05000275/LER-2015-002
| Diablo Canyon Power Plant, Unit 1 | |
| Event date: | 10-05-2015 |
|---|---|
| Report date: | 12-03-2015 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability |
| 2752015002R00 - NRC Website | |
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05000- Diablo Canyon Power Plant, Unit 1 275
I. Reportable Event Classification
This event is reportable pursuant to the following criteria:
- 10 CFR 50.73(a)(2)(i)(B), an operation or condition prohibited by Technical Specifications
- 10 CFR 50.73(a)(2)(vii), a common cause inoperability of independent trains or channels
II. Plant Conditions
At the time the condition was identified, Unit 1 was in Mode 5 (Cold Shutdown).
Ill. Problem Description
A. Background
Each of the four reactor coolant system (RCS) [AB] loops has one hot leg wide range (WR) resistance temperature detector (RTD) [TI] and one cold leg WR RTD. The RCS RTDs are part of the post-accident monitoring (PAM) [IP] Instrumentation. The primary purpose of the PAM instrumentation is to display unit variables that provide information required by the control room operators during accident situations.
The WR RTDs provide the ability to monitor core differential temperature and subcooling margin using RCS hot (outlet) and cold (inlet) leg temperatures. RCS cold leg temperature is used in conjunction with RCS hot leg temperature to verify the unit conditions necessary to establish natural circulation in the RCS. RCS hot leg temperature also provides a temperature compensating signal for the reactor vessel level instrumentation system. The RCS cold leg temperature also provides a temperature input signal for the low temperature overpressure protection system.
B. Event Description
Pacific Gas and Electric Company (PG&E) identified repeated failures of the Unit 1 RCS WR RTD (Make: Conax, Model:
7K45-10000 or 7K46-10000) elements and initiated an apparent cause evaluation (ACE). The ACE determined that the cause of the failures was an incorrect insulation configuration installed in 2010 on the thermal extension piping that housed the wires for the RTDs. The insulation configuration as installed trapped heat inside the thermal extension piping and overheated the wires, which caused the RTDs to fail. The cause of the incorrect configuration of the insulation was insufficient guidance in the associated work package instructions.
The extent of condition evaluation required verification of the installed insulation configuration and normal service temperatures of all Unit 1 WR RTDs. A walkdown performed in Mode 3 on October 4, 2015, during the Unit 1 Nineteenth Refueling Outage, found that the service temperature of six of the eight WR RTDs was in excess of the maximum expected 190 degree Fahrenheit operating temperature for the wiring. The measurements were taken for RTDs TE-413A/B, TE-423A/B and TE-443A/B. Measurements were not taken on TE-433A/B because TE-433B was inaccessible and TE-433A had failed. However, the local temperature and insulation configurations for TE-433A/B were consistent with the other six RTDs.
An engineering analysis was completed on October 5, 2015, which determined that the eight WR RCS RTDs had either failed or were operating outside the environmental qualification (EQ) temperature range prior to October 4, 2015, when the high temperatures were discovered.
Since the incorrect installation of insulation on the wiring for the eight RTDs in 2010, the minimum required RTDs were functioning and providing accurate temperature indication. However, due to the elevated temperature on the associated wiring, the RTDs had either failed or were operating outside the EQ temperature range. Therefore, PG&E determined the minimum required WR RTDs would not have been considered operable under post-accident conditions associated with Technical Specification (TS) 3.3.3, "Post Accident Monitoring (PAM) Instrumentation," following the 2010 installation of the insulation. As a result, on October 5, 2015, PG&E identified a violation of TS 3.3.3.
C. Status of Inoperable Structure, Systems, or Components That Contributed to the Event There were no other structures, systems or components that contributed to the event.
D. Other Systems or Secondary Functions Affected
No other systems or secondary functions were affected.
E. Method of Discovery
Multiple failures of the Unit 1 WR RTDs led to the performance of an ACE. A plant walkdown on October 4, 2015, completed as part of the extent of condition for the ACE identified the incorrect insulation configuration on the Unit 1 WR RTDs and the elevated temperatures on the wiring associated with the WR RTDs.
F. Operator Actions
When this condition was identified, Unit 1 was in Mode 5. TS 3.3.3 is only applicable in Modes 1, 2, and 3 and therefore no operator action was required.
G. Safety System Responses
None.
III. Cause of the Problem
A. Apparent Cause Evaluation:
PG&E completed an ACE and determined that the cause of the failures was insufficient guidance in the work package instructions which led to an incorrect insulation configuration on the wiring associated with the RTDs. The insulation configuration as installed trapped heat inside the thermal extension piping that housed the wiring associated with the RTDs. The wires overheated inside the thermal extension piping, which caused the failures. As part of the extent of condition evaluation for the failed RTDs, it was determined that the RTDs had either failed or were operating outside the EQ temperature range.
IV. Assessment of Safety Consequences
The WR RCS RTDs are not explicitly modeled in the Probabilistic Risk Assessment (PRA). However, some of the modeled operator actions use WR RCS indications to determine the need for certain actions. In these cases, there were redundant narrow range RTDs that could be used. In addition, other diverse instrumentation was available for verification of core cooling, RCS subcooling margin, and decay heat removal function via steam generators. Due to the redundancy and diversity of instrumentation and the fact that the WR RCS RTDs are not directly modeled in PRA, the impact on core damage frequency is determined to be minimal. Therefore, this event did not adversely affect the health or safety of the public and did not significantly degrade plant safety.
V. Corrective Actions
1. Revised the work order to include the correct drawing and level of details for proper installation of all WR RTDs.
2. Revised the drawings for Unit 1 WR RTDs to provide adequate level of detail for proper insulation configuration installation on all WR RTDs.
3. Installed the correct insulation configuration on all Unit 1 WR RTDs.
4. Replaced all Unit 1 WR RTDs.
VI. Additional Information
A. Failed Components
At the time of discovery on October 5, 2015, the following WR RTDs had either failed or were operating outside the EQ temperature range:
- TE-413A (Loop 1, Hot Leg)
- TE-413B (Loop 1, Cold Leg)
- TE-423A (Loop 2, Hot Leg)
- TE-423B (Loop 2, Cold Leg)
- TE-433A (Loop 3, Hot Leg)
- TE-433B (Loop 3, Cold Leg)
- TE-443A (Loop 4, Hot Leg)
- TE-443B (Loop 4, Cold Leg)
B. Previous Similar Events
A review of the past WR RTD failures or anomalies, since the incorrect insulation configurations were installed, identified the following events:
Past events:
- Unit 1 1-TE-423A failed low on February 11, 2010, and was replaced in November 2010
- Unit 1 TE-413A failed low on June 6, 2013 and was replaced during the Unit 1 Eighteenth Refueling Outage in March of 2014
- Unit 1 TE-433A erratic on July 20, 2013, and was replaced during the Unit 1 Eighteenth Refueling Outage in March of 2014
- Unit 1 TE-413A failed low on March 26, 2014, and was replaced during the Unit 1 Nineteenth Refueling Outage in October of 2015
- Unit 1 TE-433A failed low May 21, 2015, and was replaced during the Unit 1 Nineteenth Refueling Outage in October of 2015
C. Industry Reports
None.