05000423/LER-2005-002

LER-2005-002,

Docket Number
Event date:	04-17-2005
Report date:	06-16-2005
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
4232005002R00 - NRC Website

1. Event Description On April 17, 2005 at 0829, during normal 100% power operating conditions the solid state protection system (SSPS) [JC] generated an "A" Train SSPS Steam Line Pressure Low SI/MSI signal that tripped the reactor, closed the main steam isolation valves (MSIVs), and started the "A" train of the emergency core cooling system (ECCS) [BO]. Following the trip, all control rods inserted and the auxiliary feedwater system [BA] actuated automatically as expected, however the turbine driven auxiliary feedwater (TDAFW) pump tripped on startup. Immediately following the trip, control room operators entered the emergency operating procedure network, manually initiated "B" safety injection (SI) actuation so as to have a two-train SI actuation, and continued to place the plant in a stable condition. Later into the event at 0844, operators also manually initiated "B" main steam isolation (MSI) actuation.

At 0840, reactor coolant system (RCS) pressure reached 2350 psia and both pressurizer power operated relief valves (PORVs) began cycling as designed. The pressurizer safety valves remained closed throughout the event.

Safety injection was terminated at 0913. After performing their safety function, both PORVs leaked, however the leakage did not exceed the 10 gpm Technical Specification limit. Leakage was conservatively estimated to be 5.3 gpm with the plant at 2250 psia. Additionally, a field observation at 0858 reported RCS leakage from two charging system valves (3CHS*V661 and 3CHS*MV851113) into the auxiliary building. Valve 3CHS*MV8511B leakage was terminated when the valve was closed at 0905 and valve 3CHS*V661 was isolated at 1032. The leakage was determined to have commenced at 0830 and initial leakage was estimated to be approximately 60 gpm.

Regarding secondary side plant response, following the reactor trip, approximately 8 seconds after the TDAFW pump began feeding the steam generators (SGs), the TDAFW pump trip throttle valve closed. This resulted in isolating steam to the pump turbine which then began to coast down. Subsequent manual restart of the TDAFW pump occurred at 1019. The two motor driven AFW pumps provided flow to the SGs throughout the event.

Within one hour of event initiation, plant parameters were stabilized and ECCS systems were stopped and placed in a standby condition. The event was terminated at 1905.

This event is being reported pursuant to 10CFR50.73(a)(2)(iv)(A) as an event that resulted in manual or automatic actuation of the reactor protection system, multiple main steam isolation valves, emergency core cooling systems, and auxiliary feedwater system. In addition, this report satisfies the Technical Specification 3.5.2 special reporting requirement (refer to Section 6) and satisfies the 10 CFR 21 evaluation, notification and reporting obligation for reporting of defects.

2. Event Classification During the initial event, all SG pressures were above normal due to the automatic closing of all MSIVs and two of the four atmospheric steam dump valves (normal system response for a single train MSI). One of the "B" SG safety valves lifted when the "B" SG pressure reached approximately 1185 psig. The operations shift manager, based on past training on system response and interpretation of actual plant annunciator and other indications, diagnosed that the "B" SG safety valve did not reseat. Based on this evaluation, an Alert was declared at 0842 and the emergency response organization was activated. It was later determined that the "B" SG safety valve did in fact reseat (0850) as the RCS temperature decreased. Just-In-Time operator training was conducted prior to restart of the unit to address plant response associated with this event.

An NRC notification was made via the emergency notification system on April 17, 2005, at 0923 with appropriate updates (Event No. 41607).

3. Cause The root cause of this event was determined to be:

Root Cause No. 1: Diode leads on Universal Logic Board A213 were coated with a material susceptible to whisker growth that eventually shorted the card output.

Root Cause No. 2: No plan, process, or procedure exists for identifying, monitoring or addressing whisker growth on SSPS cards.

The failed component was Universal Logic Board 6056D21G01 (Serial No. 6076) located in slot A213 in "A" Logic Cabinet, local identification No. 3RSP*AA213.

4. Assessment of Safety Consequences The risk significance of this event was determined to be no greater than that of a reactor trip with inadvertent safety injection. Regarding the initial failure of the TDAFW pump, the results of a root cause investigation determined the most probable cause of the overspeed trip was stem binding of the associated control valve that occurred at the time of the event. However, this failure did not prevent manual restart. Therefore, the TDAFW pump was considered available for mitigation. Additionally, the motor driven AFW pumps were supplying adequate flow to the SGs and no loss of safety function occurred. Also, the charging pumps were able to deliver significant flow despite the valve packing leaks. For these reasons, this event is considered to be of low safety significance.

5. Corrective Action On April 18, 2005 at approximately 1200, troubleshooting of the SSPS indicated that an inadvertent "A" Train SSPS Steam Line Pressure Low SI/MSI signal was generated when a metallic thread or whisker from the anode (hard wired to ground) of a diode caused a false tripped indication. The circuit card was a Westinghouse Universal Logic Board. Additional inspection was performed on installed and spare SSPS boards and additional evidence of whiskering was observed. Based on observation, the whiskering was limited to diodes on the cards.

Universal Logic Boards with identified whiskering were replaced to the extent possible based on spare card availability. Other cards that were not replaced were cleaned and returned to service. Inspection of the Millstone Power Station Unit 2 emergency safeguards actuation system (15 boards inspected) was performed and indicated no evidence of whiskers.

A root cause investigation was conducted and appropriate corrective actions are being addressed in accordance with the Millstone Power Station Corrective Action Program.

The corrective actions to prevent recurrence of this event were determined and are summarized as follows:

• Independent lab testing was performed on suspect diodes and lab verbally confirmed diode lead material is susceptible to whiskering and that tin whiskering occurred: (Completed)

• Three separate corrective actions were established to review measures executed by Westinghouse relative to SSPS to assure 1) nonconforming materials, parts and services are controlled in order to prevent their inadvertent use, 2) conditions adverse to quality are promptly identified and corrected, and 3) purchased equipment and services conform to the procurement documents.

• A corrective action was established to develop and implement a preventive maintenance program for SSPS cards.

6. Special Reporting Pursuant to Technical Specification 3.5.2, ACTION b, the following additional information is being reported:

Millstone Power Station Unit 3 has accumulated six SI actuations to date while in Modes 1, 2, or 3. The prior five actuations occurred in 1986 as documented in LERS 86-001-00, 003-00, 019-00, 021-00, and 027-00. The total accumulated usage factor for the SI nozzles resulting from the six SI actuations is less than 0.70.

This Licensee Event Report satisfies the Technical Specification requirement for a 90-day Special Report on the safety injection actuation.

7. Previous Occurrences No previous similar events were identified related to tin whiskers.

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