05000321/LER-2002-001
| Edwin I. Hatch Nuclear Plant - Unit 1 | |
| Event date: | 02-08-2002 |
|---|---|
| Report date: | 03-26-2002 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv), System Actuation |
| Initial Reporting | |
| 3212002001R00 - NRC Website | |
FACILITY NAME (1) DOCKET LER NUMBER (6 PAGE (3)
PLANT AND SYSTEM IDENTIFICATION
General Electric - Boiling Water Reactor Energy Industry Identification System codes appear in the text as (EIIS Code XX).
DESCRIPTION OF EVENT
On 02/08/2002, at 2252 EST, Unit 1 was in the Run mode at 746 CMWT (27 percent rated thermal power). At that time, the Reactor Protection System (RPS, EIIS Code JC) was manually actuated to maintain the Main Condenser (EIIS Code Q) as the heat sink.
At 2252 EST on February 8, 2002 the reactor was scrammed at approximately 27% RTP and the Main Turbine subsequently tripped. Per off gas system recorder 1N62R602 the point 1 temperature in the Recombiner 1N62A001A began falling from 320°F to 220°F between 1700 EST and 1730 EST. Between 1700 EST and 1900 EST both points 2 and 3 began falling from 600°F to 400°F on the same recorder 1N62R602. At 1930 EST point 2 had fallen to 220°F and Off Gas Hydrogen levels began increasing per recorder1N62R603 at 1935 EST. At 2045 EST point 3 had fallen to 220°F. Per GE SIL 150 Revision 2 Supplement 3, the minimum temperature of 250°F must be maintained to have a dry catalyst surface for gas to gas catalyst contact and to have an adequate catalyst reaction rate constant in the Recombiner. At 2045 EST operators were directed to drain the Preheater (1N62B001A, EIIS Code WF) and to check the Electric Boiler (1N62D530, EIIS Code WF) with the purpose of correcting the problems associated with the Recombiner. Operators found the Electric Boiler, (EIIS Code WF) flooded and pressurized to about 280 psig.
At about 2100 EST procedure 34AB-N62-001-1S (FAILURE OF RECOMBINER AND CONTROL OF SUSTAINED COMBUSTION IN THE OFF GAS SYSTEM) was entered and a power reduction began at 100% RTP at about 2105 EST. The Steam Jet Air Ejector (SJAE, EIIS Code SH) was removed from service at about 2200 EST due to Off Gas Hydrogen concentration reaching approximately 4%. Unit 1 was scrammed to facilitate placing the Main Condenser mechanical vacuum pump in service for maintaining the Main Condenser as the heat sink. The lowest Reactor Water Level during the scram was approximately 7 inches above instrument zero and since that level was above any Engineered Safety Features (ESF) actuation settings, none were received.
The RPS actuation resulted in a rapid insertion of all control rods. As expected, following a reactor scram, void collapse occurred in the reactor core region resulting in a reduction of reactor water level. The lowest level reached during the transient was seven inches above instrument zero (165 inches above the top of the active fuel). The "B" Reactor Feedwater Pump (RFP, EIIS Code SJ) restored water level to normal. Additionally, the reactor pressure did not exceed the pre-event level of 1035 psig.
CAUSE OF EVENT
This event was the result of component failures. There are six drain lines on the Off Gas system (HIS Code WF) between the third stage Steam Jet Air Ejector and Catalytic Recombiners (1N62A001A and 1N62A001B). One of these drain lines is unique to the Catalytic Recombiner 1N62A001B that was not in service during this event. The drain line that is unique to the Catalytic Recombiner 1N62A001A that was in service was functioning as designed.
However, the four drain lines on the Off Gas system between the third stage Steam Jet Air Ejector and the Preheater (1N62B001A) were isolated due to various component failures. Each one of these drain lines has a drain pot that fills with water that actuates a level switch that in turn actuates a one inch solenoid operated valve to open and drain the line. Per the GE SIL 150 Revision 2 Supplement 1, "the system is designed to provide superheated (dry) steam to the recombiner. If the steam entering the recombiner is wet, water collects on the catalyst, inhibiting the recombining action. This results in an uncombined hydrogen/oxygen mixture passing through the recombiner with a constant increase in the hydrogen concentration in the system downstream from the recombiner." Per GE SIL 150 Revision 2 Supplement 3, "smaller sized drains are susceptible to plugging, thereby causing loss of recombination due to catalyst wetting." Although the Unit 1 drain lines are only one inch in diameter and the SIL recommends two inches, plugging was not the cause of this event. However, four of the five drain lines became isolated which resulted in the same consequences as a plugged line.
