05000446/LER-2011-001
| Comanche Peak Nuclear Power Plant Unit 2 | |
| Event date: | 04-26-2011 |
|---|---|
| Report date: | 06-27-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 4462011001R00 - NRC Website | |
I. DESCRIPTION OF THE REPORTABLE EVENT
A. REPORTABLE EVENT CLASSIFICATION
Based on preliminary review of the event, the event was reported under the following criteria:
10CFR50.73(a)(2)(v) "Any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to: (B) Remove residual heat; or (D) Mitigate the consequences of an accident" and 10CFR50.73(a)(2)(vii) "Any event where a single cause or condition caused at least one independent train or channel to become inoperable in multiple systems or two independent trains or channels to become inoperable in a single system designed to: (B) Remove residual heat; or (D) Mitigate the consequences of an accident.
Subsequent analysis of the event has shown that both trains of RHR and SI would have fulfilled their respective safety functions. However, the same analysis determined that one (1) train of containment spray would have been inoperable upon Unit 2 entry into Mode 4 contrary to Technical Specifications 3.3.6 and 3.0.4, thus this event is being reported under 10CFR50.73(a)(2)(i)(B) "Any operation or condition which was prohibited by the plant's Technical Specifications.
B. PLANT CONDITION PRIOR TO EVENT
On April 26, 2011, CPNPP Unit 2 was in Mode 2 during a planned outage.
C. STATUS OF STRUCTURES, SYSTEMS, OR COMPONENTS THAT WERE INOPERABLE AT THE START
OF THE EVENT AND THAT CONTRIBUTED TO 'THE EVENT There were no inoperable structures, systems, or components that contributed to the event.
D. NARRATIVE SUMMARY OF THE EVENT, INCLUDING DATES AND APPROXIMATE TIMES
CPNPP Unit 2 was performing a reactor power ascension from refueling outage 2RF12. Unit 2 entered Mode 2 at 1538, April 25, 2011, and was critical at 1754. At 0020, April 26, 2011, during post-outage void verification inspections, a 3.71 ft3 void was discovered in the high point piping run containing Refueling Water entered Limiting Condition for Operation Action Requirement (LCOAR) A2-11-231 for LCO 3.0.3, Two Trains of Containment Spray[EIIS: (BE)], Safety Injection [EIIS: BQ)], and Residual Heat Removal [EIIS: (BP)] Inoperable.
The void was removed by aligning temporary tubing from 2SI-8978 to a drain and venting the system until a steady stream of water was observed flowing from 2SI-8978. Void elimination was then verified via ultrasonic inspection.
LCOAR A2-11-231 was exited at 0143, April 26, 2011.
Prior to the April 2011 2RF12 refueling outage, various sections of piping were ultrasonically tested to verify the absence of gas voiding. The high point piping containing 2SI-0047 and 2SI-8978 was verified free of gas voids on March 31, 2011.
Further analysis demonstrated that the declaration of inoperability was a conservative action, and RHR and SI would have fulfilled their respective safety functions and been operable. The analysis confirmed that one (1) train of containment spray was inoperable, however the containment spray safety function would have been Willed with the remaining operable CT train.
E. THE METHOD OF DISCOVERY OF EACH COMPONENT OR SYSTEM FAILURE, OR PROCEDURAL
PERSONNEL ERROR
This condition was identified as a result of void verification inspections required by CPNPP procedure STA-698, "Gas Intrusion Program." The inspections are required to be conducted following the isolation of the Residual Heat Removal (RHR) system from the Reactor Coolant System (RCS).
H. COMPONENT OR SYSTEM FAILURES
A. CAUSE OF EACH COMPONENT OR SYSTEM FAILURE
Not applicable - No component failures were identified during this event.
B. FAILURE MODE, MECHANISM, AND EFFECTS OF EACH FAILED COMPONENT
Not applicable - No component failures were identified during this event.
C. SYSTEMS OR SECONDARY FUNCTIONS THAT WERE AFFECTED BY FAILURE OF COMPONENTS
WITH MULTIPLE FUNCTIONS
Not applicable - No component failures were identified during this event.
D. FAILED COMPONENT INFORMATION
Not applicable - No component failures were identified during this event.
Ill. ANALYSIS OF THE EVENT
A. SAFETY SYSTEM RESPONSES THAT OCCURRED
Not applicable - No safety system responses occurred as a result of this event.
B. DURATION OF SAFETY SYSTEM TRAIN INOPERABILITY
One (1) train of Containment Spray was inoperable upon Unit 2 entry into Mode 4 on April 24, 2011 at 0518 until the LCOAR was exited on April 26, 2011 at 0143.
C. SAFETY CONSEQUENCES AND IMPLICATIONS OF THE EVENT
The primary function of the ECCS is to remove the stored and fission product decay heat from the reactor core during accident conditions. The analysis methodology used to determine system operability in this event conservatively postulates that the entirety of the discovered void would enter the suction of a single pump, thereby preventing the pump from performing its designated safety function. Further analysis demonstrated that the declaration of inoperability was a conservative action, and RHR and Si would have fulfilled their respective safety functions and been operable. This analysis is based on detailed study of the void transport and piping design. The analysis confirmed that one (1) train of containment spray was inoperable, however the containment spray safety function would have been fufilled with the remaining operable CT train.
Based on the above, it is concluded that the health and safety of the public were unaffected by this condition and this event has been evaluated to not meet the definition of a safety system functional failure per 10CFR50.73(a)(2) (v).
IV. CAUSE OF THE EVENT
The direct cause of this event was the introduction, transport, and accumulation of gases in the vicinity of 2SI-0047, resulting in a void of sufficient size that very conservative analytical methods used at the time of discovery determined the Containment Spray, Safety Injection, and Residual Heat Removal systems were rendered inoperable.
The gas void was the result of the RHR system fill and vent evolution and subsequent RHR operation that transmitted the void to its final location. Contributing to this event was the fact that vent valve 2SI-8978 (located in the section of piping where the void was discovered) was not included in applicable venting procedures.
The cause of this event was the failure to identify a unique system configuration and the untimely identification and removal of a gas void that could have potentially rendered both trains of the Containment Spray, Safety Injection, and Residual Heat Removal systems inoperable.
V. CORRECTIVE ACTIONS
Immediate corrective action was the venting of the gas void and returning the ECCS systems to operability. After vent valve 2SI-8978 was discovered not to be in the applicable venting procedures, the Unit 1 vent valve counterparts were also reviewed. The review discovered Unit 1 vent valve 1SI-0240 was also not included in applicable venting procedures. Consequently, 2SI-8978 and 1SI-0240 will be incorporated into the appropriate venting procedures.
Subsequent corrective actions include revision of the post-outage void verification program to require the conduct of preliminary void verifications at locations where potentially dual-train operability-affecting voids may form after entry into Mode 5 but prior to entry into Mode 4 and revision of STA-698 to include a requirement to periodically review fill and vent procedures to ensure all known gas intrusion mechanisms (e.g., pressure damming) are considered.