05000285/LER-2010-004
| Telephone Number (Include Area Code) | |
| Event date: | |
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| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2852010004R00 - NRC Website | |
BACKGROUND
The Fort Calhoun Station (FCS) reactor coolant system (RCS) is equipped with two spring-loaded relief valves called primary (or code) safety valves which provide overpressure protection for the RCS. Code safety valve RC- 141 has a 2485 pounds per square inch gauge (psig) (2500 pounds per square inch absolute (psia)) setting, and RC- 142 has a 2530 psig (2545 psia) setting. In addition, the RCS is protected by two power operated relief valves (PORVs), PCV-102-1 and PCV-102-2.
A resistance temperature detector (RTD) is provided in the discharge pipe from each safety valve to give indication of valve leakage or lifting. The PORV pilot valve vent lines use a common pipe monitored by an RTD. Acoustic monitors provide indication of flow through valves PCV-102-1, PCV-102-2, RC-141, and RC-142. A piezoelectric (PE) accelerometer is mounted on each tail pipe. A noise generator is used on the tail pipe of the PORVs for testing the sensors. The acoustic monitors are the best indicator of flow through PORVs or safety valves.
Technical Specification (TS) 2.15, Instrumentation and Control Systems, states, in part:
The operability, permissible bypass, and Test Maintenance and Inoperable bypass specifications of the plant instrument and control systems shall be in accordance with Tables 2-2 through 2-5.
(4) In the event the number of channels of those particular systems in service not described in (3) above falls below the limits given in the columns entitled "Minimum Operable Channels" or "Minimum Degree of Redundancy," except as conditioned by the column entitled "Permissible Bypass Conditions," the reactor shall be placed in a hot shutdown condition within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />. If minimum conditions for engineered safety features or isolation functions are not met within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> from time of discovering loss of operability, the reactor shall be placed in a cold shutdown condition within the following 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.
TS TABLE 2-5 "Instrumentation Operating Requirements for Other Safety Feature Functions," reads, in part:
Minimum Minimum Permissible Functional Operable Degree of Bypass No. Unit Channels Redundancy Condition 1 CEA Position Indication Systems 1 None None 2 Pressurizer Level 1 None Not Applicable 3 PORV Acoustic Position Indication-Direct 1(a)(c) None Not Applicable 4 Safety Valve Acoustic Position Indication 1(a)(c) None Not Applicable 5 PORV/Safety Valve Tail Pipe Temperature 1(d)(b) None Not Applicable
NOTES:
a�One channel per valve.
b One RTD for both PORV's; two RTD's, one for each code safety.
c�If item 5 is operable, requirements of specification 2.15 are modified for items 3 and 4 to "Restore inoperable channels to operability within 7 days or be in hot shutdown within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />.
d�If items 3 and 4 are operable, requirements of specification 2.15 are modified for item 5 to "Restore inoperable channels to operability within 7 days or be in hot shutdown within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />.
EVENT DESCRIPTION
FE-142, Accelerometer for Pressurizer Safety Valve RC-142 Flow Detection, failed its monthly surveillance test, IC-ST-RC-0001, "Functional Test of Acoustic Flow Monitors," on May 26, 2010. The surveillance test failure occurred during performance of Section 7.2.1; verify response at FE-142. In this section, FE-142 showed no response as the impactor was operated. One month earlier, on April 28, 2010, FE-142 had also failed its monthly surveillance test, IC-ST -RC-0001. The surveillance test failure also occurred during performance of Section 7.2.1, in that, FE-142 showed no response as the impactor was operated. The failure of FE-142 on April 28, 2010, was identified in condition report (CR) 2010-2070. During the investigation for the May 26, 2010 failure, it was discovered that the direct cause for the earlier failure on April 28, 2010, was the same and had not been corrected.
As a result of this discovery, FE-142 was reclassified as inoperable from April 28, 2010, to June 2, 2010, and the TS action time was exceeded. On June 30, 2010, the failure of FE-142 was determined to be reportable in accordance with 10 CFR 50.73(a)(2)(i)(B). This LER is being reported in accordance with 10 CFR 50.73(a)(2)(i)(B).
CONCLUSION
A root cause analysis was completed that determined that the failure of FE-142 was caused by the failure of the center conductor of the softline cable connecting the charge converter to the hardline cable. The reason for this failure was that external stress when the hardline cable was pulled was applied to the softline cable. The exact time and circumstance this stress was applied could not be determined, but there are two possible scenarios:
- The hardline cable was inadvertently snagged during a containment walkdown during the plant shutdown in April 2010, or
- The hardline cable had previously been snagged or pulled which weakened the center conductor of the softline cable, and finally caused the failure prior to the completion of IC-ST-RC-0001 on April 29, 2010.
In either case, the root cause of this event was that the hardline cable for the FE-142 accelerometer was allowed to have external stress applied because there were no barriers for the protection of the hardline cable during maintenance activities in the pressurizer cubicle. This allowed stress to be applied to the softline cable, which weakened the center conductor of the softline cable and caused the failure.
CORRECTIVE ACTIONS
The softline cable was replaced making FE-142 operable. Other actions will be administered by the station's corrective action program.
SAFETY SIGNIFICANCE
The acoustic monitors are only one of several methods the operators use to determine if a PORV or safety valve is open or leaking. Since there are other methods of monitoring the status of the safety valves and PORVs, this event had no impact on the health or safety of the public.
SAFETY SYSTEM FUNCTIONAL FAILURE
This event does not result in a safety system functional failure in accordance with NEI-99-02.
PREVIOUS SIMILAR EVENTS
The station has not had any similar failure of the acoustic monitors in the past.
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