One drain line (containing valve 1N62F133A) was concluded to be isolated due to the solenoid valve being in the de- energized closed position and unable to open prior to the event. Two of the drain lines (containing valves 1N62F133B and 1N62F015A) were concluded to be isolated due to their solenoid valves being in the de-energized closed position and unable to open prior to the event and the inability of the valves' level switches (1N62N057B and 1N62N056A) to actuate at the appropriate setpoint. One drain line (containing valve 1N62F015B) was concluded to be isolated due to the inability of the valve's level switch (1N62N056B) to actuate at the appropriate setpoint.
One drain line (containing valve 1N62F126A) was concluded to be open and draining as required prior to the event.
REPORTABILITY AND SAFETY ASSESSMENT
This report is required pursuant to 10 CFR 50.73(a)(2)(iv) because a condition occurred which required a manual actuation of an engineered safety feature. Specifically, the Reactor Protection System was manually actuated as a conservative action that was not part of a pre-planned normal reactor shutdown.
The Reactor Protection System is designed to automatically shutdown the reactor by rapidly inserting the control rods into the reactor core when sensed parameters exceed predetermined setpoints. One of the parameters sensed is a Main Turbine trip or Turbine Stop Valve (TSV, HIS Code TA) closure at greater than a nominal 28 percent rated thermal power. A trip of the Main Turbine (i.e., closure of the TSVs) with the reactor at power results in a reactor pressure transient and a reactivity transient. The reactor pressure vessel and the reactor core are designed with ample margin to withstand these transients; however, when a turbine trip is anticipated conservative actions should be taken to manually shut down the reactor to avoid or lessen the transient. An automatic trip of the Main Turbine occurs on Main Condenser low vacuum to preclude damage to the Main Turbine and the Main Condenser due to an over- pressure condition.
In this event, an automatic turbine trip was not imminent; however, the RPS was manually initiated to facilitate placing the mechanical vacuum pump in service, thus preserving the main condenser as a heat sink. The reactor DOCKET FACILITY NAME (1) PAGE (3) LER NUMBER (6 � NUMBER Based on this analysis, it is concluded that this event did not adversely impact nuclear safety.
CORRECTIVE ACTIONS
pressure remained at the pre-event level of 1035 prig. As expected following the scram, void collapse occurred in the reactor core region resulting in a reactor water level transient. The lowest level reached during the transient was seven inches above instrument zero (165 inches above the top of the active fuel) before level was restored by the "B" RFP. Therefore, the reactor pressure vessel was not subjected to any abnormal pressure transients and the fuel was adequately covered during the event.
The 1N62F133A and 1N62F133B solenoid valves were replaced. The 1N62F015A solenoid coil was replaced. The 1N62N056B level switch for the 1N62F015B drain line was repaired. The 1N62N056A and 1N62N057B level switches for the 1N62F015A and 1N62F133B drain lines will be replaced during the spring 2002 refueling outage.
An interim corrective action has been established to require manually stroking the drain valves open with the control switch periodically (four of the six valves are stroked daily and two of the six drain valves are stroked weekly). An evaluation of the system will be performed to determine the reliability of the drain lines and the components associated with the drain lines by 6/01/2002. This evaluation will determine if the interim actions will continue to be necessary.
ADDITIONAL INFORMATION
No systems other than those described in this report were affected by this event.
There were no previous similar events in the last two years in which the reactor was manually scrammed because of Hydrogen concentration in the off gas system.
Master Parts List Number: 1N62F133A and 1N62F133B Type: Offgas Preheater Drain Solenoid Valves Manufacturer: Laurence Vendor Code (VPN): L087 EIIS System Code: WF EIIS Component Code: SLV Root Cause Code: X Reportable to EPIX: Yes FACILITY NAME (1) DOCKET LER NUMBER (6 PAGE (3) Master Parts List Number: 1N62F015A Type: Offgas Preheater lA Shell Drain Solenoid Valve Manufacturer: Laurence Vendor Code (VPN): L087 Model: 506WA44PSSWS EIIS System Code: WF EIIS Component Code: LSV Root Cause Code: X Reportable to EPIX: Yes Master Parts List Number: 1N62N056A and 1N62N056B Type: Offgas Preheater 1B Shell Drain Float Level Switches Manufacturer: Robertshaw Vendor Code: R290 Model: SL-326-A1X EIIS System Code: WF EIIS Component Code: LS Root Cause Code: X Reportable to EPIX: Yes Master Parts List Number: 1N62N057B Type: Condensate In Process line Float Level Switch Manufacturer: Robertshaw Vendor Code: R290 Model: SL-326-A2X EIIS System Code: WF EIIS Component Code: LS Root Cause Code: X Reportable to EPIX: Yes