this conclusion was incorrect.
There have been no similar licensee event reports in the last three years.