05000336/LER-2004-001

LER-2004-001,

Docket Number
Event date:	03-06-2004
Report date:	04-26-2004
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
Initial Reporting
ENS 40570	10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation, 10 CFR 50.72(b)(3)(iv)(A), System Actuation
3362004001R00 - NRC Website

1. Event Description On March 6, 2004, at approximately 0050 with the unit in Mode 1 at 100 percent power, and with steam generator (SG) level approaching about 55 percent, operators initiated a manual reactor trip in anticipation of the low SG water level auto trip setpoint. The decreasing SG water level was due to a reduction in feedwater [SJ] from the loss of the 'B' steam generator feedwater (SGFW) pump. The 'B' SGFW pump had tripped unexpectedly causing lowering SG water levels.

All control rods inserted into the core and all electrical busses transferred properly following the trip. Post trip procedures were followed and the plant response was as expected. When the main turbine reached approximately 1200 RPM, a Turbine High-High vibration alarm was received. The annunciator response procedure was carried out for vibration levels greater than 15 mils. Both Main Steam Isolation Valves (MSIVs) were closed and condenser vacuum was broken to slow the main turbirie. Auxiliary Feedwater (AFW) [BA] initiated as expected, on a low steam generator level following the trip from 100 percent power. Operators established decay heat removal capability using the AFW system and the atmospheric steam dump valves.

During and after the plant trip there was no challenge to the mitigating systems. Following the manual reactor trip, the standard post trip procedure actions were carried out. The plant was monitored and the crew transitioned to the post trip recovery procedure. Safety function status checks were performed successfully and, with the plant stable in MODE 3 (HOT STANDBY) at approximately 0310, the crew transitioned to the normal operation procedure set.

This event is being reported pursuant to 10 CFR 50.73(a)(2)(iv)(A) as an event that resulted in manual or automatic actuation of any of the systems listed in 50.73(a)(2)(iv)(B). This includes Reactor Protection System actuation (RPS) and Auxiliary Feedwater System initiation (AFW). A non-emergency notification was made on March 6, 2004, to the NRC Operations Center (Event Number 40570) in accordance with 10 CFR 50.72(b)(3)(iv)(A) and 50.72(b)(2)(iv)(B).

2. Cause The SGFW pumps were originally designed to operate in both automatic and manual operation utilizing a pneumatic positioner, although the automatic function was historically not utilized. In April 2002, a design change was implemented to replace the mechanical-hydraulic main feedwater pump turbine control system [JK]. The design change replaced the existing feedwater pump speed control system with a Woodward Governor digital control system (MicroNet system). The system uses a digital controller and a hydraulic actuator to control the main feedwater pumps.

The apparent cause of the trip was agitation of a relay in the feedwater pump control system. This perturbed the relay's signal to the MicroNet sufficiently for the MicroNet to sense a low pressure condition and initiate a trip of the feedwater pump, even though a low pressure condition did not exist. The resulting 'B' SGFW pump trip required the operators to initiate a manual reactor trip on low SG level.

A root cause of this event was a lack of experience, both on site and through industry review, relating to problems associated with the interfacing of a control grade electromagnetic device, (specifically a General Electric HGA type relay), with a state of the art digital microprocessor. This led to a decision to install a configuration in which very slight agitation of this control relay or its cabinet could perturb the signal to the MicroNet processor sufficiently for the microprocessor to generate a trip signal to the SGFW pump. This type of an electrical relay has been noted for its sensitivity to mechanical vibration and soft contact closure.

3. Assessment of Safety Consequences The purpose of the steam generator feedwater (SGFW) system is to provide water to the SGs for the transfer of thermal energy from the primary side of the SGs to the secondary side. After the trip, the Main Feedwater (MFW) function of the SGFW system became unavailable for decay heat removal. This function is credited in the PRA, but is not risk significant. As a result, the risk associated with this reactor trip was calculated to be slightly higher than the risk associated with the general plant transient, in which the MFW remains available. During and after the plant trip there was no challenge to the mitigating systems, and the required safety equipment was operable. The MFW function is not safety related. There was no loss of any credited safety function from structures, systems and components. Consequently, this event is considered to be of low safety significance.

4. Corrective Action As a result of the Event Investigation Team (EIT) findings in response to the immediate issue of relay contact sensitivity, a relay's operation in the speed control circuit for the SGFW pumps was changed from an energized closed to an energized open configuration with the associated software changes. This reduces the HGA relay sensitivity to agitation and will avert additional SGFW pump trips. 'Trip sensitive equipment" signs and barriers were added for the terminal box for this control relay.

Corrective actions that are related to this event, including lessons learned surrounding the considerations for use of digital interfacing, are being addressed in accordance with the Millstone Corrective Action Program.

5. Previous Occurrences No previous similar events/conditions were identified.

Energy Industry Identification System (EIIS) codes are identified in the text as [XX].