05000261/LER-2010-006

LER-2010-006, Pressurizer Heaters Inoperable Longer Than Allowed Due to Inadequate Procedure

Docket Number
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
2612010006R00 - NRC Website

I. DESCRIPTION OF EVENT

On August 4, 2010, with H. B. Robinson Steam Electric Plant (HBRSEP), Unit No. 2, operating in Mode 1, it was determined that the pressurizer heaters could not be operated from the Train B emergency power source using the existing procedural guidance under current conditions. Therefore, if Emergency Diesel Generator (EDG) 'A' [EK:DG] is inoperable, the required emergency power supply to the pressurizer heaters would not have been provided if required. Technical Specification (TS) 3.4.9, "Pressurizer," requires pressurizer heaters to be capable of being powered from an emergency power supply. The TS requires that emergency power capability be restored within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> or a plant shutdown initiated. There have been periods when EDG 'A' was inoperable for greater than 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> (e.g., December 2008) and a plant shutdown was not initiated because this condition was not known at that time. This resulted in a condition prohibited by the plant's Technical Specifications and is reportable under 10 CFR 50.73(a)(2)(i)(B).

II. CAUSE OF EVENT

The cause of this event was inadequate verification of plant changes during the implementation of TMI lessons learned.

III. ANALYSIS OF EVENT

This event was investigated using the HBRSEP, Unit No. 2, Corrective Action Program (CAP) and documented in Nuclear Condition Report 413865 and was reviewed by the Plant Nuclear Safety Committee on September 28, 2010.

The Limiting Condition for Operation regarding Technical Specification 3.4.9 "Pressurizer" requires the pressurizer heaters be operable with a capacity greater than or equal to 125 kW and capable of being powered from an emergency power supply. Condition C states that if required heaters are not capable of being powered from an emergency power supply, the capability to power the required pressurizer heaters from an emergency power supply must be restored in 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />. Condition D states that if the required action and associated completion time for Condition C cannot be met, the plant must be in Mode 3 in 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and be in Mode 4 in 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />.

On August 4, 2010, it was determined that if EDG 'A' were inoperable, energizing the required pressurizer heaters from credited emergency power supplies would not be possible. This is due to the fact that a low pressurizer level heater cutoff circuit [JL:LC], which receives power from Instrument Bus 1 [EF:BU], would be de-energized. This condition prevents closure of the Control Group Heater Breaker [ED:BKR] electrically, as specified in procedure EPP-21, "Energizing Pressurizer Heaters From Emergency Busses," to energize the heaters.

In the fall of 1979, H. B. Robinson Steam Electric Plant (HBRSEP), Unit No. 2, revised procedure EI-15, "Control Room Inaccessibility" in response to NUREG-0578, "TMI-2 Lessons Learned Task Force Status Report and Short-term Recommendations." Following this update, the method by which emergency power was supplied to the pressurizer heaters was considered to provide redundant capability without compromising independence between emergency buses. Based on the investigation of this event, it appears that the effects of Emergency Bus E-1 [EF:BU] being unavailable, causing the low pressurizer level heater cutoff circuit to be de-energized, were not recognized at that time. EDG 'A' and EDG 'B' were considered redundant and changes were validated by performing assessments of EDG 'A' only.

The investigation further concluded that the testing methodology has masked the issue with the low pressurizer level heater cutoff circuit due to maintaining Instrument Bus 1 energized by the Dedicated Shutdown Diesel Generator (DSDG) [EC:DG] in accordance with plant procedures. Although the DSDG would in most cases ensure that the pressurizer heaters could be powered from emergency power, the DSDG is not included for TS operability.

Safety Significance

The investigation concluded that the safety significance of this condition is low due to the limited time EDG 'A' has been out-of-service (other than surveillance testing). In addition, if HBRSEP, Unit No. 2, were to experience a Loss of Offsite Power with EDG 'A' unavailable, the required pressurizer heaters could have been energized by utilizing a combination of the 'B' EDG and the DSDG.

. IV. CORRECTIVE ACTIONS

Completed Corrective Actions:

• As an interim action, Operations issued a caution tag on EDG 'A' that required entering the 72 hour8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> required action for Condition C of LCO 3.4.9 if EDG 'A' was inoperable.

• EPP-21, "Energizing Pressurizer Heaters from Emergency Busses," was subsequently revised to ensure the pressurizer heaters could be powered from the 'B' EDG, which allowed removal of the caution tag.

Planned Corrective Actions:

• The changes to EPP-21 require lifting a lead to be able to power the pressurizer heaters from `B' EDG. Therefore, it is currently planned to make physical modifications to eliminate the need for lifting a lead. This modification is currently scheduled for completion by May 31, 2013.

Previous Similar Events:

License Event Reports (LERs) for HBRSEP, Unit No. 2, were reviewed from the past 10 years. No events were identified as being similar to the event described in this LER.