05000275/LER-2013-001
| Diablo Canyon Power Plant, Unit 1 | |
| Event date: | 01-03-2013 |
|---|---|
| Report date: | 04-04-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2752013001R01 - NRC Website | |
I. Plant Conditions
Diablo Canyon Power Plant (DCPP) Units 1 and 2 were in Mode 1, with reactor power at approximately 100 percent.
II Description of the Problem
A. Background
The low temperature overpressure protection system (LTOP) controls reactor coolant system (RCS) [AB] pressure at low temperatures so the integrity of the reactor coolant pressure boundary is not compromised by violating the pressure and temperature (P/T) limits of 10 CFR 50, Appendix G. The potential for vessel over-pressurization is most acute when the RCS is water solid during plant shutdown conditions. At this time, a pressure fluctuation can occur more quickly than an operator can react to relieve the condition. Exceeding the RCS P/T limits could cause brittle cracking of the reactor vessel. Technical Specification (TS) Limiting Conditions for Operation (LCO) 3.4.3, RCS P/T Limits, requires administrative control of RCS pressure and temperature during heat-up and cool-down to prevent exceeding the pressure temperature limits report (PTLR) limits.
TS 3.4.12 LCO assures RCS overpressure protection by limiting coolant input capability to no safety injection (SI) [BQ, P] pumps and a maximum of one centrifugal charging pump (CCP) [CB, P] being capable of injecting into the RCS, and isolating the RCS accumulators. However, CCPs in excess of the above limitations can be momentarily capable of injection into the RCS for swapping of inservice CCPs. Although not addressed in LCO 3.4.12, the plant design includes two emergency core cooling system (ECCS) CCPs and also a third CCP (CCP-3).
Operation of CCP-3 is controlled administratively in accordance with the PTLR.
TS 3.4.12 is applicable during the following plant modes of operation:
- Mode 4, when any RCS cold leg temperature is less than or equal to LTOP arming temperature specified in the PTLR (approximately 280°F).
- Mode 5 -Mode 6, when the reactor vessel head is on and the vessel head closure bolts are not fully detensioned.
The LTOP system for pressure relief consists of two Class I power operated relief valves (PORVs) [AB, RV] with reduced lift settings, or a depressurized RCS and an RCS vent of sufficient size. Two RCS Class I PORVs are required for redundancy. One RCS Class I PORV has adequate relieving capability to prevent over pressurization from the allowable coolant input capability.
DCPP LTOP analysis concluded that RCS limits would not be exceeded given that (1) all SI pumps were secured, (2) one ECCS CCP was secured, (3) all SI accumulators were isolated, and (4) CCP-3 was aligned for LTOP operation prior to entering the LTOP mode of operation.
B. LTOP Operating Experience and Impact on DCPP Wolf Creek Nuclear Operating Company (WCNOC) has a charging system and licensing history similar to DCPP's in that it originally had a normal-use reciprocating charging pump and two ECCS CCPs. WCNOC replaced its reciprocating pump with a CCP (similar to DCPP) without revising its LTOP TS. Similar to DCPP, WCNOC's LTOP TS specifies that only one CCP can be capable of injecting to the RCS while LTOP is required to be operable.
On January 3, 2013, the U. S. Nuclear Regulatory Commission (NRC) issued a TS interpretation clarifying that the WCNOC TS 3.4.12 does not permit more than a single CCP to be capable of injecting into the RCS while LTOP is required to be operable.
C. Event Description
On January 3, 2013, at 19:32 PST, DCPP reviewed the NRC's interpretation of WCNOC TS 3.4.12 and determined that DCPP had been in noncompliance with TS 3.4.12 (when applicable) since the positive displacement pump (PDP) replacement modifications in Unit 1 in 2005 and in Unit 2 in 2007. DCPP determined this condition was reportable pursuant to 10 CFR 50.73(a)(2)(i)(B) as an operation or condition which was prohibited by the plant's TS.
PG&E maintained CCP configurations in accordance with the LTOP analysis and the PTLR by having procedural requirements as indicated below:
A. with RCS temperature below 280°F and above 156°F, two CCPs are capable of injecting into the RCS, one of which is CCP-3 aligned for LTOP operation.
B. with RCS temperature at or below 156°F, only one CCP is capable of injecting into the RCS.
The following is a list of the approximate dates in the prior 3 years in which DCPP relied on LTOP and was in configuration A listed above and not in compliance with TS 3.4.12:
Unit 1:
Refueling outage 16 (10/03/10 at 06:50 - 10/03/10 at 17:40) (11/02/10 at 07:30 - 11/05/10 at 01:30) Refueling outage 17 (04/22/12 at 22:20 — 04/23/12 at 17:40) (06/08/12 at 13:50 — 06/10/12 at 16:00) Unit 2:
Refueling outage 16 (05/01/11 at 08:50 (05/31/11 at 09:40 — — 05/01/11 at 22:00) 06/02/11 at 12:50) D. Status of Inoperable Structures, Systems or Components that contributed to the Event None.
E. Other Systems or Secondary Functions Affected
None.
F. Method of Discovery
This condition was discovered during a review of operating experience associated with the NRC's TS Interpretation letter dated January 3, 2013, indicating that TS 3.4.12 does not permit operation of more than a single CCP while LTOP is required to be operable.
G. Operator Actions
None.
H. Safety System Responses
None.
III. Cause of the Event
A. The two apparent causes identified with this condition are summarized below:
1. A deficiency in DCPP's 10 CFR 50.59 procedure was identified as the apparent cause. The procedure did not provide guidance regarding proposed design changes that may maintain the original intent but create new literal compliance issues.
2. Human Error - The DCPP staff interpreted the operability requirements outlined in TS 3.4.12 as being equivalent with respect to the PDP to CCP design change.
IV. Assessment of Safety Consequences
The plant configuration controls using the new CCP ensured DCPP operated within the limits of the LTOP analysis, ensuring that the (P/T) limits of 10 CFR 50, Appendix G, would not be exceeded. Nuclear or radiological safety was not affected. This event had no impact to safety of the public or station personnel. This event did not impact the reliability of plant operation or production capacity.
V. Corrective Actions
A. Immediate Corrective Actions
1. Revised all affected procedures (shutdown and startup) to require disabling two CCPs consistent with TS 3.4.12 requirements.
B. Other Corrective Actions
I. Revise the Licensing Basis Impact Evaluations Procedure TS3.ID2 to provide clear guidance regarding equivalent replacements that may create new literal compliance issues.
2. Revise Current Licensing Basis Determination Procedure XI3.ID12 to discuss the importance of literal compliance with DCPP TS and license.
3. Provide lessons-learned discussion to staff associated with design changes including staff from other key departments.
VI. Previous Similar Events
There have been no similar reportable events within the last 3 years.
VII. Additional Information
None