05000353/LER-2007-001

LER-2007-001, Scram Discharge Volume Vent and Drain Valves Opened Due To Fuse Removal

Docket Number
Event date:	03-10-2007
Report date:	05-09-2007
Reporting criterion:	10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material 10 CFR 50.73(a)(2)(vii)(C), Common Cause Inoperability
3532007001R00 - NRC Website

Unit Conditions Prior to the Event Unit 2 was in Operational Condition (OPCON) 3 (Hot Shutdown).

Reactor pressure was approximately 24 psig with noble metal chemical application in progress. There were no structures, systems or components out of service that contributed to this event.

Description of the Event

On Saturday, March 10, 2007, Unit 2 was in OPCON 3 in day 1 of the 2R09 refueling outage. A clearance was being applied to perform maintenance on the reactor protection system (RPS) [EIIS:JC] scram contactors and the scram air header. The clearance required inserting a full scram signal followed by removal of the RPS logic fuses [EIIS:FU] including the "A" and "B" backup scram valve fuses. When these fuses were removed the backup scram valves were de-energized and re-positioned resulting in re-pressurizing the scram air header. This caused the scram discharge volume (SDV) vent and drain valves to open. Since the full scram was still present, reactor water in the SDV was inputted into the dirty radwaste system (DRW) [EIIS:WD] at a funnel drain on 253' elevation and a hard piped SDV drain to the equipment drain collection tank in the radwaste enclosure.

An investigation determined that the SDV vent and drain valves opened at approximately 19:42 hours when the backup scram valve fuses were removed. The SDV vent and drain valves were closed at approximately 21:48 hours by isolating and venting the air supply at the valves. The valves were open for a period of approximately 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> and 6 minutes. The event was identified at 21:03 hours when Radiation Protection (RP) personnel notified Operations of a spill on reactor enclosure 253' elevation. The Reactor Operator identified that the SDV vent and drain valves were open.

The clearance also included a step to close the scram air header manual valve at the inlet to the backup scram valves. Closure of this valve would have caused the SDV vent and drain valves to close when the scram air header depressurized, but this step was later in the clearance sequence.

This event involved a condition that could have prevented fulfillment of the safety function for the primary containment system, which is reportable per the requirements of 10CFR50.72(b)(3)(v)(C). An 8-hour NRC ENS notification was not performed since the event was not determined to be reportable within the 8-hour reporting period. This event also involved a single cause (i.e., clearance application) that caused two independent primary containment isolation valve (PCIV) trains to become inoperable in a single failure proof system designed to control the release of radioactive material. Therefore, this LER is being submitted pursuant to the requirements of 10CFR50.73(a)(2)(v)(C) and 10CFR50.73(a)(2)(vii)(C).

Analysis of the Event

There were no actual safety consequences associated with this event. The potential safety consequences of this event were minimal. There was no release of radioactive material to the environment. The coolant discharged from the SDV was collected in the equipment drain collection tank and the reactor enclosure floor drain system. Reactor coolant activity was within the limits imposed by Technical Specifications (TS) at the time of the event.

The SDV design includes a 2-inch drain line to the equipment drain collection tank and a 1-inch vent line to the reactor enclosure dirty radwaste system. Each line has two in-series air-operated valves designed to automatically close during a scram. The SDV vent and drain valves are designated as primary containment isolation valves (PCIVs) [EIIS:ISV] in the Technical Requirements Manual (TRM) and Updated Final Safety Analysis Report (UFSAR). Therefore, TS 3.6.1.1 Primary Containment, TS 3.6.3 Primary Containment Isolation Valves and TS 3.1.3.1 Control Rod Operability actions are applicable in OPCONs 1, 2 and 3. The TS action in OPCON 3 is to be in OPCON 4 (Cold Shutdown) within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. The TS actions were exited when the valves were closed.

The RPS logic for the backup scram valves energizes the 125 VDC solenoids on the backup scram valves during a scram to block and vent the scram air header. The same circuit energizes a relay that is used to de-energize the SDV vent and drain valve pilot valves during a scram. The RPS logic is otherwise designed to de-energize the scram valve pilot valve solenoids on each hydraulic control unit (HCU) during a scram.

The safety function of the SDV vent and drain valves is to close on a scram to limit the amount of reactor coolant discharged so that adequate core cooling is maintained and offsite doses remain within the limits of 10CFR100. The valves open on scram reset to maintain the SDV vent and drain path open so that there is sufficient volume to accept the reactor coolant discharged during a scram. The discharge of reactor coolant to the SDV can be terminated by scram reset. For a bounding leakage case, the offsite doses are well within the limits of 10CFR100 and adequate core cooling is maintained.

Cause of the Event

The event was caused by a failure to anticipate the system response that occurs when the backup scram valve fuses are removed.

Corrective Action Completed A System Specific Tagging Guide was implemented for backup scram valve fuse removal. The guide requires closing the manual valves in the SDV vent and drain lines prior to removing the fuses.

Previous Similar Occurrences There were no previous similar occurrences of the SDV vent and drain valves causing a primary containment breach when primary containment was required to be operable.