05000298/LER-2007-004

LER-2007-004, Cooper Nuclear Station 05000298 1 of 4

Docket Number
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
Initial Reporting
ENS 43375	10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation, 10 CFR 50.72(b)(3)(iv)(A), System Actuation
2982007004R00 - NRC Website

PLANT STATUS

Cooper Nuclear Station (CNS) was in Mode 1, Power Operation, at 50% Reactor Power in Single Loop Operation at the start of this event.

BACKGROUND

The Control Rod Drive (CRD) Hydraulic System [EllS:AA] supplies and controls pressure and flow to and from the CRDs. One supply subsystem supplies water to hydraulic control units (HCUs) [EIIS:HCU] at the correct flow. Each HCU controls the water flow to and from its associated CRD during normal operation and reactor scram.

Each HCU furnishes pressurized water, on signal, to a CRD for normal operation or a scram. The drive then positions its control rod as required. The water discharged from the drives during a scram flows through the HCUs to the scram discharge volume (SDV). The water discharged from a drive during a normal control rod positioning operation flows through its HCU and into the exhaust header.

The Reactor Manual Control System [EllS:DCC] supplies scram and normal control rod positioning signals to the HCU. The basic components in each HCU are manual, pneumatic, and electrical valves; an accumulator; related piping; electrical connections; .

filters; and instrumentation.

Each HCU has 4 Directional Control Valves (DCVs) [EllS:FSV] that are solenoid-operated valves to direct or exhaust water to or from its respective CRD drive piston. The HCU Insert Supply DCV (CRD-V-123) opens on a CRD insert signal. The valve supplies drive water to the bottom side of the associated CRD piston. The HCU Insert Exhaust DCV (CRD-V-121) also opens by solenoid on an insert signal. The valve discharges water from above the associated CRD piston to the exhaust water header.

The HCU Withdraw Supply DCV (CRD-V-122) is solenoid-operated and opens on a withdraw signal. The valve supplies drive water to the top of the associated CRD piston. At the beginning of the CRD withdrawal sequence the CRD is given a short insert signal, actuating the DCVs associated with rod insertion to unlatch the CRD collet fingers. The solenoid-operated Withdraw Exhaust DCV (CRD-V-120) opens on a withdraw signal and discharges water from below the associated CRD piston to the exhaust header. It also serves as the settle valve. The valve opens, following any normal drive movement (insert or withdraw), to allow the control rod and its drive to settle back into the nearest latch position.

EVENT DESCRIPTION

On the evening of 5/18/2007, Cooper began a scheduled down power to accomplish required preventive and corrective maintenance, including replacement of DCVs on three CRD HCUs. HCU 26-27 was hydraulically isolated for maintenance. The plant was in a single recirculation loop configuration to perform corrective maintenance on 'A' Recirculation Motor Generator [EllS:AD]. A single reactor feed pump (RFP) [EIIS:SJ] was in operation for corrective maintenance to the 'A' RFP lube oil system.

At approximately 00:25 CDT on 5/19/2007 reports from the field indicated that, when maintenance had loosened the flange bolts, a small water leak had developed from the body to bonnet seal of CRD-V-122 for HCU 26-27. The manual isolation valves for the HCU were already closed to isolate the components being worked. However, due to seat leakage past one or both of the manual isolation valves, the leakage continued and degraded to the point that a visible plume of steam was issuing from CRD-V-122, and as a precaution, the Control Room Supervisor (CRS) evacuated the Reactor Building [EllS:NG] at 01:25 CDT.

After several unsuccessful attempts to isolate the leak, operators made the conservative decision to remove the unit from service. At 02:00 CDT, the CRS entered the station reactor scram procedure and started the preparation steps of transferring electrical loads to the startup transformer [EIIS:EB]. At 02:12 CDT, the reactor was manually scrammed. Control rods inserted normally. A Group 2 containment isolation.

signal [EllS:JM] was received at a low reactor water level of 3" above instrument zero, and isolated required systems from the Primary Containment [EIIS:NH]. The leak into the Reactor Building stopped after the reactor was scrammed.

During the recovery, the running RFP was manually tripped at 50" above instrument zero in accordance with station procedures due to high reactor pressure vessel (RPV) level conditions. 'B' RFP was restarted when conditions permitted and was used to control RPV level. RPV pressure was controlled by the turbine bypass valves rejecting heat to the main condenser. Radiological conditions were assessed, and a repair and recovery plan was implemented. The scram was characterized as uncomplicated and systems functioned as required. No off-site releases occurred due to the HCU valve leak.

BASIS FOR REPORT

This report is required per 10.CFR 50.73(a)(2)(iv)(A), as an event or condition that resulted in manual or automatic actuation of the Reactor Protection System and a valid actuation of a containment isolation system. In Event Notification #43375, CNS reported this under 50.72(b)(2)(iv)(B) as a 4 Hour (Hr) Emergency Notification System (ENS) Notification and under 50.72(b)(3)(iv)(A) as an 8 Hr ENS Notification.

SAFETY SIGNIFICANCE

The manual scram and controlled shutdown that resulted from the leak in HCU 26-27 was not a nuclear risk significant event. Mitigating equipment responded as expected.

The leak did not impact functionality of any equipment and stopped after the plant shutdown. Therefore, this event resulted in negligible change in core damage frequency.

CAUSE

The root cause was that neither the Clearance Order (i.e. Tag-Out) implementation, nor the acceptance of the system for maintenance required explicit, positive verification that the system energy source was isolated, and neither process identified contingency actions in case the system was still pressurized. Leakage past one of the manual isolation valves (CRD-V-101) is the most probable direct cause of the pressure persisting internal to CRD-V-122 (HCU 26-27).

CORRECTIVE ACTION — The following actions are being tracked in the CNS Corrective Action Program.

A. Revise the Clearance Order process to require verification of hazardous, off­ normal, mode changed, or unusual system isolation prior to system release for maintenance, and to include identified contingency actions in case the system energy source cannot be isolated. Include maintenance verification by positive actions that the isolated system energy has been removed.

B. Inspect CRD-V-101(HCU 26-27) to determine if there is any mechanical degradation which could explain the leakage associated with this event. Develop and implement corrective actions for leakage based upon the results of the inspection.

PREVIOUS EVENTS

There have been no reportable events identified in the past 7 years related to HCU leaks.