05000237/LER-2009-004

LER-2009-004, Unit 2 Shutdown Cooling System Isolation during Cooldown due to Temperature Instrumentation Failure

Docket Number
Event date:	11-02-2009
Report date:	12-22-2009
Reporting criterion:	10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat
2372009004R00 - NRC Website

PLANT AND SYSTEM IDENTIFICATION

Dresden Nuclear Power Station (DNPS) Unit 2 is a General Electric Company Boiling Water Reactor with a licensed maximum power level of 2957 megawatts thermal. The Energy Industry Identification System codes used in the text are identified as [XX].

A. Plant Conditions Prior to Event:

� Unit: 02� Event Date: 11-02-2009 Event Time: 0712 hours0.00824 days <br />0.198 hours <br />0.00118 weeks <br />2.70916e-4 months <br /> Reactor Mode: 3� Mode Name: Hot Shutdown Power Level: 0 Percent

B. Description of Event:

On November 2, 2009, at 0712, shutdown cooling (SDC)[BO] function was lost on DNPS Unit 2 during cooldown for its Fall 2009 refueling outage (D2R21). The SDC system high temperature isolation logic actuated causing the SDC isolation valves to shut, resulting in a loss of cooling to the Reactor Vessel.

Reactor Recirculation (RR) system [AD] temperature was approximately 224 degrees F based on alternate indications. SDC was restarted at 0748 with Recirculation loop temperature at 260 degrees F. At approximately 0835, the reactor entered Mode 4 (i.e., Reactor Coolant temperature less than or equal to 212 degrees F and Mode Switch in Shutdown).

Troubleshooting determined that Temperature Element 2-0261-8B, a thermocouple (TC), had failed. The Resistance Temperature Detector (RTD), 2-0261-8F, housed in the same assembly, had previously failed and was out of service since December 2008. The presence of an open signal from both instruments in the same recirculation loop caused the isolation logic to activate, as designed.

The combined RTD/TC assembly is installed in the 2B RR System Pump suction line. Specifically, the RID= probe is encased in a thermo-well located in the side of the RR suction piping in the first elbow off the reactor nozzle. The instrument is a model SRT-3375 RID= assembly manufactured by Temp-Pro Inc. In this event, RTD 2-0621-8F had previously failed high meaning isolation logic already had one open contact in the logic. When TC 2-0261-8B failed, the isolation logic experienced two open contacts, one in each logic string, which generated an isolation signal as designed.

The isolation of the SDC system event is reportable in accordance with 10 CFR 50.73(a)(2)(v)(B) "Any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to remove residual heat.

C. Cause of Event:

The apparent cause was the failure at the soldered TC (2-0261-8B) junction and the RTD (2-0621-8F) element-to-lead solder joint attributed to corrosion and oxidation at the solder connection. Contributing causes included moisture intrusion aided by the hygroscopic (moisture absorbing) nature of the Aluminum Oxide (A1203) filler material and chlorine contamination, potentially due to residual flux material, in combination with high temperatures creating a corrosive environment.

�NRC FORM 366A (9-2007) PRINTED ON RECYCLED PAPER These RTD/TC assemblies have a history of failure at DNPS with at least 27 individual TE failures since 1987. This RTD/TC assembly is a commercial grade instrument with a poor performance history. The equipment is located in the suction recirculation line but has no radiation design specification. Additionally, a review of the bill of materials indicates that the epoxy used to seal the assembly has a maximum service temperature rating of 446 degrees F. Since the assembly is located inside the insulation surrounding the recirculation system piping while online, it would be exposed to temperatures at the upper range of this service temperature or greater.

The extent of condition pertains to both Unit 2 and Unit 3 Recirculation Suction line temperature elements.

Since the RTD/TC assembly is a unique part, custom made for DNPS, the extent of condition extending to other plants is unlikely. A replacement RR 2B temp instrument was installed during D2R21. Similar to the current Unit 2 configuration, logic to enhance reliability will be installed on Unit 3, which will require 2 instruments to fail in order to cause an inadvertent isolation on Unit 3.

D. Safety Analysis:

The safety significance of this event is minimal. The SDC was restored in 36 minutes, with reactor coolant temperatures rising approximately 36 degrees reaching 260 degrees F when SDC was restored. No other systems were affected and the system responded in a manner consistent with the plant design. The protection of public health and safety and personnel safety were not compromised at anytime during this event. Operator actions were appropriate and consistent with normal and emergency operating procedures.

E. Corrective Actions:

DNPS determined that the design of the existing temperature element assemblies would not ensure reliable performance, given the operational conditions (i.e., high temperature and radiation). Therefore, as the corrective action for this event, DNPS is in the process of identifying and procuring replacement temperature measurement assemblies suitable for installation in DNPS Units 2 and 3 RR suction lines.

� F.MPrevious Occurrences:

A review of DNPS Licensee Event Reports (LERs) was performed and the following events were identified.

• Licensee Event Report 237/2000-005-01, "Recirculation Loop Temperature Failure Causes Shutdown Cooling Inoperability"

G. Component Failure Data:

Manufacturer ComponentMModel Temp-Pro IncTThermocouple SRT-3375 RTDTTC M